the dura mater and bone in the anterior cranial fossa, and also anasto- limb of the internal capsule, optic radiation, optic tract, hippocampus
moses with a meningeal branch of the posterior ethmoidal artery. and the fimbria of the fornix.
Numerous small hypophysial branches supply the neurohypophysis, The combination of the petrous, cavernous and intracranial parts of
and are of particular importance because they form the pituitary portal the internal carotid artery is called the ‘carotid siphon’ because of its
system (see Fig. 23.11). sigmoid course (see Fig. 19.1). However, in infants, the parasellar region
of the internal carotid artery does not form a siphon but takes a rela-
Intracranial part
tively straight course (Weninger and Muller 1999). The cranial and
After piercing the dura mater, the internal carotid artery turns back sympathetic nerves therefore have different topographical relationships
below the optic nerve to run between it and the oculomotor nerve. It with the artery in the infant compared with that found in older children
reaches the anterior perforated substance at the medial end of the lateral and adults.
fissure and terminates by dividing into the anterior and middle cerebral
arteries (Fig. 19.2).
Anterior cerebral artery
Several preterminal vessels leave the cerebral portion of the internal
carotid. The ophthalmic artery arises from the anterior part of the inter-
nal carotid as it leaves the cavernous sinus, often at the point of piercing The anterior cerebral artery is the smaller of the two terminal branches
the dura, and enters the orbit through the optic canal. The posterior of the internal carotid (see Fig. 19.3).
communicating artery (Fig. 19.3) runs back from the internal carotid Surgical nomenclature divides the vessel into three parts: A – from
1
above the oculomotor nerve, and anastomoses with the posterior cer- the termination of the internal carotid artery to the junction with the
ebral artery (a terminal branch of the basilar artery), thereby contribut- anterior communicating artery; A – from the junction with the anterior
2
ing to the circulus arteriosus around the interpeduncular fossa. The communicating artery to the origin of the callosomarginal artery; and


posterior communicating artery is usually very small. However, some- A – distal to the origin of the callosomarginal artery; this segment is
3
times it is so large that the posterior cerebral artery is supplied via the also known as the pericallosal artery.
Anterior cerebral artery Optic nerve
Internal carotid artery
Middle cerebral artery
Anterior communicating artery
Infundibulum
Oculomotor nerve
Choroidal artery
Basilar artery Posterior cerebral artery
Abducens nerve
Superior cerebellar artery
Facial nerve
Labyrinthine artery
Anterior inferior cerebellar artery
Vertebral artery
Posterior inferior cerebellar artery Anterior spinal artery
Fig. 19.2 The arteries on the base of the brain. The anterior part of the left temporal lobe has been removed to display the initial course of the middle
cerebral artery within the lateral fissure.


VAsCulAR suPPly And dRAinAgE of THE bRAin
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noiTCEs
Olfactory tract
Anterior cerebral artery
Anterior communicating artery
Optic nerve
Medial striate artery
Internal carotid artery
Anteromedial group
Middle cerebral artery
Optic chiasma
Posterior communicating artery
Lateral striate group
Anterior choroidal artery
Infundibulum
Optic tract Posteromedial group
Posterolateral group
Oculomotor nerve
Posterior cerebral artery
Superior cerebellar artery
Trochlear nerve
Pontine rami Pons
Basilar artery
Fig. 19.3 The circulus arteriosus on the base of the brain showing the distribution of central (perforating or ganglionic) branches.
A B
Precentral Central Temporo-
Calcarine Parieto-occipital Pericallosal Callosomarginal artery artery occipital artery Postcentral
artery
artery artery artery artery
Anterior cerebral
artery Parietal
artery
Prefrontal
artery
Angular
artery
Middle cerebral
artery
Orbital
artery Anterior temporal
artery
Posterior Anterior Superior
cerebral artery cerebral artery Middle temporal cerebellar artery
artery
Basilar Anterior inferior
artery cerebellar artery
Vertebral Posterior inferior
artery cerebellar artery
Fig. 19.4 The major arteries supplying the medial (A) and lateral (B) aspects of the brain.
The anterior cerebral artery starts at the medial end of the stem of The two anterior cerebral arteries travel together in the great longi-
the lateral fissure. It passes anteromedially above the optic nerve to the tudinal fissure. They pass around the curve of the genu of the corpus
great longitudinal fissure where it connects with its fellow by a short callosum and then along its upper surface to its posterior end, where
transverse anterior communicating artery. The anterior communicating they anastomose with posterior cerebral arteries. They give off cortical
artery is about 4 mm in length and may be double. It gives off numer- and central branches.
ous anteromedial central branches that supply the optic chiasma, The cortical branches of the anterior cerebral artery are named


lamina terminalis, hypothalamus, para-olfactory areas, anterior columns according to their distribution. Two or three orbital branches ramify on
of the fornix and the cingulate gyrus (Fig. 19.4A). the orbital surface of the frontal lobe and supply the olfactory cortex,


Arteries of the brain
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A B
Precuneus Paracentral lobule Cingulate gyrus Medial frontal gyrus
Superior frontal gyrus Precentral gyrus Postcentral gyrus
Superior parietal lobule
Middle frontal gyrus
Inferior parietal lobule
Isthmus
Fornix
Uncus
Arcus parieto-
occipitalis
Inferior frontal gyrus Middle temporal gyrus
Superior temporal gyrus Inferior temporal gyrus Cuneus Lingual gyrus Corpus callosum Parahippocampal gyrus
Area supplied by anterior cerebral artery Area supplied by middle cerebral artery Area supplied by posterior cerebral artery
Fig. 19.5 The arteries supplying the left cerebral hemispheres. A, Lateral surface. B, Medial surface.
gyrus rectus and medial orbital gyrus. Frontal branches supply the VERTEBRAL ARTERY
corpus callosum, cingulate gyrus, medial frontal gyrus and paracentral
lobule. Parietal branches supply the precuneus, while the frontal and The vertebral arteries and their major branches (sometimes referred to
parietal branches both send twigs over the superomedial border of the as the ‘vertebrobasilar system’) essentially supply blood to the upper
hemisphere to supply a strip of territory on the superolateral surface spinal cord, the brainstem and cerebellum, and a significant but vari-
(Figs 19.4B, 19.5). Cortical branches of the anterior cerebral artery, able part of the posterior cerebral hemispheres (Fig. 19.6; see Fig. 19.5).
therefore, supply the areas of the motor and somatosensory cortices that The vertebral arteries are derived from the subclavian arteries
represent the lower limb. (p. 457). They ascend through the neck in the foramina transversaria
Central branches of the anterior cerebral artery arise from its proxi- of the upper six cervical vertebrae and enter the cranial cavity through
mal portion and enter the anterior perforated substance (see Fig. 19.3) the foramen magnum, close to the anterolateral aspect of the medulla


and lamina terminalis. Collectively, they supply the rostrum of (see Fig. 19.2). They converge medially as they ascend the medulla and
the corpus callosum, the septum pellucidum, the anterior part of the unite to form the midline basilar artery at approximately the level of
putamen, the head of the caudate nucleus and adjacent parts of the the junction between the medulla and pons.
internal capsule. Immediately proximal or distal to its junction with One or two meningeal branches arise from the vertebral artery near
the anterior communicating artery, the anterior cerebral artery gives the foramen magnum and ramify between the bone and dura mater in
rise to the medial striate artery, which supplies the anterior part of the the posterior cranial fossa. They supply bone, diploë and the falx
head of the caudate nucleus and adjacent regions of the putamen and cerebelli.
internal capsule. A small anterior spinal artery arises near the end of the vertebral
artery, and descends anterior to the medulla oblongata to unite with its
fellow from the opposite side at mid-medullary level. The single trunk
Middle cerebral artery
then descends on the ventral midline of the spinal cord, and is rein-
forced sequentially by small spinal rami from the vertebral, ascending
The middle cerebral artery is the larger terminal branch of the internal cervical, posterior intercostal and first lumbar arteries, which all enter
carotid. the vertebral canal via intervertebral foramina. Branches from the ante-
Surgical nomenclature divides the vessel into four parts: M – from rior spinal arteries and the beginning of their common trunk are dis-
1
the termination of the internal carotid artery to the bi-/trifurcation, this tributed to the medulla oblongata.
segment also being known as the sphenoidal; M – the segment running The largest branch of the vertebral artery is the posterior inferior
2
in the lateral (Sylvian) fissure, also known as the insular; M – coming cerebellar artery (see Fig. 19.6A). It arises near the lower end of the olive
3
out of the lateral fissure, also known as the opercular; and M – cortical and then ascends behind the roots of the glossopharyngeal and vagus
4


portions. nerves to reach the inferior border of the pons. Here it curves and
The middle cerebral artery runs at first in the lateral fissure, then descends along the inferolateral border of the fourth ventricle before it
posterosuperiorly on the insula, and divides into branches distributed turns laterally into the cerebellar vallecula between the hemispheres,
to the insula and the adjacent lateral cerebral surface (see Figs 19.3– and divides into medial and lateral branches. The medial branch runs
19.5). Like the anterior cerebral artery, it has cortical and central back between the cerebellar hemisphere and inferior vermis, and sup-
branches. plies both. The lateral branch supplies the inferior cerebellar surface as
Cortical branches send orbital vessels to the inferior frontal gyrus far as its lateral border and anastomoses with the anterior inferior and
and the lateral orbital surface of the frontal lobe. Frontal branches superior cerebellar arteries (from the basilar artery). The trunk of the
supply the precentral, middle and inferior frontal gyri. Two parietal posterior inferior cerebellar artery supplies the medulla oblongata
branches are distributed to the postcentral gyrus, the lower part of the dorsal to the olivary nucleus and lateral to the hypoglossal nucleus and
superior parietal lobule and the whole inferior parietal lobule. Two or its emerging nerve roots. It also supplies the choroid plexus of the
three temporal branches supply the lateral surface of the temporal lobe. fourth ventricle and sends a branch lateral to the cerebellar tonsil to
Cortical branches of the middle cerebral artery therefore supply the supply the dentate nucleus. The posterior inferior cerebellar artery is
motor and somatosensory cortices that represent the whole of the body sometimes absent.
(other than the lower limb), the auditory area and the insula. A posterior spinal artery usually arises from the posterior inferior
Small central branches of the middle cerebral artery, the lateral stri- cerebellar artery, but may originate directly from the vertebral artery
ate or lenticulostriate arteries, arise at its origin and enter the anterior near the medulla oblongata. It passes posteriorly and descends as two


perforated substance together with the medial striate artery. Lateral stri- branches, which lie anterior and posterior to the dorsal roots of the
ate arteries ascend in the external capsule over the lower lateral aspect spinal nerves. These are reinforced by spinal twigs from the vertebral,
of the lentiform complex, then turn medially, traverse the lentiform ascending cervical, posterior intercostal and first lumbar arteries, all of
complex and the internal capsule and extend as far as the caudate which reach the vertebral canal by the intervertebral foramina, and
nucleus. sustain the posterior spinal arteries to the lower spinal levels.


VAsCulAR suPPly And dRAinAgE of THE bRAin
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noiTCEs
Superior cerebellar Posterior cerebral Right posterior
artery artery cerebral artery
A B
Basilar Vertebral Posterior inferior Right vertebral artery Right superior Basilar artery
artery artery cerebellar artery cerebellar artery
Fig. 19.6 Vertebral arteriograms. A, Lateral projection. B, Towne’s projection.
Minute medullary arteries arise from the vertebral artery and its Posterior cerebral artery
branches, and are distributed widely to the medulla oblongata.
The paired posterior cerebral arteries are the terminal branches of the
basilar artery (see Figs 19.2–19.4). Surgical nomenclature divides the
BASILAR ARTERY vessel into three parts: P – from the basilar bifurcation to the junc-
1
tion with the posterior communicating artery; P – from the junction
2
The basilar artery is a large median vessel formed by the union of the with the posterior communicating artery to the portion in the peri-
vertebral arteries at the junction of the medulla and pons (see Figs mesencephalic cistern; and P – the portion that runs in the calcarine
3
19.2–19.3, 19.6). It lies in the pontine cistern, and follows a shallow fissure.
median groove on the ventral pontine surface, extending to the upper The posterior cerebral artery is larger than the superior cerebellar
border of the pons. It ends by dividing into two posterior cerebral arter- artery, from which it is separated near its origin by the oculomotor nerve
ies at a variable level behind the dorsum sellae, usually in the interpe- and, lateral to the midbrain, by the trochlear nerve. It passes laterally,
duncular cistern. parallel with the superior cerebellar artery, and receives the posterior
Numerous small pontine branches arise from the front and sides of communicating artery. It then winds round the cerebral peduncle and
the basilar artery along its course and supply the pons. The long and reaches the tentorial cerebral surface, where it supplies the temporal
slender labyrinthine (internal auditory) artery has a variable origin. It and occipital lobes. Like the anterior and middle cerebral arteries, the


usually arises from the anterior inferior cerebellar artery, but variations posterior cerebral artery has cortical and central branches.
in its origin include the lower part of the basilar artery, the superior The cortical branches of the posterior cerebral artery are named
cerebellar artery or, occasionally, the posterior inferior cerebellar artery. according to their distribution. Temporal branches, usually two, are
The labyrinthine artery accompanies the facial and vestibulocochlear distributed to the uncus and parahippocampal, medial occipitotempo-
nerves into the internal acoustic meatus and is distributed to the inter- ral and lateral occipitotemporal gyri. Occipital branches supply the
nal ear. cuneus, lingual gyrus and posterolateral surface of the occipital lobe.
The anterior inferior cerebellar artery (see Fig. 19.2) is given off from Parieto-occipital branches supply the cuneus and precuneus. The pos-
the lower part of the basilar artery and runs posterolaterally, usually terior cerebral artery supplies the visual areas of the cerebral cortex and
ventral to the abducens, facial and vestibulocochlear nerves. It com- other structures in the visual pathway.
monly exhibits a loop into the internal acoustic meatus below the The central branches supply subcortical structures. Several small
nerves, and when this occurs, the labyrinthine artery may arise from the posteromedial central branches arise from the beginning of the poste-
loop. The anterior inferior cerebellar artery supplies the inferior cerebel- rior cerebral artery (see Fig. 19.3) and, together with similar branches
lar surface anterolaterally and anastomoses with the posterior inferior from the posterior communicating artery, pierce the posterior perfo-
cerebellar branch of the vertebral artery. A few branches supply the rated substance to supply the anterior thalamus, subthalamus, lateral
inferolateral parts of the pons and occasionally also supply the upper wall of the third ventricle and globus pallidus. One or more posterior
medulla oblongata. choroidal branches pass over the lateral geniculate body and supply it
The superior cerebellar artery (see Figs 19.3, 19.6) arises near the before entering the posterior part of the inferior horn of the lateral


distal portion of the basilar artery, immediately before the formation ventricle via the lower part of the choroidal fissure. Branches also curl
of the posterior cerebral arteries. It passes laterally below the oculomo- round the posterior end of the thalamus and pass through the trans-
tor nerve, which separates it from the posterior cerebral artery, and verse fissure, or go to the choroid plexus of the third ventricle, or traverse
curves round the cerebral peduncle below the trochlear nerve to gain the upper choroidal fissure. Collectively these branches supply the
the superior cerebellar surface. Here it divides into branches that ramify choroid plexuses of the third and lateral ventricles and the fornix. Small
in the pia mater and supply this aspect of the cerebellum, and also posterolateral central branches arise from the posterior cerebral artery
anastomose with branches of the inferior cerebellar arteries. The supe- beyond the cerebral peduncle, and supply the peduncle and the poste-
rior cerebellar artery supplies the pons, pineal body, superior medullary rior thalamus, superior and inferior colliculi, pineal gland and medial
velum and tela choroidea of the third ventricle. geniculate body.


Arteries of the brain
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CIRCULUS ARTERIOSUS REGIONAL ARTERIES OF THE BRAIN
The circulus arteriosus (circle of Willis) is a large arterial anastomosis Brainstem
that unites the internal carotid and vertebrobasilar systems (see Figs
19.3, 19.6B). It lies in the subarachnoid space within the basal cisterns The medulla oblongata is supplied by branches of the vertebral, anterior
that surround the optic chiasma and infundibulum. The anterior cer- and posterior spinal, posterior inferior cerebellar and basilar arteries,
ebral arteries are derived from the internal carotid arteries and are entering along the ventral (anterior) median fissure and the posterior
linked by a small, but functionally important, anterior communicating median sulcus. Vessels that supply the central substance enter along the
artery. Posteriorly, the two posterior cerebral arteries, formed by the rootlets of the glossopharyngeal, vagus and hypoglossal nerves. There
division of the basilar artery, are joined to the ipsilateral internal carotid is an additional supply via a pial plexus from the same main arteries.
artery by a posterior communicating artery. The choroid plexus of the fourth ventricle is supplied by the posterior
There is considerable individual variation in the pattern and calibre inferior cerebellar arteries. The pons is supplied by the basilar artery
of vessels that make up the circulus arteriosus (Puchades-Orts et al and the anterior inferior and superior cerebellar arteries. Direct branches
1976). Although a complete circular channel almost always exists, one from the basilar artery enter the pons along the basilar sulcus. Other
vessel is usually sufficiently narrowed to reduce its role as a collateral vessels enter along the trigeminal, abducens, facial and vestibulococh-
route and the circle is rarely functionally complete. Cerebral and com- lear nerves and from the pial plexus. The midbrain is supplied by the
municating arteries individually may all be absent, variably hypoplas- posterior cerebral, superior cerebellar and basilar arteries. The crura


tic, double or even triple. The haemodynamics of the circle are cerebri are supplied by vessels entering on their medial and lateral sides.
influenced by variations in the calibre of communicating arteries and The medial vessels enter the medial side of the crus and also supply the
in the segments of the anterior and posterior cerebral arteries that lie superomedial part of the tegmentum, including the oculomotor
between their origins and their junctions with the corresponding com- nucleus. Lateral vessels supply the lateral part of the crus and the teg-
municating arteries. The greatest variation in calibre between individu- mentum. The colliculi are supplied by three vessels on each side from
als occurs in the posterior communicating artery, which is normally the posterior cerebral and superior cerebellar arteries. An additional
very small, so that only limited flow is possible between the anterior supply to the crura, and the colliculi and their peduncles, comes from
and posterior circulations. Commonly, the diameter of the pre- the posterolateral group of central branches of the posterior cerebral
communicating part of the posterior cerebral artery is larger than that artery.
of the posterior communicating artery, in which case the blood supply
to the occipital lobes is mainly from the vertebrobasilar system. Cerebellum
However, sometimes the diameter of the pre-communicating part of the
posterior cerebral artery is smaller than that of the posterior communi-
The cerebellum is supplied by the posterior inferior, anterior inferior
cating artery, in which case the blood supply to the occipital lobes is
and superior cerebellar arteries. The cerebellar arteries form superficial
mainly from the internal carotids via the posterior communicating
anastomoses on the cortical surface. Anastomoses between deeper, sub-
arteries. Since the primary purpose of the vascular circle is to provide
cortical, branches have been postulated (Duvernoy et al 1983).
anastomotic channels if one vessel is occluded, it is important to note
that a normal-sized posterior communicating artery cannot usually
Optic chiasma, tract and radiation
fulfil this role. Agenesis or hypoplasia of the initial segment of the
anterior cerebral artery is more frequent than anomalies in the anterior
communicating artery and contribute to defective circulation in about The blood supply to the optic chiasma, tract and radiation is of consid-


one-third of individuals. erable clinical importance. The chiasma is supplied in part by the
anterior cerebral arteries but its median zone depends upon rami from
the internal carotid arteries reaching it via the stalk of the hypophysis.
The anterior choroidal and posterior communicating arteries supply the
CENTRAL OR PERFORATING
optic tract, and the optic radiation receives blood through deep branches
ARTERIES
of the middle and posterior cerebral arteries.
Numerous small central (perforating or ganglionic) arteries arise from Diencephalon
the circulus arteriosus or from vessels near it (see Fig. 19.3). Many of
these enter the brain through the anterior and posterior perforated
The thalamus is supplied chiefly by branches of the posterior commu-
substances. Central branches supply nearby structures on or near the
nicating, posterior cerebral and basilar arteries (Plets et al 1970). A
base of the brain together with the interior of the cerebral hemisphere,
contribution from the anterior choroidal artery is often noted but this
including the internal capsule, basal ganglia and thalamus. These
has been disputed. The medial branch of the posterior choroidal artery
branches form four principal groups. The anteromedial group arises
supplies the posterior commissure, habenular region, pineal gland and
from the anterior cerebral and anterior communicating arteries, and
medial parts of the thalamus, including the pulvinar. Small central
passes through the medial part of the anterior perforated substance.
branches, which arise from the circulus arteriosus and its associated
These arteries supply the optic chiasma, lamina terminalis, anterior,
vessels, supply the hypothalamus. The pituitary gland is supplied by
preoptic and supraoptic areas of the hypothalamus, septum pelluci-
hypophysial arteries derived from the internal carotid artery, and the
dum, para-olfactory areas, anterior columns of the fornix, cingulate
anterior cerebral and anterior communicating arteries supply the
gyrus, rostrum of the corpus callosum and the anterior part of the
lamina terminalis.
putamen and the head of the caudate nucleus. The posteromedial
The choroid plexuses of the third and lateral ventricles are supplied
group comes from the entire length of the posterior communicating


by branches of the internal carotid and posterior cerebral arteries.
artery and from the proximal portion of the posterior cerebral artery.
Anteriorly, these arteries supply the hypothalamus and pituitary gland,
Basal ganglia
and the anterior and medial parts of the thalamus via thalamo-
perforating arteries. Caudally, branches of the posteromedial group
supply the mammillary bodies, subthalamus, the lateral wall of the The majority of the arterial supply to the basal ganglia comes from the
third ventricle, including the medial thalamus, and the globus pallidus. striate arteries, which are branches from the roots of the anterior and
The anterolateral group is mostly comprised of branches from the middle cerebral arteries. They enter the brain through the anterior per-
proximal part of the middle cerebral artery that are also known as forated substance and also supply the internal capsule. The caudate
striate, lateral striate or lenticulostriate arteries. They enter the brain nucleus receives blood additionally from the anterior and posterior
through the anterior perforated substance and supply the posterior choroidal arteries. The posteroinferior part of the lentiform complex is
striatum, lateral globus pallidus and the anterior limb, genu and pos- supplied by the thalamostriate branches of the posterior cerebral artery.
terior limb of the internal capsule. The medial striate artery, derived The anterior choroidal artery, a preterminal branch of the internal
from the middle or anterior cerebral arteries, supplies the rostral part carotid artery, contributes to the blood supply of both segments of the
of the caudate nucleus and putamen, and the anterior limb and genu globus pallidus and the caudate nucleus. Famously, the ligation of this
of the internal capsule. The posterolateral group is derived from the vessel during a neurosurgical procedure on a patient suffering from
posterior cerebral artery distal to its junction with the posterior com- Parkinson’s disease led to alleviation of the parkinsonian symptoms,
municating artery, and supplies the cerebral peduncle, colliculi, pineal presumably as a consequence of infarction of the globus pallidus. This
gland and, via thalamogeniculate branches, the posterior thalamus and chance observation led to the initiation of pallidal surgery (pallidot-


medial geniculate body. omy) for this condition (see also p. 371).


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noiTCEs
Internal capsule cortical areas involved in complex thought processes. The principal
local factors affecting regional blood flow are the local hydrogen ion
(H+) or carbon dioxide concentrations, which cause arterial dilation
The internal capsule is supplied by central, or perforating, arteries that
both by a direct action on the endothelium and through indirect media-
arise from the circulus arteriosus and its associated vessels. These
tors, such as nitric oxide.
include the lateral and medial striate arteries that come from the middle
Not all substances circulating in arterial blood have access to the
and anterior cerebral arteries and also supply the basal ganglia. The
brain parenchyma. Particulate matter, such as bacteria, is excluded. In
lateral striate arteries supply the anterior limb, genu and much of the
general, lipophilic molecules and small molecules, such as oxygen and
posterior limb of the internal capsule and are commonly involved in
carbon dioxide, can cross the blood–brain barrier but hydrophilic ones
ischaemic and haemorrhagic stroke. One of the larger striate branches
(excluding glucose) cannot. The cellular basis for the blood–brain
of the middle cerebral artery is known as ‘Charcot’s artery of cerebral
barrier is discussed on page 50.
haemorrhage’. The medial striate artery, a branch of the proximal part
of either the middle or anterior cerebral artery, supplies the anterior
Ischaemic stroke Stroke refers to the clinical syndrome of a rapidly
limb and genu of the internal capsule and the basal ganglia. The ante-
developing focal neurological deficit that is not due to seizure activity.
rior choroidal artery also contributes to the supply of the ventral part
If the cause is lack of, or reduced, blood supply to a portion of the
of the posterior limb and the retrolenticular part of the internal capsule.
brain, then the term ischaemic stroke (Fig. 19.7) is used, as opposed
Cerebral cortex
The entire blood supply of the cerebral cortex comes from cortical
branches of the anterior, middle and posterior cerebral arteries (see Figs
19.4–19.5; Duvernoy et al 1981). In general, long branches traverse the


cortex and penetrate the subjacent white matter for 3 or 4 cm without
communicating. Short branches are confined to the cerebral cortex and
form a compact network in the middle zone of the grey matter, whereas
the outer and inner zones are sparingly supplied. Although adjacent
vessels anastomose on the surface of the brain, they become end arteries
as soon as they enter it. In general, superficial anastomoses only occur
between microscopic branches of the cerebral arteries, and there is little
evidence that they can provide an effective alternative circulation after
the occlusion of larger vessels.
The lateral surface of the hemisphere is mainly supplied by the
middle cerebral artery. This includes the territories of the motor and
somatosensory cortices, which represent the whole of the body, apart
from the lower limb, and also the auditory cortex and language areas.
The anterior cerebral artery supplies a strip next to the superomedial
border of the hemisphere, as far back as the parieto-occipital sulcus.
The occipital lobe and most of the inferior temporal gyrus (excluding
the temporal pole) are supplied by the posterior cerebral artery.
Stenosis
Medial and inferior surfaces of the hemisphere are supplied by the
anterior, middle and posterior cerebral arteries. The area supplied by
the anterior cerebral artery is the largest; it extends almost to the parieto-
occipital sulcus and includes the medial part of the orbital surface. The
rest of the orbital surface and the temporal pole are supplied by the
middle cerebral artery, and the remaining medial and inferior surfaces
are supplied by the posterior cerebral artery.
Near the occipital pole, the junctional zone between the territories
of the middle and posterior cerebral arteries corresponds to the visual
(striate) cortex, which receives information from the macula. When the
posterior cerebral artery is occluded, a phenomenon known as ‘macular A
sparing’ may occur, in which vision with the central part of the retina
is preserved. Collateral circulation of blood from branches of the
middle cerebral artery into those of the posterior cerebral artery may
account for this phenomenon. In addition, in some individuals, the
middle cerebral artery may itself supply the macular area.
CEREBRAL BLOOD FLOW
The brain is devoid of either glucose stores or a means of storing oxygen
Infarction
and is, therefore, dependent minute by minute on an adequate blood


supply. It has a high metabolic rate in comparison to other organs,
which reflects the metabolic demands of constant neural activity. The
blood supply of grey matter is more copious than that of white matter.
Cerebral blood flow in the human brain is approximately
50 ml g−1 min−1. Global cerebral blood flow is autoregulated, i.e. it
remains constant in normal individuals despite variations in mean
arterial blood pressure over a range of 8.7–18.7 kPa (65–140 mmHg).
If the blood pressure falls below this range, cerebral blood flow
decreases. Alternatively, if the pressure rises above this range, cerebral B
blood flow may increase. Arterial and arteriolar intraluminal pressure
directly control contraction of intramural muscle; e.g. an increase in
Fig. 19.7 A, A frontal projection of a selective catheter angiogram of the
arterial pressure causes arterial constriction, and blood flow remains
left internal carotid artery, showing a focal narrowing at the termination of
constant. the left internal carotid artery and the origin of the left middle cerebral
Although autoregulation normally ensures that global cerebral artery. B, A coronal T2-weighted image of the brain at the level of the
blood flow remains constant, regional blood flow varies in response to basal ganglia. There is an abnormal high signal in the left putamen and
the level of neural activity and, thus, to local metabolic demand. This adjacent white matter, consistent with infarction. It is likely that the
has been demonstrated for many brain areas including the motor and infarction was due to occlusion of small perforating vessels originating in
sensory cortical regions, areas involved in convulsive activity and even the area of the stenosis and supplying the basal ganglia.


Vascular supply and drainage of the brain
286.e1
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It is difficult to make truly quantifiable measurements of regional
cerebral blood flow, particularly in young children, but this has been
attempted using, for example, 133Xe single-photon emission computed
tomography (SPECT). These studies have shown that regional cerebral
blood flow to the cortex in newborns tends to be considerably lower
than that in adults; there is a continuing increase through infancy and
by the age of 5–6 years the cortical flow is at least 50% higher than in
adults (Chiron et al 1992). After that age, the cortical blood flow
decreases and adult levels are attained at between 15 and 19 years. It
should be appreciated that cerebral blood flow, in its strictest sense of
microvascular perfusion, cannot be measured by less invasive methods
such as ultrasonography. That said, it is possible, using these tech-
niques, to estimate blood velocity and/or macroscopic bulk flow in
large vessels such as the internal carotid artery or middle cerebral artery,
and to demonstrate a significant increase in blood flow to the brain
with increasing gestational age in premature and term babies.


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to haemorrhagic stroke, e.g. subarachnoid haemorrhage. The lack of
blood flow can be due to pathology in the vessel lumen, such as throm-
bosis or embolus (common); pathology outside the blood vessel, such
as occlusion from the mass effect of a tumour or haematoma (rare); or
pathology of the vessel wall, such as inflammatory or infective arter-
itides (rare). The symptoms and signs of ischaemic stroke depend on
the location and extent of the arterial infarction (Bogousslavsky and
Caplan 2002). In certain locations, even a small-volume stroke can have
devastating effects. For example, the internal capsule and most of the
adjacent basal ganglia are supplied by small perforating arteries, which
are a common site for ischaemic stroke. Corticonuclear and corticospi-
nal motor pathways and third-order thalamocortical sensory fibres all
pass through the internal capsule, so that it is effectively the only route
connecting the cerebral cortex with other regions of the neuraxis. The
neurological deficits that result from stroke in the internal capsule,
therefore, may include contralateral spastic hemiparesis, contralateral
hemisensory loss and psychological deficits.
It is possible for a major vessel, such as the internal carotid artery in
the neck, to become occluded and for the individual to remain asymp-
tomatic, providing an appropriate flow of blood can be recruited to
supply the affected brain region from other sources (collateral supply).
The primary role of the circulus arteriosus is to allow for this eventual-
ity by redirecting blood from other sources, such as the contralateral
internal carotid and vertebrobasilar system in the example given above.
If the circulus arteriosus is complete, two or even three of the four
major arteries supplying the brain can occlude without producing an
ischaemic stroke. Unfortunately, the circulus arteriosus is rarely haemo-
dynamically complete; ischaemic stroke is, therefore, often caused
because the collateral supply is inadequate. For example, an individual Aneurysm Posterior communicating artery
in whom neither anterior nor posterior communicating arteries are Fig. 19.8 An intra-arterial digital subtraction angiogram of the right


patent will suffer an ischaemic stroke if there is occlusion of one inter- internal carotid artery in a patient with a complete right oculomotor nerve
nal carotid artery because the proximal anterior and middle cerebral palsy, lateral projection.
arteries cannot receive collateral flow from the incomplete circulus
arteriosus. Some collateral flow may be available from anastomoses
between distal branches of the external carotid artery (e.g. middle
meningeal artery) but invariably this is not sufficient to supply the
whole hemisphere.
Somewhat paradoxically, the more distally the occlusion occurs in
the cerebral vasculature, the more likely it is to cause an ischaemic
stroke (although the volume of infarcted tissue will be less). This is Subarachnoid Subarachnoid
because the major capacity to form collateralization is proximal, mainly haemorrhage haemorrhage in
at the circulus arteriosus. For example, an embolus formed in the proxi- in suprasellar interhemispheric
mal internal carotid artery may travel to the M portion of the ipsilateral cistern fissure
1
middle cerebral artery and occlude the vessel. There is very little scope Subarachnoid
for collateralization at this site and an ischaemic stroke is unavoidable, haemorrhage in
unless the embolus can be cleared and flow restored in good time. Sylvian fissure
Watershed areas Regions of the brain lying at the extreme edges of
the major cerebral arterial territories are called watershed areas; they are
the first to be deprived of sufficient blood flow in the event of cerebral
hypoperfusion. Ischaemic infarcts of the cortex and adjacent subcortical
white matter in the border zones between these territories are known
as watershed, boundary-zone or border-zone infarcts. There is some
evidence that a second arterial border zone, the internal arterial border
zone, exists within the white matter of the centrum semiovale and Fig. 19.9 An axial CT image (without intravenous contrast agent) at the
corona radiata (between the lenticulostriate perforators and the deep level of the posterior clinoids. There is abnormally high attenuation in the
penetrating branches of the distal middle cerebral artery). basal cisterns and subarachnoid space of the interhemispheric fissure,


indicating an acute subarachnoid haemorrhage, subsequently shown to
Cerebral aneurysms Aneurysms are balloon-like swellings that be due to a ruptured aneurysm on the anterior communicating artery.
occur on arteries as a result of defects in the vessel wall. They are most
commonly found on the vessels of the circulus arteriosus, particularly
at or near the junctions of vessels. Aneurysms on the internal carotid an X-ray CT examination, which has a very high sensitivity for detecting
artery near its termination may compress the lateral aspect of the optic fresh haemorrhage. Blood will be seen in the basal cisterns and entering
chiasma and compromise axons derived from the temporal side of the the depths of the cortical sulci, i.e. delineating the anatomy of the
ipsilateral retina, which causes a defect in the nasal visual field. Aneu- subarachnoid space, a presentation that permits an accurate diagnosis
rysms in the vicinity of the oculomotor nerve, e.g. on the posterior (Fig. 19.9). In marked contrast, the spread of blood in a subdural
communicating artery, superior cerebellar artery or the tip of the basilar haemorrhage is limited by the arachnoid mater on its deep surface, and
artery, can cause damage to the oculomotor nerve by compression the blood therefore remains on the surface of the brain.
(Fig. 19.8, Video 19.1).
Arteriovenous malformations Arteriovenous malformations
Subarachnoid haemorrhage Bleeding into the subarachnoid (AVMs) of the brain are developmental abnormalities of the blood
compartment, subarachnoid haemorrhage, is the most common pathol- vessels supplying the brain and are thought to occur in 1–2/10,000
ogy that involves the subarachnoid space (Sengupta and McAllister people. In the normal brain, blood flows from a cerebral artery into a
1986). There are many causes: in adults, the most common is rupture capillary system and then into a draining vein(s). When an AVM is
of an aneurysm of the intracranial vessels that run within the subarach- present, arterial blood passes into a tangle (nidus) of abnormal blood


noid space. A person who has had a subarachnoid haemorrhage usually vessels that allows passage into the venous system without passing
complains of a headache of very sudden onset that is frequently through normal capillaries (Fig. 19.10). As a result, there is often rapid,
described as being their ‘worst ever headache’ or ‘like being hit on the high-volume blood flow through the nidus, which, coupled with
head with a hammer’. When this is suspected, the first investigation is abnormal vessel walls, leads to a high risk of rupture. The resulting


VAsCulAR suPPly And dRAinAgE of THE bRAin
288
3
noiTCEs
haemorrhage from a cerebral AVM is associated with high mortality/ VEINS OF THE CEREBRAL HEMISPHERE
morbidity, and recurrent haemorrhages are common in survivors. Not
all brain AVMs present with rupture but they may cause headaches, Superficial and deep cerebral veins drain the surfaces and the interior
seizures or focal neurological deficits and some brain AVMs are discov- of the cerebral hemisphere respectively (Figs 19.11–19.14). The super-
ered incidentally. There are several treatment options for brain AVMs, ficial veins may be divided into three groups, namely: superior, middle
including surgical resection, endovascular obliteration and stereotactic and inferior. Between 8 and 12 superior cerebral veins drain the supero-
radiosurgery. In some circumstances, conservative management is lateral and medial surfaces of each hemisphere. They mainly follow the
appropriate. sulci, although some pass across gyri. They ascend to the superomedial
border of the hemisphere, where they receive small veins from the
medial surface, and then open into the superior sagittal sinus. Superior
VEINS OF THE BRAIN
cerebral veins in the anterior part of the hemisphere join the sinus
almost at right angles. The larger posterior veins are directed obliquely
The venous drainage of the brain occurs through a complex system of forwards, against the direction of flow in the sinus, an arrangement that
deep and superficial veins (Andeweg 1996) devoid of valves and with may resist their collapse when intracranial pressure is raised.
thin walls that lack muscular tissue. They pierce the arachnoid mater The superficial middle cerebral vein drains most of the lateral surface
and the inner layer of the dura mater to open into the dural venous of the hemisphere, and follows the lateral fissure to end in the cavern-
sinuses. ous sinus. A superior anastomotic vein runs posterosuperiorly between
VEINS OF THE POSTERIOR FOSSA
Thalamostriate Superior anastomotic vein Superior sagittal sinus
The veins of the brainstem form a superficial venous plexus deep to the and choroidal veins
arteries.
Anterior


Veins of the medulla oblongata drain into the veins of the spinal cerebral vein Inferior sagittal
cord or the adjacent dural venous sinuses, or into variable radicular sinus
veins that accompany the last four cranial nerves to either the inferior Internal
petrosal or occipital sinuses, or to the superior bulb of the jugular vein. cerebral
Anterior and posterior median medullary veins may run along the vein
ventral (anterior) median fissure and posterior median sulcus, to
Great
become continuous with the spinal veins in corresponding positions.
cerebral
Pontine veins, which may include a median vein and a lateral vein on vein
each side, drain into the basal vein, cerebellar veins, the petrosal or
Basal vein
transverse sinuses or the venous plexus of the foramen ovale. Veins
of the midbrain join the great cerebral vein (of Galen) or the basal Superficial middle Straight
vein. cerebral vein sinus
The veins of the cerebellum drain mainly into sinuses adjacent to
them or, from the superior surface, into the great cerebral vein. The Deep middle Inferior
cerebral vein anastomotic
cerebellar veins course on the cerebellar surface, and consist of supe-
vein
rior and inferior groups. Superior cerebellar veins either run anterome- Cavernous sinus
Transverse sinus
dially across the superior vermis to the straight sinus or great cerebral
Superior petrosal sinus
vein, or they run laterally to the transverse and superior petrosal Occipital sinus
sinuses. Inferior cerebellar veins include a small median vessel that Inferior petrosal sinus Sigmoid sinus
runs backwards on the inferior vermis to enter the straight or sigmoid Fig. 19.11 The cerebral venous system showing the principal superficial
sinus. Laterally coursing vessels join the inferior petrosal and occipital and deep veins of the brain and their relationship to the dural venous
sinuses. sinuses, viewed from the left side.
Vein (Rolandic) of the central sulcus
Superior sagittal sinus Superior anastomotic vein
(Trolard)
Superior cerebral veins
Superior cerebral veins
Inferior anastomotic vein
(Labbé)
Inferior cerebral veins
Inferior cerebral veins


Superficial middle cerebral vein Straight sinus
Occipital sinus
Temporal cerebral veins
Inferior petrosal sinus Transverse sinus
Cerebellar veins
A Superior petrosal sinus Sigmoid sinus
Fig. 19.12 A, Veins and sinuses of the brain, viewed from the left side. Continued


Vascular supply and drainage of the brain
288.e1
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A B
C D
Fig. 19.10 Selective catheter angiograms showing an arteriovenous malformation (AVM) in the left cerebral hemisphere. A, Injection of contrast medium
into the left internal carotid artery shows the nidus of an AVM being supplied by enlarged posterior branches of the middle cerebral artery (arrow). B, An
image taken one-third of a second later shows better filling of the nidus (arrow) and early filling of a draining vein (arrowheads). C, One second after the
original image was taken, abnormally large cortical veins can be seen draining towards the superior sagittal sinus. D, Two seconds after the original, the
superior sagittal sinus is opacified but the remainder of the dural venous system is not filled with contrast, indicating rapid shunting through the AVM.


Veins of the brain
289
91
RETPAHC
Anterior intercavernous sinus
Ophthalmic vein Anterior cerebral vein
Sphenoparietal sinus
Sphenoparietal sinus
Superficial middle cerebral vein
Superficial middle cerebral vein
Deep middle cerebral vein
Cavernous sinus
Basal vein (of Rosenthal)
Superior petrosal sinus Posterior intercavernous sinus
Inferior petrosal sinus
Sigmoid sinus
Transverse sinus Straight sinus
Cerebellar veins
B
Confluence of sinuses
Fig. 19.12, cont’d B, Veins and sinuses on the anterior (ventral) surface of the hemisphere. The cerebellum, pons and caudal midbrain have been
removed. For clarity, the petrosal sinuses are shown only on the left and the basal vein (of Rosenthal) only on the right sides of the figure. In life, these
vessels are bilateral. (With permission from Haines, D, Fundamental Neuroscience for Basic and Clinical Applications, 4th Edition, 2013, Saunders,
Elsevier.)
by the anterior cerebral artery and the central branches that enter the
anterior perforated substance. The basal veins pass back alongside the
interpeduncular fossa and midbrain, receive tributaries from this vicin-
ity and join the great cerebral vein.
Corpus callosum
Inferior cerebral veins on the orbital surface of the frontal lobe join
Septum
the superior cerebral veins and thus drain to the superior sagittal sinus.
pellucidum
Those on the temporal lobe anastomose with basal veins and middle
Caudate cerebral veins, and drain to the cavernous, superior petrosal and trans-
Thalamostriate nucleus verse sinuses.
vein The basal vein begins at the anterior perforated substance by the
union of a small anterior cerebral vein, which accompanies the anterior
cerebral artery; a deep middle cerebral vein, which receives tributaries
Thalamus
Choroidal from the insula and neighbouring gyri, and runs in the lateral cerebral
Internal vein fissure; and striate veins, which emerge from the anterior perforated
cerebral vein substance. The basal vein passes back round the cerebral peduncle to
Choroid
the great cerebral vein and receives tributaries from the interpeduncular
plexus


fossa, inferior horn of the lateral ventricle, parahippocampal gyrus and
Basal vein midbrain.
The internal cerebral vein drains the deep parts of the hemisphere
and the choroid plexuses of the third and lateral ventricles. It is formed
Great near the interventricular foramen, behind the column of the fornix,
cerebral vein primarily by union of the thalamostriate and choroidal veins, although
numerous smaller veins from surrounding structures also converge
here. The thalamostriate vein runs anteriorly, between the caudate
nucleus and thalamus, and receives many tributaries from both. The
choroidal vein runs a convoluted course along the whole choroid
plexus, and receives veins from the hippocampus, fornix, corpus cal-
losum and adjacent structures. After their formation, the two internal
Fig. 19.13 The internal (deep) cerebral veins, viewed from above after cerebral veins travel back parallel to one another beneath the splenium
removal of the central portion of the corpus callosum. of the corpus callosum, where they unite to form the great cerebral vein.
The great cerebral vein is a short median vessel that curves sharply up
around the splenium of the corpus callosum and opens into the ante-
the superficial middle cerebral vein and the superior sagittal sinus, thus rior end of the straight sinus after receiving the right and left basal veins.
connecting the superior sagittal and cavernous sinuses. An inferior anas-
tomotic vein courses over the temporal lobe and connects the superfi- Intracranial venosinus thrombosis One of the most potentially
cial middle cerebral vein to the transverse sinus. The deep middle serious pathologies affecting the intracranial venous system is veno-
cerebral vein drains the insular region and joins the anterior cerebral sinus thrombosis (Fig. 19.15). In this condition, blood clots within the
and striate veins to form a basal vein. Regions drained by the anterior dural sinuses and/or intracranial veins, and the resulting obstruction of
cerebral and striate veins correspond approximately to those supplied venous drainage may produce life-threatening complications.


Vascular supply and drainage of the brain
289.e1
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RETPAHC
In many cases, the cause of the venosinus thrombosis is not found; Clinicians distinguish between thrombosis that affects the ‘superfi-
trauma involving the dural sinuses and infections in the vicinity of cial’ and ‘deep’ venous systems, although they may occur together (see
the sinuses (e.g. mastoiditis) present significant risk. Conditions that Fig. 19.11). Superficial thrombosis usually occurs in the superior sagittal
increase the coagulability of the blood, such as the oral contraceptive sinus (with or without involvement of the cortical veins), whilst deep
pill, pregnancy, dehydration and inborn abnormalities of the coagula- intracranial thrombotic disease usually affects the great cerebral vein
tion pathways, are also risk factors. and the straight sinus. The parts of the cerebral hemispheres affected
Individuals with intracranial venosinus thrombosis at its most severe are different in the two situations: superficial thrombosis affects the
present moribund, in coma or with major seizure activity; in these situ- cerebral hemispheres (particularly the white matter) and deep throm-
ations the prognosis is poor. The clot within the dural sinuses and/or bosis affects the thalami and basal ganglia. However, one common
cortical veins obstructs the drainage of venous blood from the brain feature is the tendency for bilateral involvement because all of the main
and causes venous hypertension. As the pressure in the venous system draining venous channels are in the midline and accept venous drain-
approaches the arterial pressure, the blood flow perfusing the affected age from both hemispheres.
parts of the brain decreases. If it dips below a critical level, the brain If the extent of the intracranial venosinus thrombosis is less severe,
tissue will become ischaemic and may die (venous infarction). As or if alternative venous drainage has been accommodated by reasonable
the arterial supply to the brain is still present, the infarcted areas of collateral channels, the patient may have more modest symptoms such
the brain frequently undergo major haemorrhage and this is often the as headaches or visual disturbance. Those symptoms are due to raised
factor that kills the patient. intracranial pressure and, if untreated, may become chronic.
A B


Fig. 19.15 Venosinus thrombosis. Images from a magnetic resonance study in an adult patient with a 6-day history of headache and increasingly
severe seizures. A, The sagittal T1-weighted image shows abnormal high signal along the course of the superior sagittal sinus (arrows). B, The axial
T2-weighted image also shows high signal in the enlarged superior sagittal sinus (arrow). This pattern of abnormal signal indicates extensive subacute
thrombus in the sinus. There are abnormal high signal changes in the left paracentral lobule (arrowheads), indicating venous infarction. This was shown
to be bilateral and haemorrhagic on other images.


VAsCulAR suPPly And dRAinAgE of THE bRAin
290
3
noiTCEs
Superior
sagittal sinus
Cortical
Superior sagittal
veins
sinus
Great cerebral
vein
Straight
sinus
Sigmoid Confluence of Transverse
sinus sinuses sinus Confluence of
sinuses
Sigmoid
A Transverse sinus
sinus
B
C veo inrt sical S siu np ue srior sagittal I sn af ge ir ti to ar
l sinus
S s sa iu ngp uie t st r aio lr
Internal
cerebral vein
Confluence
of sinuses
Sphenoidal
Sigmoid sinus
sinus
D Atlanto-axial
joint
Transverse
sinus
Confluence of
C
sinuses
Fig. 19.14 Phase contrast magnetic resonance venograms taken at 3.0T in lateral (A), frontal (B) and superior (C) projections. D, A CT venogram,
midsagittal section.
Bonus e-book images and video
Fig. 19.10 Selective catheter angiograms Fig. 19.15 Venosinus thrombosis. Video 19.1 Rotational angiography of an
showing an arteriovenous malformation intracranial aneurysm.
(AVM) in the left cerebral hemisphere.
KEY REFERENCES
Andeweg J 1996 The anatomy of collateral venous flow from the brain and Plets C, De Reuck J, Vander Eecken H et al 1970 The vascularization of the
its value in aetiological interpretation of intracranial pathology. Neuro- human thalamus. Acta Neurol Belg 70:687–770.
rad 38:621–8. A detailed analysis of the blood supply to the thalamus.
A brief historical review is included.
Puchades-Orts A, Nombela-Gomez M, Ortu-o-Pacheco G 1976 Variation in
Bogousslavsky J, Caplan L 2002 Stroke Syndromes, 2nd ed. Cambridge: form of the circle of Willis. Some anatomical and embryological con-
Cambridge University Press. siderations. Anat Rec 185:119–23.
A classic textbook encompassing all aspects of stroke. A postmortem study of 62 cases.


Duvernoy H, Delon S, Vannson JL 1983 The vascularization of the human Sengupta RP, McAllister VL 1986 Subarachnoid Haemorrhage. Berlin:
cerebellar cortex. Brain Res Bull 11:419–80. Springer.
A detailed study of pial and intracortical vessels of the cerebral cortex. Details on variations in the circle of Willis are included.
Duvernoy HM, Delon S, Vannson JL 1981 Cortical blood vessels of the Weninger WJ, Muller GB 1999 The parasellar region of infants: cavernous
human brain. Brain Res Bull; 7:519–79. sinus topography and surgical approaches. J Neurosurg 90:484–90.
A detailed study of pial and intracortical vessels of the cerebellar cortex. A postmortem study highlighting the differences between the vascular
anatomy of infants and adults.
Kaplan HA, Ford DH 1966 The Brain Vascular System. Amsterdam:
Elsevier.
A detailed overview of the subject.


Vascular supply and drainage of the brain
290.e1
91
RETPAHC
REFERENCES
Andeweg J 1996 The anatomy of collateral venous flow from the brain and Plets C, De Reuck J, Vander Eecken H et al 1970 The vascularization of the
its value in aetiological interpretation of intracranial pathology. Neuro- human thalamus. Acta Neurol Belg 70:687–770.
rad 38:621–8. A detailed analysis of the blood supply to the thalamus.
A brief historical review is included.
Puchades-Orts A, Nombela-Gomez M, Ortu-o-Pacheco G 1976 Variation in
Bogousslavsky J, Caplan L 2002 Stroke Syndromes, 2nd ed. Cambridge: form of the circle of Willis. Some anatomical and embryological con-
Cambridge University Press. siderations. Anat Rec 185:119–23.
A classic textbook encompassing all aspects of stroke. A postmortem study of 62 cases.
Chiron C, Raybaud C, Maziere B et al 1992 Changes in regional cerebral Ray B, Gupta N 2006 Caroticoclinoid foramen with interclinoid osseous bar.
blood flow during brain maturation in children and adolescents. Clin Anat 19:732–3.
J Nuclear Med 33:696–703. Sengupta RP, McAllister VL 1986 Subarachnoid Haemorrhage. Berlin:
Duvernoy H, Delon S, Vannson JL 1983 The vascularization of the human Springer.
cerebellar cortex. Brain Res Bull 11:419–80. Details on variations in the circle of Willis are included.
A detailed study of pial and intracortical vessels of the cerebral cortex.
Weninger WJ, Muller GB 1999 The parasellar region of infants: cavernous
Duvernoy HM, Delon S, Vannson JL 1981 Cortical blood vessels of the sinus topography and surgical approaches. J Neurosurg 90:484–90.
human brain. Brain Res Bull; 7:519–79. A postmortem study highlighting the differences between the vascular
A detailed study of pial and intracortical vessels of the cerebellar cortex. anatomy of infants and adults.


Kaplan HA, Ford DH 1966 The Brain Vascular System. Amsterdam:
Elsevier.
A detailed overview of the subject.


CHAPTER
20
Spinal cord: internal organization
The spinal cord provides innervation for the trunk and limbs via spinal Dorsal median septum
nerves and their peripheral ramifications. It receives primary afferent
Cervical
fibres from peripheral receptors located in widespread somatic and
visceral structures, and sends motor axons to skeletal muscle. It also
contains the cell bodies of all the preganglionic neurones responsible Tract of
for the sympathetic innervation of cardiac and smooth muscle and Lissauer
secretory glands, and for the parasympathetic innervation of smooth
muscle in the distal part of the hindgut, the pelvic viscera and the
erectile tissues of the external genitalia. Many bodily functions are regu-
lated at an unconscious level by intraspinal reflex connections between
afferent and efferent neurones. Profuse ascending and descending path-
ways link the spinal cord with the brain, allowing higher centres to
monitor and perceive external and internal stimuli and to modulate
and control spinal efferent activity.
A
EXTERNAL FEATURES AND RELATIONS Ventral median fissure
The topographical anatomy of the spinal cord and its external features Dorsal funiculus Thoracic
and relations are described in more detail in Chapter 45. In brief, the
cord lies within the vertebral canal. It is continuous rostrally with the
medulla oblongata, just below the level of the foramen magnum and
it terminates caudally as the conus medullaris, which is continuous
with the filum terminale and is anchored to the dorsum of the coccyx.
The cord is ensheathed by spinal meninges that are continuous with
the cranial meninges through the foramen magnum. Although it is
approximately circular in cross-section, the diameter of the spinal Lateral funiculus
cord varies according to level; it bears two enlargements, cervical and
B
lumbar.
Ventral funiculus
The spinal cord is essentially a segmental structure, giving rise to 31
bilaterally paired spinal nerves. These attach to the cord as a linear series
Dorsal and ventral
of smaller dorsal and ventral nerve rootlets. Dorsal rootlets contain Lumbar
commissures
afferent nerve fibres and ventral rootlets contain efferent fibres (see Fig.
Dorsal horn


16.2). Groups of adjacent rootlets coalesce to form dorsal or ventral
nerve roots that cross the subarachnoid space and unite to form func-
tionally mixed spinal nerves as they pass through the intervertebral
foramina. The dorsal roots bear dorsal root ganglia that contain the cell
bodies of primary afferent neurones.
Central
INTERNAL ORGANIZATION canal
In transverse section, the spinal cord is incompletely divided into sym-
metrical halves by a dorsal (posterior) median septum and a ventral
(anterior) median fissure (sulcus) (Fig. 20.1). It consists of an outer
layer of white matter and an inner core of grey matter; their relative
sizes and configuration vary according to level. The amount of grey C
Ventral horn
matter reflects the number of neuronal cell bodies present; it is propor-
tionately largest in the cervical and lumbar enlargements, which contain
the neurones that innervate the limbs. The absolute amount of white Sacral
matter is greatest at cervical levels and decreases progressively at lower
levels because descending tracts shed fibres as they descend and ascend-
ing tracts accumulate fibres as they ascend.
A diminutive central canal, lined by columnar, ciliated epithelium
(ependyma) and containing cerebrospinal fluid (CSF), extends the
whole length of the spinal cord, lying in the centre of the spinal grey
matter. Rostrally, the central canal extends into the caudal half of the
medulla oblongata and then opens into the fourth ventricle.
D
SPINAL GREY MATTER Fig. 20.1 Transverse sections through the spinal cord at representative
levels. Approximately ×5. (Figure enhanced by B Crossman.)
In transverse section, the spinal grey matter is often described as being
‘butterfly-shaped’ or resembling the letter ‘H’ (see Fig. 20.1). It consists 291


Spinal cord: internal organization
292
3
noitceS
A
I Dorsal horn
II
III
IV
V
VI
X
VII
VIII
IX IX
IX Ventral horn
B I
II
III
IV
V
Fig. 20.2 A transverse section of a spinal cord at a mid-lumbar level. T
Groups of the larger motor neurones in the ventral grey column are X
prominent. Stained with cresyl fast violet. CC IM IL
VIII
of four linked cellular masses: the right and left dorsal horns, which
project dorsolaterally, and the right and left ventral horns which project
ventrolaterally towards the surface. The grey matter that immediately IX
surrounds the central canal and unites the two sides constitutes the
dorsal and ventral grey commissures. The dorsal horn is the site of IX
termination of the primary afferent fibres that enter the cord via the
dorsal roots of spinal nerves. The tip of the dorsal horn is separated
Fig. 20.3 Rexed’s laminae shown in the cervical cord (A) and in the
from the dorsolateral surface of the cord by a thin fasciculus or tract (of
thoracic cord (B). Abbreviations: CC, central canal; IL, intermediolateral;
Lissauer), in which primary afferent fibres ascend and descend for a
IM, intermediomedial; T, posterior thoracic nucleus. (A, Redrawn with
short distance before terminating in the subjacent grey matter. The
permission from Crossman AR, Neary D 2010 Neuroanatomy, 4th edn.
ventral horn contains efferent neurones whose axons leave the spinal
Edinburgh: Churchill Livingstone; B, Redrawn with permission from
cord in ventral nerve roots. A small intermediate, or lateral, horn is
Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore:
present at thoracic and upper lumbar levels; it contains the cell bodies
Williams and Wilkins.)
of preganglionic sympathetic neurones.
Spinal grey matter (Fig. 20.2) is a complex mixture of neuronal cell
bodies, their processes and synaptic connections, neuroglia and blood


vessels. Neurones in the grey matter are multipolar. They vary in size the almost total lack of myelinated fibres. Lamina II corresponds
and features such as the length and the arrangement of their axons and approximately to the substantia gelatinosa. Lamina III consists of
dendrites. Neurones may be intrasegmental, i.e. contained within a somata that are mostly larger, more variable and less closely packed
single segment, or intersegmental, i.e. their ramifications spread through than those in lamina II. It also contains many myelinated fibres. The
several segments. ill-defined nucleus proprius of the dorsal horn corresponds to some of
the cell constituents of laminae III and IV. Lamina IV is a thick, loosely
Neuronal cell groups of the spinal cord
packed, heterogeneous zone permeated by fibres. Its neuronal somata
vary considerably in size and shape, from small and round, through
The grey matter of the spinal cord may be divided, on the basis of intermediate and triangular, to very large and stellate (Schoenen and
cytoarchitecture, into 10 zones, known as Rexed’s laminae, which are Faull 1990).
numbered sequentially from dorsal to ventral. The laminae are defined Laminae V and VI lie at the base of the dorsal horn. They receive
on the basis of neuronal size, shape, cytological features and density. most of the terminals of proprioceptive primary afferents, profuse cor-
Some of these lamina are equated with cell groupings of particular ticospinal projections from the motor and sensory cortex, and input
functional types (Fig. 20.3) (Rexed 1952). from subcortical levels, suggesting their involvement in the regulation
Laminae I–IV correspond to the dorsal part of the dorsal horn, and of movement. Lamina V is a thick layer, divisible into a lateral third
are the main site of termination of cutaneous primary afferent termi- and medial two-thirds. Both have a mixed cell population but the
nals and their collaterals. Many complex polysynaptic reflex paths (ipsi- former contains many prominent well-staining somata interlaced by


lateral, contralateral, intrasegmental and intersegmental) start from numerous bundles of transverse, dorsoventral and longitudinal fibres.
this region, as also do many long ascending tract fibres, which pass to Lamina VI is most prominent in the limb enlargements. It has a
higher levels. Lamina I (lamina marginalis) is a very thin layer with an densely staining medial third of small, densely packed neurones and a
ill-defined boundary at the dorsolateral tip of the dorsal horn. It has a lateral two-thirds containing larger, more loosely packed, triangular or
reticular appearance, reflecting its content of intermingling bundles of stellate somata.
coarse and fine nerve fibres. It contains small, intermediate and large Lamina VII occupies the region just ventral to lamina VI and extend-
neuronal somata, many of which are fusiform in shape. The much ing across the spinal grey matter on each side. This region is known as
larger lamina II consists of densely packed small neurones, responsible the intermediate zone and within the thoracic cord includes the lateral
for its dark appearance in Nissl-stained sections. With myelin stains, horn. The size of lamina VII varies at different spinal levels. In the cervi-
lamina II is characteristically distinguished from adjacent laminae by cal and lumbar enlargements, lamina VII extends laterally and ventrally


internal organization
293
02
retpaHc
throughout the ventral horn containing cell groups of lamina IX neu- to the preganglionic sympathetic fibres; and the intermediomedial
rones embedded within it. In the thoracic region, lamina VII occupies nucleus, which extends the full length of the cord and may be involved
the intermediate zone and the dorsal part of the ventral horn only. in the control of visceral motor neurones (Fig. 20.4).
Three important nuclear groups are contained within lamina VII: the Lamina VIII spans the base of the ventral horn in the thoracic region
posterior thoracic nucleus (nucleus thoracicus posterior, Clarke’s but is restricted to the medial aspect of the ventral horn in the cervical
column), which extends throughout the thoracic and upper lumbar and lumbar enlargements. Its neurones display a heterogeneous mixture
segments and gives rise to the dorsal spinocerebellar tract; the interme- of sizes and shapes from small to moderately large. Lamina VIII is a
diolateral nucleus, which is located between T1 and L2 and gives rise mass of propriospinal interneurones. It receives terminals from the
Dorsal column Lateral column Ventral column
Medial Lateral Central Accessory
Substantia gelatinosa
Nucleus proprius
Posterior thoracic
nucleus
Visceral grey
Intermediolateral
Intermediomedial
Retrodorsolateral
Dorsomedial
Dorsolateral
Phrenic nucleus
Ventrolateral
Ventromedial
Accessory
C 1
2
3
Dorsal funiculus 4
5
6
7
Dorsal grey column 8
T 1
2
3
Central canal 4
5
6
7
Lateral funiculus 8
9
10
11
Lateral grey column 12
L 1
2
3
Ventral grey column 4
5
S 1
2
Ventral funiculus 3
4
5
C 1
Substantia gelatinosa
Nucleus proprius
Posterior thoracic
nucleus
Visceral grey
Intermediolateral
Intermediomedial
Sacral parasympathetic
Retrodorsolateral
Dorsomedial
Dorsolateral
Lumbosacral
Ventrolateral


Ventromedial
Fig. 20.4 The groups of nerve cells in the grey columns of the spinal cord. The relative positions of these columnar groups and their extent through
spinal segments are indicated. (Modified with permission of Simon & Schuster from Correlative Anatomy of the Nervous System by E Crosby,
T Humphrey, E Lauer. Copyright © 1962 Macmillan Publishing Company.)


Spinal cord: internal organization
294
3
noitceS
adjacent laminae, many commissural fibres from the contralateral in turn inhibited the T cells, i.e. the gate was closed. Activity in small-
lamina VIII, and descending connections from the interstitiospinal, calibre nociceptive afferents had the opposite effect of inhibiting the
reticulospinal and vestibulospinal tracts and the medial longitudinal SG interneurones and opening the gate (Fig. 20.5). Since all primary
fasciculus. The axons from these interneurones influence α motor afferents are excitatory, the model implied that inhibition of the SG
neurone activity bilaterally, perhaps directly but more probably by exci- cells was the product of interneuronal circuitry, although the participat-
tation of small γ motor neurones supplying efferent fibres to muscle ing cells had yet to be identified.
spindles. More is now known about the dorsal horn circuitry that generates
Lamina IX consists of several distinct groups of somatic motor neu- the gate. Interneurones in laminae I–IV are morphologically, topo-
rones located throughout the ventral horn. In the thoracic cord, these graphically and neurochemically heterogeneous: islet, central, radial
nuclear groups are embedded within lamina VIII, whereas in the cervi- and vertical cells have been identified in experimental animal studies.
cal and lumbar enlargements, the lamina IX cell groups are embedded Islet cells are predominantly GABAergic (transmitting or secreting
within both lamina VII and lamina VIII. Lamina IX is a complex array γ-aminobutyric acid); a subset co-express glycine and are presumed to
of cells consisting of α and γ motor neurones and many interneurones. be exclusively inhibitory. Radial and vertical cells are predominantly
The large α motor neurones supply motor end-plates of extrafusal glutamatergic and presumed to be excitatory, and central cells include
muscle fibres in striated muscle. Recording techniques have demon- both inhibitory and excitatory subsets. The input from C nociceptors is
strated tonic and phasic α motor neurones. The former have a lower concentrated in lamina I and lamina II; recent evidence has refocused


rate of firing and lower conduction velocity, and tend to innervate type attention on the concept of labelled lines (each sensory modality is
S muscle units. The latter have higher conduction velocity and tend to processed via a fixed path from skin to brain) for noxious heat, cold,
supply fast twitch (type FR, FF) muscle units. The smaller γ motor neu- noxious mechanical stimulation and pruritogens, at least at the level of
rones give rise to small-diameter efferent axons (fusimotor fibres), the primary afferent input. The main elements in the circuitry by which
which innervate the intrafusal muscle fibres in muscle spindles. There noxious and innocuous primary afferent inputs are thought to influence
are several functionally distinct types of γ motor neurone. The ‘static’ projection neurones within the dorsal horn of the spinal cord are
and ‘dynamic’ responses of muscle spindles have separate controls shown in Figure 20.6. (For further reading, see Ma (2010), Moayedi and
mediated by static and dynamic fusimotor fibres, which are distributed Davis (2013), Todd and Koerber (2013), Zhang et al (2013), Braz et al
variously to nuclear chain and nuclear bag fibres (Boyd and Gladden (2014), Mendell (2014).)
1985). Onward transmission in the lateral spinothalamic tract is likely to
Lamina X surrounds the central canal and consists of the dorsal and evoke the perception of pain at supraspinal levels, although the activity
ventral grey commissures. of spinothalamic tract neurones may be modified by descending pain
modulatory systems. In the brainstem, the regions inducing such effects
correspond to a number of midbrain and rhombencephalic nuclei,
Dorsal horn
which, with their connections, constitute an endogenous analgesic
The dorsal horn is a major zone of termination of primary afferent
fibres, which enter the spinal cord through the dorsal roots of spinal
nerves. Dorsal root fibres contain numerous molecules, which are either Aδ
known, or suspected, to fulfil a neurotransmitter or neuromodulator C C Aδ
role. These include glutamic acid, substance P, calcitonin gene-related Aβ


peptide (CGRP), bombesin, vasoactive intestinal polypeptide (VIP),
+
cholecystokinin (CCK), somatostatin, dynorphin and angiotensin II.
Dorsal root afferents carry exteroceptive, proprioceptive and interocep- I + NK1 +
tive information. Laminae I–IV are the main cutaneous receptive areas; - +
lamina V receives fine afferents from the skin, muscle and viscera; and I-i
lamina VI receives proprioceptive and some cutaneous afferents. Most, IIo - +
V
if not all, primary afferent fibres divide into ascending and descending +
branches on entering the cord. These then travel for variable distances
in the tract of Lissauer, near the surface of the cord, and send collaterals + C +
into the subjacent grey matter. IIi - +
I
The lamina marginalis (lamina I) is a thin lamina of neurones at the
+
dorsolateral tip of the dorsal horn, deep to the tract of Lissauer. Beneath -
I
it lies the substantia gelatinosa (laminae II), which is present at all III-IV +
levels, and consists mostly of small neurones, together with some larger + E +
neurones. The substantia gelatinosa receives afferents via the dorsal
Fig. 20.6 Routes by which noxious and innocuous primary afferent inputs
roots, and its neurones give rise to fibres that form the contralateral
may engage projection neurones in lamina I. Note that the excitatory
spinothalamic tract. The large propriospinal neurones of the nucleus
activation circuits have a ventral-to-dorsal organization. Nociceptive C
proprius lie ventral to the substantia gelatinosa within lamina III and
and Aδ fibres (purple) activate vertical cells (V) of the outer part of lamina
lamina IV; they link segments for the mediation of intraspinal coordina- II (IIo) directly and these, in turn, excite neurokinin 1 (NK1)-expressing
tion (see Fig. 20.2). projection neurones of lamina I (green axon). Nociceptive C fibres also


The posterior thoracic nucleus lies at the base of the dorsal horn activate central cells (C) of lamina II (IIi); the C cells contact, and probably
within lamina VII. At most levels, it is near the dorsal white funiculus excite, V cells in lamina IIo. The projection neurones in lamina I also
and may project into it. It can usually be identified from the eighth receive direct nociceptive C and Aδ input. Non-nociceptive Aβ fibres
cervical to the third or fourth lumbar segments. Neurones of the pos- (blue) project to deeper laminae (III–IV), where they establish
terior thoracic nucleus vary in size but most are large, especially in the monosynaptic connections with local excitatory interneurones (E), which
lower thoracic and lumbar segments. Some send axons into the dorsal directly activate V cells. The majority of the inhibitory neurones (I) in
spinocerebellar tracts and others are interneurones. lamina II are islet cells (I-i). These dorsally directed circuits are the route
through which both noxious and innocuous primary afferent input can
Pain pathways engage the projection neurones of lamina I. The excitatory circuits in the
Fifty years ago, a paper was published that not only had a profound superficial dorsal horn are subject to profound inhibitory controls (red).
The majority of the inhibitory neurones (I) in lamina II are islet cells (I-i)
effect on thinking about sensory processing and perception at the time
and these can be directly engaged by input from low-threshold,
but also continues to influence research in the field of pain perception.
mechanoreceptive C fibres (blue). The islet cells in turn establish
The authors described a mechanism by which nociceptive input from
monosynaptic inhibitory connections with both vertical cells of lamina IIo
the skin was modulated (‘gated’) by local circuits within the dorsal horn
and NK1 receptor-expressing projection neurones of lamina I. Low-
and by descending inputs from the brain; the theory was known as the
threshold Aδ (D-hair; blue) and Aβ fibres also directly engage inhibitory
gate control theory (GCT; Melzack and Wall 1965). Interneurones in


interneurones in laminae IIi–IV. The latter in turn exert inhibitory control of
the substantia gelatinosa, SG cells, were cast in the role of the meta- a variety of excitatory interneurones, including vertical (V) and central (C)
phorical gate, modulating patterns of afferent input to noci-responsive cells. The Aβ-to-inhibitory cell circuit presumably underlies the circuit
projection neurones (T cells, tract cells), found mainly in lamina I of through which the ‘gate’ of the gate control theory may be closed.
the dorsal horn, and responsible for onward transmission of noxious (Redrawn with permission from Braz J, Solorzano C, Wang X, et al 2014
messages to the brain. According to the GCT, increased activity in the Transmitting pain and itch messages: a contemporary view of the spinal
collaterals of large-calibre innocuous afferents excited SG cells, which cord circuits that generate gate control, Neuron 82:522–536.)


Spinal cord: internal organization
294.e1
02
retpaHc
Higher
centres
Descending tracts
Large-diameter
afferent fibres
Substantia Reflex response
Lamina IV
gelatinosa
neurone
neurone
Small-diameter
afferent fibres
Fig. 20.5 The basic arrangement of the sensory ‘gate’ mechanism in the
dorsal laminae of the grey matter of the spinal cord. (Redrawn with
permission from Melzack R, Wall PD. Pain mechanisms: a new theory.
Science 150:971–979. Copyright 1965 American Association for the
Advancement of Science.)


internal organization
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02
retpaHc
system. In the midbrain, these regions are the periaqueductal grey C8
matter, dorsal raphe nucleus and part of the cuneiform nucleus. Neu-
rones in these sites contain 5-HT (5-hydroxytryptamine, serotonin),
GABA, substance P, CCK, neurotensin, enkephalin and dynorphin. The
periaqueductal grey matter receives afferents from the frontal somato-
sensory and cingulate neocortex, the amygdala, numerous local reticu-
lar nuclei and the hypothalamus. Afferents from the latter are separate Muscles of the hand
bundles, which carry histamine, luteinizing hormone-releasing hor-
mone (LHRH), vasopressin, oxytocin, adrenocorticotrophic hormone
Intrinsic limb muscles
(ACTH), melanocyte-stimulating hormone (γ-MSH), endorphin and
angiotensin II. Some fibres descend from the periaqueductal grey matter
to rhombencephalic centres, while others pass directly to the spinal Limb muscles
originating from the trunk
cord.
In the rhombencephalon, the nucleus raphe magnus and the medial
reticular column constitute an important multineuromediator centre. Axial muscles
Neurones in these sites contain 5-HT, substance P, CCK, thyrotrophin-
releasing hormone (TRH), enkephalin and dynorphin; some neurones Fig. 20.7 The approximate location of motor neurone groups at the C8
contain two or even three neuromediators. Descending corticospinal segmental level of the spinal cord.
fibres pass to the nucleus of the spinal tract of the trigeminal nerve and
its continuation, the substantia gelatinosa. The latter extends through-
out the length of the cord and contains populations of neurones that gated into two major longitudinal columns: medial and lateral. In
express many different neuromediators, e.g. GABA, substance P, neuro- transverse section these form the medial and lateral cell groups in the
tensin, enkephalin and dynorphin. There is abundant physiological ventral horn (Fig. 20.7). The motor neurones of lamina IX are soma-


and pharmacological evidence that all of these regions are intimately totopically organized. The basic arrangement is that medial cell groups
concerned with the control of nociceptive (and probably other modal- innervate the axial musculature, and lateral cell groups innervate the
ity) inputs. limbs.
Stimulation of forebrain sites, including the periventricular grey The medial nuclear group is divisible into a posteromedial and
matter, the ventral posterolateral nucleus of the thalamus, and the anteromedial group. This group innervates the muscles attached to the
primary sensory (S1) and posterior parietal cortices, inhibit spinotha- axial skeleton. The medial nuclear group extends throughout the length
lamic tract cells. In contrast, some spinothalamic tract cells are excited of the spinal cord. Its neurones innervate epaxial muscle groups (which
by stimulation of the medullary reticular formation and the primary extend the head and vertebral column) and hypaxial muscle groups
motor cortex (the latter effect probably being mediated by the cortico- (which flex the head and vertebral column). Epaxial muscles include
spinal tract). the erector spinae group, while hypaxial muscles include prevertebral
Modern neuroimaging and molecular, genetic and behavioural muscles of the neck, intercostal and anterior abdominal wall muscles.
studies confirm that the experience of pain involves a complex network The epaxial muscles are innervated by branches of the dorsal primary
of responsive brain regions. The somatosensory cortices (S1 and S2) rami of the spinal nerves, and the hypaxial muscles by branches of
and the insula are thought to encode the sensory features of pain, the ventral primary rami. In the medial column, motor neurones sup-
whereas the prefrontal cortex and components of the limbic system plying epaxial muscles are sited ventral to those supplying hypaxial
(anterior cingulate cortex, amygdala, ventral tegmental area and nucleus muscles.
accumbens) mediate the affective and contextual aspects of pain. Mech- The lateral nuclear group innervates the rest of the body muscula-
anisms underlying ‘deep’ visceral, muscular, neuropathic and persistent, ture. In the thoracic region, it is small and innervates the lateral trunk


chronic pain remain even less clear than those mediating cutaneous muscles, including the intercostal muscles. In the cervical and lumbar
pain (Devor 2013). Patients with chronic pain may have anatomical enlargements, the lateral nuclear group is enlarged and a number of
alterations within the regions involved in the cognitive and emotional subgroups may be distinguished, including the anterolateral (ventrola-
modulation of pain, e.g. the dorsolateral and medial prefrontal cortex, teral), posterolateral (dorsolateral), central and retroposterolateral (ret-
anterior cingulate gyrus and insula (Bushnell et al 2013, Denk et al rodorsolateral) groups.
2014) and dysregulation of the descending pain modulation systems In the cervical and lumbar enlargements, the lateral nuclear group
(Ossipov et al 2014). innervates muscles of the upper and lower limbs, respectively. These
neurones are somatotopically organized. Passing round the cervical
Lateral horn ventral horn clockwise from ventromedial to dorsolateral, α motor
neurones successively innervate muscles of the trunk, shoulder girdle,
upper arm and lower arm (see Fig. 20.4). The proximal muscles of the
The lateral horn is a small lateral projection of grey matter located
limb are supplied from motor cell groups located more rostrally in the
between the dorsal and ventral horns in lamina VII. It is present from
enlargement than those supplying the distal muscles, e.g. motor neu-
the eighth cervical or first thoracic segment to the second or third
rones in C8 and T1 innervate the intrinsic muscles of the hand, and
lumbar segment. It contains the cell bodies of preganglionic sympa-
motor neurones in C5 and C6 innervate shoulder muscles. There is a
thetic neurones. These develop in the embryonic cord dorsolateral to
similar overall arrangement in the lumbosacral cord for motor neu-
the central canal and migrate laterally, forming the intermediolateral
rones innervating the muscles in the lower limb (Fig. 20.8).
cell columns. Their axons travel via ventral spinal roots and white rami
The spinal nucleus of the accessory nerve (SAN) contains motor


communicantes to the sympathetic trunk. A similar cell group is found
neurones that innervate sternocleidomastoid and trapezius. Described
in the second to fourth sacral segments but, unlike the thoracolumbar
as a curvilinear band of motor neurones extending within the ventral
lateral cell column, it does not form a visible lateral projection. It is the
horn from C1/2 to C5/6 (Orhan et al 2009; Saylam et al 2009) in line
source of the sacral outflow of parasympathetic preganglionic nerve
rostrally with the nucleus ambiguus, the spinal nucleus of the accessory
fibres.
nerve is unusual in that it not only lies between the most medial
column in the ventral horn (innervating axial muscles) and the lateral
Ventral horn
column (innervating upper limb muscles) (Routal and Pal 2000) but
also its rootlets exit the spinal cord just dorsal to the ligamentum den-
Lamina IX neurones in the ventral horn vary in size. The largest cell ticulatum (Jenny et al 1988). They subsequently combine to form the
bodies, which may exceed 25 µm in diameter, are those of α motor intraspinal portion of the accessory nerve, which ascends the vertebral
neurones, the axons of which emerge in ventral roots to innervate canal to enter the posterior cranial fossa via the foramen magnum. The
extrafusal fibres in striated skeletal muscles. Large numbers of smaller supranuclear innervation of the spinal nucleus of the accessory nerve is
neurones, 15–25 µm in diameter, are also present. Some of these are γ uncertain. The bulk of current evidence favours bilateral hemispheric
motor neurones, which innervate intrafusal fibres of muscle spindles, innervation of motor neurones in the rostral spinal nucleus of the acces-
and the rest are interneurones. Motor neurones utilize acetylcholine as sory nerve (innervating sternocleidomastoid), although opinion varies
their neurotransmitter. as to the extent of the ipsilateral input, but predominantly contralateral


The basic building block of the motor neurones in lamina IX is supranuclear control of the neurones in the caudal spinal nucleus of
represented by a longitudinally disposed group of neurones, which the accessory nerve (innervating the upper and middle fibres of trape-
innervate a given muscle, and in which the α and γ motor neurones are zius). This pattern is analogous to the supranuclear innervation of the
intermixed. The various groups innervating different muscles are aggre- facial nerve motor nucleus.


Spinal cord: internal organization
296
3
noitceS
Fibres in the spinal white matter vary in calibre. Many are small and
L1 L2 L3 L4 L5 S1 S2 S3 lightly myelinated or unmyelinated. Most regions contain a wide spec-
trum of fibre diameters, from 1 µm or less to 10 µm. Some tracts typi-
Iliopsoas
Sartorius cally contain only small fibres, e.g. the dorsolateral tract, fasciculus
Pectineus gracilis and central part of the lateral funiculus. The fasciculus cuneatus,
Gracilis ventral funiculus and peripheral zone of the lateral funiculus all contain
Adductor longus many large-diameter fibres.
Adductor brevis Whilst the ascending and descending tracts are, to a large extent,
Adductor magnus discrete and regularly located, significant overlap between adjacent
Quadriceps femoris tracts does occur. Their general disposition is shown in Figure 20.9
Obt. ext.
(Parent and Carpenter 1996).
Tib. ant.
Tib. post.
Ten. fas. latae Ascending pathways
Glut. med. and min.
Semimembranosus
Dorsal columns
Semitendinosus
Ext. hall. l. The dorsal funiculus consists of two large ascending tracts, the fasciculus
Ext. dig. l. gracilis and fasciculus cuneatus (Fig. 20.10), which are also known as
Fib. tert. the dorsal columns. They are separated by a posterointermediate
Fib. brevis septum. The dorsal columns contain a high proportion of myelinated
Fib. longus
fibres carrying proprioceptive (position sense and kinaesthesia), extero-
Lat. hip. rot.
ceptive (touch-pressure) and vibratory sensation to higher levels. These
Gastrocn.
fibres come from several sources: long primary afferent fibres, which
Soleus and Plant.
Biceps femoris enter the cord in the dorsal roots of spinal nerves and ascend to the
Gluteus max. dorsal column nuclei in the medulla oblongata; shorter primary affer-
Flex. hall. l. and b. ent fibres projecting to neurones of the posterior thoracic nucleus and


Flex. dig. l. and b. other spinal neurones; and axons from secondary neurones of the
Foot intrinsics spinal cord ascending to the dorsal column nuclei. The dorsal columns
also contain axons of propriospinal neurones.
Fig. 20.8 The segmental arrangement of motor neurones innervating
The fasciculus gracilis begins at the caudal end of the spinal cord. It
muscles of the lower limb. Obt. ext. = obturator externus; Tib. ant. =
contains long ascending branches of primary afferents, which enter the
tibialis anterior; Tib. post. = tibialis posterior; Ten. fas. latae = tensor
cord through ipsilateral dorsal spinal roots and ascending axons of
fasciae latae; Glut. med. and min. = gluteus medius and minimus; Ext.
secondary neurones in laminae IV–VI of the ipsilateral dorsal horn. As
hall. l. = extensor hallucis longus; Fib. tert. = fibularis tertius; Fib. brevis =
the fibres ascend, they are joined by axons of successive dorsal roots.
fibularis brevis; Fib. longus = fibularis longus; Lat. hip. rot. = lateral hip
Fibres entering in coccygeal and lower sacral regions are shifted medi-
rotators; Gastrocn. = gastrocnemius; Soleus and plant. = soleus and
plantaris; Gluteus max. = gluteus maximus; Flex. hall. l. and b. = flexors ally by successive additions of fibres entering at higher levels.
hallucis longus and brevis; Flex. dig. l. and b. = flexors digitorum longus The fasciculus gracilis lies medial to the fasciculus cuneatus in the
and brevis. upper spinal cord (see Fig. 20.9). At upper cervical levels, the fasciculus
gracilis contains a larger proportion of afferents from cutaneous recep-
tors than from deep proprioceptors because many of the latter leave the
fasciculus at lower segments to synapse in the posterior thoracic
The nucleus of Onuf, which is thought to innervate the perineal nucleus. Indeed, proprioception from the lower limb mostly reaches


striated muscles, is an anterolateral group of lamina IX cells in the the thalamus by relaying in the posterior thoracic nucleus and then
first and second sacral segments. The centrally situated phrenic again in nucleus Z. Axons of the fasciculus gracilis, from both primary
nucleus, containing the motor neurones that innervate the dia- and secondary neurones, terminate in the nucleus gracilis of the dorsal
phragm, lies in the third to seventh cervical segments. The lumbosac- medulla.
ral nucleus is also centrally located and its axons have an unknown The fasciculus cuneatus (see Fig. 20.9) begins at mid-thoracic level
distribution. and lies lateral to the fasciculus gracilis. It is composed mostly of
The main afferent connections to motor neurones are: direct mono- primary afferent fibres of the upper thoracic and cervical dorsal roots.
synaptic connections from proprioceptive dorsal root afferents in the At upper cervical levels, it contains a large population of afferents from
same or nearby segments; connections from axonal collaterals of dorsal both deep and cutaneous receptors of the upper limb. In addition, some
horn and other interneurones; direct monosynaptic connections from of its axons arise from secondary neurones in laminae IV–VI of the
the vestibulospinal and corticospinal tracts. ipsilateral dorsal horn. Many axons (both primary and secondary) that
ascend in the fasciculus cuneatus terminate in the nucleus cuneatus of
the dorsal medulla. Some also end in the lateral (external or accessory)
SPINAL WHITE MATTER cuneate nucleus; neurones in this nucleus project to the cerebellum via
the cuneocerebellar pathway.
The spinal white matter surrounds the central core of grey matter. The Many ascending fibres of the fasciculus gracilis and fasciculus cunea-
white matter consists primarily of longitudinally running nerve fibres. tus terminate by synapsing on neurones of the dorsal column nuclei
Fibres subserving related functions, or those with common origins or (nucleus gracilis and nucleus cuneatus, respectively) in the medulla
destinations, are generally grouped together anatomically to form tracts oblongata. (The connections of the dorsal column nuclei are described


(fasciculi), which may be ascending, descending and propriospinal. further with the medulla oblongata; Ch. 21.) Axons arising from neu-
Ascending tracts consist either of primary afferent fibres, which enter rones in the dorsal column nuclei arch ventromedially round the
the cord via the dorsal roots of spinal nerves, or fibres derived from central grey matter of the medulla as internal arcuate fibres (see Fig.
intrinsic spinal neurones, which carry afferent impulses to supraspinal 21.7) and decussate in the great sensory decussation to form the medial
levels. Descending tracts contain fibres that descend from the cerebral lemniscus. They ascend to the ventral posterolateral nucleus of the
cortex or brainstem nuclei to control the activity of spinal neurones. thalamus, from where neurones project to the somatosensory cortex in
Propriospinal tracts contain the axons of neurones that are localized the postcentral gyrus of the parietal lobe (areas 1, 2 and 3). Some neu-
entirely to the spinal cord; they contain both ascending and descending rones of the dorsal column nuclei form posterior external arcuate fibres
components, collectively mediating intersegmental coordination. that enter the cerebellum.
The spinal white matter is conventionally described as being arranged The high degree of somatotopic organization that is present in the
into three large, bilaterally paired masses, the dorsal, lateral and ventral dorsal columns is preserved as the pathways ascend through the dorsal
funiculi, each of which contains a number of specific tracts (see Fig. column nuclei and thalamus to reach the primary somatosensory
20.1; Fig. 20.9). The dorsal (posterior) funiculus lies between the dorsal cortex. In the dorsal column nuclei, the lower limb is represented in
horn and the posterior median septum. The lateral funiculus lies the nucleus gracilis, the upper limb in the nucleus cuneatus, and the
between the dorsal root entry zone and the site where the ventral roots trunk in an intermediate position between them. Fibres are also segre-
emerge. The ventral (anterior) funiculus lies between the ventral median gated by modality in the dorsal columns; fibres from hair receptors are


fissure and the site where the ventral roots emerge. Narrow dorsal and most superficial, while those from tactile and vibratory receptors lie in
ventral white commissures run between the two halves of the cord. deeper layers.


internal organization
297
02
retpaHc
A
Descending pathways Fasciculus interfascicularis Fasciculus gracilis
From cerebral cortex
Dorsolateral fasciculus
From brainstem (of Lissauer)
Ascending pathways Dorsal spinocerebellar tract
Fasciculus cuneatus
Dorsal columns
Fasciculus proprius
Spinocerebellar tracts
Spinothalamic and
spinoreticular tracts
Lateral corticospinal tract
Rubrospinal tract
Lateral reticulospinal tract
Medial longitudinal fasciculus
Medial reticulospinal tract Ventral spinocerebellar tract
Tectospinal tract Spinothalamic and spinoreticular tracts
Ventrolateral reticulospinal tract Spino-olivary tract
Lateral (ventrolateral)
vestibulospinal tract
Ventral reticulospinal tract Ventral corticospinal tract
B Fasciculus septomarginalis Fasciculus gracilis
Dorsolateral fasciculus (of Lissauer)
Fasciculus proprius
Lateral corticospinal tract
Tegmentospinal tract
Lateral reticulospinal tract
Ventral spinocerebellar tract
Lateral spinothalamic and spinotectal tract
Medial reticulospinal tract
Spino-olivary tract
Ventrolateral vestibulospinal tract
Ventral reticulospinal tract
Ventral spinothalamic tract
Ventral corticospinal tract Sulcomarginal fasciculus
Fig. 20.9 The approximate positions of nerve fibre tracts in the spinal cord at mid-cervical (A) and lumbar (B) levels.
Spinocerebellar tracts oblongata to reach the upper pontine level, from where it descends in
The dorsal (posterior) and ventral (anterior) spinocerebellar tracts the dorsal part of the superior cerebellar peduncle to terminate, mainly
occupy the periphery of the lateral aspect of the spinal white matter contralaterally, in the anterior cerebellar vermis.
(see Fig. 20.9; Fig. 20.11) and carry proprioceptive and cutaneous infor- The rostral spinocerebellar tract originates from cell groups of the