mation to the cerebellum for the coordination of movement. intermediate zone and horn of the contralateral cervical enlargement
Both tracts contain large-diameter myelinated fibres but there are more and is considered to be the upper limb and upper trunk equivalent of
in the dorsal tract. Finer-calibre fibres are associated with the ventral the ventral spinocerebellar tract.
tract. Experimental tracing studies in animals have demonstrated a cervico-
The dorsal spinocerebellar tract lies lateral to the lateral corticospinal spinocerebellar tract arising from a central cervical nucleus in the upper
tract. It begins at about the level of the second or third lumbar segment cervical spinal cord (C1–C4); the central cervical nucleus, possibly
and enlarges as it ascends. Axons of the tract originate ipsilaterally from Cajal’s nucleus intermedius, receives primary afferents from the vestibu-
the larger neurones of the posterior thoracic nucleus, in lamina VII lar labyrinth and muscle afferents from the deep muscles of the neck.
throughout spinal segments T1–L2. The posterior thoracic nucleus A similar pathway may exist in humans where the nucleus intermedius
receives input from collaterals of long ascending primary afferents of of Cajal receives fibres from only the upper two cervical roots (Marani
the dorsal columns and terminals of shorter ascending primary affer- and Schoen 2005).
ents of the dorsal columns. Many of these afferent fibres ascend from The dorsal and ventral spinocerebellar tracts are organized such that
segments caudal to L2. In the medulla, the dorsal spinocerebellar tract fibres from lower spinal segments are most superficial. Both tracts
passes through the restiform body and thence the inferior cerebellar convey proprioceptive and exteroceptive information, but they are func-
peduncle to terminate ipsilaterally in the rostral and caudal parts of the tionally different. Neurones of the posterior thoracic nucleus are excited
cerebellar vermis. monosynaptically by Ia and Ib primary afferent fibres (from muscle
The ventral spinocerebellar tract lies immediately ventral to the spindles and tendon organs, respectively) and also by group II muscle


dorsal tract. The cells of origin are in laminae V–VII of the lumbosacral afferents, and cutaneous touch and pressure afferents. The propriocep-
cord and the tract carries information from the lower limb. Most of the tive impulses often arise from a single muscle or from synergistic
axons forming the tract decussate but some remain ipsilateral. The tract muscles acting at a common joint. Thus, the dorsal spinocerebellar tract
begins in the upper lumbar region and ascends through the medulla transmits modality-specific and space-specific information that is used


Spinal cord: internal organization
298
3
noitceS
Cerebral cortex (postcentral gyrus) Dentatothalamic fibres Cerebellorubral fibres
Thalamus Leg area
Anterior spinocerebellar Dentatoreticular fibres
Internal capsule
tract on surface of superior
Trunk area cerebellar peduncle
Decussation of superior
cerebellar peduncle
Arm area
Medial lemniscus
Upper pons
Head area
Face area
Vermis of cerebellum
Dentate nucleus
Posterior
Superior spinocerebellar fibres
cerebellar peduncle in inferior cerebellar
peduncle
Ventral
posterolateral
Basal ganglia
nucleus of Middle
Midbrain thalamus Pons cerebellar
peduncle
Medial lemniscus
Cuneocerebellar tract
Pons
Accessory
cuneate nucleus
Trigeminal nerve
Medulla
Ventral (anterior)
spinocerebellar tract
Nucleus gracilis Medial lemniscus
Nucleus cuneatus C4
Spinal nucleus of
Medulla trigeminal nerve Dorsal (posterior)
spinocerebellar tract
Internal arcuate fibres Sensory decussation
Dorsal root
ganglion neurone Fasciculus gracilis C8
Posterior thoracic
nucleus
C7
Fasciculus cuneatus
Golgi tendon organ
L3 (Quadriceps femoris)
T4
Dorsal root
Fasciculus gracilis Cells of the dorsal ganglion
spinocerebellar tract neurones
Sacral and cuneocerebellar
Lumbar L4 tract S2 Golgi tendon organ
Thoracic (Semitendinosus)
Cells of the ventral
Cervical spinocerebellar tract
Fig. 20.10 The dorsal columns. Primary afferent fibres from different Fig. 20.11 The spinocerebellar tracts. (Redrawn with permission from
levels and their associated second- and third-order neurones are depicted Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore:
in different colours. Williams and Wilkins.)


in the fine coordination of individual limb muscles. On the other hand, Since the posterior thoracic nucleus diminishes rostrally (see Fig.
the cells of the ventral tract are activated monosynaptically by Ib affer- 20.4) and does not extend above the lowest cervical segment, it follows
ents and transmit information from large receptive fields that include that the dorsal spinocerebellar tract carries information from the trunk
different segments of a limb. The ventral tract lacks subdivisions for and lower limb. Proprioceptive and exteroceptive information from the
different modalities and transmits information for the coordinated upper limb travel in primary afferent fibres of the fasciculus cuneatus.
movement and posture of the entire lower limb. These fibres end somatotopically in the accessory (external or lateral)


internal organization
299
02
retpaHc
cuneate nucleus and the adjoining part of the cuneate nucleus situated Cerebral cortex (postcentral gyrus)
in the medulla oblongata. Cells of these nuclei give rise to the posterior
Corpus callosum Foot Axons of neurones in posterior
external arcuate fibres that form the cuneocerebellar tract (see Fig. limb of internal capsule
Thalamus
20.11), which enters the cerebellum via the ipsilateral inferior cerebellar Leg
peduncle. The cells in the accessory cuneate nucleus and the lateral part Internal capsule
of the cuneate nucleus are considered to be homologous to the cells in
Arm
the posterior thoracic nucleus. The cuneocerebellar tract is, therefore,
H
functionally allied to the dorsal spinocerebellar tract, and is its upper- a n
d
limb equivalent.
Axons of all the spinocerebellar tracts and the cuneocerebellar tract
form part of the ‘mossy-fibre system’. They end in the cerebellar cortex
in a highly organized, somatotopical and functional pattern.
Spinothalamic tracts
The spinothalamic tracts (see Fig. 20.9) consist of second-order neu-
rones that convey pain, temperature, coarse (non-discriminative) touch
and pressure information to the somatosensory region of the thalamus.
The cells of origin lie in various laminae of all segments of the spinal
cord. Fibres decussate in the ventral white commissure to reach the
contralateral spinothalamic tracts; pain and temperature fibres do so
promptly, within about one segment of their origin, whilst fibres carry-
ing other modalities may ascend for several segments before crossing.
Spinothalamic fibres mostly ascend in the white matter ventrolateral to
the ventral horn, partly intermingled with ascending spinoreticular Basal ganglia Ventral posterolateral
nucleus of thalamus
fibres and descending reticulospinal fibres. Some authorities describe
Midbrain
two spinothalamic tracts (lateral and ventral) with more or less distinct
anatomical locations and functions. However, physiological studies in Substantia nigra
animals support the notion that these tracts may be best considered as


a structural and functional continuum. Crus cerebri
The lateral spinothalamic tract (Fig. 20.12) is sited in the lateral
funiculus, lying medial to the ventral spinocerebellar tract. Clinical
evidence indicates that it subserves pain and temperature sensations.
The ventral spinothalamic tract (Fig. 20.13) lies in the ventral funiculus Pons
medial to the point of exit of the ventral spinal nerve roots and dorsal
to the vestibulospinal tract, which it overlaps. On the basis of clinical Medial lemniscus
evidence, it subserves coarse tactile and pressure modalities.
A dorsolateral spinothalamic tract has been described in animals.
The axons arise mainly from neurones in lamina I, and cross to ascend
in the contralateral dorsolateral funiculus. These neurones respond
maximally to noxious, mechanical and thermal cutaneous stimuli. Reticular formation
Examples of clinical pain relief following dorsolateral cordotomy Medulla
suggest that a similar projection exists in humans.
On reaching the lower brainstem, spinothalamic tract axons sepa- Lateral
rate. Axons in the ventral tract join the medial lemniscus. Axons in the spinothalamic tract
Temperature
lateral tract continue as the spinal lemniscus.
There is clear somatotopic organization of the fibres in the spinotha- Dorsal root
lamic tracts throughout their extent. Fibres crossing at any cord level ganglion neurone
join the deep aspect of those that have already crossed, which means Sacral fibres
that both tracts are segmentally laminated (Fig. 20.14). Somatotopy is C8 Lumbar fibres
maintained throughout the medulla oblongata and pons. In the mid- Thoracic fibres
brain, fibres in the spinal lemniscus conveying pain and temperature Cervical fibres
sensation from the lower limb extend dorsally, while those from the Pain
trunk and upper limb are more ventrally placed. Both lemnisci ascend Dorsolateral fasciculus
to end in the thalamus. The major spinothalamic projections in humans (of Lissauer)
are to the ventral posterolateral nucleus, and also to the centrolateral
T4
intralaminar nucleus.


neurones of the spinothalamic tracts
The specific localization of spinothalamic tract cell bodies is poorly
documented in humans. In animals, about one-third are localized to
L2
the upper three cervical segments. About 20% are located in lower cervi-
cal segments, 20% in the thoracic region (mostly in segments T1–3),
20% in the lumbar region and 10% in the sacrococcygeal cord. Cells
are located in laminae I and IV–VIII, the greatest concentration being
in laminae VI and VII. Cell bodies giving rise to spinothalamic tract
axons are predominantly contralateral, although a relatively small S2
number (10%), mostly in the upper three cervical segments, are Axons crossing
ipsilateral. to opposite side in
ventral white commissure
Neurones of the spinothalamic tracts have varied receptive fields.
Specificity of separate types of afferents, as it exists in the dorsal column Fig. 20.12 The lateral spinothalamic tract. (Redrawn with permission from
nuclei, is absent in the spinothalamic nuclei. Convergence of different Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore:
functional types of afferent fibres on to an individual tract cell is a Williams and Wilkins.)
common feature in the cord. On the basis of laminar site, functional
properties, and specific thalamic termination of their axons, spinotha- Lamina I cells project to the thalamus. They respond maximally to
lamic tract neurones may be divided into three separate groups. These noxious or thermal cutaneous stimulation and consist mainly of high-
are the apical cells of the dorsal grey column (lamina I), deep dorsal threshold but also some wide-dynamic-range units (responding to
column cells (laminae IV–VI), and cells in the ventral grey column various different intensities of stimulus). Their receptive fields are
(laminae VII and VIII). There are species differences and the description usually small, representing a part of a digit or a small area of skin
below is derived from studies in non-human primates. involving several digits. Lamina I spinothalamic tract neurones receive


Spinal cord: internal organization
300
3
noitceS
Cerebral cortex (postcentral gyrus) Axons of neurones in posterior Fasciculus Fasciculus
limb of internal capsule cuneatus gracilis
Third ventricle Leg C4C8T2T6T7T12L3S1S5
Ar m Pressure
Internal Vibration
capsule H a n d
eral
corticospinal tract
Leg
M
P
Tooo usv cie
t
him onent
Lat Trunk
Arm
C1
C5
S5 C6
S1 L1 T12T2 T1 C8 C7
eT
m
p
e
ra tu C4
re C8
T2
T6
Thalamus T7
Ventral posterolateral
Midbrain nucleus (VPL) T12
Pain SL3
1
Superior colliculus
S5
Spinothala
mic tracts Touch
Pressure
Pons
Anterior spinothalamic Fig. 20.14 The general plan of the segmental organization of fibres in the
tract and medial dorsal funiculus, the lateral corticospinal tract and the spinothalamic
lemniscus
tracts. The probable cross-sectional areas of these tracts are
schematically enlarged. This general plan applies to all segmentally
organized tracts, whether ascending, descending, ipsilateral or
contralateral.
Medulla
cord (Combarros et al 2002). Lamina I spinothalamic tract neurones
project preferentially to the ventral posterolateral nucleus of the thala-
mus, with limited projections to the centrolateral intralaminar and
Nucleus gracilis
mediodorsal thalamic nuclei.
The population of deep dorsal column (laminae IV–VI) spinotha-
Medulla
lamic neurones of the lumbar cord contains units of the wide-dynamic-
range (60%), high-threshold (30%) and low-threshold (10%) types.
Axon of neurone They can accurately code both innocuous and noxious cutaneous
decussating to ascend Pyramid
stimuli. Some cells also respond to input from deep somatic and vis-
in medial lemniscus
ceral receptors. In the lumbar cord, their receptive fields are small or


medium-sized; they are larger than the area of the foot but smaller than
the entire leg. In the thoracic cord, the fields of these laminar cells are
C8 larger and often include the entire upper limb plus part of the chest.
Many of the deep dorsal grey column spinothalamic tract neurones in
the thoracic segments receive convergent input from sympathetic affer-
ent fibres.
Laminae IV–VI spinothalamic tract units project either to the ventral
T12
posterolateral nucleus or to the centrolateral intralaminar nucleus of
the thalamus, and sometimes to both. Units projecting to the ventral
posterolateral nucleus receive input from all classes (Aβ, Aδ and C) of
cutaneous fibres.
Ventral grey column (laminae VII and VIII) spinothalamic tract cells
Dorsal root ganglion neurone respond mainly to deep somatic (muscle and joint) stimuli but also to
innocuous and/or noxious cutaneous stimuli. In the thoracic regions
of the spinal cord, they also receive convergent input from visceral
Axon of neurone sources. The majority of laminae VII and VIII spinothalamic tract neu-
crossing in ventral L3 rones have large, complex receptive fields (often bilateral), which
white commissure
encompass widespread areas of the body. Cells of this group, which
to ascend in anterior
spinothalamic tract project exclusively to the medial thalamus, receive input from Aβ, Aδ
and C classes of afferent fibres, and many respond to convergent input
Fig. 20.13 The ventral (anterior) spinothalamic tract. (Redrawn with
from receptors of deep structures. This population of neurones contains
permission from Carpenter MB 1991 Core Text of Neuroanatomy,
wide-dynamic-range (25%), high-threshold (63%) and low-threshold
4th edn. Baltimore: Williams and Wilkins.)
(12%) units. Most of the spinothalamic tract cells in the ventral grey
column project to the intralaminar nuclei of the thalamus. The wide-
input from Aδ and C fibres, and some respond to convergent input from dynamic-range type of neurones are particularly effective for discrimi-


deep somatic and visceral receptors. Spinothalamic tract cells in the nating between different intensities of painful stimulation.
thoracic cord display marked viscerosomatic convergence. Hence pain
Spinoreticular pathway
from cardiac disease such as angina or a myocardial infarction is com-
monly felt as a tightness around the chest wall (referred pain). This is Spinoreticular fibres are intermingled with those of the spinothalamic
very similar to the pain that may be felt after an infarct of the spinal tracts and ascend in the ventrolateral quadrant of the spinal cord


internal organization
301
02
retpaHc
Ascending reticular serving pain perception. Like other ascending pathways, the tract cells
Superior colliculus projections to are influenced by descending control. For example, electrical stimula-
intralaminar thalamic tion of the periaqueductal grey matter inhibits the responses of certain
nuclei and hypothalamus
spinoreticular cells to input from cardiopulmonary afferents. Stimula-
Red nucleus
tion of the reticular formation also alters the activity of spinoreticular
neurones.
Oculomotor nerve
Spinocervicothalamic pathway
Midbrain The lateral cervical nucleus is small in humans. It lies in the lateral
funiculus, ventrolateral to the dorsal horn in the upper two cervical
Inferior colliculus Ascending reticular segments. In some human cord specimens, the nucleus is not distinctly
fibre system defined and is possibly incorporated into the dorsal horn. It receives
axons from the spinocervical tract, which ascends in the dorsolateral
Tegmentum of midbrain
funiculus. The tract cells are found in laminae III–V at all levels of the
Medial lemniscus
spinal cord, ipsilateral to the nucleus. Most neurones of the nucleus
project to the contralateral thalamus via the medial lemniscus, and
some project to the contralateral midbrain. Specific thalamic targets
include the ventral posterolateral nucleus and part of the posterior
complex. Spinocervical tract neurones respond to hair movement,
pressure, pinch and thermal stimuli and to high-threshold muscle
input; many also respond to noxious stimuli. Like tract cells of other
Pontine reticular formation ascending pathways, they are under tonic descending inhibitory
Collateral endings in
(nucleus reticularis control.
reticular formation
pontis oralis)
Spinomesencephalic pathway
The spinomesencephalic pathway consists of a number of tracts ascend-
ing from the spinal cord to various regions of the midbrain. It includes
Ascending
Trigeminal nerve the spinotectal tract projecting to the superior colliculus, neurones
collateral fibres
synapsing in the periaqueductal grey matter, and other spinal cord
Pons projections that terminate in the parabrachial nucleus, the pretectal


nuclei and the nucleus of Darkschewitsch. Cells of origin are located
throughout the length of the spinal cord, particularly in the cervical
Pontine reticular formation
(nucleus reticularis Collateral endings in segments and the lumbosacral enlargement, mostly in lamina I, but
pontis caudalis) reticular formation they are also present in laminae IV–VIII, where they are concentrated
in lamina V. Most are contralateral, but a prominent ipsilateral group
is also found at upper cervical levels. Spinomesencephalic fibres are
Facial nerve
mostly myelinated and ascend in the white matter of the ventrolateral
quadrant of the spinal cord, in association with the spinothalamic and
Abducens nerve
spinoreticular tracts.
Spinomesencephalic neurones are of low-threshold, wide-dynamic-
Ascending range or high-threshold classes. Their receptive fields may be small, or
Medullary reticular collateral fibres very complex and encompass large surface areas of the body. Many
formation
spinomesencephalic cells are nociceptive and are likely to be involved
(nucleus reticularis
gigantocellularis) Nucleus of spinal in the motivational–affective component of pain. Electrical stimulation
tract of trigeminal of their site of termination in the periaqueductal grey matter results in
Medulla nerve severe pain in humans. Furthermore, the cells of the deeper layers of
the superior colliculus, where spinotectal fibres synapse, are activated
Lateral reticular by noxious stimuli.
nucleus
Spino-olivary tract
Pontine reticulospinal fibres
Hypoglossal nerve The spino-olivary tract is described in animals as arising from neurones
Medullary in the deeper laminae of grey matter. Axons forming the tract cross and
reticulospinal fibres then ascend superficially at the junction of the ventral and lateral white
funiculi, to end in the dorsal and medial accessory olivary nuclei. The
Ascending
Ascending tract spinoreticular fibres tract carries information from muscle and tendon proprioceptors, and
Descending tract also from cutaneous receptors. A functionally similar route, the dorsal
Medullary tract spino-olivary tract, ascends in the dorsal white funiculi, and relays in


the dorsal column nuclei to the contralateral inferior olivary nucleus.
Fig. 20.15 The reticular tracts. (Redrawn with permission from Carpenter
Information on these tracts in primates is scant, but postmortem evi-
MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and
dence following cordotomies in humans has revealed degenerating
Wilkins.)
axonal terminals in the inferior olivary nucleus.
Descending tracts
(Fig. 20.15). Evidence from animal studies suggests that cells of origin
occur at all levels of the spinal cord, particularly in the upper cervical Descending pathways to the spinal cord originate primarily from the
segments. Most neurones are in lamina VII, some are in lamina VIII, cerebral cortex and from numerous sites within the brainstem (see Fig.
and others are in the dorsal horn, especially lamina V. Most axons in 20.15; Figs 20.16–20.17). They are concerned with the control of move-
the lumbar and cervical enlargements cross the midline, but there is a ment, muscle tone and posture, the modulation of spinal reflex mecha-
large uncrossed component in cervical regions. Most axons are myeli- nisms and the modulation of transmission of afferent information to
nated. The pattern of anterograde degeneration, in both human post- higher levels. They also mediate control over spinal autonomic neu-
mortem studies and in experimental animals following anterolateral rones (Kuypers 1981).
cordotomy, indicates the existence of spinoreticular projections to many
Corticospinal and corticonuclear tracts
nuclei of the medial pontomedullary reticular formation. There is also
a projection to the lateral reticular nucleus (a precerebellar relay Corticospinal and corticonuclear fibres arise from neurones in the cere-
nucleus). These projections do not appear to be somatotopically organ- bral cortex. They project, in a somatotopically organized fashion, to
ized. Spinoreticular neurones respond to inputs from the skin or deep neurones that are mostly located in the contralateral spinal cord or
tissues. Innocuous cutaneous stimuli may inhibit or excite a particular brainstem respectively (see Fig. 20.16). The majority of corticospinal


cell, whereas noxious stimuli are often excitatory. A spino-reticulo- and corticonuclear fibres arise from cells situated in the primary motor
thalamo-cortical pathway has been proposed as an important route cortex (area 4) and the premotor cortex (area 6). A small contribution


Spinal cord: internal organization
302
3
noitceS
Motor cortex (precentral gyrus) Lateral corticospinal tract
Thalamus
Leg
Trunk
R Veu sb tr io bs up loin sa pl
i
nt ara
l
c trt
acts
Posterior limb
Reticulospinal tracts
internal capsule Ar
m
Large
pyramidal
cells of Betz
Lentiform Fibres for
complex lower
extremity
External capsule
Fibres
Claustrum for trunk
Extreme Fibres for
capsule upper
extremity
Cortex of insula
Genu of
Anterior limb of internal
internal capsule capsule
Caudate nucleus (head) Corticospinal tract
Midbrain
Temporopontine tract
Crus cerebri
Frontopontine tract
Oculomotor nerve
Fig. 20.17 A simplified scheme of some of the major descending
pathways of the spinal cord, including their overlapping zones of
Pons
termination in the grey matter. Within the grey matter the dotted lines
Longitudinal fibres show the laminar pattern, while within the white matter they are an
in basilar portion approximate guide to the topography of the tracts.
of pons
Abducens nerve
are clustered into groups or strips. The largest neurones (giant pyrami-
dal neurones, Betz cells) are located in the primary motor cortex of the
precentral gyrus.
Medulla
Corticospinal and corticonuclear fibres descend through the subcor-
tical white matter to enter the genu and posterior limb of the internal
capsule. They then pass through the ventral part of the midbrain in the
Hypoglossal nerve Pyramidal crus cerebri. As they continue caudally through the pons, they are sepa-
rated from its ventral surface and fragmented into fascicles by trans-
versely running pontocerebellar fibres. Corticonuclear fibres leave to
terminate in association with the cranial nerve motor nuclei of the
Medulla midbrain, pons and medulla. In the medulla oblongata, the residual
corticospinal fibres form a discrete bundle, the pyramid, which is visible
Motor decussation
as a prominent longitudinal column on the ventral surface of the


medulla (see Fig. 21.4); the corticospinal tract is, therefore, also referred
Lateral corticospinal tract Ventral corticospinal tract to as the pyramidal tract. Each pyramid contains about a million axons
of varying diameter. The majority are myelinated. Most have a diameter
of 1–4 µm; approximately 10% have diameters of 5–10 µm; and a very
C8 few have diameters of 11–22 µm. The largest-diameter axons arise from
To motor endings the giant pyramidal neurones.
in muscles of Just rostral to the level of the spinomedullary junction, 75–90% of
forearm and hand
the corticospinal fibres in the pyramid cross the median plane in the
motor decussation (decussation of the pyramids) and continue cau-
To motor endings dally as the lateral corticospinal tract. The rest of the fibres continue
T4
in intercostal and uncrossed as the ventral corticospinal tract. The lateral corticospinal
segmental tract (see Fig. 20.16) descends in the lateral funiculus throughout most
Ventral root fibre
back muscles of the length of the spinal cord until about the fourth sacral segment.
It occupies an oval area, ventrolateral to the dorsal horn and medial to
To motor endings L4 the dorsal spinocerebellar tract (see Fig. 20.9). In the lumbar and sacral
in gluteus medius regions, where the dorsal spinocerebellar tract is absent, the lateral
and tibialis anterior corticospinal tract reaches the dorsolateral surface of the cord. The
lateral corticospinal tract is somatotopically organized such that axons
To sacral segments of cord destined for spinal segments innervating the upper limbs are located
Fig. 20.16 The corticospinal tracts. (Redrawn with permission from more medially than those destined for cord segments innervating the
Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: lower limbs (see Fig. 20.14). As it descends, its fibres terminate in pro-
Williams and Wilkins.) gressively lower spinal segments, which means that the tract diminishes


in size. The lateral corticospinal tract also contains some uncrossed
corticospinal fibres.
comes from cells in the postcentral gyrus (somatosensory cortex; areas The smaller ventral corticospinal tract (see Fig. 20.16) descends in
3, 1 and 2) and the adjacent parietal cortex (area 5). In the monkey, the ventral funiculus. It lies close to the ventral median fissure, and is
30% of corticospinal fibres arise from area 4, 30% from area 6, and separated from it by the sulcomarginal fasciculus (see Fig. 20.9). The
40% from the parietal regions. The cells of origin of corticospinal and ventral corticospinal tract diminishes as it descends and usually disap-
corticonuclear fibres vary in size according to their cortical origins and pears completely at mid-thoracic cord levels. It may either be absent or,


internal organization
303
02
retpaHc
very rarely, contain almost all the corticospinal fibres. Near their termi- cervical cord segments. Rubrospinal fibres are distributed to the lateral
nation, most fibres of the tract cross the median plane in the ventral parts of laminae V–VI and the dorsal part of lamina VII of the spinal
white commissure to synapse on contralateral neurones. The vast grey matter. The terminal zones of the tract correspond to those of
majority of corticospinal fibres, irrespective of the tract in which they corticospinal fibres from the motor cortex. Animal studies demonstrate
descend, therefore terminate in the spinal cord on the side contralateral that the effects of rubrospinal fibres on α and γ motor neurones are
to their cortical origin. similar to those of corticospinal fibres.
Knowledge of the detailed termination of corticospinal fibres is
Tectospinal tract
based largely on animal studies, but is supplemented by data from
postmortem studies on human brains using anterograde degeneration The tectospinal tract arises from neurones in the intermediate and
methods. Most corticospinal fibres are believed to terminate contralat- deep layers of the superior colliculus of the midbrain. It crosses ventral
erally on interneurones in the lateral parts of laminae IV–VI and both to the periaqueductal grey matter in the dorsal tegmental decussation
lateral and medial parts of lamina VII; some are also distributed to and descends in the medial part of the ventral funiculus of the spinal
lamina VIII bilaterally. Terminals are also associated with contralateral cord (see Fig. 20.9). Fibres of the tract project only to the upper cervi-
motor neuronal cell groups in lamina IX, in the dorsolateral group and cal cord segments, ending in laminae VI–VIII. They make polysynaptic
the lateral parts of both central and ventrolateral groups (see Fig. connections with motor neurones serving muscles in the neck, facili-
20.17). tating those that innervate contralateral muscles and inhibiting those
Corticospinal fibres from the frontal cortex, including motor and that innervate ipsilateral ones. In animals, unilateral electrical stimula-


premotor areas 4 and 6, terminate mostly on interneurones in laminae tion of the superior colliculus causes turning of the head to the
V–VIII, with the densest concentration ending in the lateral part of contralateral side, an effect mainly mediated through the tectospinal
lamina VI. They influence α and γ motor neurones of lamina IX, via tract.
these interneurones. As the widespread dendrites of multipolar neu-
Vestibulospinal tracts
rones in lamina IX penetrate lamina VII, direct monosynaptic axoden-
dritic contacts also occur on large α motor neurones. Direct termination The vestibular nuclear complex lies in the lateral part of the floor of the
on motor neurones is most abundant in the spinal enlargements. fourth ventricle, at the level of the pontomedullary junction. It gives
Experimental evidence shows that precentral corticospinal axons rise to the lateral and medial vestibulospinal tracts, which are function-
influence the activities of both α and γ motor neurones, facilitating ally and topographically distinct (Fig. 20.18).
flexor muscles and inhibiting extensors, which are the opposite effects The lateral vestibulospinal tract arises from small and large neu-
to those mediated by lateral vestibulospinal fibres. Evidence from rones of the lateral vestibular nucleus (Deiters’ nucleus). It descends
animal studies shows that direct projections from the precentral cortical ipsilaterally, initially in the periphery of the ventrolateral spinal white
areas to spinal motor neurones are concerned with highly fractionated, matter, but subsequently migrating into the medial part of the ventral
precision movements of the limbs. Accordingly, in primates, precentral funiculus at lower spinal levels. Fibres of this tract are somatotopically
corticospinal fibres are mainly distributed to motor neurones supplying organized. Thus, fibres projecting to the cervical, thoracic and lum-
the distal limb muscles. Corticospinal projections may use glutamate bosacral segments of the cord arise from neurones in the rostroventral,
or aspartate, often co-localized, as excitatory neurotransmitters. central and dorsocaudal parts, respectively, of the lateral vestibular


Corticospinal fibres from parietal sources end mainly in the contra- nucleus. Lateral vestibulospinal fibres end ipsilaterally, mostly in the
lateral dorsal horn, in the lateral parts of laminae IV–VI and lamina VII. medial part of the ventral horn in lamina VIII and the medial part of
Phylogenetically these fibres represent the oldest part of the corticospi- lamina VII.
nal system. Axons from the sensory cortex terminate chiefly in laminae The medial vestibulospinal tract (see Fig. 20.18) arises mainly from
IV and V. They are concerned with the supraspinal modulation of the neurones in the medial vestibular nucleus, but some are also located
transmission of afferent impulses to higher centres, including the motor in the inferior and lateral vestibular nuclei. It descends in the medial
cortex. longitudinal fasciculus into the ventral funiculus of the spinal cord,
Experimental studies in primates indicate that isolated transection where it lies close to the midline in the so-called sulcomarginal fas-
of corticospinal fibres at the level of the pyramid (pyramidotomy) ciculus (see Fig. 20.9). Unlike the lateral tract, it contains both crossed
results in flaccid paralysis or paresis of the contralateral limbs and loss and uncrossed fibres, and does not extend beyond the mid-thoracic
of independent hand and finger movements. Destruction of corticospi- cord level. Fibres of the medial tract project mainly to the cervical
nal fibres at the level of the internal capsule, commonly caused by a cord segments, ending in lamina VIII and the adjacent dorsal part of
cerebral vascular accident or ‘stroke’, results in a contralateral hemiple- lamina VII.
gia. The paralysis is initially flaccid but later becomes spastic, and is The vestibular nuclei exert a strong excitatory influence upon the
most marked in the distal muscles of the extremities, especially those antigravity muscles by way of the medial and lateral vestibulospinal
concerned with individual movements of the fingers and hand. Associ- tracts. The antigravity muscles include the epaxial muscles of the verte-
ated signs on the paralysed side are: hyperactive deep tendon reflexes; bral column and the extensor muscles of the lower limbs.


increased tone; the loss of superficial abdominal and cremasteric
Reticulospinal tracts
reflexes; and the appearance of dorsiflexion of the toes (Babinski’s sign)
in response to stroking the sole of the foot. The latter is usually inter- The reticulospinal tracts pass from the brainstem reticular formation to
preted as pathognomonic of corticospinal damage, but it is not always the spinal cord. The pontine reticulospinal tract (see Fig. 20.15) origi-
present in patients with confirmed corticospinal lesions. More over, Bab- nates from the pontine tegmentum. The medullary reticulospinal tract
inski’s sign is normally present in human infants up to about 2 years arises from the nucleus gigantocellularis, which lies dorsal to the infe-
of age; its subsequent disappearance may reflect the completion of rior olivary complex. The terminals of reticulospinal fibres are distrib-
myelination of the corticospinal fibres and/or the establishment of uted to lamina VIII, and the central and medial parts of lamina VII. The
direct cortical connections to lower motor neurones. medullary reticulospinal terminals are more widely distributed, ending
Some of the sequelae of stroke damage in the internal capsule, in additionally in the lateral parts of laminae VI and VII, and also directly
particular hyperreflexia and hypertonia, are due to the involvement of on motor neurones. Terminations of reticulospinal fibres that originate
other pathways in addition to the corticospinal tract. These include in the medulla are, in general, more dorsally placed than those that
descending cortical fibres to brainstem nuclei, such as the vestibular originate in the pons, although there is considerable overlap.
and reticular nuclei, which also give rise to descending projections that The course and location of the reticulospinal tracts are poorly
influence motor neurone activity. defined in humans. Some anatomical texts classify the reticulospinal
tracts into medial, ventral and ventrolateral tracts, but this is based
Rubrospinal tract
mainly on studies in animals (see Fig. 20.9). Studies in humans have


The rubrospinal tract arises from neurones in the caudal magnocellular reported that the reticulospinal fibres in general do not form well-
part of the red nucleus (an ovoid mass of cells situated centrally in the defined tracts but are scattered throughout the ventral and lateral
midbrain tegmentum; p. 324). This part of the nucleus contains some columns (Nathan et al 1996).
150–200 large neurones, interspersed with smaller neurones. Both α and γ motor neurones are influenced by reticulospinal fibres,
The origin, localization, termination and functions of rubrospinal through polysynaptic and monosynaptic connections. The pontine
connections are poorly defined in humans, and the tract appears to be reticulospinal tract appears to be tonically active and is excitatory to the
rudimentary. Rubrospinal fibres cross in the ventral tegmental decussa- antigravity muscles, including the epaxial muscles of the vertebral
tion and descend in the lateral funiculus of the cord, where they lie column and the extensor muscles of the lower limbs.
ventral to, and intermingled with, fibres of the lateral corticospinal tract On the other hand, the medullary reticulospinal tract is inhibitory
(see Fig. 20.9). In animals, the tract descends as far as lumbosacral to antigravity muscles. The cells of origin of the medullary reticulospi-
levels, whereas in humans it appears to project only to the upper three nal tract receive input from the corticospinal tract and the rubrospinal


Spinal cord: internal organization
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Vestibular nuclei: Solitariospinal tract
Medial nucleus
The solitariospinal tract is a small group of mostly crossed fibres that
Superior nucleus
Fourth ventricle arises from neurones in the ventrolateral part of the nucleus solitarius
Lateral nucleus
Abducens nucleus of the medulla. Descending in the ventral funiculus and ventral part of
Spinal nucleus of the lateral funiculus of the cord, these axons terminate on phrenic
trigeminal nerve motor neurones supplying the diaphragm and thoracic motor neurones
Pontomedullary Restiform body that innervate intercostal muscles. A pathway with somewhat similar
course and terminations to that of the solitariospinal tract originates
junction
Brachium pontis
from the nucleus retroambiguus. Both pathways subserve respiratory
Facial nerve Vestibular nerve activities by driving inspiratory muscles, and some descending axons
and nucleus from the nucleus retroambiguus facilitate expiratory motor neurones.
There is clinical evidence that bilateral ventrolateral cordotomy at high
cervical levels abolishes rhythmic ventilatory movements.
Nucleus of Hypothalamospinal fibres
superior olive
Hypothalamospinal fibres exist in animals. They arise from the paraven-
Medial vestibular nucleus Medial lemniscus tricular nucleus and other areas of the hypothalamus, and descend
Inferior vestibular nucleus ipsilaterally, mainly in the dorsolateral region of the cord, to be distrib-
uted to sympathetic and parasympathetic preganglionic neurones in the
Restiform body
Medulla intermediolateral column. Fibres from the paraventricular nucleus
show oxytocin and vasopressin immunoreactivity. They are also distrib-
Medial lemniscus uted to laminae I and X. Descending fibres from the dopaminergic cell
group (A11) situated in the caudal hypothalamus innervate sympathetic
Pyramid
preganglionic neurones and neurones in the dorsal horn. That similar
pathways exist in humans may be inferred from ipsilateral sympathetic
deficits (e.g. Horner’s syndrome), which follow lesions of the hypotha-
lamus, the lateral tegmental brainstem, or the lateral funiculus of
Medulla


the cord.
Accessory nerve
Ventral grey column Motor decussation Monoaminergic spinal pathways
Monoaminergic cell groups utilize dopamine, adrenaline (epineph-
rine), noradrenaline (norepinephrine) and 5-HT as neurotransmitters.
Medial vestibulospinal tract Lateral vestibulospinal tract
They occur widely throughout the brainstem and in the hypothalamus.
They project rostrally to many forebrain areas and caudally to the spinal
cord, and appear to be concerned with the modulation of sensory
C3 transmission, and the control of autonomic and somatic motor neuro-
Ventral horn cell nal activities.
The projections to the spinal cord arise from several sources. Coer-
Motor end-plates
uleospinal projections originate from noradrenergic cell groups A4 and
in trapezius and
Ventral root nerve fibre scalene muscles A6 in the locus coeruleus complex in the pons and descend via the
ventrolateral white matter to innervate all cord segments bilaterally.
They end in the dorsal grey matter (laminae IV–VI) and the intermedi-
T4
ate and ventral horns. They also project extensively to preganglionic
Motor end-plates parasympathetic neurones in the sacral cord. Descending noradrenergic
in intercostal fibres, which arise from the lateral tegmental cell groups A5 and A7 of
and segmental the pons, travel in the dorsolateral white matter. They are distributed
back muscles
to laminae I–III, and particularly to the intermediate grey horn.
L3 Descending fibres from adrenergic cell groups C1 and C3 of the medulla
oblongata have been traced into the ventral funiculus of the cord and
Motor end-plates
in quadriceps are extensively distributed to the intermediolateral column. Dopamin-
femoris ergic fibres projecting to the spinal cord travel in the hypothalamospi-
nal pathway.
S2
The raphe nuclei pallidus (B1), obscurus (B2) and magnus (B3) in
Motor end-plates
the brainstem give rise to two serotoninergic descending bundles. The
in gastrocnemius
lateral raphe spinal bundle, from B3 neurones, is concerned with the


Fig. 20.18 The vestibulospinal tracts. (Redrawn with permission from
control of nociception. It descends close to the lateral corticospinal
Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore:
tract and ends in the dorsal horn (laminae I, II and V). The ventral
Williams and Wilkins.)
bundle, composed mainly of axons from B1 neurones, travels in the
medial part of the ventral white column and ends in the ventral horn
(laminae VIII and IX). It facilitates extensor and flexor motor neurones.
Some descending serotoninergic fibres project to sympathetic pregan-
tract. The reticulospinal tracts normally function in a coordinated, bal-
glionic neurones and are concerned with the central control of cardio-
anced fashion to control muscle tone. However, if the descending
vascular function.
control from higher centres is lost, such as occurs after a stroke in the
internal capsule, then the natural excitability of the pontine reticulo-
Propriospinal pathways
spinal tract, combined with the activity of the vestibulospinal tracts,
causes the antigravity muscles to become hypertonic and hyperreflexic
(spasticity) (Guyton and Hall 2006). Propriospinal pathways (fasciculi proprii) consist of the ascending and
descending fibres of intrinsic spinal neurones. They contact other neu-
Interstitiospinal tract rones within the same segment and/or in more distant segments of the
The interstitiospinal tract arises from neurones in the interstitial nucleus spinal cord and so subserve intrasegmental and intersegmental integra-
(of Cajal) and the immediate surrounding area, and descends via the tion and coordination. The majority of spinal neurones are propriospi-
medial longitudinal fasciculus into the ventral funiculus of the spinal nal neurones, most of which lie in laminae V–VIII. Propriospinal fibres
cord. Its fibres project, mainly ipsilaterally, as far as lumbosacral levels, are mainly concentrated around the margins of the grey matter (see Fig.
and are mostly distributed to the dorsal part of lamina VIII and the 20.9), but are also dispersed diffusely in the white funiculi.


dorsally adjoining part of lamina VII. They establish some monosyn- The propriospinal system plays important roles in spinal functions.
aptic connections with motor neurones supplying neck muscles, but Descending pathways end on specific subgroups of propriospinal neu-
their main connections are disynaptic with motor neurones supplying rones and these, in turn, relay to motor neurones and other spinal
limb muscles. neurones. The system mediates all those automatic functions that


Spinal reflexes
305
02
retpaHc
continue after transection of the spinal cord, e.g. sudomotor and vaso- Dorsal root Dorsal root
motor activities, and bowel and bladder functions. ganglion
Some propriospinal axons are very short and span only one segment,
while others run the entire length of the cord. The shortest axons lie
immediately adjacent to the grey matter and the longer ones are situ-
ated more peripherally. Propriospinal neurones can be categorized
according to the length of their axons as long, intermediate or short
neurones. Long propriospinal neurones distribute their axons through-
out the length of the cord, mainly via the ventral and lateral funiculi;
their cell bodies are in lamina VIII and the dorsally adjoining part of
lamina VII. Axons from the long propriospinal neurones of the cervical
cord descend bilaterally, whereas those from the corresponding lum-
bosacral neurones ascend mainly contralaterally. Most of the fibres are
Ventral root
fine (less than 3 µm in diameter). Some are the first spinal tract axons
to become myelinated. Intermediate propriospinal neurones occupy the
central and medial parts of lamina VII and project mainly ipsilaterally. Leg extensor
muscles
Short propriospinal neurones are found in the lateral parts of laminae
V–VIII and their axons run ipsilaterally in the lateral funiculus.
Propriospinal fibres in the different parts of the white funiculi are Muscle spindle
distributed preferentially to specific regions of the spinal grey matter. Golgi tendon
Motor
In the spinal enlargements, the propriospinal fibres in the dorsolateral end-plates organ
funiculus project to the dorsal and lateral parts of the intermediate
zone, and also to spinal motor neurones that supply distal limb muscles, Leg flexor muscles
especially those of the hand and the foot. The propriospinal fibres in
the ventral part of the ventrolateral funiculus are distributed to the Patellar tendon
central and medial parts of lamina VII and to motor neurones of proxi-
mal limb and girdle muscles. Other propriospinal fibres run in the


medial part of the ventral funiculus and travel mainly to the ventrome-
dial part of the intermediate zone, which characteristically contains
long propriospinal neurones, and to motor neurones innervating axial
and girdle muscles.
The tract of Lissauer
Fig. 20.19 The pathway for the patellar tendon reflex and reciprocal
The tract of Lissauer (dorsolateral tract) lies between the apex of the inhibition (left) and autogenic inhibition (right). The inhibitory glycinergic
dorsal horn and the surface of the spinal cord, where it surrounds the interneurones are represented by the red open cell bodies. (Redrawn with
incoming dorsal root fibres. It is present throughout the spinal cord and permission from Haines D, Fundamental Neuroscience for Basic and
is most developed in the upper cervical regions. Clinical Applications, 4th Edition, 2013, Saunders, Elsevier.)
The tract consists of fine myelinated and unmyelinated axons. Many
are the branches of axons in the lateral bundles of the dorsal roots.
These axons bifurcate into ascending and descending branches as they
enter the cord. The branches travel in the tract of Lissauer for one or efferent neurone, and it is therefore also known as a monosynaptic
two segments and give off collaterals that end on and around neurones stretch reflex or a myotactic reflex. The afferent component arises from
in the dorsal horn. The tract also contains propriospinal fibres, some stretch receptors associated with intrafusal muscle fibres located within
being short axons of small substantia gelatinosa neurones, which muscle spindles. The primary or anulospiral endings of these receptive
re-enter the dorsal horn. cells give rise to primary afferent fibres that enter the spinal cord, where
they make excitatory synaptic contact directly on to α motor neurones
innervating the same muscle (Fig. 20.19). The α motor neurones of
SPINAL REFLEXES
antagonistic muscles are simultaneously inhibited via collateral connec-
tions to inhibitory interneurones (reciprocal innervation).
The intrinsic connections of the spinal cord and brainstem subserve a
number of reflexes by which the functions of peripheral structures are gamma reflex


modulated in response to afferent information in a relatively automatic As well as α motor neurones innervating extrafusal muscle fibres, mus-
or autonomous fashion. The fundamental components of such reflex cles also receive γ motor neurones, which innervate intrafusal muscle
‘arcs’ are an afferent and an efferent neurone, although in all but the fibres. Activation of γ motor neurones increases the sensitivity of the
simplest of reflexes, interneurones intervene between the afferent and intrafusal fibres to stretch; changes in γ activity have a profound effect
efferent components, conferring increased versatility and complexity on on the stretch reflex and on muscle tone. Like α motor neurones, γ
reflex responses. Reflexes may be confined to a single cord level motor neurones are under the influence of descending pathways from
(intrasegmental) or involve several cord segments (intersegmental). the brainstem and cerebral cortex. Changes in muscle tone and in the
Reflexes, by their very nature, are relatively fixed and stereotyped in activity of the stretch reflex are commonly found in disorders of the
form. Nevertheless, they are strongly influenced and modulated by central nervous system as well as the peripheral nervous system.
descending connections. In the case of spinal reflexes, these descending
controls come from both the brainstem and the cerebral cortex. Pathol- Flexor reflex and crossed extensor reflex
ogy of descending supraspinal pathways commonly causes abnormali- Painful stimulation of the limbs leads to flexion withdrawal, mediated
ties of spinal reflex activity, which are routinely tested for in neurological by a polysynaptic reflex (Fig. 20.20) in which interneurones link the
examination; reflexes may be absent (areflexia), diminished (hypore- afferent and efferent neurones. Thus, activation of nociceptive primary
flexia) or exaggerated (hyperreflexia). During development, descending afferents indirectly causes activation of limb flexor motor neurones.
control mechanisms suppress what may be regarded as ‘primitive’ Collateralization of fibres to nearby spinal segments mediates flexion
spinal reflex responses, such as the extensor plantar reflex and the grasp of a limb at several joints, depending on the intensity of the stimulus.
reflex. When the higher control mechanisms become damaged, these Decussating connections to the contralateral side of the cord activate α


reflexes are released and reappear as a sign of central nervous system motor neurones innervating corresponding extensor muscles, which
pathology (e.g. the Babinski reflex). produces a crossed extensor reflex (Fig. 20.21). In principle, virtually
any cutaneous stimulus has the potential to induce a flexor reflex but,
Muscle stretch reflex other than in the case of noxious stimuli, this response is normally
The muscle stretch reflex (also known as the tendon reflex or deep inhibited by descending pathways. When descending influences are
tendon reflex) is the mechanism by which stretch applied to a muscle lost, even harmless cutaneous stimulation can elicit flexion of the limbs.
elicits its reflex contraction. It is essential for the maintenance of both The Babinski (extensor plantar) reflex, which is generally regarded as
muscle tone and an upright stance (via the innervation of the postural pathognomonic of damage to the corticospinal tract, is part of a flexion
muscles of the neck, back and lower limbs). Anatomically it is the withdrawal of the lower limb in response to stimulation of the sole of
simplest of reflexes, since it is mediated solely by an afferent and an the foot.


Spinal cord: internal organization
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Posterolateral tract
Dorsal root
Dorsal root
ganglion
Flexor motor neurone
Extensor motor neurone
Ventral root
Extensor muscles of leg Leg extensors
Flexor muscles of leg Leg flexors
Muscles of foot
Flexor muscles of foot Dorsiflexors
Plantar flexors
Nociceptive input Nociceptive input
Fig. 20.20 The pathway for the flexor reflex. The inhibitory glycinergic Fig. 20.21 The pathway for the crossed extensor reflex. Glycinergic
interneurone is represented by the red open cell and the excitatory interneurones (inhibitory) are represented by the open red cell bodies and
glutaminergic interneurone by the green closed cell. In addition to being glutaminergic interneurones (excitatory) by the closed green cells.
involved in reflexes, this nociceptive input is also relayed to higher levels (Redrawn with permission from Haines D, Fundamental Neuroscience for
of the neuraxis via the anterolateral system (ascending black fibre). (With Basic and Clinical Applications, 4th Edition, 2013, Saunders, Elsevier.)
permission from Haines D, Fundamental Neuroscience for Basic and
Clinical Applications, 4th Edition, 2013, Saunders, Elsevier.)
and absent abdominal reflexes. Descending pathways to the bladder are
SPINAL CORD LESIONS
interrupted and this produces a ‘neurogenic bladder’.
The same principles apply to lesions at other levels of the spinal cord
Mechanical compression and secondary ischaemic damage to underly- and they are illustrated in Figure 20.22.
ing nervous tissue cause surgically relevant spinal cord disease (mye- The time course of spinal cord disease is very important in suggesting
lopathy). The site and the level of damage to the cord determine the the likely cause, as the rate of evolution of a disorder varies depending
particular clinical syndrome, e.g. whether the lesion involves the upper on the aetiology. Disorders with sudden or rapid onset are usually
or lower cervical, thoracic or lumbosacral spinal cord. At each of these caused by trauma or by a vascular accident (i.e. stroke). When a condi-


levels, symptoms and signs are determined by direct destruction of tion develops over days it is often due to an inflammatory disorder such
segmental tissue, i.e. transversely distributed damage, and disconnec- as multiple sclerosis. Neoplastic lesions such as a spinal cord meningi-
tion of suprasegmental ascending and descending tracts above and oma often progress over months. Degenerative problems usually
below the level of a lesion, i.e. longitudinally distributed damage (Fig. progress over months (such as amyotrophic lateral sclerosis or motor
20.22). For example, a lower cervical spinal cord lesion damages the neurone disease) or even years (e.g. hereditary spastic paraparesis).
segmental sensory and motor contributions to the nerve roots and In the online supplementary data there are case histories that illus-
brachial plexus, causing sensory loss, weakness and wasting of the trate typical patterns of spinal cord disease (Boxes 20.1–20.5). A good
muscles and loss of tendon reflexes in the upper limbs. Disruption of knowledge of spinal cord anatomy is vital in making the correct
the ascending sensory pathways in the lateral and dorsal columns of diagnosis.
the cervical spinal cord leads to loss of sensation to pain and tempera-
ture (lateral spinothalamic tracts) and touch and proprioception (dorsal
columns) below the ‘sensory level’ corresponding to the segment of the
Bonus e-book image
spinal cord. Damage to the descending corticospinal tracts in the lateral
columns of the spinal cord produces a spastic paraparesis, i.e. increased
tone of the muscles, weakness that is of an upper motor neurone pattern
(weakness of extensors more than flexors in the upper limbs; weakness Fig. 20.5 The basic arrangement of the sensory ‘gate’ mechanism
of flexors more than extensors in the lower limbs), exaggerated tendon in the dorsal laminae of the grey matter of the spinal cord.
reflexes and abnormal superficial reflexes, e.g. extensor plantar responses


Spinal cord: internal organization
306.e1
02
retpaHc
Box 20.1 case history 1: complete transection of the spinal cord Box 20.4 case history 4: posterior cord syndrome
A 29-year-old man fractured his cervical spine after a road traffic A 73-year-old man presented with a 6-month history of
accident, leading to transection of the spinal cord at C5. When unsteadiness on walking. When examined, he had an unsteady gait,
examined 6 months later, he had paralysis of all four limbs but a tendency to fall when asked to stand still with his eyes closed
shoulder abduction and elbow flexion were spared (i.e. all muscles (positive Romberg’s test), and impaired proprioception in all four
innervated by nerve fibres arising below C5 were paralysed). Tone limbs. Spinothalamic sensation (pinprick sensation) was intact and
was increased in the limbs with brisk reflexes and extensor plantar his power was normal.
responses (i.e. the weakness was upper motor neurone or pyramidal
Investigations revealed signal change in the posterior cervical cord
weakness). He had complete loss of sensation below C5 and a
on magnetic resonance imaging and his vitamin B levels were very
neurogenic bladder. 12
low. A diagnosis of posterior cord syndrome secondary to B
12
deficiency was made. Posterior cord syndromes involve the dorsal
columns, leading to impaired proprioception. Although not illustrated
in this case, B deficiency sometimes affects the corticospinal tract, Box 20.2 case history 2: Hemisection of the spinal cord giving 12
causing weakness in the limbs, and may also sometimes affect the
rise to the Brown–Séquard syndrome
peripheral nerves.
A 20-year-old woman presented with a 4-day history of gradually
worsening right leg weakness, burning pain and loss of sensation in
the left leg. On examination she had pyramidal pattern weakness of
the right leg (i.e. hip and knee flexion and ankle dorsiflexion were
Box 20.5 case history 5: central cord syndrome
mainly affected). Tone was increased in the right leg: there were
brisk right leg reflexes and a right extensor plantar response (i.e. A 20-year-old woman presented with a 4-month history of painless


right pyramidal weakness). There was impaired proprioception in the injuries to her hands. For example, after gardening she would
right leg. The patient had reduced pinprick sensation affecting the sometimes come inside and notice later that there were rose thorns
left leg and side up to the umbilicus. sticking into her hands. Examination showed reduced pinprick
sensation (spinothalamic sensation) over the upper limbs.
This case illustrates a Brown–Séquard syndrome resulting from
hemisection of the cord caused by a right-sided cord lesion in the Magnetic resonance imaging of her cervical spine showed a
thoracic spine. Involvement of the pyramidal tract causes pyramidal collection of fluid in the centre of the cervical spinal cord
weakness below the level of the lesion on the same side as the (syringomyelia). A diagnosis of central cord syndrome secondary to
lesion (because the pyramidal tract crosses in the medulla). syringomyelia was made. As spinothalamic tract fibres cross over
Involvement of the dorsal columns causes impaired proprioception from the dorsal horn to run in the contralateral spinothalamic tract,
below the level of the lesion on the same side as the lesion they pass very close to the centre of the cord. Central cord
(because the fibres in the dorsal columns also cross in the medulla). syndromes thus initially damage the spinothalamic tracts, resulting
Involvement of the spinothalamic tract causes loss of pinprick in altered pain sensation at the level of the lesion. As the lesion
sensation below the lesion on the contralateral side (because the expands to involve the ventral horns, patients may develop lower
spinothalamic tract crosses the midline soon after entering the motor neurone weakness of the upper limbs. They may
cord). The progression of symptoms over several days suggests an subsequently experience upper motor neurone problems in the legs
inflammatory problem. Magnetic resonance imaging of the cord if the lesion expands further to involve the corticospinal tracts.
showed a right-sided area of cord inflammation (myelitis) at T10.
The patient had previously had an episode of inflammation affecting
the optic nerve (optic neuritis) and the diagnosis was therefore one
of multiple sclerosis.
Box 20.3 case history 3: anterior cord syndrome
A 75-year-old man with diabetes and hypertension presented with


sudden onset of weakness and sensory disturbance in both legs.
When examined 6 months later, he had pyramidal weakness of both
legs with increased tone, brisk reflexes and extensor plantar
responses, and a complete loss of pinprick sensation below T10.
Proprioception was normal.
The acute onset suggested a vascular problem. The diagnosis was
one of an anterior spinal artery infarction. Proprioception was
spared because the anterior spinal artery supplies only the anterior
cord, whereas the posterior cord, including the dorsal columns, is
supplied by the posterior spinal arteries, and so was spared.


Spinal cord lesions
307
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retpaHc
Cranial nerves and motor system Reflexes Sensation Coordination
++++++
+++ +++
– –
+++ +++
Upper cervical cord lesion
A high cervical cord lesion causes spastic tetraplegia with hyperreflexia, extensor plantar responses (upper motor neurone lesion), incontinence, sensory loss below the level of the lesion and 'sensory' ataxia.
Cranial nerves and motor system Reflexes Sensation Coordination
– – – –
– –
– –
+++ +++
+++ +++
Lower cervical cord lesion
A lower cervical cord lesion causes weakness, wasting and fasciculation of muscles and areflexia of the upper limbs (lower motor neurone lesion). In addition, there is spastic paraparesis, hyperreflexia and
extensor plantar responses (upper motor neurone lesion) in the lower limbs, incontinence, sensory loss below the level of the lesion and 'sensory' ataxia.
Cranial nerves and motor system Reflexes Sensation Coordination
+ +
+ +
+ – – +
+++ +++
++++++
Thoracic cord lesion
A thoracic cord lesion causes a spastic paraparesis, hyperreflexia and extensor plantar responses (upper motor neurone lesion), incontinence, sensory loss below the level of the lesion and 'sensory' ataxia.
Fig. 20.22 Lesions of the spinal cord. Continued


Spinal cord: internal organization
308
3
noitceS
Cranial nerves and motor system Reflexes Sensation Coordination
+ +
+ +
+ + + +
– –
– –
Lumbar cord lesion
A lumbar cord lesion causes weakness, wasting and fasciculation of muscles, areflexia of the lower limbs (lower motor neurone lesion), incontinence, sensory loss below the level of the lesion and 'sensory' ataxia.
Cranial nerves and motor system Reflexes Sensation Coordination
+ +
+ +
+ + + +
+++ +
+++ + Proprioception Pain and
and touch loss temperature loss
Hemisection of the cord gives rise to the Brown-Séquard syndrome
This is characterized by ipsilateral loss of proprioception and upper motor neurone signs (hemiplegia/monoplegia) plus contralateral loss of pain and temperature sensation.
Fig. 20.22, cont’d Lesions of the spinal cord.(With permission from Crossman AR, Neary D 2010 Neuroanatomy, 4th edn. Edinburgh: Churchill
Livingstone.)
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The original description of the gate theory of pain.


Spinal cord: internal organization
308.e1
02
retpaHc
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A detailed classic review of the structure and function of muscle spindles. Pain 155:210–6.
Braz J, Solorzano C, Wang X et al 2014 Transmitting pain and itch messages: Moayedi M, Davis KD 2013 Theories of pain: from specificity to gate control.
a contemporary view of the spinal cord circuits that generate gate J Neurophysiol 109:5–12.
control. Neuron 82:522–36. Nathan PW, Smith M, Deacon P 1996 Vestibulospinal, reticulospinal and
Bushnell MC, Čeko M, Low LA 2013 Cognitive and emotional control of descending propriospinal nerve fibres in man. Brain 119:1809–33.
pain and its disruption in chronic pain. Nat Rev Neurosci 14:502–11. An analysis of the vestibulospinal, reticulospinal and propriospinal pathways
in humans, illustrating that these pathways are scattered throughout the
Combarros O, Vadillo A, Gutierrez-Perez R et al 2002 Cervical spinal cord
ventral and lateral columns rather than forming discrete tracts.
infarction simulating myocardial infarction. Eur Neurol 47:185–6.
Denk F, McMahon SB, Tracey I 2014 Pain vulnerability: a neurobiological Orhan M, Saylam CY, Ikiz ZAA et al 2009 Connections between the acces-
perspective. Nat Neurosci 17:192–200. sory nerve and the posterior root of the first cervical nerve. Surg Radiol
A discussion of the potential mechanisms that have been proposed as Anat 31:107–11.
underlying vulnerability and resilience towards developing chronic pain. Ossipov MH, Morimura K, Porreca F 2014 Descending pain modulation and
chronification of pain. Curr Opin Support Palliat Care 8:143–51.
Devor M 2013 Neuropathic pain: pathophysiological response of nerves to


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Melzack’s Textbook of Pain, 6th ed. Philadelphia: Elsevier, Saunders; Williams & Wilkins.
Ch. 61, pp. 861–88. Rexed B 1952 The cytoarchitectonic organization of the spinal cord in the
Guyton A, Hall J 2006 Cortical and brain stem control of motor function. cat. J Comp Neurol 96:415–95.
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Elsevier, Saunders, pp. 685–97. Routal RV, Pal GP 2000 Location of the spinal nucleus of the accessory nerve
A review of what is currently understood to be the function of the in the human spinal cord. J Anat 196:263–8.
descending motor pathways, including the vestibulospinal and reticulospinal
Saylam CY, Orhan M, Ikiz ZAA et al 2009 Connection types between the
tracts.
spinal root of the accessory nerve and the posterior roots of the C2–C6
Jenny A, Smith J, Decker J 1988 Motor organization of the spinal accessory spinal nerves. Surg Radiol Anat 31:419–23.
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Motor Control, pt 1. Bethesda, MD: Am Physiol Soc, pp. 597–666. Todd A, Koerber R 2013 Neuroanatomical substrates of spinal nociception.


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The original description of the gate theory of pain. 1097–110.


CHAPTER
21
Brainstem
The brainstem consists of the medulla oblongata, pons and midbrain.
It is situated in the posterior cranial fossa, and its ventral surface lies
on the clivus. It contains numerous intrinsic neuronal cell bodies and
their processes, some of which are the brainstem homologues of
spinal cell groups. Some brainstem cell groups are the nuclei of cranial
nerves and are concerned with the sensory, motor and autonomic
innervation of the head and neck. Other autonomic (visceromotor)
Anterolateral system
fibres that arise from the brainstem are distributed widely within the
thorax and abdomen via branches of the vagus nerve. The brainstem
also contains a complex and sometimes ill-defined network of neu-
rones, known as the reticular formation (Haines 2013, Olszewski and
Baxter 1954), which extends throughout its length, and is continuous
caudally with its spinal counterpart. Some reticular nuclei are referred
A
to as vital centres because they are concerned with regulation of
cardiac and respiratory activities; other parts of the reticular formation
are essential for cerebral cortical arousal and the maintenance of con- Spinal tract of trigeminal nerve Spinal nucleus of trigeminal nerve
sciousness, or are involved in the regulation of muscle tone, posture
and reflex activities. The brainstem is the site of origin and/or termi-
nation of numerous ascending and descending fibres and is traversed 1 2
V1
by many others. The anterolateral system (spinal lemniscus), medial
lemniscus and the trigeminothalamic tracts all ascend through the 3
brainstem to reach the thalamus. Prominent corticospinal projections Spinal tract of trigeminal nerve 4
descend through the brainstem and corticonuclear projections end
within it. Spinal nucleus of trigeminal nerve V2
5
Damage to the brainstem is often devastating and life-threatening 8
(Posner et al 2007). This is because it is a structurally and functionally 6
compact region, where even small lesions can destroy vital centres 7
(cardiac and respiratory), disconnect forebrain motor areas from brain- 9
11
stem and spinal motor neurones, and sever incoming sensory fibres


from higher centres of consciousness, perception and cognition. Irre- V1 V3 10
versible cardiac and respiratory arrest follows complete destruction of
the neural respiratory and cardiac centres in the medulla. Clinically, this
V2
is part of a complex event called brain death (cerebral death), a condi- V3
tion that includes cessation of brainstem function. Accurate diagnosis
i ps
i
re as ts oe rn yt i aa nl db e cc aa ru ds ie
a
cb r fa ui nn cs tt ie om
n
sd mea ath
y
m bea y
a
ro tic fic cu ir
a
li ln
y
p ma ati ie nn tats
i
nw eh do is ne
d
r ee fis-
-
Caudal
nitely on life support.
1
THE BRAINSTEM IN A CLINICAL CONTEXT
2
Rostral
The advent of computed tomography (CT) and magnetic resonance
3
imaging (MRI) has revolutionized clinical medicine and forever altered
4
how we look at the brainstem in an educational setting. This is espe-
5
cially the case when the goal is to teach neuroanatomical concepts in 8
a
v
iem
w
Ta hen
d
e
n
ta
ie mnr
d
em
-u
ho
s
oes nt
d
o
a
i
unp
r
p ethl di ec
a
a
c
pb
l
pil ne
r
oict ao
a cl
h
t
e
h
tn
oe
v i
tw
r eo
aa
n
cy
hm
ii nen
n
g
tw
b.
rh ai ic nh
s
teth me s ne
e
uc ro on bc ie op lt os
g
ya r ie
s
V1
7
6
10
9
Posterior
face,
scalp,
ear
in an anatomical orientation, where dorsal/posterior is ‘up’ in the image V2
and ventral/anterior is ‘down’ (Figs 21.1–21.2). Location of internal
structures and the arrangement of somatotopic maps follow accord- 11
ingly. Learning neuroanatomy in this orientation is correct but may not


be the most appropriate for the clinical setting. Moreover, there is no
V3 Perioral
standard of laterality in brainstem images viewed anatomically; the
observer’s right may be either the right or the left side. These inconsist- Input from surface of the pinna,
encies disappear when the brainstem is taught using a clinical orienta- external auditory meatus, and outer
tion: the shape of the image (stained section or line drawing) matches surface of the tympanic membrane
that of the corresponding MRI or CT (see Figs 21.1–21.2); the orienta-
tion of all images, irrespective of their source (stained section, drawing,
CT, MRI), is the same, i.e. the observer’s right is the patient’s left and B
the observer’s left is the patient’s right; and so the concept of laterality,
Fig. 21.1 Somatotopy of the spinal tract of the trigeminal nerve and its
which is enormously important in diagnosis, is reinforced for the termination in the spinal nucleus, mainly the pars caudalis, in clinical
student. When learned in a clinical orientation, somatotopically organ- orientation. Note the dorsal–ventral arrangement of the tract and the
ized tracts and pathways map exactly on to their counterparts in a rostral–caudal distribution of its terminals in the nucleus as correlated with
patient (see Fig. 21.1). In recognition of the inherent value of contex- the facial dermatomes. (Redrawn with permission from Haines D,
tualizing neuroanatomical concepts within clinical reality, examples Fundamental Neuroscience for Basic and Clinical Applications, 4th
will be integrated into this chapter. Edition, 2013, Saunders, Elsevier.) 309


Brainstem
310
3
nOitCes
Basis pedunculi Interpeduncular cistern
Crus cerebri Crural cistern
Substantia
nigra
Tegmentum
Tectum Ambient cistern
A B
Quadrigeminal cistern
Fig. 21.2 The topographic relationship of the midbrain regions and the adjacent cisterns, with comparable (A) T2- and (B) T1-weighted MR images.
(Redrawn from Haines D, Fundamental Neuroscience for Basic and Clinical Applications, 4th Edition, 2013, Saunders, Elsevier.)
A Fig. 21.3 A, The cranial
nerve nuclei. The
components of the
vestibular nuclei are
indicated by the dotted
lines. B, The components
Edinger-Westphal preganglionic nucleus of the vestibular and
cochlear nuclei shown in
Oculomotor nucleus more detail.
Trochlear nucleus Mesencephalic nucleus of trigeminal nerve
Principal sensory nucleus of trigeminal nerve
Motor nucleus of trigeminal nerve
Abducens nucleus
Spinal nucleus of trigeminal nerve
Facial motor nucleus
Superior
Salivatory nuclei
Inferior
Superior
Dorsal vagal motor nucleus
Nucleus ambiguus Nucleus of tractus solitarius Lateral
Hypoglossal nucleus
Ventral
Efferent cranial nerve nuclei Medial cochlear
Dorsal
cochlear
Inferior
Afferent cranial nerve nuclei
B Vestibular and cochlear nuclei
the lateral geniculate nucleus of the thalamus (the visual pathways are
OVERVIEW OF CRANIAL NERVES AND CRANIAL
discussed in Ch. 42). The oculomotor (III) and trochlear (IV) nerves
NERVE NUCLEI
arise in, and are attached to, the midbrain. The trigeminal nerve (V)
attaches to the pons, medial to the middle cerebellar peduncle. The
The cranial nerves are the routes by which the brain receives informa- abducens (VI), facial (VII) and vestibulocochlear (VIII) nerves attach


tion directly from, and controls the functions of, structures that are to the brainstem at, or close to, the pontomedullary junction. The glos-
located mainly, although not exclusively, within the head and neck. All sopharyngeal (IX), vagus (X) and hypoglossal (XII) nerves all attach to
but 3 of the 12 pairs of cranial nerves are regarded as being attached to the medulla. The accessory nerve (XI) arises from cervical levels C1–C5/
the brainstem, the exceptions being the olfactory, optic and accessory C6, enters the cranium through the foramen magnum and then exits
nerves. via the jugular foramen, together with the glossopharyngeal and vagus
The cranial nerves are individually named and numbered, using nerves.
Roman numerals, in a rostro–caudal sequence, reflecting their order of Cranial nerves III–X and XII are associated with brainstem cell
attachment to the brain (Haines 2015) (see Table 16.1). The olfactory groupings referred to collectively as the cranial nerve nuclei (Fig. 21.3).
nerve (I) arises from the olfactory epithelium in the nasal cavity and The latter are either the origin of efferent cranial nerve fibres or the site
terminates directly in cortical and subcortical areas of the frontal and of termination of cranial nerve or descending cerebral cortical afferents.
temporal lobes; it is closely associated functionally with the limbic They are considered to be organized into six discontinuous longitudinal
system and is described in that context. The axons of the optic nerve cell columns corresponding to columns that may be identified in the
(II) pass into the optic chiasma, where medially positioned axons dec- embryo (see Fig. 16.1). Three columns are ‘sensory’ and three are
ussate; all of the axons emerge as the optic tract, which terminates in ‘motor’ in function.


Brainstem
310.e1
12
retPaHC
For well over a century, the accessory nerve has been described as anatomy in order to minimize potential complications during surgery
having two roots: a cranial root composed of a variable number of small in the region. Paradoxically, meticulous cadaveric dissection studies,
rootlets that emerge from the postolivary groove of the dorsolateral often undertaken stereomicroscopically, have failed to reach a consen-
medulla, caudal to the rootlets of the vagus, and a spinal root composed sus, finding either no connections between rootlets arising from the
of fibres derived from the cervical segments of the spinal cord. Accord- dorsolateral medulla and the spinal root of the accessory nerve because
ing to this description, the cranial rootlets carry axons of motor neu- the cranial root was absent, or significant variations in the arrangement
rones located in the caudal part of the nucleus ambiguus, and the spinal and size of plexiform connections between the cranial rootlets and the
rootlets carry axons of motor neurones located in a nucleus in the spinal root. Plexiform interconnections between neighbouring cranial
ventral horn of the cervical spinal cord in line with the nucleus ambig- nerves are not uncommon and are certainly not confined to the acces-
uus (Ch. 18). The spinal root ascends the vertebral canal and enters the sory and vagus nerves (Shoja et al 2014); their functional significance
skull via the foramen magnum. The cranial and spinal roots join, either is often uncertain. (For further reading, see Krammer et al (1987),
before or within the jugular foramen, and the bundle so formed then Lachman et al (2002), Ryan et al (2007), Wiles et al (2007), Benninger
divides into internal and external branches on leaving the jugular and McNeil (2010), Skinner (2011), Liu et al (2014), Tubbs et al (2014).)
foramen. The internal branch joins the vagus nerve and its fibres inner- In addition to the interaction between fibres of the corticospinal and
vate branchiomeric muscles via the pharyngeal and laryngeal branches rubrospinal systems at the spinal level, there is yet another factor that


of the vagus nerve. The external branch descends into the neck to inner- relates to the functional characteristics described above. Experimental
vate sternocleidomastoid and the upper and middle fibres of trapezius. lesions in primates that target either of these tracts are usually stereo-
In recent years, this description has been challenged: the cranial root taxically placed, damaging very specific fibre populations. On the other
has been redesignated as anatomically and functionally part of the hand, spontaneous lesions in humans invariably damage, in addition
vagus nerve, and the spinal root has become, de facto, the accessory to corticospinal axons, other axon populations that directly, or indi-
nerve. The issue is not simply one of contentious anatomical terminol- rectly, influence spinal alpha motor neurones. This is also a major
ogy; rather, it reflects the need to clarify the relevant microsurgical contributor to the differential results seen in primates versus humans.


medulla oblongata
311
12
retPaHC
The sensory nucleus of the trigeminal nerve, which extends through-
out the length of the brainstem and into the cervical spinal cord, rep-
resents a general somatic afferent cell column (Haines 2013, Haines
2015, Olszewski 1950). Its principal afferents are carried in the trigemi-
nal nerve. General visceral afferents carried by the facial, glossopharyn-
geal and vagus nerves end in the nucleus solitarius of the medulla. The
12
special somatic afferent column corresponds to the vestibular and coch-
lear nuclei, which are located beneath the vestibular area of the floor 13
of the fourth ventricle.
The general somatic efferent cell column consists of four nuclei that 14
lie near the midline and give rise to motor fibres that run in nerves of
1
the same name. From rostral to caudal, these are the oculomotor, tro- 15
chlear and abducens nuclei, which innervate the extraocular muscles, 2
and the hypoglossal nucleus, which innervates all but one of the 16
muscles of the tongue. The general visceral efferent, or parasympathetic, 3
cell column is made up of the Edinger–Westphal preganglionic nucleus 17
of the midbrain, the salivatory nuclei of the pons, and the vagal nucleus 4
of the medulla. Cells in the special visceral efferent column innervate
muscles derived from the branchial arches and lie in the trigeminal 5 18
motor nucleus, the facial nucleus and the nucleus ambiguus.
6 19
MEDULLA OBLONGATA 7 20
8
EXTERNAL FEATURES AND RELATIONS 21
9
The medulla oblongata extends from just above the first pair of cervical 22
spinal nerves to the lower border of the pons (Haines 2013). It is
10
approximately 3 cm in length and 2 cm in diameter at its widest. The 23
ventral surface of the medulla is separated from the basilar part of the
occipital bone and apex of the dens by the meninges and occipito-axial 11
24


ligaments. Caudally, the dorsal surface of the medulla is adjacent to
the cisterna magna, a subarachnoid cistern between the cerebellar
hemispheres.
The ventral and dorsal surfaces of the medulla (Figs 21.4–21.5)
possess a longitudinal median fissure and sulcus, respectively, which
are continuous with their spinal counterparts. Caudally, the ventral
(anterior) median fissure is interrupted by the obliquely crossing fasci-
cles of the motor decussation (pyramidal decussation). Rostrally, it
ends at the pontine border in a diminutive depression, the foramen
caecum. A prominent elongated ridge, the pyramid, which contains
descending corticospinal axons, lies immediately lateral to the ventral
median fissure. The lateral margin of the pyramid is indicated by a Fig. 21.4 The ventral aspect of the brainstem. Key: 1, infundibulum; 2,
shallow ventrolateral sulcus. A linear series of rootlets forming the tuber cinereum; 3, mammillary body; 4, basilar pons; 5, abducens nerve;
6, foramen caecum; 7, olive; 8, glossopharyngeal nerve; 9, vagus nerve;
hypoglossal nerve emerge from this sulcus in line with the ventral
10, rootlets of hypoglossal nerve; 11, accessory nerve; 12, olfactory tract;
spinal nerve roots. The abducens nerve emerges at the caudal border of
13, optic nerve; 14, optic chiasma; 15, optic tract; 16, oculomotor nerve;
the pons generally in line with the rootlets of the hypoglossal nerve.
17, uncus; 18, trochlear nerve; 19, trigeminal nerve; 20, facial nerve; 21,
Caudally, most fibres of the pyramid taper, decussate and enter the
vestibulocochlear nerve; 22, flocculus; 23, pyramid; 24, motor decussation
lateral funiculus of the spinal cord. Lateral to the pyramid and the
(decussation of pyramids).


ventrolateral sulcus is an oval prominence, the olive (see Fig. 21.4),
which contains the inferior olivary nuclear complex. The posterolateral
small contribution from the juxtarestiform body (long Purkinje cell
sulcus is lateral to the olive; the glossopharyngeal and vagus nerves join
axons and efferents from the fastigial nuclei that traverse the vestibular
the brainstem along the line of this sulcus, in line with the dorsal spinal
nuclei; see Fig. 21.12). The inferior cerebellar peduncles form the ante-
nerve roots.
rior and rostral boundaries of the lateral recesses of the fourth ventricle;
The spinal central canal extends into the caudal half of the medulla,
these are continuous with the subarachnoid space through the lateral
migrating progressively more dorsally until it opens out into the lumen
apertures of the fourth ventricle (foramina of Luschka). A tuft of choroid
of the fourth ventricle at the obex. This divides the medulla into a closed
plexus, continuous with that of the fourth ventricle, protrudes from the
part, which contains the central canal, and an open part, which contains
foramina on either side.
the caudal half of the fourth ventricle (see Fig. 21.5).
In the closed part of the medulla, a shallow dorsal intermediate
sulcus, on either side of the dorsal median sulcus, is continuous with
INTERNAL ORGANIZATION
its cervical spinal counterpart and indicates the location of the dorsal
(posterior) columns (fasciculi gracilis and cuneatus). The ascending
Transverse section of the medulla at the
fasciculi are parallel to each other, but in the caudal medulla they
level of the motor (pyramidal) decussation
diverge slightly to end in elongated swellings, the gracile and cuneate
tubercles, produced by the subjacent nuclei gracilis and cuneatus respec-
tively (see Fig. 21.5; Figs 21.6–21.7). Most fibres in the fasciculi synapse A transverse section of the lower medulla oblongata (see Fig. 21.6)


with neurones in their respective nuclei. The latter project to the con- intersects the dorsal, lateral and ventral funiculi, which are continuous
tralateral thalamus, which, in turn, projects to the primary somaesthetic with their counterparts in the spinal cord. The ventral funiculi are sepa-
cortex (see Fig. 20.10). The restiform body lies dorsolaterally in the rated from the central grey matter by corticospinal fibres, which cross
medulla, forming a rounded ridge between the caudal part of the fourth in the motor (pyramidal) decussation to reach the contralateral lateral
ventricle and the glossopharyngeal and vagal rootlets on each side. It funiculi (see Fig. 21.16). The decussation displaces the central grey
contains a wide variety of fibres (dorsal spinocerebellar, cuneocerebel- matter and central canal dorsally (Haines 2015). Continuity between
lar, olivocerebellar, reticulocerebellar, and other cerebellar afferents) the ventral grey column and central grey matter, which is maintained
(see Fig. 21.9). The restiform bodies on the two sides diverge and incline throughout the spinal cord, is lost. The column subdivides into the
to enter the cerebellar hemispheres as the major component of the supraspinal nucleus (continuous above with that of the hypoglossal
inferior cerebellar peduncles. They are crossed at this point by the striae nerve), which is the efferent source of the first cervical nerve, and the
medullares, which run medial to lateral on the ventricular floor (see nucleus of the accessory nerve, which is in line rostrally with the nucleus
Fig. 21.5). Usually, each inferior cerebellar peduncle also contains a ambiguus.


Brainstem
312
3
nOitCes
Anterior thalamic tubercle
Third
Taenia chorioidea ventricle Lamina affixa
Habenular commissure
Stria terminalis
Habenular trigone
Stria medullaris thalami Pineal gland
Superior colliculus Brachium of superior colliculus
Medial geniculate body
Pulvinar
Brachium of inferior colliculus
Lateral geniculate body
Crus cerebri Brachium conjunctivum
(superior cerebellar peduncle)
Inferior colliculus
Brachium pontis
Trochlear nerve (middle cerebellar peduncle)
Superior medullary velum Lateral recess of fourth ventricle
Median sulcus Trigeminal nerve
Medial eminence
Facial nerve
Superior fovea Vestibulocochlear nerve
Facial colliculus Glossopharyngeal nerve
Striae medullares of fourth
ventricle over vestibular area Vagus nerve
Hypoglossal trigone
Area postrema
Gracile tubercle
Cuneate tubercle
Obex
Posterior intermediate sulcus Fasciculus cuneatus
Posterior median fissure Fasciculus gracilis
Fig. 21.5 The dorsal aspect of the brainstem. The floor of the fourth ventricle has been exposed by cutting the cerebellar peduncles and removing the
cerebellum. (With permission from Neuroanatomy by FA Mettler (1948) 2nd edn; St Louis: The CV Mosby Company.)
Dorsal
intermediate sulcus
Dorsal median sulcus Fasciculus
cuneatus
Nucleus gracilis Nucleus
cuneatus
Spinal tract of
trigeminal nerve
Lateral
corticospinal
Spinal nucleus of
tract
trigeminal nerve
Dorsal
spinocerebellar tract
Ventral
spinocerebellar tract
Central
canal
Nucleus of the
accessory nerve
Motor decussation
(pyramidal decussation)
Ventral horn
(Medial motor cell column)
Ventral median fissure


Fig. 21.6 A transverse section through the medulla oblongata at the level of the motor decussation. Figures 21.6–21.8, 21.10–21.12 and 21.15–21.16 are
all prepared using a stain for myelinated nerve fibres (blue).


medulla oblongata
313
12
retPaHC
Fasciculus Fig. 21.7 A transverse section through the
Dorsal median sulcus gracilis Dorsal inter- medulla oblongata at the level of the sensory
Nucleus mediate sulcus decussation.
gracilis
Fasciculus
Internal cuneatus
arcuate fibres Nucleus
Spinal cuneatus
tract of Central
trigeminal nerve canal
Spinal nucleus of
trigeminal nerve
Dorsal spino-
cerebellar tract
Reticular
Ventral spino-
formation
cerebellar tract
Sensory
decussation
Anterior external Medial accessory
arcuate fibres olivary nucleus
Olivary complex
Pyramid
Medial lemniscus Ventral median fissure
The dorsal grey column is also modified at this level as the nucleus sally to enter the restiform body and then the inferior cerebellar pedun-
gracilis appears in the ventral part of the fasciculus gracilis. The nucleus cle at a higher level.
gracilis begins caudal to the nucleus cuneatus; the latter invades the
fasciculus cuneatus from its ventral aspect in similar fashion.
Transverse section of the medulla at the
The spinal nucleus and tract of the trigeminal nerve are visible ven-
trolateral to the dorsal columns and are continuous with the substantia caudal end of the fourth ventricle
gelatinosa and tract of Lissauer of the spinal cord.
A transverse section at the caudal end of the fourth ventricle (Fig. 21.8)
Transverse section of the medulla at the shows some new features together with most of those already described.
The total area of grey matter is increased by the presence of the large
level of the sensory decussation of the
inferior olivary nuclear complex and the nuclei of the vestibulocochlear,
dorsal column–medial lemniscus system
glossopharyngeal and vagus nerves.
A smooth, oval elevation, the olivary eminence, or olive, lies between
The medullary white matter is rearranged above the level of the motor the ventrolateral and dorsolateral sulci of the medulla. It is formed by


decussation (see Fig. 21.7). The pyramids form two large ventral bundles the underlying inferior olivary complex and lies lateral to the pyramid,
flanking the ventral median fissure on the ventral surface of the medulla; separated from it by the ventrolateral sulcus and emerging hypoglossal
they contain corticospinal fibres of ipsilateral origin. nerve fibres. The roots of the facial nerve emerge between its rostral end
The nucleus gracilis is prominent on the dorsal medullary aspect, and the lower pontine border, in the cerebellopontine angle. The
with diminishing numbers of fibres of the fasciculus gracilis located on arcuate nuclei are curved, interrupted bands, located on the pyramids,
its margins. The nucleus cuneatus is well developed. Both nuclei retain and are said to be displaced pontine nuclei. Arcuatocerebellar fibres and
continuity with the central grey matter at this level but this is lost more those of the striae medullares are derived from them. They project
rostrally. First-order afferent fibres contained within the fasciculi gracilis mainly to the contralateral cerebellum through the inferior cerebellar
and cuneatus synapse on neurones in their respective nuclei (Millar and peduncle (Fig. 21.9).
Basbaum 1975). Second-order axons emerge from the nuclei as internal The inferior olivary complex (dorsal accessory, medial accessory,
arcuate fibres, at first curving ventrolaterally around the central grey principal nuclei) is an irregularly crenated mass of grey matter with a
matter and then ventromedially between the spinal tract of the trigemi- medially directed hilum, through which numerous fibres enter and
nal nerve and the central grey matter. The fibres decussate in the midline, leave the nucleus (Nieuwenhuys et al 2008). It has prominent connec-
as the sensory decussation, and thereafter form the medial lemniscus, tions with the cerebellum and is described more fully in Chapter 22.
which ascends to the thalamus (Haines 2012). The decussation of these The central grey matter at this level constitutes the ventricular floor.


internal arcuate fibres is located dorsal to the pyramids and ventral to It contains (sequentially from medial to lateral): the hypoglossal
the central grey matter, which is therefore more dorsally displaced than nucleus, dorsal motor nucleus of the vagus, nucleus solitarius, and the
in the previous section. caudal ends of the inferior and medial vestibular nuclei.
The medial lemniscus ascends from the sensory decussation as a The tractus and nucleus solitarius extend throughout the length of
flattened tract near the median raphe. The pyramidal tract lies ventrally, the medulla. The tract is composed of general visceral afferents from
and the medial longitudinal fasciculus and the tectospinal tract lie the vagus and glossopharyngeal nerves. The nucleus and its central con-
dorsally. Decussating internal arcuate fibres are rearranged as they cross, nections with the reticular formation subserve the reflex control of
so that those derived from the nucleus gracilis lie ventral to those from cardiovascular, respiratory and cardiac functions (Ciriello 1983). The
the nucleus cuneatus. Above this level, the medial lemniscus is further rostral portion of the nucleus solitarius (the gustatory nucleus) receives
rearranged so that gracile fibres migrate laterally, while cuneate fibres taste fibres from cranial nerves VII, IX and X. These same cranial nerves
migrate medially. At pontine levels, the medial lemniscus is somato- send general visceral sensation to the caudal portion of the nucleus
topically organized with C1–S4 spinal segments represented sequen- solitarius (the cardiorespiratory nucleus).
tially from medial to lateral (Nieuwenhuys et al 2008). The medial longitudinal fasciculus is a small compact tract near the
The spinal nucleus of the trigeminal nerve is separated from the midline, ventral to the hypoglossal nucleus. It contains fibre bundles
central grey matter by internal arcuate fibres, and from the lateral med- including medial vestibulospinal tract fibres. At this medullary level, it
ullary surface by the spinal tract of the trigeminal nerve and by some is displaced dorsally by the pyramidal and lemniscal decussations. It


dorsal spinocerebellar tract fibres. The latter progressively incline dor- ascends in the pons and midbrain, maintaining its relationship to the


Brainstem
314
3
nOitCes
Dorsal vagal nucleus Floor of fourth Fig. 21.8 A transverse section through the
Hypoglossal nucleus
ventricle medulla oblongata at the caudal end of the fourth
Medial vestibular
nucleus ventricle.
Medial
longitudinal
Inferior vestibular fasciculus
nucleus
Dorsal spino-
cerebellar tract
Restiform body
Tectospinal
tract
Nucleus solitarius
and tractus
Medial
solitarius
lemniscus
Spinal tract of
trigeminal nerve
Spinal nucleus of
trigeminal nerve
Olivocerebellar
fibres Dorsal accessory
olivary nucleus
Reticular
formation Medial
accessory
olivary nucleus
Fibres of hypoglossal
nerve Principal olivary nucleus
Pyramid
External arcuate fibres Arcuate nucleus
Vestibular nuclei directions. This is the reticular formation, which exists throughout the
Striae medullares medulla and extends into the pontine tegmentum and midbrain
(Olszewski and Baxter 1954).
Corticospinal (pyramidal) tract
Each pyramid contains descending corticospinal fibres, derived from
the ipsilateral cerebral cortex, which have traversed the internal capsule,
Restiform body
midbrain and pons (see Fig. 20.16). Some 75–90% of the axons in the
medullary pyramid cross the midline internal to the ventral median
fissure as the motor (pyramidal) decussation. In the rostral part of the
Inferior olivary Reticular decussation, fibres cross ventromedially, whereas more caudally they
nucleus formation pass dorsally, crossing ventral to the central grey matter. The decussation
is orderly: fibres to cervical levels cross in its rostral part, whereas fibres
Vestibular nerve
to lumbosacral levels cross in its caudal portion. Fibres continue to pass
Anterior external dorsolaterally as they descend, to reach the contralateral lateral funicu-
arcuate fibres lus, where they form the crossed lateral corticospinal tract. A modest