BASAl gAngliA
372
3
nOiTCES
ee PPMM lliinnee
SSCC
eennttrriiccll
hh
vv
IICC uurrtt
oo
B oooorr
ff
FFll
A
Fig. 24.15 Stimulation locations represented in Montreal Neurological Institute space in pedunculopontine nucleus deep-brain stimulation for Parkinson’s
disease. The relative extent of the pedunculopontine nucleus has been outlined based on choline acetyltransferase immunohistochemistry. A, Coronal
view. B, Sagittal view. Abbreviations: IC, inferior colliculus; PM line, pontomesencephalic line connecting the pontomesencephalic junction to the caudal
end of the inferior colliculi; SC, superior colliculus. (Courtesy of Mr J A Hyam, Departments of Neurosurgery, Oxford University Hospitals and adapted
from Thevathasan et al., Alpha oscillations in the pedunculopontine nucleus correlate with gait performance in parkinsonism. Brain. 2012 Jan;135(Pt
1):148–60.)
KEY REFERENCES
Alexander GE, DeLong MR, Strick PL 1986 Parallel organization of function- A review that highlights some limitations of the anatomical model of basal
ally segregated circuits linking basal ganglia and cortex. Ann Rev Neu- ganglia function.
rosci 9:357–82.
Mena-Segovia J, Bolam JP, Magill PJ 2004 Pedunculopontine nucleus and
A landmark publication setting out a conceptual framework for the way in
basal ganglia: distant relatives or part of the same family? Trends Neu-
which the basal ganglia and cerebral cortex process different types of
rosci 27:585–8.
information through largely distinct parallel circuits based on known
A review presenting a persuasive argument for functional similarities and
anatomical connectivity.
intimate reciprocal connections between pedunculopontine nucleus and other
Crossman AR 1990 A hypothesis on the pathophysiological mechanisms basal ganglia structures.
that underlie levodopa- or dopamine agonist-induced dyskinesia in
Penney JB Jr, Young AB 1986 Striatal inhomogeneities and basal ganglia
Parkinson’s disease: implications for future strategies in treatment. Mov


function. Mov Disord 1: 3–15.
Disord 5:100–8.
A landmark publication, introducing some of the basic concepts behind
A seminal paper outlining the limitations of dopaminergic treatments of
current models of the pathophysiology of Parkinson’s disease and
Parkinson’s disease and translating an anatomical model of basal ganglia
Huntington’s disease.
function into potential neurosurgical treatment targets.
Pereira EA, Aziz TZ 2006 Parkinson’s disease and primate research: past,
Hallett M 2006 Pathophysiology of dystonia. Neural Transm Suppl 70:
present, and future. Postgrad Med J 82:293–9.
485–8.
A review of the non-human primate experiments that led to contemporary
An explanation of the abnormalities in basal ganglia function involved in
functional neurosurgery for Parkinson’s disease that discusses the anatomical
dystonia.
model of basal ganglia function.
Laitinen LV, Bergenheim AT, Hariz MI 1992 Ventroposterolateral pallidot-
Rodriguez-Oroz MC, Moro E, Krack P 2012 Long-term outcomes of surgical
omy can abolish all Parkinsonian symptoms. Stereotact Funct Neuro-
therapies for Parkinson’s disease. Mov Disord 27:1718–28.
surg 58:14–21.
A contemporary review and results of five-year, long-term follow-up of
A key paper that ignited widespread interest in functional neurosurgery for
patients receiving deep-brain stimulation for Parkinson’s disease.
Parkinson’s disease.
Marsden CD, Obeso JA 1994 The functions of the basal ganglia and the
paradox of stereotaxic surgery in Parkinson’s disease. Brain 117:877–97.


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Alam M, Schwabe K, Krauss J 2011 The pedunculopontine nucleus area: Marsden CD, Obeso JA 1994 The functions of the basal ganglia and the
critical evaluation of interspecies differences relevant for its use as a paradox of stereotaxic surgery in Parkinson’s disease. Brain 117:
target for deep brain stimulation. Brain 134:11–23. 877–97.
Alexander GE, DeLong MR, Strick PL 1986 Parallel organization of function- A review that highlights some limitations of the anatomical model of basal
ally segregated circuits linking basal ganglia and cortex. Ann Rev Neu- ganglia function.
rosci 9:357–82. Massey LA, Miranda MA, Zrinzo L et al 2012 High resolution MR anatomy
A landmark publication setting out a conceptual framework for the way in of the subthalamic nucleus: imaging at 9.4 T with histological valida-
which the basal ganglia and cerebral cortex process different types of tion. Neuroimage 59:2035–44.
information through largely distinct parallel circuits based on known
Massey LA, Yousry TA 2010 Anatomy of the substantia nigra and subthalamic
anatomical connectivity.
nucleus on MR imaging. Neuroimaging Clin N Am 20:7–27.
Crossman AR 1990 A hypothesis on the pathophysiological mechanisms Mena-Segovia J, Bolam JP, Magill PJ 2004 Pedunculopontine nucleus and
that underlie levodopa- or dopamine agonist-induced dyskinesia in basal ganglia: distant relatives or part of the same family? Trends Neu-
Parkinson’s disease: implications for future strategies in treatment. Mov rosci 27:585–8.
Disord 5:100–8. A review presenting a persuasive argument for functional similarities and
A seminal paper outlining the limitations of dopaminergic treatments of intimate reciprocal connections between pedunculopontine nucleus and other
Parkinson’s disease and translating an anatomical model of basal ganglia basal ganglia structures.
function into potential neurosurgical treatment targets.
Penney JB Jr, Young AB 1986 Striatal inhomogeneities and basal ganglia


Hallett M 2006 Pathophysiology of dystonia. Neural Transm Suppl 70: function. Mov Disord 1: 3–15.
485–8. A landmark publication, introducing some of the basic concepts behind
An explanation of the abnormalities in basal ganglia function involved in current models of the pathophysiology of Parkinson’s disease and
dystonia. Huntington’s disease.
Hamani C, Stone S, Laxton A et al 2007 The pedunculopontine nucleus and Pereira EA, Aziz TZ 2006 Parkinson’s disease and primate research: past,
movement disorders: anatomy and the role for deep brain stimulation. present, and future. Postgrad Med J 82:293–9.
Parkinsonism Relat Disord 13:S276–S280. A review of the non-human primate experiments that led to contemporary
Jankovic J 2012 Movement disorders. In: Daroff RB, Fenichel GM, Jankovic functional neurosurgery for Parkinson’s disease that discusses the anatomical
J et al (eds) Bradley’s Neurology in Clinical Practice, 6th ed. Philadel- model of basal ganglia function.
phia: Elsevier, Saunders; Ch.71, pp. 1762–801.
Rodriguez-Oroz MC, Moro E, Krack P 2012 Long-term outcomes of surgical
Laitinen LV, Bergenheim AT, Hariz MI 1992 Ventroposterolateral pallidot- therapies for Parkinson’s disease. Mov Disord 27:1718–28.
omy can abolish all Parkinsonian symptoms. Stereotact Funct Neuro- A contemporary review and results of five-year, long-term follow-up of
surg 58:14–21. patients receiving deep-brain stimulation for Parkinson’s disease.
A key paper that ignited widespread interest in functional neurosurgery for
Parkinson’s disease. Stein JF, Aziz TZ 2012 Basal ganglia output to the PPN, a commentary.
Exp Neurol 233:745–6.
Little S, Brown P 2014 The functional role of beta oscillations in Parkinson’s
disease. Parkinsonism Relat Disord 20 Suppl 1:S44–8.


CHAPTER
25
Cerebral hemispheres
The cerebral hemispheres are the largest and most developed part trostimulation and behavioural analyses (Alarcon et al 2014, Jin et al
of the human brain. They contain the primary motor and sensory cor- 2014). Some details of the anatomy of the gyri, sulci, association fibres
tices, the highest levels at which motor activities are controlled and to and the amygdaloid nuclear complex are described online only (Figs
which general and special sensory systems project, and which provide 25.2; 25.9–25.13; 25.14; 25.17; 25.18; 25.20; 25.22; 25.27; 25.36;
the neural substrate for the conscious experience of sensory stimuli. 25.37; 25.46–25.49).
Association areas are both modality-specific and multimodal, enabling
complex analyses of the internal and external environment and of the
CEREBRAL HEMISPHERE SURFACES,
relationship of the individual with the external world. The elements of
SULCI AND GYRI
the limbic system are particularly concerned with memory and the
emotional aspects of behaviour, and provide an affective overtone to
conscious experience as well as an interface with subcortical areas such Each hemisphere has superolateral, medial and inferior (basal) surfaces,
as the hypothalamus, through which widespread physiological activities separated by superomedial, inferolateral, medial orbital and medial
are integrated. Other cortical areas, primarily within the frontal region, occipital margins respectively.
are concerned with the highest aspects of cognitive function and con- The superolateral surface is convex and lies beneath the bones of the
tribute to personality, judgment, foresight and planning. cranial vault; the frontal, parietal, temporal and occipital lobes corre-
The external surface of each hemisphere is highly convoluted into a spond approximately in surface extent to the overlying bones from
series of folds or gyri, separated by furrows or sulci (Fig. 25.1). The which they take their names. The frontal and parietal lobes are sepa-
configuration of the main cerebral sulci and gyri provides the basis for rated from the temporal lobe by the prominent lateral (Sylvian) fissure.


dividing the hemispheres into frontal, parietal, occipital, temporal, The inferior surface is divided by the anterior part of the lateral
insular and limbic lobes. The internal white matter contains associa- fissure into a small anterior orbital part and a larger posterior tentorial
tion fibres limited to each hemisphere, commissural fibres linking part. The orbital part is the concave orbital surface of the frontal lobe
corresponding areas of both hemispheres, and projection fibres con- and rests on the floor of the anterior cranial fossa. The posterior part is
necting the cerebral cortex of each hemisphere with subcortical, brain- formed by the basal aspects of the temporal and occipital lobes, and
stem and spinal cord nuclei. Some of these bundles (tracts, fasciculi) rests on the floor of the middle cranial fossa and the upper surface of
are relatively well defined macroscopically and microscopically, while the tentorium cerebelli, which separates it from the superior surface
others are less easy to identify. A detailed knowledge of the three- of the cerebellum. The medial surface is flat and vertical, separated from
dimensional anatomical interrelationships of white matter tracts is a the opposite hemisphere by the longitudinal fissure and the falx cerebri.
requisite for the planning, intraoperative monitoring and execution of Anteriorly, the cerebral hemisphere terminates at the frontal and tem-
neurosurgical resective procedures, e.g. for tumour surgery, and epi- poral poles, and posteriorly at the occipital pole.
lepsy and deep brain stimulation procedures. Current understanding The cerebral sulci delineate the brain gyri and are extensions of the
of these relationships owes much to the seminal work of Josef Klingler subarachnoid space (Butler and Hodos 2005, Sarnat and Netsky 1981,
and his meticulous dissection of white matter tracts using formalin- Park et al 2007, Chi et al 1977, Nishikuni and Ribas 2013, Ono et al
fixed, freeze-thawed brains (Agrawal et al 2011). Contemporary neuro- 1990, Catani and Thiebaut de Schotten 2012, Duvernoy 1991, Naidich


surgical anatomical studies seek to define and delineate these fibre et al 2013). When they are deep and anatomically constant, they are
bundles, particularly in areas of complexity such as fibre crossing, by referred to as fissures. The main sulci have depths of 1–3 cm, and their
correlating anatomical findings obtained using Klingler’s dissection walls harbour small gyri that connect with each other (transverse gyri).
techniques with the results obtained from diffusion-weighted mag- Sulci that separate the transverse gyri vary in length and depth, and may
netic resonance imaging, functional MRI (fMRI), intraoperative elec- become visible as incisures at the surface of the brain. The indentations
Precentral gyrus Postcentral sulcus Fig. 25.1 The lateral aspect of the left cerebral
Central sulcus hemisphere, indicating the major gyri and sulci.
Precentral sulcus (Dissection by EL Rees; photograph by Kevin
Postcentral gyrus
Fitzpatrick on behalf of GKT School of Medicine,
Superior Supramarginal London; figure enhanced by B Crossman.)
frontal gyrus gyrus
Middle
frontal gyrus
Intraparietal
Inferior sulcus
frontal gyrus
Angular gyrus
Ascending
ramus of
lateral fissure
Lateral fissure
Superior
Posterior ramus
temporal gyrus
of lateral fissure
Middle Inferior temporal
temporal gyrus gyrus
373


Cerebral hemispheres
374
3
NOiTCes
Paracentral Body of corpus callosum Fig. 25.3 A sagittal section of the brain, with the
lobule brainstem removed, showing the major gyri and
Fornix Cingulate sulcus sulci on the medial aspect of the left cerebral
hemisphere. (Photograph by Kevin Fitzpatrick on
Precuneus Superior frontal
gyrus behalf of GKT School of Medicine, London; figure
Parieto-occipital enhanced by B Crossman.)
sulcus
Cingulate gyrus
Cuneus
Genu of corpus
Splenium of callosum
corpus callosum
Calcarine
sulcus
Lingual gyrus
Rostrum of
Lateral occipitotemporal gyrus corpus callosum
Isthmus Subcallosal area
Parahippocampal gyrus Uncus
caused by cortical arteries can have an appearance similar to that of the two types – granular and agranular – are regarded as virtually lacking
incisures. The sulci of the superolateral and inferior surfaces of the certain laminae, and are referred to as heterotypical. Homotypical vari-
hemisphere are usually orientated towards the nearest ventricular cavity. ants, in which all six laminae are found, are called frontal, parietal and
On the brain surface, the sulci can be long or short, interrupted or polar, names that link them with specific cortical regions in a somewhat
continuous. Sulci that are usually continuous include the lateral fissure misleading manner (e.g. the frontal type also occurs in parietal and
and the callosal, calcarine, parieto-occipital, collateral and, generally, temporal lobes).
the central sulcus. The agranular type is considered to have diminished, or absent,
On the superolateral surface of the hemisphere, the frontal and granular laminae (II and IV) but always contains scattered stellate
temporal regions are each composed of three horizontal gyri (superior, somata. Large pyramidal neurones are found in the greatest densities
middle and inferior frontal and temporal gyri). The central area is in agranular cortex, which is typified by the numerous efferent projec-


composed of two slightly oblique gyri (pre- and postcentral gyri). The tions of pyramidal cell axons. Although it is often equated with motor
parietal region is comprised of two semicircular lobules (superior and cortical areas such as the precentral gyrus (area 4), agranular cortex also
inferior parietal lobules, the inferior being formed by the supramarginal occurs elsewhere, e.g. areas 6, 8 and 44, and parts of the limbic system.
and angular gyri) (see Fig. 25.1). The occipital region is composed of In the granular type of cortex the granular layers are maximally
two or three less well-defined gyri (superior, middle and inferior occipi- developed and contain densely packed stellate cells, among which small
tal gyri). The insula, which lies deep in the floor of the lateral fissure, pyramidal neurones are dispersed. Laminae III and IV are poorly devel-
consists of 4–5 diagonal gyri (short and long insular gyri) (Fig. 25.2). oped or unidentifiable. This type of cortex is particularly associated with
The orbital part of the inferior surface is covered by the orbital gyri afferent projections. However, it does receive efferent fibres, derived
and the basal aspect of the rectus gyri, and the tentorial part of the from the scattered pyramidal cells, although they are less numerous
inferior surface is covered by the basal aspects of the inferior temporal, than elsewhere. Granular cortex occurs in the postcentral gyrus (soma-
inferior occipital and lingual gyri, and the fusiform gyrus. The medial tosensory area), striate area (visual area) and superior temporal gyrus
surface of the hemisphere is characterized by a very well defined (acoustic area), and in small areas of the parahippocampal gyrus.
C-shaped inner ring composed primarily of two continuous gyri (cin- Despite its very high density of stellate cells, especially in the striate
gulate and parahippocampal gyri), surrounded by a much less well area, it is almost the thinnest of the five main types. In the striate cortex,


defined outer ring of gyri (medial aspects of rectus and superior frontal the external band of Baillarger (lamina IV) is well defined as the stria
gyri, paracentral lobule, precuneus, cuneus, and medial aspect of lingual (white line) of Gennari.
gyrus) (Fig. 25.3). The other three types of cortex are intermediate forms. In the frontal
type, large numbers of small- and medium-sized pyramidal neurones
appear in laminae III and V, and granular layers (II and IV) are less
MICROSTRUCTURE OF THE CORTEX prominent. The relative prominence of these major forms of neurone
varies reciprocally wherever this form of cortex exists.
The microscopic structure of the cerebral cortex is an intricate complex The parietal type of cortex contains pyramidal cells, which are mostly
of nerve cells and fibres, neuroglia and blood vessels. The neocortex smaller in size than in the frontal type. In marked contrast, the granular
essentially consists of three neuronal cell types. Pyramidal cells are the laminae are wider and contain more of the stellate cells: this kind of
most abundant. Non-pyramidal cells, also called stellate or granule cortex occupies large areas in the parietal and temporal lobes. The polar
cells, are divided into spiny and non-spiny types. All types have been type is classically identified with small areas near the frontal and occipi-
further subdivided on the basis of size and shape (Fig. 25.4; see tal poles, and is the thinnest form of cortex. All six laminae are repre-
Fig. 3.3). sented, but the pyramidal layer (III) is reduced in thickness and not so
extensively invaded by stellate cells as it is in the granular type of cortex.
In both polar and granular types, the multiform layer (VI) is more
LAMINAR ORGANIZATION highly organized than in other types.
It is customary to refer to some discrete cortical territories not only
The most obvious microscopic feature of a thin section of the neocor- by their anatomical location in relation to gyri and sulci, but also in
tex stained to demonstrate cell bodies or fibres is its horizontal lami- relation to their cytoarchitectonic characteristics (Brodmann’s areas)


nation. The extent to which this organization aids the understanding (Fig. 25.7). Some of the areas so defined, e.g. the primary sensory and
of cortical functional organization is debatable, but the use of cytoar- motor cortices, have clear relevance in terms of anatomical connections
chitectonic description to identify regions of cortex is common. Typical and functional significance, others less so.
neocortex is described as having six layers, or laminae, lying parallel to
the surface (Fig. 25.5). These are the molecular or plexiform layer;
CORTICAL LAMINATION AND
external granular lamina; external pyramidal lamina; internal granular
lamina; internal pyramidal (ganglionic) lamina; and multiform (or CORTICAL CONNECTIONS
fusiform/pleiomorphic) layer.
The cortical laminae represent, to some extent, horizontal aggregations
of neurones with common connections. This is most clearly seen in
NEOCORTICAL STRUCTURE the lamination of cortical efferent (pyramidal) cells. The internal
pyramidal lamina, layer V, gives rise to cortical projection fibres, most
Five regional variations in neocortical structure are described (Fig. notably corticostriate, corticobulbar (including corticopontine) and
25.6). While all are said to develop from the same six-layered pattern, corticospinal axons. In addition, a significant proportion of feedback


Cerebral hemispheres
374.e1
52
reTpahC
Four main types of sulci have been described: large primary sulci
(e.g. central, precentral, postcentral and continuous sulci); short primary
sulci (e.g. rhinal, olfactory, lateral and occipital sulci); short sulci com-
posed of several branches (e.g. orbital and subparietal sulci); and short,
free supplementary sulci (e.g. medial frontal and lunate sulci) (Ono
et al 1990). Sulci often have side branches that may be unconnected or
connected (with end-to-side, end-to-end or side-to-side connections
that can also join two neighbouring parallel sulci).
Pyramidal cells have a flask-shaped or triangular cell body ranging
from 10 to 80 µm in diameter. The soma gives rise to a single thick
apical dendrite and multiple basal dendrites. The apical dendrite
ascends towards the cortical surface, tapering and branching, to end in
a spray of terminal twigs in the most superficial lamina, the molecular
layer. From the basal surface of the cell body, dendrites spread more
horizontally, for distances up to 1 mm for the largest pyramidal cells.
Like the apical dendrite, the basal dendrites branch profusely along
their length. All pyramidal cell dendrites are studded with a myriad of A
dendritic spines. These become more numerous as distance from the
parent cell soma increases. A single slender axon arises from the axon
hillock, which is usually situated centrally on the basal surface of the
pyramidal neurone. Ultimately, in the vast majority of cases, if not in
all, the axon leaves the cortical grey matter to enter the white matter.
Pyramidal cells are thus, perhaps universally, projection neurones. They
use an excitatory amino acid, either glutamate or aspartate, as their
neurotransmitter.
Spiny stellate cells are the second most numerous cell type in the
neocortex and, for the most part, occupy lamina IV. They have relatively
small multipolar cell bodies, commonly 6 to 10 µm in diameter. Several
primary dendrites, profusely covered in spines, radiate for variable dis-


tances from the cell body. Their axons ramify within the grey matter
predominantly in the vertical plane. Spiny stellate cells probably use
glutamate as their neurotransmitter.
The smallest group comprises the heterogeneous non-spiny or
sparsely spinous stellate cells. All are interneurones, and their axons are
confined to grey matter. In morphological terms, this is not a single B
class of cell but a multitude of different forms, including basket, chan-
delier, double bouquet, neurogliaform, bipolar/fusiform and horizon- Fig. 25.2 The basic organization of the main cerebral gyri.
A, Superolateral surface, left side. B, Medial and basal surfaces, right
tal cells. Various types may have horizontally, vertically or radially
side. Red lines indicate constant gyri. The frontal and temporal regions
ramifying axons.
each consist of three horizontal gyri; the central area consists of two
Neurones with mainly horizontally dispersed axons include basket
slightly oblique gyri; the parietal region consists of two lobules (a
and horizontal cells. Basket cells have a short, vertical axon, which
quadrangular superior lobule and an inferior lobule consisting of two
rapidly divides into horizontal collaterals, and these end in large termi-
semicircular gyri); the occipital region consists of three irregular, less well
nal sprays synapsing with the somata and proximal dendrites of pyrami-
defined, predominantly longitudinal gyri that converge towards the
dal cells. The cell bodies of horizontal cells lie mainly at the superficial occipital pole; and the insula is composed of four or five diagonal gyri.
border of lamina II, occasionally deep in lamina I (the molecular or Medially, the external lateral gyri and lobules extend along the superior
plexiform layer). They are small and fusiform, and their dendrites and inferolateral borders of each hemisphere. Together, they constitute
spread short distances in two opposite directions in lamina I. Their an outer medial ring that surrounds a well-defined, C-shaped inner ring
axons often stem from a dendrite, then divide into two branches, which composed of two continuous gyri. (With permission from Ribas GC, The


travel away from each other for great distances in the same layer. cerebral sulci and gyri, Neurosurg Focus 2010 Feb;28(2):E2.)
Neurones with an axonal arborization predominantly perpendicular
to the pial surface include chandelier, double bouquet and bipolar/
fusiform cells. Chandelier cells have a variable morphology, although
The principal recognizable neuronal type is the neurogliaform or
most are ovoid or fusiform and their dendrites arise from the upper and
spiderweb cell. These small spherical cells, 10–12 µm in diameter, are
lower poles of the cell body. The axonal arborization, which emerges
found mainly in laminae II–IV, depending on cortical area. Seven to
from the cell body or a proximal dendrite, is characteristic and identifies
ten thin dendrites typically radiate out from the cell soma, some
these neurones. A few cells in the more superficial laminae (II and IIIa)
branching once or twice to form a spherical dendritic field of approxi-
have descending axons, deeper cells (laminae IIIc and IV) have ascend-
mately 100–150 µm diameter. The slender axon arises from the cell
ing axons, and intermediate neurones (IIIb) often have both. The axons
body or a proximal dendrite. Almost immediately, it branches profusely
ramify close to the parent cell body and terminate in numerous verti-
within the vicinity of the dendritic field (and usually somewhat
cally orientated strings, which run alongside the axon hillocks of
beyond), to give a spherical axonal arbor up to 350 µm in diameter.
pyramidal cells, with which they synapse. Double bouquet (or bitufted)
The majority of non-spiny or sparsely spinous non-pyramidal cells
cells are found in laminae II and III and their axons traverse laminae II
probably use γ-aminobutyric acid (GABA) as their principal neurotrans-
and V. Generally, these neurones have two or three main dendrites,
mitter. This is almost certainly the case for basket, chandelier, double
which give rise to a superficial and deep dendritic tuft. A single axon
bouquet, neurogliaform and bipolar cells. Some are also characterized


arises usually from the oval or spindle-shaped cell soma and rapidly
by the coexistence of one or more neuropeptides, including neuropep-
divides into an ascending and descending branch. These branches col-
tide Y, vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK),
lateralize extensively, but the axonal arbor is confined to a perpendicu-
somatostatin and substance P. Acetylcholine is present in a subpopula-
larly extended, but horizontally confined, cylinder, 50–80 µm across.
tion of bipolar cells, which may additionally be GABAergic and contain
Bipolar cells are ovoid with a single ascending and a single descending
VIP.
dendrite, which arise from the upper and lower poles, respectively.
These primary dendrites branch sparsely and their branches run verti-
cally to produce a narrow dendritic tree, rarely more than 10 µm across,
which may extend through most of the cortical thickness. Commonly,
the axon originates from one of the primary dendrites, and rapidly
branches to give a vertically elongated, horizontally confined axonal
arbor, which closely parallels the dendritic tree in extent.


Cerebral hemispheres
374.e2
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NOiTCes
The molecular or plexiform layer is cell-sparse, containing only scat-
tered horizontal cells and their processes enmeshed in a compacted
mass of tangential, principally horizontal axons and dendrites. These
are afferent fibres, which arise from outside the cortical area, together
with intrinsic fibres from cortical interneurones, and the apical den-
dritic arbors of virtually all pyramidal neurones of the cerebral cortex.
In histological sections stained to show myelin, layer I appears as a
narrow horizontal band of fibres. The external granular lamina contains
a varying density of small neuronal cell bodies, including both small
pyramidal and non-pyramidal cells; the latter may predominate. Myelin
fibre stains show mainly vertically arranged processes traversing the
layer. The external pyramidal lamina contains pyramidal cells of varying
sizes, together with scattered non-pyramidal neurones. The size of the
pyramidal cells is smallest in the most superficial part of the layer and
greatest in the deepest part. This lamina is frequently further subdivided
into IIIa, IIIb and IIIc, with IIIa most superficial and IIIc deepest. As in
layer II, myelin stains reveal a mostly vertical organization of fibres. The
internal granular lamina is usually the narrowest of the cellular laminae.
It contains densely packed, small, round cell bodies of non-pyramidal
cells, notably spiny stellate cells and some small pyramidal cells. Within
the lamina, in myelin stained sections, a prominent band of horizontal
fibres (outer band of Baillarger) is seen. The internal pyramidal (gan-
glionic) lamina typically contains the largest pyramidal cells in any
cortical area, though actual sizes vary considerably from area to area.
Scattered non-pyramidal cells are also present. In myelin stains, the
lamina is traversed by ascending and descending vertical fibres, and also
contains a prominent central band of horizontal fibres (inner band of
Baillarger). The multiform (or fusiform/pleiomorphic) layer consists of
neurones with a variety of shapes, including recognizable pyramidal,
spindle, ovoid and many other shapes of somata. Typically, most cells
are small to medium in size. This lamina blends gradually with the


underlying white matter, and a clear demarcation of its deeper bound-
ary is not always possible.


Cerebral lobes
375
52
reTpahC
A Fig. 25.4 A, Characteristic
H neocortical neurones.
I From left to right are
shown Martinotti,
neurogliaform, basket,
II
horizontal, fusiform,
S
stellate and pyramidal
N types of neurone. B, The
III most frequent types of
neocortical neurone,
showing typical
connections with each
IV F other and with afferent
B P fibres. The right and left
afferent fibres are
association or
V corticocortical
connections; the central
afferent is a specific
sensory fibre. Neurones
VI M
are shown in their
characteristic lamina but
many have somata in
B
more than one layer.
Afferent fibres
H Efferent neurones
I
Neurones limited to cortex
Neurone types:
II
B Basket
F Fusiform
P
H Horizontal
III S M Martinotti
F N Neurogliaform
P Pyramidal
S Stellate
IV B P
P
V P
N
M
VI
corticocortical axons arise from cells in this layer, as do some cortico- (ocular dominance columns). Adjacent orientation columns aggregate
thalamic fibres. Layer VI, the multiform lamina, is the major source of within an ocular dominance column to form a hypercolumn, respond-
corticothalamic fibres. Supragranular pyramidal cells, predominantly ing to all orientations of stimulus for both eyes for one point in the
layer III but also lamina II, give rise primarily to both association and visual field. Similar functional columnar organization has been
commissural corticocortical pathways. Generally, short corticocortical described in widespread areas of neocortex, including motor cortex and
fibres arise more superficially, and long corticocortical (both association association areas.
and commissural) axons come from cells in the deeper parts of layer
III. Major afferents to a cortical area tend to terminate in layers I, IV
CEREBRAL LOBES
and VI. Quantitatively lesser projections end either in the intervening
laminae II/III and V, or sparsely throughout the depth of the cortex.


Numerically, the largest input to a cortical area tends to terminate Each cerebral hemisphere is divided into six lobes: frontal, parietal,
mainly in layer IV. This pattern of termination is seen in the major occipital, temporal, insular and limbic lobes. The surface features of the
thalamic input to visual and somatic sensory cortex. In general, non- hemispheres exhibit considerable inter-individual variation in terms of
thalamic subcortical afferents to the neocortex, which are shared by the depth and size of their sulci and the resulting patterns of gyral sepa-
widespread areas, tend to terminate throughout all cortical layers, but ration (Ribas 2010). Connections between sulci are common; differing
the laminar pattern of their endings still varies considerably from area interpretations of these patterns of connectivity continue to contribute
to area. to inconsistencies in the literature, e.g. the use of different boundaries
to demarcate the temporal, parietal and occipital lobes (Fig. 25.8).
In what follows, each lobe will be described in terms of its external
COLUMNS AND MODULES sulci and gyri, internal cortical structure and connectivity. Unless other-
wise indicated in the caption, the dissections in this chapter display
Experimental physiological and connectional studies have demon- features in left cerebral hemispheres.
strated an internal organization of the cortex, which is at right angles
to the pial surface, with vertical columns or modules running through
the depth of the cortex. The term ‘column’ refers to the observation that FRONTAL LOBE
all cells encountered by a microelectrode penetrating and passing per-
pendicularly through the cortex respond to a single peripheral stimulus, The frontal lobe is the largest part of the cerebral hemisphere. It con-
a phenomenon first identified in the somatosensory cortex. In the visual tains the primary motor area (MI) within the precentral gyrus, the
cortex, narrow (50 µm) vertical strips of neurones respond to a bar supplementary motor area (SMA) anteriorly and medially, and the
stimulus of the same orientation (orientation columns), and wider premotor areas anteriorly and laterally. While movement is thought to
strips (500 µm) respond preferentially to stimuli detected by one eye be initiated from within MI, the supplementary motor and premotor


Cerebral hemispheres
376
3
NOiTCes
Plexiform (molecular) Internal granular and external band of Baillarger superior, middle and inferior frontal gyri are disposed longitudinally,
anterior to the precentral gyrus, and are separated by superior and
External granular Ganglionic layer, containing inner band of Baillarger inferior frontal sulci (see Fig. 25.1); they are frequently referred to as
Pyramidal Multiform (polymorphous) F 1, F 2 and F 3 respectively.
On its medial surface, the frontal lobe is limited inferiorly by the
cingulate sulcus, which starts within the subcallosal region and extends
over the cingulate gyrus. The cingulate sulcus may occasionally be
double anteriorly and enclose a connection of the anterior aspect of the
cingulate gyrus and the medial and anterior aspects of the superior
frontal gyrus. It frequently has side branches that are directed inferiorly
(see Fig. 25.10).
Frontal lobe internal structure
and connectivity
Primary motor cortex
The primary motor cortex (MI) corresponds to the precentral gyrus
(area 4), and is the area of cortex with the lowest threshold for eliciting
contralateral muscle contraction by electrical stimulation. It contains a
detailed topographically organized map (motor homunculus) of the
opposite body half, with the head represented most laterally, and the
leg and foot represented on the medial surface of the hemisphere in
the paracentral lobule (Fig. 25.15). A striking feature is the dispropor-
tionate representation of body parts in relation to their physical size:
large areas represent the muscles of the face and hand, which are
capable of finely controlled or fractionated movements.
The cortex of area 4 is agranular, and layers II and IV are difficult to
identify. The most characteristic feature is the presence in lamina V of
some extremely large pyramidal cell bodies, Betz cells, which may
approach 80 µm in diameter. These neurones project their axons into
the corticospinal and corticonuclear tracts.
The major thalamic connections of area 4 are with the ventral
posterolateral nucleus, which in turn receives afferents from the deep
cerebellar nuclei. The ventral posterolateral nucleus also contains a


topographic representation of the contralateral body half, which is
preserved in its point-to-point projection to area 4, where it terminates
largely in lamina IV. Other thalamic connections of area 4 are with the
centromedian and parafascicular nuclei. These appear to provide the
only route through which output from the basal ganglia, routed via
the thalamus, reaches the primary motor cortex, since the projection of
Golgi Nissl Weigert
the internal segment of the globus pallidus to the ventrolateral nucleus
Fig. 25.5 The layers of the cerebral cortex. The three vertical columns of the thalamus is confined to the anterior division, and there is no
represent the disposition of cellular elements, as revealed by the staining
overlap with cerebellothalamic territory. The anterior part of the vent-
techniques of Golgi (impregnating whole neurones), Nissl (staining cell
rolateral nucleus projects to the premotor and supplementary motor
bodies) and Weigert (staining nerve fibres).
areas of cortex with no projection to area 4.
The ipsilateral somatosensory cortex (SI) projects in a topographi-
areas are believed to instruct the MI area. The most anterior and basal cally organized way to area 4, and the connection is reciprocal. The
aspects of the frontal lobes are related to judgement and complex projection to the motor cortex arises in areas 1 and 2, with little or no
aspects of volitional behaviour. contribution from area 3b. Fibres from SI terminate in layers II and III
of area 4, where they contact mainly pyramidal neurones. Evidence
Frontal lobe sulci and gyri
suggests that neurones activated monosynaptically by fibres from SI, as
well as those activated polysynaptically, make contact with layer V
The frontal lobe is delimited posteriorly by the central sulcus, medially pyramidal cells, including Betz cells, which give rise to corticospinal
by the (great) longitudinal fissure and inferolaterally by the lateral fibres. Movement-related neurones in the motor cortex that can be


fissure (Figs 25.9–25.10; see Figs 25.1, 25.8). The area of the frontal activated from SI tend to have a late onset of activity, mainly during the
lobe anterior to the precentral gyrus is divided into longitudinal supe- execution of movement. It has been suggested that this pathway plays
rior, middle and inferior frontal gyri; the frontal pole lies anterior to a role primarily in making motor adjustments during a movement.
these gyri (see Figs 25.1, 25.8, 25.21B). Its superolateral (dorsal) surface Additional ipsilateral corticocortical fibres to area 4 from behind the
is covered by the frontal bone. Its basal (ventral) surface lies over the central sulcus come from the second somatic sensory area (SII).
orbital part of the frontal bone and the cribriform plate of the ethmoid Neurones in area 4 are responsive to peripheral stimulation, and
bone, and displays the orbital and rectus gyri. The medial surface faces have receptive fields similar to those in the primary sensory cortex. Cells
the falx cerebri. located posteriorly in the motor cortex have cutaneous receptive fields,
The central sulcus is the boundary between the frontal and parietal whereas more anteriorly situated neurones respond to stimulation of
lobes. It demarcates the primary motor and somatosensory areas of deep tissues.
the cortex, located in the precentral and postcentral gyri respectively. It The motor cortex receives major frontal lobe association fibres from
starts in or near the superomedial border of the hemisphere, a little the premotor cortex and the supplementary motor area, and also fibres
behind the midpoint between the frontal and occipital poles, and from the insula. It is probable that these pathways modulate motor
runs sinuously, resembling a lengthened letter S, downwards and for- cortical activity in relation to the preparation, guidance and temporal
wards, to end usually a little above the posterior ramus of the lateral organization of movements. Area 4 sends fibres to, and receives fibres
sulcus. The central sulcus is usually a continuous sulcus in both from, its contralateral counterpart, and also projects to the contralateral
hemispheres. supplementary motor cortex.


The precentral gyrus lies obliquely over the superolateral surface of Apart from its contribution to the corticospinal tract, the motor
the cerebral hemisphere, its upper aspect extending on to the medial cortex has diverse subcortical projections. The connections to the stria-
surface. It is continuous superiorly and inferiorly with the postcentral tum and pontine nuclei are heavy. It also projects to the subthalamic
gyrus along connections that encircle both extremities of the central nucleus. The motor cortex sends projections to all nuclei in the brain-
sulcus. stem, which are themselves the origin of descending pathways to the
The pre- and the postcentral gyri are roughly parallel to the coronal spinal cord: namely, the reticular formation, the red nucleus, the supe-
suture; the precentral sulcus is located slightly posterior to it. The rior colliculus, the vestibular nuclei and the inferior olivary nucleus.


Cerebral hemispheres
376.e1
52
reTpahC
A SFS PreCS CS
IPS
MFS
PostCS
IFS
ISJ
ASCR
SyF
SOS
IOS
STS ITS PSCR
B SFG
MFG
SPLob
PreCG
PostCG
SMG
IFG
Tr AG
SOG
Orb
Op
MOG
IFG
STG MTG ITG
Fig. 25.9 The main sulci (A) and gyri (B) of the superolateral surface of the brain. Abbreviations: AG, angular gyrus; ASCR, anterior subcentral ramus of
Sylvian fissure; CS, central sulcus; IFG, inferior frontal gyrus; IFS, inferior frontal sulcus; IOS, inferior occipital sulcus; IPS, intraparietal sulcus; ISJ,
intermediary sulcus of Jensen; ITG, inferior temporal gyrus; ITS, inferior temporal sulcus; MFG, middle frontal gyrus; MFS, middle frontal sulcus; MOG,
middle occipital gyrus; MTG, middle temporal gyrus; Op, opercular part of inferior frontal gyrus; Orb, orbital part of inferior frontal gyrus; PostCG,
postcentral gyrus; PostCS, postcentral sulcus; PreCG, precentral gyrus; PreCS, precentral sulcus; PSCR, posterior subcentral ramus of Sylvian fissure;
SFG, superior frontal gyrus; SFS, superior frontal sulcus; SMG, supramarginal gyrus; SOG, superior occipital gyrus; SOS, superior occipital sulcus;
SPLob, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; SyF, lateral or Sylvian fissure; Tr, triangular part of inferior
frontal gyrus. (Adapted with permission from Ribas GC. The cerebral sulci and gyri. Neurosurg Focus 2010, 28(2):E2.)
The inferior connection corresponds to the subcentral gyrus, deline- The localization of motor and sensory hand areas has been studied
ated anteriorly and posteriorly by the anterior and posterior subcentral by correlating imaging cortical stimulation and postmortem cadaveric


rami of the lateral fissure. It can either be situated completely over the studies. The motor hand area has been localized by fMRI to a protrusion
lateral fissure or be in part internal to the fissure, in this situation giving of the precentral gyrus that corresponded precisely to the middle genu
the false impression that the central sulcus is a branch of the lateral of the central sulcus, at the distal end of the superior frontal sulcus
fissure. The superior connection corresponds to the paracentral lobule (Yousry et al 1997, Ribas 2010) (Figs 25.11–25.12). Postmortem studies
(of Ecker) disposed along the medial surface of the hemisphere inside revealed that this protrusion was delimited by two anteriorly directed
the interhemispheric fissure, delineated anteriorly by the paracentral fissures that deepened towards the base of the protrusion. Hand sensory
sulcus and posteriorly by the ascending and distal part (marginal function has been localized to the postcentral component of the middle
ramus) of the cingulate sulcus. Broca described a middle connection connection of the pre- and postcentral gyri (Boling and Olivier 2004,
between the pre- and postcentral gyri (pli de passage moyen of Broca) Boling et al 2008).
that may be present as a gyral bridge, usually hidden within the central The precentral gyrus is delimited anteriorly by the precentral sulcus,
sulcus; on the cortical surface, this corresponds to the classic, posteriorly itself divided into superior and inferior precentral sulci by the connec-
convex, middle genu of the central sulcus. When this middle connec- tion of the middle frontal gyrus with the precentral gyrus. Further con-
tion is sufficiently developed so that it reaches the brain surface, it nections of the superior, middle and inferior frontal gyri may divide the
interrupts the central sulcus (Régis et al 2005). superior and the inferior precentral sulci into additional segments. The


Cerebral hemispheres
376.e2
3
NOiTCes
A RoCC CaS CiS PreCS PaCS CS MaCiS POS
SubPS
SupRosS
CaF
InfRosS
Spl
Ant and PostOlfS
PaTeG RhiS OTS ColS
B SFG (MedFG) CaNCCCiG Fo IVeFoTha PaCLob PreCu Cu
LatV
CiG
RoCC
PaOlfG
GRe
PaTeG
CiPo AntCom TePo ITGUn IIIV PHG FuG Spl Ist LiG
Fig. 25.10 The main sulci (A) and gyri (B) of the medial and basal temporo-occipital surfaces of the right side of the brain. Abbreviations: AntCom,
anterior commissure; Ant and PostOlfS, anterior and posterior paraolfactory sulcus; CaF, calcarine fissure; CaN, caudate nucleus; CaS, callosal sulcus;
CC, corpus callosum; CiG, cingulate gyrus; CiPo, cingulate pole; CiS, cingulate sulcus; ColS, collateral sulcus; CS, central sulcus; Cu, cuneus; Fo, fornix;
FuG, fusiform gyrus; GRe, gyrus rectus; IIIV, third ventricle; InfRosS, inferior rostral sulcus; Ist, isthmus of cingulate gyrus; ITG, inferior temporal gyrus;
IVeFo, interventricular foramen of Monro; LatV, lateral ventricle; LiG, lingual gyrus; MaCiS, marginal ramus of the cingulate sulcus; MedFG, medial frontal
gyrus; OTS, occipitotemporal sulcus; PaCLob, paracentral lobule; PaCS, paracentral sulcus; PaOlfG, paraolfactory gyri; PaTeG, paraterminal gyrus; PHG,
parahippocampal gyri; POS, parieto-occipital sulcus; PreCS, precentral sulcus; PreCu, precuneus; RhiS, rhinal sulcus; RoCC, rostrum of the corpus


callosum; SFG, superior frontal gyrus; Spl, splenium of corpus callosum; SubPS, subparietal sulcus; SupRosS, superior rostral sulcus; TePo, temporal
pole; Tha, thalamus; Un, uncus. (Adapted with permission from Ribas GC. The cerebral sulci and gyri. Neurosurg Focus 2010, 28(2):E2.)
superior part of the precentral sulcus is very often interrupted superiorly 2012). Superiorly, the inferior frontal gyrus is crossed by various small
by a connection between the superior frontal and precentral gyri, pro- branches of the interrupted inferior frontal sulcus; the triangular sulcus
ducing a more medial segment, the medial precentral sulcus, that cor- typically pierces the superior aspect of the triangular part. In the domi-
responds to the sulcus precentralis medialis of Eberstaller. More dorsally, nant hemisphere, the opercular and triangular parts of the inferior gyrus
within the precentral region, the marginal precentral sulcus (sulcus correspond to Broca’s area, which is responsible for the production of
precentralis marginalis of Cunningham) may merge with the superior spoken language (Fig. 25.13) (Quiñones-Hinojosa et al 2003). The
precentral or central sulci. The inferior segment of the precentral sulcus most posterior aspect of the inferior frontal gyrus, identifiable by the
always ends inside the opercular part of the inferior frontal gyrus, pro- connection of its opercular part with the precentral gyrus, corresponds
ducing its characteristic U shape. to the ventral premotor cortical area; its bilateral stimulation causes
The superior frontal gyrus is continuous anteriorly and inferiorly speech arrest (Duffau 2011).
with the rectus gyrus; it may also be connected to the orbital gyri and The superior frontal sulcus separates the superior and middle frontal
the middle frontal gyrus. Posteriorly, it is connected to the precentral gyri. It is very deep and is frequently continuous, ending posteriorly by
gyrus by at least one fold, which most commonly lies medially along encroaching on the precentral gyrus at the level of its omega region


the interhemispheric fissure. Usually the superior longitudinal gyrus is (corresponding to the motor cortical representation of the contralateral
subdivided into two longitudinal portions by a medial frontal sulcus; hand). The superior frontal sulcus therefore tends to point the way to
its medial portion is sometimes termed the medial frontal gyrus. The the middle frontoparietal pli de passage, as well as to the middle genu
supplementary motor area is located along the most medial portion of of the precentral gyrus, where there is also a motor representation of
the superior frontal gyrus, immediately facing the precentral gyrus; it the hand (Boling et al 1999).
varies between individuals and has poorly defined borders. The middle The inferior frontal sulcus is always interrupted by the multiple con-
frontal gyrus is usually the largest of the frontal gyri, frequently con- nections running between the middle and inferior gyri and usually has
nected superficially to the precentral gyrus by a prominent root that lies three parts: orbital, triangular and opercular. The orbital part is the most
between the extremities of a marked interruption in the precentral prominent. The triangular part is usually more retracted, such that there
sulcus. It harbours a complex of multiple shallow sulcal segments is a small widening of the lateral fissure at its base corresponding to the
known collectively as the middle or intermediate frontal sulcus (Petrides anterior Sylvian point. It is characterized by horizontal and anterior


Cerebral hemispheres
376.e3
52
reTpahC
ascending rami of the lateral fissure that consistently divide the lateral
fissure into anterior and posterior branches. The opercular part is always
U-shaped and harbours the inferior aspect of the precentral sulcus; it
is continuous posteriorly with the basal aspect of the precentral gyrus
over the anterior subcentral ramus of the lateral fissure (Fig. 25.14).
The anterior basal portion of the opercular part is sometimes divided
by another branch of the lateral fissure, the diagonal sulcus of
Eberstaller.
Inferiorly, the orbital part continues with the lateral orbital gyrus, at
times passing under a shallow sulcus known as the fronto-orbital
sulcus. The basal apex of the triangular part is always superior to the
SFS lateral fissure; the base of the opercular part can be located either supe-
PreCS riorly or within the fissure. Anteriorly, the inferior frontal gyrus termi-
nates by merging with the anterior portion of the middle frontal gyrus.
PreCG
All of the frontal gyri are delineated anteriorly by the frontomarginal
sulcus (frontomarginal sulcus of Wernicke), which lies superior and
parallel to the supraciliary margin, separating the superolateral and
orbital frontal surfaces. Posteriorly, the inferior frontal gyrus is con-
nected to the precentral gyrus along the posterior aspect of its opercular
part.
The olfactory sulcus lies longitudinally in a paramedian position on
the frontobasal or orbital surface of each frontal lobe. It accommodates
the olfactory tract and bulb. Posteriorly, the olfactory tract is divided
into medial and lateral striae, which delineate the most anterior aspect
of the anterior perforated substance (see Figs 25.20B, 25.32). The
narrow gyrus rectus, medial to the olfactory sulcus, is considered to be
the most anatomically constant of the cerebral gyri. The orbital gyri,
lateral to the olfactory sulcus, account for the greatest proportion of the
frontobasal surface. The anterior, posterior, medial and lateral orbital
gyri are delineated by the lateral, medial and transverse orbital sulci and


the cruciform sulcus of Rolando, which together form a characteristic
H shape. The posterior orbital gyrus lies anterior to the anterior perfo-
rated substance and typically presents a configuration similar to a
Fig. 25.11 The hand motor activation site corresponds to a knob-like tricorn hat, a feature that may facilitate its identification in anatomical
cortical area of the contralateral precentral gyrus, which in MRI axial specimens where the H-shaped orbital sulcus is less obvious. The
planes usually resembles an inverted omega shape (the area within the remaining orbital gyri are connected to the superior, middle and infe-
red circle) and may be identified by its relationship to the posterior end of rior frontal gyri along the frontal pole.
the superior frontal sulcus. Abbreviations: PreCG, precentral gyrus;
PreCS, precentral sulcus; SFS, superior frontal sulcus (non-continuous,
interrupted). (Courtesy of Professor Edson Amaro Jr MD, Department of
Radiology, University of São Paulo Medical School.)
Fig. 25.12 A reconstruction of the short U-shaped
(red) and long projection (green) tracts of the
hand-knob motor region in the left hemisphere.
A, Left lateral view. B, Top view. C, Posterior view.
The connections of the hand region resemble a
‘poppy flower’ with a green stem and four red
‘petals’ (1, posterior; 2, inferior; 3, anterior; 4,
superior). The posterior (1) and inferior (2) petals
correspond to the frontoparietal U-tracts between
the precentral (PrCG) and postcentral (PoCG) gyri
(i.e. hand superior and hand middle, respectively).
The anterior (3) and superior (4) petals correspond
to the U-shaped connections between the
precentral gyrus and the middle frontal gyrus
(MFG), and the middle frontal gyrus and superior
frontal (SFG) gyrus, respectively. The ‘green stem’
is formed by ascending thalamocortical projection
fibres and descending projections to the putamen
(corticostriatal), pons (corticopontine) and spinal
A C
cord (corticospinal tract). (With permission from
Catani M, Dell’acqua F, Vergani F et al; Short


frontal lobe connections of the human brain.
Cortex 2012 Feb;48(2):273–91.)
Putamen Thalamus
Pons
Spinal cord
B


Cerebral hemispheres
376.e4
3
NOiTCes
MFG
SOG
Tr STS
Orb Op
MFG
Tr SOG
STS
Orb Op
MFG
Op
STS
Orb
A
B
C
Fig. 25.13 Functional magnetic resonance images (fMRI) of the language
cortical areas (left cerebral hemisphere) activated by rhyme tasks (A, red),
semantic tasks (B, blue) and fluency tasks (C, green). Abbreviations: MFG,
middle frontal gyrus; Op, opercular part of inferior frontal gyrus; Orb,
orbital part of inferior frontal gyrus; SOG, superior occipital gyrus; STS,
superior temporal sulcus; Tr, triangular part of inferior frontal gyrus.
(Courtesy of Prof. Edson Amaro Jr MD, Department of Radiology,
University of São Paulo Medical School.)
The paracentral lobule, bounded posteriorly by the marginal ramus
and anteriorly by the paracentral sulcus (a branch of the cingulate
sulcus), contains the distal part of the central sulcus and, inferior to it,
the so-called paracentral fossa. Anterior to the paracentral lobule, the
medial aspect of the superior frontal gyrus lies over the cingulate sulcus
and the cingulate gyrus, merging inferiorly with the gyrus rectus. The
latter is bounded superiorly by the superior rostral sulcus and accom-
modates the shallower inferior rostral sulcus along its surface. The
cingulate gyrus systematically connects with the gyrus rectus around the
posterior end of the superior rostral sulcus by a prominent U-shaped
cortical fold known as the cingulate pole, which is located immediately
anterior to the subcallosal gyri. Small supraorbital sulci lie within the
medial surface of the frontal pole, superior to the superior rostral sulcus
at the level of the genu of the corpus callosum.


Cerebral hemispheres
376.e5
52
reTpahC
A IFS IFS/PreCS PreCS CS PostCS
ASyP IRP
B
Fig. 25.14 Components of the frontoparietal operculum. A, A cadaveric specimen. B, MRI. The triangular part of the inferior frontal gyrus usually
contains a descending branch of the inferior frontal sulcus (IFS). Three U-shaped convolutions are formed by the opercular part of the inferior frontal
gyrus, which always harbours the inferior part of the precentral sulcus (PreCS); the subcentral gyrus or Rolandic operculum (the inferior connection of the
pre- and postcentral gyri enclosing the inferior part of the central sulcus (CS)); and the connection between the postcentral and supramarginal gyri that
contains the inferior part of the postcentral sulcus (PostCS). The most distal component of the operculum is a C-shaped convolution that connects the
supramarginal and superior temporal gyri, and encircles the posterior end of the lateral (Sylvian) fissure. The bases of the U-shaped convolutions and
their related sulcal extremities may be either superior to the fissure, as indicated in this specimen, or inside the fissure. Other abbreviations: ASyP,
anterior Sylvian point; IRP, inferior Rolandic point. (Adapted with permission from Ribas GC, Ribas EC, Rodrigues CJ: The anterior sylvian point and the
suprasylvian operculum. Neurosurg Focus 18:E1–E6, 2005.)


Cerebral lobes
377
52
reTpahC
A 3 B
4
8 6 5 7A
5
7B 7
9
31 24 9
23
19 32
46 1 2 40 18 23 90 33 10
10 39
26
44 18 17
45 19 11
18 36 34
11 22 37 17 19 37 28 25
38
38 21
47
42 20
20 41 27
52
35
43
Fig. 25.7 The lateral (A) and medial (B) surfaces of the left cerebral hemisphere depicting Brodmann’s areas.
Corticospinal tract nate in the ventral horn of the spinal cord. In cord segments mediating
The corticospinal or pyramidal tract provides direct control by the cere- dexterous hand and finger movements, they terminate in the lateral part
bral cortex over motor centres of the spinal cord (Ch. 20). A homolo- of the ventral horn, in close relationship to motor neuronal groups. A
gous pathway to the brainstem, the corticonuclear projection, fulfils a small percentage establish direct monosynaptic connections with α
similar function in relation to motor nuclei of the brainstem (Ch. 21). motor neurones. Between 40 and 60% of pyramidal tract axons arise
The percentage of corticospinal fibres that arise from the primary from parietal areas, including area 3a, area 5 of the superior part of the
motor cortex may be in the region of 20–30%. They arise from pyrami- parietal lobe, and SII in the parietal operculum. The majority of parietal
dal cells in layer V and give rise to the largest-diameter corticospinal fibres to the spinal cord terminate in the deeper layers of the dorsal
axons. There is also a widespread origin from other parts of the frontal horn.
lobe, including the premotor cortex and the supplementary motor area. Motor cortical neurones are active in relation to the force of contrac-
Many axons from the frontal cortex, notably the motor cortex, termi- tion of agonist muscles; their relation to amplitude of movement is less
8


A B 1 Agranular 2 Frontal 3 Parietal 4 Granular 5 Polar
I
I I I I
II
II II II II
1
4 2 III
2
III
III III
3 3 III IV
5
1 4 4 IV
4
IV
IV V IV
2 V
V V IV
VI
V
VI
VI
VI VI
Fig. 25.6 The distribution (A) and characteristics (B) of the five major types of cerebral cortex.
2 1 3
4 6


Cerebral hemispheres
378
3
NOiTCes
1 A
F P
2
6
TA
oenK
k
sn le
eepiH Trunk Shoulder Elbow Wrist
FingeHa rnd
s
Little Rin Mig
d Idl ne Td he ux m Nb e Bc rk
ow
T
O
Eyelid and
eye Fb aa cll
e
Lips N
O IT
A
Z
IL
A 7
9
3
1
5 4 Swallo MwT io Ann
g
Sg TIu CJ e Aa TIw
ON
SALIVATI N OVOCA
2
P
F L
10
B
O
FiB
g. 25.8
Lobar7
boundariesT
and
nom1 e1
nclature. A,
Lat4
eral
s8
urface, left
FaN co
eE
seyeThuI mnd
beM
xiddR leingLittleH a n d FW ri ist ngF eo re a r rm sE lb o wA rmS h o u ld e rH e a dN e ckT ru n k GpiH eniTF tog aoe le iL o st
a
side. The central sulcus (1) separates the frontal (F) from the parietal (P) Upper
lip
lobes. The Sylvian fissure (3) separates the frontal from the temporal (T) Lips
lobes. The demarcation of the temporal, parietal and occipital lobes
according to two different systems is shown. In one system, a
Lower lip
parietotemporal line is drawn from the lateral edge of the parieto-occipital
sulcus (2) to the preoccipital notch (temporo-occipital incisure) (4). This Teeth, gums and jaw
line sets the arbitrary anterior border of the occipital lobe (O), separating it
Tongue
from the parietal and temporal lobes anterior to it. A second arbitrary
t oe fm thp eo r So y- lo vc iac nip fiit sa sl uli rn ee
(
3(5
)
)
t
ois td hr ea w mn
id
f dro lem
o
ft h te
h


ep pos at re ier tio or
t
ed me psc oe ran ld li in ng
e
r (6a )m
.
Tu hs
is
Phar Iy nn trx a-abdominal
line sets the arbitrary parietotemporal boundary. B, Medial surface, right
side. The central sulcus (1) usually curves onto the medial surface
perpendicular to the marginal segment of the cingulate sulcus. A line
drawn from the central sulcus to the cingulate sulcus establishes the
frontoparietal border. The deep parieto-occipital sulcus (2) demarcates
the parietal lobe from the occipital lobe. An arbitrary basal
parietotemporal line (8) drawn from the inferior end of the parieto-occipital
Fig. 25.15 A, The motor homunculus showing proportional somatotopical
sulcus to the preoccipital notch establishes the temporal (T)/occipital (O)
representation in the main motor area. B, The sensory homunculus
border. The limbic lobe (L) is delimited by the cingulate sulcus (9), the
showing proportional somatotopical representation in the somaesthetic
subparietal sulcus (10) and the collateral sulcus (11). The number 7
cortex.
indicates the orbital surface. (With permission from Naidich TP, Tang CY,
Ng JC, Delman BN, Surface Anatomy of the Cerebrum. In: Naidich TP,
Castillo M, Cha S, Smirniotopoulos JG (eds) Imaging of the Brain, 2013,
Elsevier, Saunders.)
The major thalamic connections of the premotor cortex are with the
anterior division of the ventrolateral nucleus and with the centrome-
clear. Their activity precedes the onset of electromyographic activity by dian, parafascicular and centrolateral components of the intralaminar
50–100 milliseconds, suggesting a role for cortical activation in generat- nuclei. Subcortical projections to the striatum and pontine nuclei are
ing rather than monitoring movement. prominent, and this area also projects to the superior colliculus and the
reticular formation. Both dorsal and ventral areas contribute to the
Premotor cortex


corticospinal tract. Commissural connections are with the contralateral
Brodmann’s area 6 lies immediately in front of the primary motor premotor, motor and superior parietal (area 5) cortex. Ipsilateral corti-
cortex (Fig. 25.16). It extends on to the medial surface, where it becomes cocortical connections with area 5 in the superior parietal cortex, and
contiguous with area 24 in the cingulate gyrus, anterior and inferior to inferior parietal area 7b, are common to both dorsal and ventral sub-
the paracentral lobule. A number of functional motor areas are con- divisions of the premotor cortex, and both send a major projection to
tained within this cortical region. Lateral area 6, the area over most of the primary motor cortex. The dorsal premotor area also receives fibres
the lateral surface of the hemisphere, corresponds to the premotor from the posterior superior temporal cortex and projects to the sup-
cortex. plementary motor cortex. The frontal eye field (area 8) projects to the
The premotor cortex is divided into a dorsal and a ventral area (PMd dorsal subdivision. Perhaps the greatest functionally significant differ-
and PMv respectively) on functional grounds, and on the basis of ipsi- ence in connectivity between the two premotor area subdivisions is
lateral corticocortical association connections. that the dorsal premotor area receives fibres from the dorsolateral


Cerebral lobes
379
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reTpahC
nucleus, the intralaminar nuclei – notably the centrolateral and centro-
medial nuclei, and also from the mediodorsal nucleus. The connections
with the thalamus are reciprocal. The supplementary motor cortex
receives connections from widespread regions of the ipsilateral frontal
lobe, including from the primary motor cortex, the dorsal premotor
8 6
area, the dorsolateral and ventrolateral prefrontal, medial prefrontal
9 40 and orbitofrontal cortex, and the frontal eye field. These connections
39 are reciprocal but the major ipsilateral efferent pathway is to the motor
cortex. Parietal lobe connections of the supplementary motor cortex are
45 44 with the superior parietal area 5 and possibly inferior parietal area 7b.
Contralateral connections are with the supplementary motor area, and
22
motor and premotor cortices of the contralateral hemisphere. Subcorti-
cal connections, other than with the thalamus, pass to the striatum,
37
subthalamic nucleus and pontine nuclei, the brainstem reticular forma-
tion and the inferior olivary nucleus. The supplementary motor area
makes a substantial contribution to the corticospinal tract, contributing
as much as 40% of the fibres from the frontal lobe.
Fig. 25.16 The lateral surface of the left cerebral hemisphere showing the
The supplementary motor area contains a representation of the body
frontal eye field (parts of areas 6, 8, 9), the motor speech (Broca’s) area
in which the leg is posterior and the face anterior, with the upper limb
(areas 44, 45) and Wernicke’s area. The perimeter of these areas is
between them. Its role in the control of movement is primarily in
delineated by an interrupted line to indicate uncertainty as to their precise
complex tasks that require temporal organization of sequential move-
extent. Wernicke’s area is variously depicted by different authorities as
ments and in the retrieval of motor memory.
encompassing a large parietotemporal area that includes areas 39 and
Stimulation of the supplementary motor area in conscious patients
40. Areas 22 and 37 are considered by some to be respectively auditory


and visuo-auditory areas associated with speech and language. has been reported to elicit the sensation of an urge to move, or of
anticipation that a movement is about to occur. A region anterior to the
supplementary motor area for face representation (areas 44, 45) is
important in vocalization and speech production (see Fig. 25.16).
prefrontal cortex, whereas the ventral subdivision receives fibres from Prefrontal cortex
the ventrolateral prefrontal cortex. All of these association connections The prefrontal cortex on the lateral surface of the hemisphere com-
are likely to be, or are known to be, reciprocal. prises predominantly Brodmann’s areas 9, 46 and 45 (see Fig. 25.16).
Neuronal activity in the premotor cortex in relation to both prepara- In non-human primates, two subdivisions of the lateral prefrontal
tion for movement and movement itself has been extensively studied cortex are recognized: a dorsal area equivalent to area 9, and perhaps
experimentally. Direction selectivity for movement is a common feature including the superior part of area 46; and a ventral area, consisting of
of many premotor neurones. In behavioural tasks, neurones in the the inferior part of area 46 and area 45. Areas 44 and 45 are particularly
dorsal premotor cortex show anticipatory activity and task-related dis- notable in humans since, in the dominant hemisphere, they constitute
charge as well as direction selectivity, but little or no stimulus-related the motor speech area (Broca’s area) within the opercular and triangu-
changes. The dorsal premotor cortex is probably important in establish- lar parts of the inferior frontal gyrus, immediately anterior to the most
ing a motor set or intention, contributing to motor preparation in inferior aspect of the precentral gyrus. Both the dorsolateral and vent-
relation to internally guided movement. In contrast, ventral premotor rolateral prefrontal areas receive their major thalamic afferents from the
cortex is more related to the execution of externally (especially visually) medio dorsal nucleus, and there are additional contributions from the
guided movements in relation to a specific external stimulus. medial pulvinar, from the ventral anterior nucleus and from the para-
central nucleus of the anterior intralaminar group. The dorsolateral
Frontal eye field


area receives long association fibres from the posterior and middle
The frontal eye field corresponds to parts of Brodmann’s areas 6, 8 and superior temporal gyrus (including auditory association areas), from
9 (see Fig. 25.16). As its name implies, it is important in the control of parietal area 7a, and from much of the middle temporal cortex. From
eye movements. It receives its major thalamic projection from the par- within the frontal lobe it also receives projections from the frontal pole
vocellular mediodorsal nucleus, with additional afferents from the (area 10), and from the medial prefrontal cortex (area 32) on the
medial pulvinar, the ventral anterior nucleus and the suprageniculate– medial surface of the hemisphere. It projects to the supplementary
limitans complex, and connects with the paracentral nucleus of the motor area, the dorsal premotor cortex and the frontal eye field. All
intralaminar group. The thalamocortical pathways to the frontal eye these thalamic and corticocortical connections are reciprocal. Commis-
field form part of a pathway from the superior colliculus, the substantia sural connections are with the homologous area and with the contral-
nigra and the dentate nucleus of the cerebellum. The frontal eye field ateral inferior parietal cortex. The ventrolateral prefrontal area receives
has extensive ipsilateral corticocortical connections, receiving fibres long association fibres from both area 7a and area 7b of the parietal
from several visual areas in the occipital, parietal and temporal lobes, lobe, from auditory association areas of the temporal operculum, from
including the medial temporal area (V5) and area 7a. There is also a the insula and from the anterior part of the lower bank of the superior
projection from the superior temporal gyrus, which is auditory rather temporal sulcus. From within the frontal lobe it receives fibres from the
than visual in function. From within the frontal lobe, the frontal eye anterior orbitofrontal cortex and projects to the frontal eye field and
field receives fibres from the ventrolateral and dorsolateral prefrontal the ventral premotor cortex. It connects with the contralateral homolo-


cortices. It projects to the dorsal and ventral premotor cortices and to gous area via the corpus callosum. These connections are probably all
the medial motor area, probably to the supplementary eye field adja- reciprocal.
cent to the supplementary motor area proper. It projects prominently The cortex of the frontal pole (area 10) receives thalamic input from
to the superior colliculus, to the pontine gaze centre within the pontine the mediodorsal nucleus, the medial pulvinar and the paracentral
reticular formation, and to other oculomotor related nuclei in the nucleus. It is reciprocally connected with the cortex of the temporal
brainstem. pole, the anterior orbitofrontal cortex and the dorsolateral prefrontal
cortex. The orbitofrontal cortex connects with the mediodorsal, antero-
Supplementary motor cortex
medial, ventral anterior, medial pulvinar, paracentral and midline
The supplementary motor area (SMA; MII) lies medial to area 6, and nuclei of the thalamus. Cortical association pathways come from the
extends from the most superolateral part to the medial surface of the inferotemporal cortex, the anterior superior temporal gyrus and the
hemisphere. Area 24 in the cingulate gyrus adjacent to area 6 contains temporal pole. Within the frontal lobe it has connections with
several motor areas, which are termed cingulate motor areas. An addi- the medial prefrontal cortex, the ventrolateral prefrontal cortex and
tional functional subdivision, the pre-supplementary motor area, lies medial motor areas. Commissural and other connections follow the
anterior to the supplementary motor area on the medial surface of the general pattern for all neocortical areas.
cortex. In the present discussion, these additional medial motor areas The medial prefrontal cortex is connected with the mediodorsal,
are included with the supplementary motor cortex. ventral anterior, anterior medial pulvinar, paracentral, midline and
The supplementary motor area receives its major thalamic input suprageniculate–limitans nuclei of the thalamus. It receives fibres from
from the anterior part of the ventral lateral nucleus, which in turn is the anterior cortex of the superior temporal gyrus. Within the frontal


the major recipient of fibres from the internal segment of the globus lobe, it has connections with the orbitofrontal cortex, and the medial
pallidus. Additional thalamic afferents are from the ventral anterior motor areas of the dorsolateral prefrontal cortex.


Cerebral hemispheres
380
3
NOiTCes
PARIETAL LOBE modality and somatotopic properties. They appear to project to
restricted focal patches in SI of approximately 0.5 mm width, which
The parietal lobe lies posterior to the central sulcus on the superolateral form narrow strips mediolaterally along SI. The laminar termination of
and medial surfaces of the cerebral hemisphere (see Figs 25.1, thalamocortical axons from the ventral posterior nucleus is different in
25.8–25.10). This region is part of the complex language network (see the separate cytoarchitectonic subdivisions of SI. In 3a and 3b, these
below). Posterior to the supramarginal gyrus and again in the dominant axons terminate mainly in layer IV and the adjacent deep part of layer
hemisphere, the cortex of the angular gyrus is related to neuronal III, whereas in areas 1 and 2 they end in the deeper half of layer III,
processing associated with reading and writing. avoiding lamina IV. Additional thalamocortical fibres to SI arise from
the intralaminar system, notably the centrolateral nucleus.
There is a complex internal connectivity within SI. An apparently
Parietal lobe sulci and gyri
stepwise hierarchical progression of information processing occurs
from area 3b through area 1 to area 2. Outside the postcentral gyrus,
The parietal gyri are morphologically poorly defined and tortuous; SI has ipsilateral corticocortical association connections with a second
some are termed lobules. Posteriorly, the parietal lobe is delineated on somatosensory area (SII); area 5 in the superior part of the parietal lobe;
the medial aspect by the parieto-occipital sulcus and on the lateral area 4, the motor cortex, in the precentral gyrus; and the supplementary
aspect by an imaginary line running from the point where the parieto- motor cortex in the medial part of area 6 of the frontal lobe.
occipital sulcus emerges on to the superolateral border to the preoc- SI has reciprocal commissural connections with its contralateral
cipital notch (a small sulcus situated on the inferolateral border homologue, with the exception that the cortices containing the repre-


approximately 5 cm anterior to the occipital pole). The inferior bound- sentation of the distal extremities are relatively devoid of such con-
ary is the posterior ramus of the lateral fissure and its imaginary poste- nections. Callosal fibres in SI arise mainly from the deep part of
rior prolongation. layer III and terminate in layers I–IV. Pyramidal cells contributing cal-
The lateral aspect of the parietal lobe is divided into three areas by losal projections receive monosynaptic thalamic and commissural
the postcentral and intraparietal sulci. The intraparietal sulcus lies connections.
predominantly longitudinally along the midportion of the parietal SI has reciprocal subcortical connections with the thalamus and
superolateral surface (see Fig. 25.1). It delineates the superior parietal claustrum, and receives afferents from the nucleus basalis (basal nucleus
lobule, continuous medially with the precuneus, and the inferior of Meynert), the locus coeruleus and the midbrain raphe. It has other
parietal lobule, made up of the supramarginal and angular gyri and a prominent subcortical projections. Corticostriatal fibres, arising in layer
more posterior convolution continuous with the occipital lobe. The V, pass mainly to the putamen of the same side. Corticopontine and
inferior aspect of the supramarginal gyrus within the inferior parietal corticotectal fibres from SI arise in layer V. SI projects to the main
lobule of the dominant hemisphere corresponds to Wernicke’s area, pontine nuclei and to the nucleus reticularis tegmenti pontis (pontine
which extends along the posterior aspect of the superior temporal tegmental reticular nucleus). In addition, axons arising in SI pass to the
gyrus. dorsal column nuclei and the spinal cord. Corticospinal pyramidal cells
The postcentral gyrus lies posterior to the precentral gyrus and is are found in layer V of SI. The topographical representation in the cortex
connected to it along the superior and inferior extremities of the is preserved in terms of the spinal segments to which different parts of


central sulcus. It is usually narrower than the precentral gyrus. Both the postcentral gyrus project. Thus, the arm representation projects to
gyri are located obliquely on the superolateral surface of the hemi- the cervical enlargement, the leg representation to the lumbosacral
sphere, just superior to the lateral fissure; their midportions corre- enlargement, and so on. Within the grey matter of the spinal cord, fibres
spond approximately to the anteroposterior centre of each cerebral from SI terminate in the dorsal horn, in Rexed’s laminae 3–5; fibres
hemisphere. The superior portions of the pre- and postcentral gyri, from 3b and 1 end more dorsally, and those from area 2 more
which constitute the paracentral lobule on the medial surface of the ventrally.
cerebral hemisphere, are topographically related to the ventricular The second somatosensory area (SII) lies along the upper bank of
atrium, situated posterior to the thalamus. The inferior portions of the lateral fissure, posterior to the central sulcus. SII contains a soma-
both gyri cover the posterior half of the insula and are topographically totopic representation of the body, with the head and face most ante-
related to the body of the lateral ventricle, situated superior to the riorly, adjacent to SI, and the sacral regions most posteriorly. SII is
thalamus. The portion of the subcentral gyrus corresponding to the reciprocally connected with the ventral posterior nucleus of the thala-
base of the postcentral gyrus consistently lies over the transverse gyri of mus in a topographically organized fashion. Some thalamic neurones
Heschl, situated on the opercular surface of the temporal lobe (Wen probably project to both SI and SII via axon collaterals. Other thalamic
et al 1999). connections of SII are with the posterior group of nuclei and with the
intralaminar central lateral nucleus. SII also projects to laminae IV–VII
Parietal lobe internal structure of the dorsal horn of the cervical and thoracic spinal cord, the dorsal
column nuclei, the principal trigeminal nucleus, and the periaqueductal
and connectivity
grey matter of the midbrain.


Within the cortex, SII is reciprocally connected with SI in a topo-
Somatosensory cortex
graphically organized manner and projects to the primary motor cortex.
The postcentral gyrus corresponds to the primary somatosensory cortex SII also projects in a topographically organized way to the lateral part
(SI; Brodmann’s areas 3a, 3b, 1 and 2). Area 3a lies most anteriorly, of area 7 (area 7b) in the superior part of the parietal lobe, and makes
apposing area 4, the primary motor cortex of the frontal lobe; area 3b connections with the posterior cingulate gyrus. Both right and left SII
is buried in the posterior wall of the central sulcus; area 1 lies along the areas are interconnected across the corpus callosum, although distal
posterior lip of the central sulcus; and area 2 occupies the crown of the limb representations are probably excluded. There are additional cal-
postcentral gyrus. losal projections to SI and area 7b.
The primary somatosensory cortex contains within it a topographical Experimental studies show that neurones in SII respond particularly
map of the contralateral half of the body. The face, tongue and lips are to transient cutaneous stimuli, e.g. brush strokes or tapping, which are
represented inferiorly, the trunk and upper limb are represented on the characteristic of the responses of Pacinian corpuscles in the periphery.
superolateral aspect, and the lower limb on the medial aspect of the They show little response to maintained stimuli.
hemisphere, giving rise to the familiar ‘homunculus’ map (see Fig.
25.15). Superior and inferior parietal lobules
The somatosensory properties of SI depend on its thalamic input Posterior to the postcentral gyrus, the superior part of the parietal lobe
from the ventral posterior nucleus of the thalamus, which in turn is composed of areas 5, 7a and 7b (see Fig. 25.7). Area 5 receives a
receives the medial lemniscal, spinothalamic and trigeminothalamic dense feed-forward projection from all cytoarchitectonic areas of SI in


pathways. The nucleus is divided into a ventral posterolateral part, a topographically organized manner. The thalamic afferents to this area
which receives information from the trunk and limbs, and a ventral come from the lateral posterior nucleus and from the central lateral
posteromedial part, in which the head is represented. Within the ventral nucleus of the intralaminar group. Ipsilateral corticocortical fibres from
posterior nucleus, neurones in the central core respond to cutaneous area 5 go to area 7, the premotor and supplementary motor cortices,
stimuli and those in the most dorsal anterior and posterior parts, which the posterior cingulate gyrus and the insular granular cortex. Commis-
arch as a ‘shell’ over this central core, respond to deep stimuli. This is sural connections between area 5 on both sides tend to avoid the areas
reflected in the differential projections to SI: the cutaneous central core of representation of the distal limbs. The response properties of cells in
projects to 3b, the deep tissue-responsive neurones send fibres to areas area 5 are more complex than in SI, with larger receptive fields and
3a and 2, and an intervening zone projects to area 1. Within the ventral evidence of submodality convergence. Area 5 contributes to the corti-
posterior nucleus, anteroposterior rods of cells respond with similar cospinal tract.


Cerebral hemispheres
380.e1
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reTpahC
The postcentral sulcus delineates the posterior boundary of the post-
central gyrus. It is frequently interrupted by connections with the supe-
rior and inferior parietal lobules. The inferior part of the postcentral
SFG
sulcus always ends at a basal connection between the postcentral and
the supramarginal gyri (Ribas 2010). The intraparietal sulcus, which
originates from around the midpoint of the postcentral sulcus, is prom-
inent on the superolateral surface of the parietal lobe, running parallel PreCG
CS
with the superior margin of the hemisphere. Anteriorly, the intraparietal PostCG
sulcus is usually continuous with the inferior portion of the postcentral
sulcus and posteriorly it passes into the occipital lobe as the intra- PaCLob PosCS
occipital sulcus (superior occipital sulcus), which continues more pos- SMG
SPaLob
teriorly into the transverse occipital sulcus. The intraparietal sulcus
IPaLob
divides the superolateral parietal surface into superior and inferior
parietal lobules; along its length, it typically gives rise to superior and AG
PreCu
inferior vertical sulcal branches. The superior vertical sulcal branch
(transverse parietal sulcus of Brissaud) divides the superior parietal POS
POArc
lobule. The inferior vertical sulcal branch (intermediate sulcus of IPS
Jensen; sulcus intermedius primus of Jensen) separates the supramar- PreOccNo
ginal gyrus anteriorly from the angular gyrus posteriorly.
The supramarginal gyrus is always a very well defined curved gyrus.
It surrounds the distal portion of the lateral fissure (its posterior ascend-
ing branch) and becomes continuous with the posterior portion of the
superior temporal gyrus. Above the distal end of the lateral fissure, the
supramarginal gyrus is connected anteriorly to the postcentral gyrus
through a fold that runs underneath the inferior aspect of the postcen-
tral sulcus. Posteriorly, it occasionally rounds the inferior extremity of IOGMOG SOG CaS


the intermediate sulcus and connects to the angular gyrus. The angular
Fig. 25.17 A superior view of the cerebral hemispheres. Abbreviations:
gyrus is a curved gyrus, often poorly defined morphologically. It always
AG, angular gyrus; CaS, calcarine sulcus; CS, central sulcus; IOG, inferior
surrounds one of the distal segments of the superior temporal sulcus,
occipital gyrus; IPaLob, inferior parietal lobule; IPS, intraparietal sulcus;
usually the middle one (angular sulcus), and its most inferior portion MOG, middle occipital gyrus; PaCLob, paracentral lobule; POArc,
is continuous with the middle temporal gyrus. The configuration of the parieto-occipital arch; POS, parieto-occipital sulcus; PosCS, postcentral
angular gyrus is defined by the distal branching of the superior temporal sulcus; PostCG, postcentral gyrus; PreCG, precentral gyrus; PreCu,
sulcus, which typically ends as three continuous or interrupted caudal precuneus; PreOccNo, pre-occipital notch; SFG, superior frontal gyrus;
branches. SMG, supramarginal gyrus; SOG, superior occipital gyrus; SPaLob,
The most superior distal branch of the superior temporal sulcus has superior parietal lobule. (Adapted with permission from Ribas GC. The
an ascending course; it may either penetrate the supramarginal gyrus or cerebral sulci and gyri. Neurosurg Focus 2010, 28(2):E2.)
coincide with the intermediate sulcus of Jensen separating the supra-
marginal and angular gyri. The second branch is usually more horizon-
tal and enters the angular gyrus as the angular sulcus. The most inferior
caudal branch of the superior temporal sulcus is less evident and less In non-human primates, the inferior parietal lobule is area 7. In
constant. It courses underneath a posterior fold that frequently connects humans, this area is more superior; areas 39 and 40 intervene inferiorly.
the angular gyrus and the most lateral aspect of the occipital lobe, and The counterparts for the latter areas in monkeys are unclear and little


is usually continuous with the anterior occipital sulcus, lying predomi- experimental evidence is available on their connections and functions.
nantly vertically along the anterior edge of the middle occipital gyrus. Their role in human cerebral processing is discussed below. In the
The bulge of the supramarginal and angular gyri is responsible for the monkey, area 7b receives somatosensory inputs from area 5 and SII.
cranial parietal tuberosity (bossa). Connections pass to the posterior cingulate gyrus (area 23), insula and
The superior parietal lobule has a quadrangular shape. It is deline- temporal cortex. Area 7b is reciprocally connected with area 46 in the
ated anteriorly by the superior aspect of the postcentral sulcus and later- prefrontal cortex and the lateral part of the premotor cortex. Commis-
ally by the intraparietal sulcus; medially, it is continuous with the sural connections of area 7b are with the contralateral homologous area
precuneus gyrus along the superomedial border (Fig. 25.17). Anteri- and with SII, the insular granular cortex and area 5. Thalamic connec-
orly, it is typically connected to the postcentral gyrus via a fold that tions are with the medial pulvinar nucleus and the intralaminar para-
transects the most superior portion of the postcentral sulcus and, occa- central nucleus.
sionally, via a fold that interrupts the postcentral sulcus more inferiorly. In monkeys, area 7a is not related to the cortical pathways for
Posteriorly, the superior parietal lobule continues to the superior occipi- somatosensory processing but instead forms part of a dorsal cortical
tal gyrus via the prominent parieto-occipital arcus. pathway for spatial vision. The major ipsilateral corticocortical connec-
On the medial surface of each hemisphere, the precuneus lies pos- tions to area 7a are derived from visual areas in the occipital and tem-
terior to the paracentral lobule as a medial extension of the superior poral lobes. In the ipsilateral hemisphere, area 7a has connections


parietal lobule. The precuneus and the medial aspect of the postcentral with the posterior cingulate cortex (area 24) and with areas 8 and 46
gyrus correspond to the medial portion of the parietal lobe (see Figs of the frontal lobe. Commissural connections are with its contralateral
25.3, 25.10B). The precuneus is quadrangular (quadrangular lobule of homologue. Area 7a is connected with the medial pulvinar and intra-
Foville), delineated anteriorly by the marginal branch of the cingulate laminar paracentral nuclei of the thalamus. In experimental studies,
sulcus, posteriorly by the parieto-occipital sulcus and inferiorly by mul- neurones within area 7a are visually responsive; they relate largely to
tiple Y-shaped sulcal segments that constitute the subparietal sulcus. peripheral vision, respond to stimulus movement, and are modulated
Inferior to the subparietal sulcus, the precuneus is connected to the by eye movement.
isthmus of the cingulate gyrus, which is continuous with the parahip-
pocampal gyrus.
The parieto-occipital sulcus separating the precuneus from the
cuneus is deep and contains many small sulci and gyri. The precuneal
limiting sulcus and the cuneal limiting sulcus delineate the inferior
(posterior) limit of the precuneus and the superior (anterior) limit of
the cuneus, respectively. They lie along the most superficial aspects of
the superior and inferior margins of the parieto-occipital sulcus. Small
cuneal gyri lie along its inner surfaces. The superior parietal lobule and
the precuneus are also referred to as P and the supramarginal and
1
angular gyri are referred to as P and P 3 or P respectively.
2 c 3


Cerebral lobes
381
52
reTpahC
OCCIPITAL LOBE It occupies the upper and lower lips and depths of the posterior part of
the calcarine sulcus and extends into the cuneus and lingual gyrus.
On the superolateral cerebral surface, the occipital lobe is situated pos- The primary visual cortex receives afferent fibres from the lateral
terior to an imaginary line connecting the point of emergence of the geniculate nucleus via the optic radiation (Fig. 25.19). The latter curves
parieto-occipital fissure on the superomedial border of the cerebral posteriorly and spreads through the white matter of the occipital lobe.
hemisphere with the preoccipital notch of Meynert (approximately Its fibres terminate in strict point-to-point fashion in the striate area.
5 cm anterior to the occipital pole) (see Fig. 25.8). On the medial The cortex of each hemisphere receives impulses from two hemi-retinae,
surface, the occipital lobe is limited anteriorly by the parieto-occipital which represent the contralateral half of the binocular visual field.
sulcus and by its prolongation towards the tentorium cerebelli. Along Superior and inferior retinal quadrants are connected with correspond-
the inferior cerebral surface, the base of the occipital lobe is continuous ing areas of the striate cortex. Thus, the superior retinal quadrants
with the base of the temporal lobe. The superolateral surface of the (representing the inferior half of the visual field) are connected with
occipital lobe lies mostly beneath the squamous part of the occipital the visual cortex above the calcarine sulcus, and the inferior retinal
bone; its medial surface faces the most posterior aspect of the falx quadrants (representing the upper half of the visual field) are connected
cerebri, and the inferior surface lies over the tentorium cerebelli. The with the visual cortex below the calcarine sulcus. The peripheral parts
sulci and gyri of the occipital lobe, especially of its superolateral surface, of the retinae activate the most anterior parts in the visual cortex. The
exhibit a greater degree of anatomical variation than is seen in the other macula impinges on a disproportionately large posterior part around


lobes: this is reflected in an often confusing literature where the same the occipital pole.
sulcus may be identified using different names, and two obviously dif- The striate cortex is granular. Layer IV, bearing the stria of Gennari,
ferent sulci may be called by the same name. The particular complexity is commonly divided into three sublayers. Passing from superficial to
of sulcal, and therefore gyral, variability in this part of the occipital lobe deep, these are IVA, IVB (which contains the stria) and IVC. The densely
has been attributed to variations in regional cortical folding that are cellular IVC is further subdivided into a superficial IVCα and a deep
reflected in the diversity of sulcal origin, degree of segmentation, length, IVCβ. Layer IVB contains only sparse, mainly non-pyramidal neurones.
depth and intrasulcal geometry. Regarding the major named sulci, there The input to area 17 from the lateral geniculate nucleus terminates
appears to be reasonable consensus that the parieto-occipital, calcarine, predominantly in layers IVA and IVC. Other thalamic afferents, from
paracalcarine, lingual, transverse occipital, inferior lateral occipital and the inferior pulvinar nucleus and the intralaminar group, pass to layers
anterior occipital sulci, together with the occipital parts of the collateral I and VI. Geniculocortical fibres terminate in alternating bands. Axons
and lateral occipitotemporal sulci, are constant features of the surface from geniculate laminae that receive information from the ipsilateral
of the occipital lobe, whereas the superior occipital, superior lateral eye (laminae 2, 3 and 5) are segregated from those of laminae that
occipital, inferior occipital, lunate and occipitopolar sulci are less con- receive input from the contralateral eye (laminae 1, 4 and 6). Neurones
stant. (For further details, consult Iaria and Petrides (2007), Iaria et al within layer IVC are monocular, i.e. they respond to stimulation of


(2008), Alves et al (2012), Malikovic et al (2012).) either the ipsilateral or contralateral eye, but not both. This horizontal
segregation forms the anatomical basis of the ocular dominance
Occipital lobe sulci and gyri column in that neurones encountered in a vertical strip from pia to
white matter, although binocular outside layer IV, exhibit a preference
for stimulation of one or other eye. The other major functional basis
There are two or three gyri (superior, middle and inferior, or O1, O2
for visual cortical columnar organization is the orientation column.
and O3, respectively) on the superolateral cerebral surface of the occipi-
This describes the observation that an electrode passing through the
tal lobe (see Figs 25.9B, 25.17). They converge posteriorly to form the
depth of the cortex, at right angles to the plane from pia to white
occipital pole. Commonly, only the superior and inferior gyri are
present; the area corresponding to the middle occipital gyrus lies
between the inferior extension of the intra-occipital (or superior occipi-
tal or transverse occipital) sulcus and the lateral (or inferior occipital)
sulcus.
The fusiform or lateral temporo-occipital gyrus lies along the
temporo-occipital transition. Its posterior or occipital part is bounded
medially by the collateral sulcus and laterally by the occipitotemporal
sulcus; hence it lies between the lingual gyrus medially and the inferior
Genu of
occipital gyrus laterally. The occipital part of the fusiform gyrus (O 4) corpus callosum
lies over the tentorium cerebelli just posterior to the petrous part of the
Extreme Anterior horn
temporal bone. Topographically, it corresponds to the floor of the ven- capsule of lateral ventricle
tricular atrium; the temporal part of the gyrus lies underneath the Caudate nucleus (head)
temporal or inferior horn of the lateral ventricle. External Septum pellucidum
The occipitotemporal sulcus rarely extends posteriorly as far as the capsule Anterior limb
occipital pole; both the collateral and occipitotemporal sulci frequently of internal capsule


have secondary side branches and merge anteriorly. The inferior or basal Column of fornix
aspect of the inferior occipital gyrus lies lateral to the fusiform gyrus Claustrum Genu of internal capsule
and constitutes the most inferior portion of the lateral aspect of the Putamen
Insula
occipital lobe. Along the inferolateral border of the hemisphere, the Globus pallidus
inferior temporal gyrus is continuous with the inferior occipital gyrus Posterior limb
over the preoccipital notch, and the inferior occipital gyrus is continu- of internal capsule
ous with the lingual gyrus along the occipital pole. Along the parietal Thalamus
and occipital aspects of the superomedial border of the hemisphere, the
Tail of caudate nucleus
superior parietal lobule is continuous with the precuneus, and the
Hippocampus
superior occipital gyrus is continuous with the cuneus above the cal- Optic
Inferior horn
carine sulcus and with the lingual gyrus below the calcarine sulcus. radiation
of lateral ventricle
Occipital lobe internal structure
and connectivity
Striate cortex
The occipital lobe is composed almost entirely of Brodmann’s areas 17,
18 and 19. Area 17, the striate cortex, is the primary visual cortex (VI).
Posterior horn of
A host of other distinct visual areas reside in the occipital and temporal lateral ventricle
cortex. Functional subdivisions V2, V3 (dorsal and ventral) and V3A lie
within Brodmann’s area 18. Other functional areas at the junction of
the occipital cortex with the parietal or temporal lobes lie wholly or
partly in area 19.
The primary visual cortex is mostly located on the medial aspect of
the occipital lobe and is coextensive with the subcortical nerve fibre Fig. 25.19 The superior aspect of a horizontal section through the left
stria of Gennari in layer IV; hence its alternative name, the striate cortex. cerebral hemisphere.