Lateral aspect (Figs 3.1 and 3.4A)

The precentral gyrus ribbons forward over the cerebral convexity, immediately anterior to the central sulcus. It corresponds to the primary motor cortex (BA 4) which contains an inverted, point-to-point or somatotopic map of the opposite half of the body (Greek: soma, body; topos, place). The representation of each body part in the motor strip is proportional to the precision of movement control. This means that the areas for the hands, face and tongue are disproportionately large (Fig. 3.5). A useful landmark for identifying the motor cortex is the motor hand area which resembles an inverted capital omega (Fig. 3.6).

The remainder of the lateral frontal lobe consists of the superior, middle and inferior frontal gyri which run from anterior to posterior. The region in front of the motor strip is the lateral premotor area (BA 6) but it does not correspond to any particular gyral or sulcal boundaries. The premotor cortex also contains an inverted body map and is concerned with preparation and execution of movement sequences, particularly those that occur in response to an external trigger (e.g. catching a ball, rather than throwing one). More anteriorly, the frontal eye field (BA 8) is a cortical centre for attention and gaze which directs both eyes towards the contralateral visual field. The frontal eye field and the remainder of the lateral frontal lobe belong to the prefrontal cortex, which is discussed separately below.

Medial aspect (Figs 3.2 and 3.4B)

The superior frontal gyrus and precentral gyrus both continue onto the medial surface of the hemisphere. The boundary between the superior frontal gyrus and the underlying limbic lobe is the cingulate sulcus. This runs parallel to the corpus callosum before turning upwards to the superior margin of the hemisphere as the pars marginalis (marginal part). The central sulcus lies immediately anterior to the pars marginalis and slopes downwards and backwards towards it at an angle of approximately 90 degrees. The paracentral lobule is a U-shaped convolution on the medial surface of the hemisphere that loops underneath the central sulcus. It therefore straddles the boundary between the frontal and parietal lobes. The paracentral lobule includes the primary motor and sensory areas for the lower limbs and genitalia.

The supplementary motor area (SMA) is just in front of the paracentral lobule. It contains a map of both sides of the body and tends to be recruited with its counterpart in the opposite hemisphere (e.g. when the hands are working together to manipulate an object). In contrast to the lateral premotor area, the SMA (or ‘medial premotor area’) is particularly concerned with internally generated (self-initiated) actions, rather than those that occur in response to an external event. The SMA is underactive in Parkinson’s disease, in which voluntary movements are initiated with effort and performed slowly (Ch. 13). Just anterior to the SMA, there is a small medial extension of the frontal eye field.

Inferior (orbitofrontal) aspect (Fig. 3.3)

The inferior part of the frontal lobe is the orbital cortex, which overlies the orbital cavity. The olfactory sulcus divides the orbital region into two parts. There is a small medial portion that is narrow and rectangular, called the gyrus rectus (Latin: rectus, straight) and a much larger lateral orbital region that contains an H-shaped sulcus. The orbital region belongs to the prefrontal cortex.

Prefrontal cortex (Figs 3.3 and 3.4)

The large portion of the frontal lobe anterior to the motor and premotor areas is the prefrontal cortex and is involved in personality, behaviour, language and intellect. It is divided into lateral, orbital and medial parts:

The prefrontal cortex includes Broca’s area (discussed below, in the context of language) and the frontal eye fields (see above). The effects of prefrontal cortex lesions are discussed in Clinical Box 3.1.

Lateral aspect (Figs 3.1 and 3.4A)

The postcentral gyrus is immediately posterior to the central sulcus, behind and parallel to the motor strip. It corresponds to the primary somatosensory cortex (BA 3, 1 and 2). The sensory strip contains an inverted map of the opposite side of the body that mirrors that of the motor strip, but the relative proportions of the body parts reflect the degree of tactile sensitivity.

The remainder of the lateral parietal lobe is divided into superior and inferior parietal lobules by the intraparietal sulcus. This is a deep cleft at right angles to the central sulcus. The somatosensory association cortex (BA 5) is a small area in the superior parietal lobule, just behind the sensory strip. Lesions here may lead to astereognosia: the inability to recognize objects by touch (Greek: a-, without; stereos, solid; gnosis, knowledge). The posterior parietal cortex (BA 7) has close links with the occipital lobe and is concerned with visuospatial perception and attention (Clinical Box 3.2). This includes the representation and manipulation of objects (e.g. using a knife and fork) and the perception of movement (e.g. judging the approach of a moving vehicle). Certain semi-automatic movements are initiated by projections from the parietal cortex to the lateral premotor area (Clinical Box 3.3).

The inferior parietal lobule is a multimodal association area which lies at the junction of the visual, auditory and somatosensory cortices. It consists of the supramarginal gyrus (BA 40) anteriorly and the angular gyrus (BA 39) posteriorly. The inferior parietal lobule contributes to aspects of receptive language such as phonology, reading and spelling, particularly in the language-dominant hemisphere. It is also involved in spatial and symbolic representation of abstract concepts including quantity and number.

Medial aspect (Figs 3.2 and 3.4B)

The superior parietal lobule continues onto the medial surface of the hemisphere as the precuneus. This rectangular-shaped area is involved in mental imagery and recall of personal experiences. Like the medial prefrontal cortex, it is part of the ‘default network’ of the brain and is engaged during activities such as daydreaming and introspection. The postcentral gyrus (‘sensory strip’) also continues onto the medial surface of the hemisphere, making up the posterior part of the paracentral lobule (representing the lower half of the body).

Medial aspect (Figs 3.2 and 3.4B)

The calcarine sulcus follows an undulating course from the parieto-occipital sulcus anteriorly to the occipital pole posteriorly. It is a deep cleft which extends to the occipital horn of the lateral ventricle. The wedge-shaped region above the calcarine sulcus is the cuneus (Latin: cuneus, wedge) which represents the lower quadrant of the opposite visual field. The tongue-like lingual gyrus is below the calcarine sulcus (Latin: lingua, tongue) and represents the upper quadrant of the opposite visual field.

Much of the primary visual cortex (BA 17) is hidden from view within the banks of the calcarine sulcus. Central vision is represented towards the occipital pole, peripheral vision more anteriorly. The primary visual cortex is flanked above and below by visual association cortex (BA 18 and 19) located within the cuneus and lingual gyrus.

Lateral and inferior aspects (Figs 3.1–3.4)

The gyral pattern is variable in the lateral occipital region but it is possible to identify superior, middle and inferior occipital gyri which converge on the occipital pole. These consist mainly of visual association cortex. The inferior occipital surface is described together with the inferior aspect of the temporal lobe, with which it is continuous.

The optic chiasm (Fig. 3.7)

The central visual pathways are crossed. This means that the right visual field is represented in the left occipital lobe and vice versa. Since light travels in straight lines and enters the eye via the small aperture of the pupil, objects in the right visual field project to the left half of each retina (coloured red in Fig. 3.7). Axons originating from the left half of each retina must therefore project to the left cerebral hemisphere. For this to happen, nerve fibres from the inner half of the retina must cross the midline to enter the opposite optic tract. This takes place at the optic chiasm, named for its resemblance to the Greek letter chi.

The primary visual cortex

Visual information is segregated into three separate ‘channels’ (concerned with form, motion and colour) by the primary visual cortex (V1) and visual association cortices (V2, V3… and higher). Two parallel visual ‘streams’ dealing with different aspects of visual perception arise from the occipital lobe. The dorsal or parietal lobe stream is concerned with the location and movement of objects and their positions relative to the body (the ‘where’ pathway). The ventral or temporal lobe stream synthesizes information about form and colour, allowing objects to be recognized (the ‘what’ pathway). Central visual pathway lesions are discussed in Clinical Box 3.4.

Superior aspect (Fig 3.10)

The superior surface of the temporal lobe is hidden within the lateral sulcus and includes the transverse temporal gyri. These finger-like convolutions run obliquely (posteriorly and medially) and include the primary auditory cortex (BA 41 and 42). The auditory cortex contains a tonotopic map that represents the audible frequency spectrum (low frequencies laterally, high frequencies medially).

Projections reach the auditory cortex from the medial geniculate nucleus (MGN) of the thalamus via the auditory radiations. The primary auditory cortex receives projections from both ears. This means that a temporal lobe lesion would not be expected to cause contralateral deafness in the same way that an occipital lobe lesion might cause contralateral loss of sight.

The surrounding cortex, which continues onto the lateral surface of the temporal lobe is the auditory association area (BA 22). In the language-dominant hemisphere this is specialized for the comprehension of speech sounds.

Lateral aspect (Figs 3.1 and 3.4A)

The lateral temporal lobe has superior, middle and inferior temporal gyri, separated by the superior and inferior temporal sulci. The superior temporal sulcus is parallel to the lateral sulcus and is generally uninterrupted, whilst the inferior temporal sulcus is usually broken up into segments. The lateral temporal lobe consists of visual and auditory association cortex.

Inferior aspect (Fig. 3.3)

On the inferior aspect of the hemisphere, the occipital and temporal lobes form a continuous, uninterrupted surface that is composed of the medial and lateral occipitotemporal gyri. The medial occipitotemporal gyrus is medial to the collateral sulcus. The portion lying within the occipital lobe is also known as the lingual gyrus, whereas the part contained in the temporal lobe is the parahippocampal gyrus. The lateral occipitotemporal gyrus runs alongside, between the collateral sulcus medially and occipitotemporal sulcus laterally. The anterior and posterior ends of the lateral occipitotemporal gyrus are usually tapered to give it a ‘spindle’ shape. It is therefore also known as the fusiform gyrus (Latin: fusiform, shaped like a spindle). It receives projections from the occipital lobe (part of the ‘what pathway’) and appears to be involved in the recognition of complex visual patterns. It contributes to reading (in the language-dominant hemisphere) and face recognition (in the non-dominant hemisphere) (Clinical Box 3.5).

Broca’s area

Broca’s area is involved in the expressive aspects of spoken and written language (production of sentences constrained by the rules of grammar and syntax). It corresponds to the opercular and triangular parts of the inferior frontal gyrus (BA 44 and 45) (Fig. 3.12A). As illustrated in the figure, these are defined by two rami (branches) of the lateral sulcus (one ascending, one horizontal) which ‘slice into’ the inferior frontal gyrus. In keeping with its role in speech and language, Broca’s area is immediately anterior to the motor and premotor representations of the face, tongue and larynx. A homologous area in the opposite hemisphere is involved in non-verbal communication such as facial expression, gesticulation and modulation of the rate, rhythm and intonation of speech.

Wernicke’s area

Wernicke’s area (pronounced: VER-nikker) corresponds to the posterior third of the superior temporal gyrus and is part of the auditory association cortex (Fig. 3.12B). It lies at the junction of the visual and auditory cortices and is involved in transforming the visual impression of letters (graphemes) into mental representations of speech sounds (phonemes). It is therefore important for speech comprehension and reading. The non-dominant homologue of Wernicke’s area is involved in understanding intonation and emphasis (the ‘music’ of speech) which can alter the meaning of words considerably. Some patients with non-dominant temporal lobe lesions may therefore have monotone, ‘robotic’ speech or fail to grasp nuances of intonation (termed aprosodia).

Language pathways

The two main language areas are connected by the arcuate fasciculus, a large white matter bundle that arches around the lateral sulcus (Latin: arcus, bow) (Fig. 3.12B).

One component, the long segment, passes directly between Broca’s and Wernicke’s areas. There is also an indirect pathway (composed of two short segments) which connects the anterior and posterior language areas via the inferior parietal lobule. Collectively, these pathways make up the dorsal language stream which is concerned with the phonological aspects of language.

A second language-associated pathway interconnects the anterior and lateral temporal lobe with the inferior frontal lobe via the hook-shaped uncinate fasciculus (Latin: uncus, hook) (see Ch. 1, Fig. 1.18). This has been referred to as the ventral language stream and is more concerned with semantic aspects of language (the meaning of words and concepts). Language disorders are discussed in Clinical Box 3.6.

Parts of the limbic lobe

The limbic lobe includes the cingulate and parahippocampal gyri, connected by an underlying core of white matter called the cingulum (or cingulum bundle). The cingulate gyrus wraps around the corpus callosum on the medial surface of the cerebral hemisphere (Latin: cingulum, belt). It is separated from the overlying frontal lobe by the cingulate sulcus. The boundary with the parietal lobe is the subparietal sulcus. Posteriorly, the cingulate gyrus tapers to a narrow isthmus (pronounced: IZ-muss) behind the splenium of the corpus callosum. The limbic lobe continues into the medial temporal region as the parahippocampal gyrus (medial to the collateral sulcus). This ends in a hook-shaped fold of cortex called the uncus (Latin: hook).

The insula

The insula is an ‘island’ of cortex hidden in the depths of the lateral sulcus (Latin: insula, island) which can be exposed by retracting the overhanging opercula of the frontal, parietal and temporal lobes (Latin: operculum, lid) (Fig. 3.14; see also Ch. 2, Fig. 2.10). It is divided into anterior (visceral) and posterior (somatic) parts by the central sulcus of the insula. The anterior insula receives projections from the olfactory bulb and is part of the primary olfactory cortex. It is also involved in nausea, vomiting, disgust and pain perception, including the accompanying visceral and autonomic phenomena. Studies using functional magnetic resonance imaging (fMRI) suggest that the anterior insula, which is immediately adjacent to Broca’s area, may also be involved in language. The posterior insula integrates non-visceral (somatic) information related to touch, vision and hearing.

Parts of the hippocampus

The hippocampus is formed by an S-shaped fold of relatively simple three-layered cortex that is continuous with the neighbouring parahippocampal gyrus (Greek: para-, beside). It is composed of allocortex (see Ch. 5) which is thinner and less complex than the six-layered neocortex that is found in 90% of the cerebral hemisphere. Its relationship to the lateral ventricle and parahippocampal gyrus are illustrated in Figure 3.16A. The hippocampus consists of the dentate gyrus and Ammon’s horn (within the ventricle) together with the subiculum below (Latin: subicere, to support) (Fig. 3.16B). Ammon’s horn (also referred to as the hippocampus proper) contains large pyramidal neurons arranged in three zones called CA1, CA2 and CA3. The terminology derives from the Latin form of Ammon’s horn, cornu ammonis (CA) and refers to an Egyptian deity with ram’s horns.

Afferent and efferent connections

The major afferent projection into the hippocampus is the perforant path. This originates in the entorhinal cortex (BA 28) in the anterior part of the parahippocampal gyrus. It terminates on granule cells in the dentate gyrus of the hippocampus. The axons of dentate granule cells (called mossy fibres) project in turn to CA3 pyramidal cells. These give rise to long axons that project out of the hippocampus (to the hypothalamus and brain stem). The intrinsic connections of the hippocampus also project back to the entorhinal cortex to form a closed loop that is important in memory formation. The term hippocampal formation is used to describe the entorhinal cortex together with the hippocampus.

Fimbria and fornix

Hippocampal efferent fibres gather in the fimbria, which runs along the medial border of the hippocampus (Latin: fimbria, fringe) (see Fig. 3.16). The fimbria emerges from the posterior aspect of the hippocampus to be renamed the fornix. This is a white, cord-like structure that sweeps up to the midline and forms an arch below the corpus callosum (Latin: fornix, arch) (see Fig. 3.15A). The fornix terminates in the mamillary bodies (of the hypothalamus) in the floor of the third ventricle (Latin: mamilla, nipple). The mamillary bodies project in turn to the cingulate gyrus (via a ‘relay station’ in the anterior thalamus) and a complete loop can be traced back to the entorhinal cortex via the cingulum bundle (Fig. 3.17).

Functions of the hippocampus

The hippocampus is involved in the formation of new memories and is particularly important for the recollection of personal experiences, termed episodic memory; this is in contrast to semantic memory which has to do with abstract knowledge and facts (Clinical Box 3.7). The hippocampus also stores spatial maps of the environment (e.g. familiar towns, streets and buildings) that we use to navigate. In keeping with this role, it has been found to be significantly larger in licensed London taxi drivers. The effects of hippocampal lesions are discussed in Clinical Boxes 3.8 and 3.9.

Caudate nucleus

The caudate nucleus is a large C-shaped structure with a head, body and tail, that nestles into the inner curvature of the lateral ventricle (Latin: cauda, tail). On coronal sections the head and body of the caudate nucleus can be seen bulging into the side wall of the lateral ventricle, whilst its slender tail sweeps down into the temporal lobe to occupy the roof of the temporal horn.

Lentiform nucleus

The lentiform nucleus lies beneath the insula. It is said to resemble a lens (Latin: lentiform, lens-shaped) but is best regarded as a cone with the base underlying the insula and a blunt apex pointing towards the midline (Fig. 3.21). The outer portion of the lentiform nucleus, immediately beneath the insula, is the putamen (Latin: putamen, husk or shell). The inner part is the globus pallidus which has internal and external segments. The globus pallidus is so named because of its pallid (pale) appearance in comparison to the dark grey colour of the caudate nucleus and putamen. This is due to the presence of myelinated fibres forming the internal connections of the basal ganglia.

Internal capsule

The internal capsule is a thick sheet of white matter consisting of projection fibres (see Ch. 1, Fig. 1.17) passing to and from the cerebral cortex. It makes a sharp ‘knee-bend’ or genu around the apex of the lentiform nucleus (Latin: genu, knee). This gives it a V-shape when viewed in axial (horizontal) section (Fig. 3.22). There is therefore an anterior limb (between the lentiform nucleus and the head of the caudate nucleus) and a posterior limb (between the lentiform nucleus and the thalamus).

The voluntary motor loop

The motor loop is the best understood component of the basal ganglia and is disturbed in movement disorders such as Parkinson’s disease. Projections arise from the supplementary motor area (SMA) in the medial frontal lobe and project into the putamen. For this reason the putamen is regarded as the ‘motor part’ of the striatum. The putamen gives rise to direct and indirect connections that converge on the internal pallidum. This projects in turn to the thalamus, which completes the loop via a thalamocortical projection back to the SMA. Dopamine deficiency in Parkinson’s disease leads to underactivity of the motor loop and SMA, interfering with the initiation of voluntary actions. The control of the motor loop (by the direct and indirect pathways) is discussed further in Chapter 13, in the context of Parkinson’s disease.

Cognitive-executive loops

Basal ganglia loops passing through the caudate nucleus arise and terminate in the prefrontal cortex. They influence cognition and behaviour, but also contribute to the control of visual attention and voluntary gaze via projections that arise and terminate in the frontal eye fields. Over-activity in caudate-prefrontal connections has been implicated in obsessive-compulsive disorder (Clinical Box 3.10).

Limbic-affective loops

Anteriorly, the caudate nucleus and putamen are fused underneath the anterior limb of the internal capsule (see Fig. 3.20) to form the ventral striatum. Due to its proximity to the septal area (Fig. 3.24) it is also known as the nucleus accumbens septi or nucleus accumbens (Latin: accumbens, leaning against). Projections taking part in the ‘limbic loops’ of the basal ganglia arise in the limbic lobe or amygdala and project to the ventral striatum. This region is rich in opiate receptors and has been implicated in motivation, reward-based learning and addictive behaviours.

Thalamic nuclei

Almost all ascending pathways synapse in a thalamic nucleus in order to reach the cerebral cortex. The thalamus is therefore described as the ‘gateway’ to the cortex. It is composed of more than a dozen nuclei, separated into anterior, medial and lateral groups by a Y-shaped internal medullary lamina. This thin sheet of white matter contains a few small intralaminar nuclei which are involved in arousal, wakefulness and pain (see Ch. 4). The lateral nuclear group of the thalamus has dorsal and ventral tiers. The ventral tier contains specific nuclei, which project to discrete cortical zones such as the primary sensory and motor areas. The non-specific nuclei form diffuse, reciprocal connections with large regions of the cortex.

Pineal gland

The pineal gland is above and behind the thalamus in the roof of the third ventricle. It secretes the sleep-inducing hormone melatonin when light levels are low (Greek: melas, black). This helps to control sleep–wake cycles and circadian rhythms (Latin: circa, about; dies, day). Melatonin release is influenced by a projection from the suprachiasmatic nucleus of the hypothalamus, which acts as a biological clock that is adjusted by the amount of light falling on the retina.

Tectum (3.26C and 3.27B)

The tectum bears four smooth elevations or colliculi (Latin: colliculus, little hill). The superior colliculi (collectively: the optic tectum) give rise to the tectospinal tracts which co-ordinate head, neck and eye movements during orientation reflexes (e.g. involuntary turning towards a novel or unexpected stimulus). The inferior colliculi are part of a complex pathway between the cochlea and primary auditory cortex. The role of the midbrain in the control of pupil size is discussed in Clinical Box 3.11.

Cerebral peduncles (3.27B)

When viewed from the front, the cerebral peduncles resemble two stout Roman pillars, separated by the interpeduncular fossa (Latin: fossa, ditch). A transverse (horizontal) slice through the midbrain shows the deeply pigmented substantia nigra (Latin: nigra, black). This supplies dopamine to the basal ganglia via the nigro-striatal tract. The much smaller ventral tegmental area (which provides the ventral striatum with dopamine) is just medial to the nigra, but cannot be seen with the naked eye. The tegmentum is the portion of the cerebral peduncle posterior to the substantia nigra. The part in front of the nigra is the crus cerebri (plural: crura). The crura transmit axons of the primary motor pathway descending from the cerebral cortex to the spinal cord (the corticospinal tract).

Basilar pons

The anterior two thirds of the pons is called the base (or basilar pons). It transmits bundles of descending corticospinal tract fibres that have already passed through the internal capsule and crus cerebri (above) on their way to the spinal cord (below). It also contains the pontine nuclei, which give rise to axons that project to the opposite cerebellar hemisphere via the middle cerebellar peduncle (crossing the midline as the transverse pontine fibres).

Pontine tegmentum

The dorsal third of the pons is the tegmentum, the posterior surface of which forms part of the floor of the fourth ventricle. The tegmentum contains the pontine reticular formation, together with several cranial nerve nuclei and ascending tracts. The loci coerulei (singular: locus coeruleus) are found in the rostral pons, just beneath the floor of the fourth ventricle on each side. These pigmented nuclei (Latin: locus, place; coeruleus, dark-blue) give rise to a diffuse projection for noradrenaline (see Ch. 1).

Basilar region

The anterior medulla contains two tapering columns of white matter called the pyramids, which are equivalent to the ‘basilar region’ in the midbrain and pons. The pyramids transmit the primary motor pathway (the corticospinal tract) which is therefore also known as the pyramidal tract (Fig. 3.29). Just lateral to the pyramids in the upper medulla are the olives (see Fig. 3.26), which send an ascending projection to the opposite cerebellum. This olivo-cerebellar pathway contributes to motor learning by signalling unexpected events (e.g. dropping a ball whilst learning to juggle) and provides a ‘training signal’ to the cerebellum which alters synaptic strengths in such a way that the error is less likely to be repeated.

Medullary tegmentum

The tegmentum of the medulla contains the medullary reticular formation, together with cranial nerve nuclei and ascending tracts. It also includes a number of ‘vital centres’ which subserve cardiorespiratory functions and airway-protective reflexes (e.g. cough, sneeze, gag).

Functional divisions

The cerebellum has three functional divisions, corresponding to: (i) the flocculonodular lobe; (ii) the vermis/paravermal region; and (iii) the cerebellar hemispheres (Fig. 3.30). It is important to note that the functional regions do not coincide exactly with the gross anatomical (lobar) boundaries.

Role in movement control

The cerebellum acts as a comparator. The motor and premotor areas of the frontal lobe provide information about intended movements whereas the spinocerebellar tracts (and other pathways) provide information about actual movements. Any discrepancy between the two creates an ‘error signal’ that is fed back to the motor cortex, permitting continuous modification and improvement of on-going movements. Cerebellar disease therefore leads to incoordination (Clinical Box 3.12).