External features (Fig. 234; see Fig. 236)

The cerebellum is the largest part of the hindbrain and occupies most of the posterior cranial fossa. It is made up of the right and left cerebellar hemispheres and a median vermis. Inferiorly, the vermis is clearly separated from the two hemispheres and lies at the bottom of a deep cleft, the vallecula; superiorly, it is marked off from the hemispheres only as a low median elevation. A small ventral portion of each cerebellar hemisphere lies on the middle cerebellar peduncle of its side, and is almost completely separated from the rest of the cerebellum. It is termed the flocculus. The surface of the cerebellum is divided into numerous narrow folia and, by a few deep fissures, into a number of lobules. The effect of this fissuring is to give the cerebellum in section the appearance of a many‐branched tree (the arbor vitae) (see Fig. 243).

Frontal lobe

This includes all the cortex anterior to the central sulcus of Rolando. Its important cortical areas are as follows:

1. The motor cortex. The primary motor area occupies a large part of the precentral gyrus. It receives afferents from the premotor cortex, thalamus and cerebellum and is concerned with voluntary movements. Stimulation of this area results in discrete muscle movements. Details of localization of function in the motor cortex are considered on page 372.
   
2. The premotor cortex. This lies anterior to the precentral gyrus and the adjoining lower part of the frontal gyri. It, too, is concerned with voluntary movement, but its stimulation results in movements of groups of muscles with a common function, and thus lacks the precise definition of muscle movement that is a feature of stimulation of the precentral gyrus.
   
3. Eye motor field. This lies in the middle frontal area anterior to the premotor cortex. Lesions of this area result in impaired eye movement with deviation of gaze to the side of the lesion.
   
4. Broca’s speech area. Lesions of the area around the posterior part of the inferior frontal gyrus of the dominant hemisphere (the left hemisphere in 98% of individuals) were shown by Broca to affect the motor element in speech.
   
5. Frontal association cortex (clinically called the prefrontal cortex). This comprises a considerable part of the frontal lobe and is one of the remarkable, evolutionary advances seen in the human brain. Its afferents are derived from the thalamus, limbic area and also from other cortical areas; it probably sends efferents to the thalamus and hypothalamus. From a functional point of view the lateral aspect of the frontal lobe appears to be related to ‘intellectual activity’ (i.e. cognitive functions – analysis, judgement and planning), the medial and orbital surfaces to affective (or emotional) behaviour and the control of autonomic activity.

Parietal lobe

The parietal lobe is bounded anteriorly by the central sulcus and behind by a line drawn from the parieto‐occipital sulcus to the posterior end of the lateral (Sylvian) sulcus. The important cortical areas of the parietal lobe are as follows.

1. The primary somatosensory cortex. The postcentral gyrus receives afferent fibres from the thalamus and is concerned with all forms of somatic sensation. Details of localization along the sensory cortex are considered on pages 370 and 371.
   
2. The parietal association cortex, comprising the remainder of the parietal lobe, is concerned largely with the recognition of somatic sensory stimulation and its integration with other forms of sensory information. It also receives afferents from the thalamus and, when damaged, gives rise to more complex defects than simple loss of sensation. An example of this is the inability to recognize somatic stimuli, which is called astereognosis; put a pen or a coin in the patient’s hand – he is aware of the object but is unable to recognize what it is. The lower part of the parietal lobe in the subject’s dominant hemisphere interacts with the somatosensory visual and auditory associations and has a key role in language.

The temporal lobe

This is arbitrarily separated from the occipital lobe by a line drawn vertically downwards from the upper end of the lateral sulcus.

The important cortical areas of the temporal lobe are the following.

1. The auditory cortex. This lies in the superior temporal gyrus on the lateral and superior surfaces of the hemisphere. Its afferent fibres are from the medial geniculate body and it is concerned with the perception of auditory stimuli.
   
2. The temporal association cortex. The area surrounding the auditory cortex is responsible for the recognition of auditory stimuli and for their integration with other sensory modalities. Lesions of this area result in auditory agnosia, i.e. the inability to recognize or to understand the significance of meaningful sounds. The cortical region just above and behind this area on the dominant hemisphere (Wernicke’s area) is of considerable importance in the sensory aspects of language comprehension. This visual area of the occipital lobe connects with the temporal lobe and is concerned with visual recognition. The antero‐inferior aspect of the frontal lobe connects with the medial aspect of the temporal lobe and is concerned with behaviour.

The parahippocampal gyrus  The cortex of the most medial part of the undersurface of the temporal lobe is known as the parahippocampal gyrus, much of which is referred to as the entorhinal cortex. It receives widespread association cortical afferents and is a significant source of inputs to the hippocampus. Anteriorly, it is related to the olfactory cortex of the uncus. Medially, it is in direct continuity with the layer of in‐rolled cortex which is the hippocampus and which is one of the most important sources of afferents to this structure. The hippocampus occupies the whole length of the floor of the inferior horn of the lateral ventricle and extends to the amygdala. It sends its efferents into the overlying layer of white matter known as the alveus. The fibres of the alveus collect on the medial margin of the hippocampus to form a compact bundle, the fimbria, which, as it arches under the corpus callosum, becomes known as the fornix. The fornix passes forwards and then downwards in front of the interventricular foramen and finally backwards into the hypothalamus to terminate in the mamillary body. It also gives fibres to the thalamus and the hypothalamus.

Projection of the hippocampus to the hypothalamus is part of the limbic system. This is an important substrate for emotions, behaviour and memory. The circuit is completed by projections of the hypothalamus to the thalamus, from the thalamus to the cingulate gyrus and from thence back to the hippocampus. Bilateral hippocampal damage results in inability to form new long‐term memories.

The amygdaloid nuclear complex  The amygdaloid nuclear complex is also a prominent temporal lobe structure, situated immediately rostral to the hippocampus. It is conveniently divided into three groups of nuclei: corticomedial, central and basolateral, which receive largely olfactory, gustatory and association cortical afferents, respectively. These divisions also have more or less separable projections to the hypothalamus and septum, brainstem autonomic centres and ventral striatum. The amygdala is involved in the control of emotional behaviour and conditioned reflexes. Its neuroanatomical connections are clearly appropriate for such a role, since it is in a position to affect emotional responses in endocrine, autonomic and motor domains. Destruction of the amygdala is particularly associated with reduced aggressive behaviour, whereas the very high density of benzodiazepine receptors here has suggested amygdaloid involvement in anxiety and stress and their treatments.

Occipital lobe

The occipital lobe lies behind the parietal and temporal lobes. On its medial aspect it presents the Y‐shaped calcarine and postcalcarine sulci (Fig. 236). The following cortical areas are noteworthy.

1. The visual cortex surrounds the calcarine and postcalcarine sulci and receives its afferent fibres from the lateral geniculate body of the thalamus of the same side; it is concerned with vision of the opposite (contralateral) half field of sight (Fig. 237).
   
2. The occipital association cortex lies anterior to the visual cortex. This area is particularly concerned with the recognition and integration of visual stimuli.

The insula (Figs 235, 237)

If the lips of the lateral sulcus are separated, it is seen that there is a considerable area of cortex buried in the floor of this sulcus. This area is known as the insula of Reil. It is divided into a number of small gyri and is crossed by the middle cerebral artery. Apart from its upper part, which abuts on the sensory cortex and probably represents the taste area of the cerebral cortex, the function of the insula is unknown. Its stimulation excites visceral effects such as belching, increased salivation, gastric movements and vomiting.

The connections of the cerebral cortex

As has been indicated, most areas of the cerebral cortex receive their main afferent input from the thalamus, but, in addition to this, there are well‐established commissural connections with the corresponding area of the opposite hemisphere by way of the corpus callosum. Intracortical association fibres also link neighbouring cortical areas on the same side and, in some cases, connect distant cortical areas; thus, the frontal, occipital and temporal lobes within the same hemisphere are directly connected by long association pathways.

The corpus striatum

The caudate nucleus is a large homogeneous mass of grey matter consisting of a head, anterior to the interventricular foramen and forming the lateral wall of the anterior horn of the lateral ventricle; a body, forming the lateral wall of the body of the ventricle; and an elongated tail, which forms the roof of the inferior (temporal) horn of the ventricle. It is largely separated from the putamen by the internal capsule, but the two structures are connected anteriorly. The putamen is a roughly ovoid mass closely applied to the lateral aspect of the globus pallidus; together, they are called the lentiform nucleus. The corpus striatum receives afferent connections from the cerebral cortex and sends efferents to the globus pallidus. From there, fibres project to the thalamus and, thence, back to the premotor cortex. Dopaminergic fibres project from the substantia nigra to the corpus striatum and efferent fibres also pass to the thalamus, hypothalamus, red nucleus, substantia nigra and the inferior olivary nucleus.

The somatic afferent pathways (Fig. 238)

1. Proprioceptive and tactile impulses pass uninterruptedly through the posterior root ganglia, through the ipsilateral posterior columns of the spinal cord to the gracile and cuneate nuclei in the lower part of the medulla. In the posterior columns there is a fairly precise organization of the afferent fibres; those from sacral and lumbar segments are situated medially in the tracts whereas fibres from thoracic and cervical levels are successively added to their lateral aspect. This arrangement according to body segments is maintained in the gracile and cuneate nuclei and in the efferents from these nuclei to the contralateral thalamus. The fibres arising from the gracile and cuneate nuclei immediately cross over to the opposite side in the sensory decussation of the medulla and continue up to the thalamus as a compact contralateral bundle – the medial lemniscus.
   
2. Dorsal root fibres subserving pain and temperature, together with some tactile afferents, end ipsilaterally in the substantia gelatinosa of the posterior horn. They then synapse and cross to the contralateral anterior lateral columns of the cord and are relayed to the contralateral thalamus. The fibre crossing occurs in the anterior white commissure of the spinal cord. In the brainstem these fibres come to lie immediately lateral to the medial lemniscus and are sometimes known as the spinal lemniscus (Fig. 238). They terminate in the thalamus.

These somatic afferents are relayed from the thalamus, through the posterior limb of the internal capsule (Fig. 237) to the somatic sensory cortex of the postcentral gyrus. In the internal capsule the fibres are arranged in the sequence ‘face, arm, trunk and leg’ from before backwards, and this segregation persists in the sensory cortex, where the leg is represented on the dorsal and medial part of the cortex, the trunk and arm in its middle portion and the face most inferiorly. Since the size of the area of cortical representation reflects the density of the peripheral innervation and hence complexity of the function being performed rather than the area of the receptive field, there is a good deal of distortion of the body image in the cortex, the cortical representation of the face and hand being much greater than that of the limbs and trunk.

The motor pathways (Fig. 239)

It is customary to divide the motor pathways of the brain and spinal cord into pyramidal and extrapyramidal systems. Although the latter is an imprecise term, it nevertheless provides a useful collective term for the many motor structures not confined to the pyramidal tracts in the medulla.

The pyramidal tract  The pyramidal system is the main ‘voluntary’ motor pathway and derives its name from the fact that projections to the motor neurones in the spinal cord are grouped together in the medullary pyramids. The fibres in this pathway arise from a wide area of the cerebral cortex. About two‐thirds derive from the motor and premotor cortex of the frontal lobes; however, about one‐third arise from the primary somatosensory cortex. In both the motor and premotor cortex there is an organization comparable to that seen in the sensory area. Again, the body is inverted so that the ‘leg area’ is situated in the dorsomedial part of the precentral gyrus encroaching on the medial surface of the hemisphere, supplied by the anterior cerebral artery. The ‘face area’ is near the lateral sulcus, while the ‘arm area’ occupies a central position, both supplied by the middle cerebral artery. Again, the body image is greatly distorted; the areas representing the hand, lips, eyes and foot are exaggerated out of proportion to the rest of the body and in accordance with the complexity of the tasks they perform.

From the cortex, the motor fibres pass through the posterior limb of the internal capsule (Fig. 237) where they are again organized in the sequence of ‘face, arm, leg’, anteroposteriorly. From the internal capsule the fibres form a compact bundle that occupies the central third of the cerebral peduncle. From there they pass through the ventral pons, where they are broken up into a number of small bundles between the cells of the pontine nuclei and the transversely disposed pontocerebellar fibres. Near the lower end of the pons they again collect to form a single bundle, which comes to lie on the ventral surface of the medulla and forms the elevation known as the ‘pyramid’. As it passes through the brainstem, the pyramidal system gives off, at regular intervals, contributions to the somatic and branchial arch efferent nuclei of the cranial nerves. Most of these corticobulbar fibres cross over in the brainstem, but many of the cranial nerve nuclei are bilaterally innervated.

Near the lower end of the medulla the great majority of the pyramidal tract fibres cross over to the opposite side and come to occupy a central position in the lateral white column of the spinal cord. This is the so‐called ‘crossed pyramidal tract’ shown in Fig. 245. A small proportion of the fibres of the medullary pyramid, however, remain uncrossed until they reach the segmental level at which they finally terminate. This is the direct or uncrossed pyramidal tract, which runs downwards close to the anteromedian fissure of the cord, with fibres passing from it at each segment to the opposite side.

In view of the frequent involvement of the pyramidal tract in cerebrovascular accidents, its blood supply is listed here in some detail:

• motor cortex – leg area: anterior cerebral artery; face and arm areas: middle cerebral artery;
  
• internal capsule – branches of the middle cerebral artery;
  
• cerebral peduncle – posterior cerebral artery;
  
• pons – pontine branches of basilar artery;
  
• medulla – anterior spinal branches of vertebral artery;
  
• spinal cord – segmental branches of anterior and posterior spinal arteries.

The extrapyramidal motor system  This should, by definition, include all those motor projections that do not pass physically through the medullary pyramids. It was once thought to control movement in parallel with and, to a large extent, independently of the pyramidal motor system and the pyramidal/extrapyramidal division was used clinically to distinguish between two motor syndromes: one characterized by spasticity and paralysis whereas the other involved involuntary movements, or immobility without paralysis. It is now clear that many ‘extrapyramidal’ structures, particularly the basal ganglia, actually control movement by altering activity in the premotor cortex and, thus, the pyramidal motor projections. This clearly emphasizes the blur between the two systems.

Components of the extrapyramidal system include the red nuclei, vestibular nuclei, superior colliculus and reticular formation in the brainstem, all of which project via discrete pathways to influence spinal cord motor neurones. Cerebellar projections (see page 361) are also included since they influence not only these brainstem motor pathways but also the motor cortex itself via the dentatothalamic projection.

Perhaps the most important structures to retain an extrapyramidal definition are the basal ganglia (see page 370). The neostriatum (caudate and putamen) receives widespread cortical afferents, including those from high‐order sensory association and motor areas, and projects mainly to the globus pallidus. The latter nucleus is the major outflow for the basal ganglia and, via the ventral anterior thalamus, exerts its major influence on premotor and hence the motor cortices. This pattern of connections suggests that the basal ganglia are involved in complex aspects of motor control, including motor planning and the initiation of movement.

A variety of motor disorders are associated with basal ganglia pathology and, in some instances, neuroanatomically discrete deficits in specific neurotransmitters. For example, Parkinson’s disease involves the degeneration of dopaminergic neurones in the substantia nigra in the midbrain. This pigmented nucleus provides the neostriatum with a dense dopaminergic innervation which may be completely lost in severe cases of Parkinsonism. Knowledge of this selective chemical neuropathology has resulted in the development of a treatment of the disease which involves the oral administration of the dopamine precursor L‐dopa.