Rostral to the midbrain lies the forebrain (prosencephalon, cerebrum; Fig. 1.10). The forebrain consists of the bilaterally paired diencephalon and cerebral hemisphere on each side and is by far the largest derivative of the three basic embryological divisions of the brain (pp. 5–6). The diencephalon is continuous with the rostral part of the midbrain and lies between the brain stem and the cerebral hemisphere. The diencephalon is comprised of – from dorsal to ventral: the epithalamus, thalamus, subthalamus and hypothalamus, of which the thalamus is the largest. The thalamus consists of numerous nuclei, most of which have extensive reciprocal connections with the cerebral cortex. Of particular note are:
nuclei that transmit general and special sensory information to corresponding regions of the sensory cortices
nuclei that receive impulses from the cerebellum and basal ganglia and interface with motor regions of the frontal lobe
nuclei that have connections with associative and limbic areas of the cerebral cortex.
The diencephalon is almost entirely surrounded by the cerebral hemisphere; consequently, little of its structure can be seen externally, apart from the ventral portion of the hypothalamus, which can be seen on the base of the brain (Fig. 12.1). Immediately caudal to the optic chiasma is a small midline elevation, the tuber cinereum. From its apex extends the infundibulum or pituitary stalk, which attaches to the pituitary gland. Caudal to the tuber cinereum, a pair of rounded eminences, the mammillary bodies, are located on either side of the midline. These contain the mammillary nuclei of the hypothalamus. The hypothalamus lies below the thalamus, extending medial and ventral to the subthalamus. It has important connections with the limbic system, a controlling influence upon the activity of the autonomic nervous system and a central role in neuroendocrine function, partly through its relationship with the pituitary gland. The hypothalamus is discussed further in Chapter 16.
Figure 12.1 Ventral aspect of the diencephalon.
The other parts of the diencephalon can be seen in sagittal and coronal sections of the brain (Figs 12.2, 12.3). The diencephalon forms the lateral wall of the third ventricle. The dorsal part of the ventricular wall is formed by the thalamus and the ventral part by the hypothalamus. The epithalamus is a relatively small part of the diencephalon located in its most caudal and dorsal region, immediately rostral to the superior colliculus of the midbrain. It consists principally of the pineal gland and the habenula (habenular nuclei). The pineal gland is an endocrine organ. It synthesises the hormone melatonin. The pineal gland has been implicated in control of the sleep/waking cycle (circadian rhythm) and in regulation of the onset of puberty. The habenular nuclei have connections with the limbic system (Ch. 16).
Figure 12.2 Median sagittal section of the brain showing the relationships of the diencephalon.
Figure 12.3 Coronal sections through the diencephalon. Luxol fast blue stain for myelin.
The subthalamus lies beneath the thalamus and dorsolateral to the hypothalamus, with its ventrolateral aspect against the internal capsule. It contains two notable cell groups, the subthalamic nucleus and the zona incerta. The subthalamic nucleus is located in the ventrolateral part of the subthalamus, immediately medial to the internal capsule. It has the shape of a biconvex lens in coronal section. The subthalamic nucleus has prominent connections with the globus pallidus and the substantia nigra and is important in the control of movement. It is discussed in more detail in Chapter 14. The zona incerta is a rostral extension of the brain stem reticular formation. Several important fibre systems traverse the subthalamus en route to the thalamus. These include ascending sensory projections (medial lemniscus, spinothalamic tracts, trigeminothalamic tracts), cerebellothalamic fibres from the dentate nucleus and pallidothalamic fibres from the medial segment of the globus pallidus. The last envelop the zona incerta as the lenticular fasciculus and thalamic fasciculus (see Fig. 14.8).
Topographical anatomy of the thalamus
The thalamus has been likened in size and shape to a small hen’s egg. Together with the hypothalamus it forms the lateral wall of the third ventricle, the transition between the two being marked by a faint groove, the hypothalamic sulcus. In most individuals, the two thalami are joined across the thin slit of the ventricle by the interthalamic adhesion or massa intermedia. A fascicle of nerve fibres, the stria medullaris, which has limbic connections, courses along the dorsomedial margin of the thalamus (Fig. 12.2). Along this line, the ependymal lining of the third ventricle spans the narrow lumen to form the ventricular roof.
The anterior pole of the thalamus extends as far as the interventricular foramen, through which the third and lateral ventricles are in continuity. Lateral to the thalamus lies the posterior limb of the internal capsule and anterolateral lies the head of the caudate nucleus (Fig. 12.4). The dorsal aspect of the thalamus thus forms part of the floor of the body of the lateral ventricle. Another fascicle of nerve fibres with limbic connections, the stria terminalis, marks the boundary between thalamus and caudate (Fig. 12.4). Ventral to the thalamus lie the subthalamus and hypothalamus; caudal to it lies the midbrain.
Figure 12.4 Dorsal aspect of the diencephalon. The choroid plexus has been removed on the right side.
Within the thalamus is located a thin layer of nerve fibres composed of some of the afferent and efferent connections of thalamic nuclei. This is called the internal medullary lamina (Figs 12.3, 12.5, 12.6). The lamina is roughly Y-shaped when viewed from above and provides the basis for dividing the main part of the thalamus into three nuclear masses: anterior, medial and lateral. Each of these cellular complexes is further subdivided into a number of individually named nuclei. Embedded within the internal medullary lamina are several cell groups, known collectively as the intralaminar nuclei (Figs 12.5, 12.6). Lateral to the main mass of thalamic nuclei lies another sheet of nerve fibres, the lateral medullary lamina, which consists of thalamocortical and corticothalamic fibres. Between this and the internal capsule is located a thin stratum of neurones that constitute the reticular nucleus of the thalamus (Fig. 12.3).
Figure 12.5 The left thalamus viewed from the anterolateral aspect (A,C) and in coronal section (B,D) showing the principal nuclear groups (A,B) and the divisions of the lateral nuclear group (C,D).
Figure 12.6 Organisation of thalamic nuclei and their principal relationships with the cerebral cortex. The thalamus is viewed from its anterolateral aspect (A) and in coronal section (B). Colours indicate the relationships between thalamic nuclei and corresponding cerebral cortical regions on the lateral (C) and medial (D) aspects of the cerebral hemisphere.
Anatomy of the thalamus
The thalamus is the largest component of the diencephalon, which is situated between the brain stem and the cerebral hemisphere.
Almost all thalamic nuclei have rich reciprocal connections with the cerebral cortex.
The thalamus is divided into three principal nuclear masses (anterior, medial and lateral) by the internal medullary lamina.
Embedded within the internal medullary lamina lie intralaminar nuclei.
On the lateral aspect of the thalamus lies the thin reticular nucleus.
Functional organisation of thalamic nuclei
All the nuclei of the thalamus, with the exception of the reticular nucleus, project to the ipsilateral cerebral cortex and the whole of the cortex receives input from the thalamus. Similarly, all thalamic nuclei receive corticofugal fibres in a basically reciprocal fashion. In some cases, precise, point-to-point projections exist between individual thalamic nuclei and restricted cortical zones with well-defined sensory or motor functions. This typifies the relationship between the thalamic nuclei and cortical regions that subserve the general and special senses and the motor regions that receive cerebellar and basal ganglia input (Fig. 12.6).
Such thalamic nuclei are often referred to as the ‘specific’ nuclei. The specific nuclei all lie within the ventral part (tier) of the lateral nuclear group. Other thalamic nuclei receive less functionally distinct afferent input that does not include overtly sensory or motor pathways; in turn, these connect with wider areas of cortex, including associative and limbic domains. These are often referred to as the ‘non-specific’ nuclei. Non-specific thalamic nuclei include the nuclei of the dorsal tier of the lateral nuclear complex as well as the whole of the anterior and medial complexes.
Strokes and tumours destroying the thalamus lead to loss of sensation in the contralateral face and limbs, accompanied by a distressing discomfort in the paradoxically anaesthetic areas (thalamic pain).
Thalamic lesions may mimic focal cortical defects because of the richness of thalamocortical connections.
Lateral nuclear group
The lateral nuclear group contains all of the so-called ‘specific’ thalamic nuclei. These are located in the ventral part of the complex and include the ventral anterior, ventral lateral, ventral posterior, lateral geniculate and medial geniculate nuclei (Figs 12.5, 12.6).
Ventral posterior nucleus
The ventral posterior (VP) nucleus lies between the ventrolateral nucleus and the pulvinar. Within the ventral posterior nucleus there is termination of all the ascending pathways from the spinal cord and brain stem that carry general sensory information from the contralateral half of the body to a conscious level. These pathways include the spinothalamic tracts, medial lemniscus and trigeminothalamic tracts. The termination of these fibres in the ventral posterior nucleus is highly organised somatotopically.
An extensive, lateral portion of the nucleus receives information from the trunk and limbs via the spinothalamic tracts and medial lemniscus. This is referred to as the ventral posterolateral (VPl) division (Fig. 12.6). A smaller, medial portion of the ventral posterior nucleus receives information from the head, via the trigeminothalamic tract, and is termed the ventral posteromedial nucleus (VPm). This area also receives taste information from the nucleus solitarius of the medulla and vestibular information from the vestibular nuclei. The ventral posterior nucleus projects to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe.
Lateral geniculate nucleus
The geniculate nuclei are located near the posterior pole of the thalamus, ventral to the pulvinar. Here they form small eminences on the surface, known as the geniculate bodies. The lateral geniculate nucleus is a part of the visual system and receives further discussion in Chapter 15. It is the site of termination of the optic tract, which carries the axons of retinal ganglion cells. As a result of hemidecussation of optic nerve fibres in the optic chiasma, each nucleus receives axons that have originated in the ipsilateral temporal hemiretina and the contralateral nasal hemiretina and is thus provided with visual information relating to the contralateral half of the visual field. The lateral geniculate nucleus sends fibres, via the retrolenticular part of the internal capsule and the optic radiation, to the primary visual cortex of the occipital lobe.
Medial geniculate nucleus
The medial geniculate nucleus is part of the auditory system, which is described in Chapter 10. It receives ascending fibres from the inferior colliculus of the midbrain, via the inferior brachium. The medial geniculate nucleus projects, via the retrolenticular part of the internal capsule and the auditory radiation, to the primary auditory cortex of the temporal lobe.
Ventral anterior nucleus
The ventral anterior (VA) nucleus occupies the rostral part of the lateral nuclear mass. It consists of two subdivisions: the larger, principal part (VApc) and the smaller, magnocellular part (VAmc). The principal subcortical afferents to this region are output fibres of the ipsilateral basal ganglia system, which originate from the medial segment of the globus pallidus and its homologue, the pars reticulata of the substantia nigra. Fibres from the globus pallidus terminate in VApc, while those from the substantia nigra end in VAmc. The ventral anterior nucleus of the thalamus has reciprocal connections with motor regions of the frontal lobe, particularly the premotor and supplementary motor cortices. It is, therefore, an important part of the mechanism by which the basal ganglia exert their influence on normal movement and through which abnormalities of movement are mediated in basal ganglia disorders.
Ventral lateral nucleus
The ventral lateral (VL) nucleus lies immediately caudal to the ventral anterior nucleus in the ventral tier of the lateral nuclear complex. It consists of three subdivisions: pars oralis (VLo), pars medialis (VLm) and pars caudalis (VLc). Subcortical afferents to the ventral lateral nucleus originate mainly from the ipsilateral globus pallidus and substantia nigra, and from the contralateral dentate nucleus of the cerebellum. Pallidal and nigral afferents terminate in VLo and VLm, while those from the cerebellum terminate in VLc. The ventral lateral nucleus, like the ventral anterior nucleus, has reciprocal connections with motor areas of the frontal lobe and especially with the primary motor cortex of the precentral gyrus.
Surgical treatment of disorders of the basal ganglia and cerebellum
Neurosurgically placed lesions in the region of the ventral anterior and ventral lateral nuclei have been used to alleviate some of the motor symptoms associated with disorders of the basal ganglia (rigidity, tremor at rest, dyskinesias) and cerebellum (intention tremor). Such thalamotomy has been largely superseded by drug therapy in basal ganglia disease but is still used to relieve cerebellar tremor, e.g. in multiple sclerosis.
Functional organisation of the lateral nuclear group
’Specific’ thalamic nuclei have well-defined sensory or motor functions and highly organised connections with sensory and motor regions of the cerebral cortex. They all lie within the ventral tier of the lateral nuclear mass and include:
Ventral posterior nucleus: receives general sensory afferents in the medial lemniscus, spinothalamic tract and trigeminothalamic tract; sends efferents to the primary somatosensory cortex of the parietal lobe.
Lateral geniculate nucleus: receives visual afferents in the optic tract; projects to the primary visual cortex of the occipital lobe.
Medial geniculate nucleus: receives auditory afferents from the inferior colliculus; sends efferents to the primary auditory cortex of the temporal lobe.
Ventral anterior and ventral lateral nuclei: receive afferents from the cerebellum and basal ganglia; send efferents to motor cortical areas of the frontal lobe.
‘Non-specific’ nuclei connect with wider areas of cortex, including associative and limbic regions.
Within the dorsal tier of the lateral nuclear complex lie a number of so-called ‘non-specific’ nuclei. Among these, the lateral dorsal nucleus is part of the limbic system. It receives afferents from the hippocampus and sends efferents to the cingulate gyrus. The lateral posterior nucleus has connections with the sensory association cortex of the parietal lobe. The pulvinar is a large region at the most posterior part of the thalamus. It has extensive connections with association cortices of the parietal, temporal and occipital lobes.
Anterior nuclear group
The most anterior portion of the thalamus, extending to its rostral pole, is the anterior nuclear complex. It consists of three subdivisions: the anteroventral, anteromedial and anterodorsal nuclei. The individual connections of these will not be discussed. The anterior nuclear group is part of the limbic system. It receives a large afferent projection from the mammillary body of the hypothalamus via the mammillothalamic tract. The anterior complex projects principally to the cingulate gyrus on the medial surface of the cerebral hemisphere (Fig. 16.11). It is involved in the control of instinctive drives, in the emotional aspects of behaviour and in memory.
Medial nuclear group
The medial nuclear group forms a large region consisting primarily of the mediodorsal nucleus (dorsomedial nucleus) and some much smaller components, such as the nucleus reuniens. Subcortical afferents to the mediodorsal nucleus come from the hypothalamus, amygdala and from other thalamic nuclei, including the intralaminar nuclei and nuclei of the lateral complex. Extensive reciprocal connections exist between the mediodorsal nucleus and the prefrontal cortex. It is concerned mainly with the control of mood and the emotions.
Several nuclei lie embedded within the internal medullary lamina of the thalamus. These include the centromedian and parafascicular nuclei, the centromedian being the largest intralaminar nucleus in humans. The intralaminar nuclei receive ascending afferents from the brain stem reticular formation and also from the spinothalamic and trigeminothalamic systems. In turn, they project to widespread regions of the cerebral cortex and to the caudate nucleus and putamen of the basal ganglia. The intralaminar nuclei are part of the mechanism for activation of the cerebral cortical mantle. When they are stimulated, alpha rhythm activity, which is associated with repose and sleep, is disrupted and the electroencephalogram (EEG) becomes desynchronised. Lesions of the intralaminar nuclei reduce the perception of pain and the level of consciousness.
The reticular nucleus is a thin layer of cells located on the lateral aspect of the thalamus between the external medullary lamina and the internal capsule. This nucleus receives collaterals of both thalamocortical and corticothalamic fibres, which pass between other thalamic nuclei and the cerebral cortex.
Functional organisation of anterior, medial, intralaminar and reticular nuclei
The anterior nuclear group of the thalamus is part of the limbic system. This region receives fibres from the mammillary body of the hypothalamus and projects to the cingulate gyrus.
Within the medial nuclear group, the mediodorsal nucleus has extensive reciprocal connections with the cortex of the frontal lobe.
The intralaminar nuclei receive input from the reticular formation and ascending sensory systems. They project to the cerebral cortex and the striatum and are responsible for activation of the cerebral cortex.
The reticular nucleus receives collaterals of thalamocortical and corticothalamic fibres.