Diencephalon

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CHAPTER 21 Diencephalon

The diencephalon is part of the prosencephalon (forebrain), which develops from the foremost primary cerebral vesicle and differentiates into a caudal diencephalon and rostral telencephalon. The cerebral hemispheres develop from the sides of the telencephalon, each containing a lateral ventricle. The sites of evagination become the interventricular foramina, through which the two lateral ventricles and midline third ventricle communicate. The diencephalon corresponds largely to the structures that develop lateral to the third ventricle. The lateral walls of the diencephalon form the epithalamus most superiorly, the thalamus centrally and the subthalamus and hypothalamus most inferiorly.

THALAMUS

The thalamus is an ovoid nuclear mass, about 4 cm long, which borders the dorsal part of the third ventricle (Figs 21.1, 21.2). The narrow anterior pole lies close to the midline, and forms the posterior boundary of the interventricular foramen. Posteriorly, an expansion, the pulvinar, extends beyond the third ventricle to overhang the superior colliculus (see Fig. 19.3). The brachium of the superior colliculus (superior quadrigeminal brachium) separates the pulvinar above from the medial geniculate body below. A small oval elevation, the lateral geniculate body, lies lateral to the medial geniculate.

Fig. 21.1 Coronal T1 weighted inversion recovery MRI image at the level of the thalamus and third ventricle in a normal volunteer.

(By courtesy of Professor Alan Jackson, Wolfson Molecular Imaging Centre, The University of Manchester, Manchester, UK.)

Fig. 21.2 The principal parts of the diencephalon and basal ganglia, coronal section.

The superior (dorsal) surface of the thalamus (see Fig. 19.3) is covered by a thin layer of white matter, the stratum zonale. It extends laterally from the line of reflection of the ependyma (taenia thalami), and forms the roof of the third ventricle. This curved surface is separated from the overlying body of the fornix by the choroid fissure with the tela choroidea within it. More laterally, it forms part of the floor of the lateral ventricle. The lateral border of the superior surface of the thalamus is marked by the stria terminalis and overlying thalamostriate vein, which separate the thalamus from the body of the caudate nucleus. Laterally, a slender sheet of white matter, the external medullary lamina, separates the main body of the thalamus from the reticular nucleus. Lateral to this, the thick posterior limb of the internal capsule lies between the thalamus and the lentiform complex.

The medial surface of the thalamus is the superior (dorsal) part of the lateral wall of the third ventricle. It is usually connected to the contralateral thalamus by an interthalamic adhesion behind the interventricular foramina (see Fig. 16.7). The boundary with the hypothalamus is marked by an indistinct hypothalamic sulcus, which curves from the upper end of the cerebral aqueduct to the interventricular foramen. The thalamus is continuous with the midbrain tegmentum, the subthalamus and the hypothalamus.

Internally, the greater part of the thalamus is divided into anterior, medial and lateral nuclear groups by a vertical Y-shaped sheet of white matter, the internal medullary lamina (Figs 21.3, 21.4). In addition, intralaminar nuclei lie embedded within the internal medullary lamina and midline nuclei abut the lateral walls of the third ventricle. The reticular nucleus forms a shell-like lateral covering to the main nuclear mass, separated from it by an external medullary lamina of nerve fibres.

Fig. 21.3 The main nuclear aggregations of cell bodies in the diencephalon (coronal sections). A, At the level of the mammillary bodies. B, At the level of the tuber cinereum. Note the variations in cell size, shape and packing density, which characterize the nuclear masses of the thalamus, subthalamus and hypothalamus at these levels.

Fig. 21.4 The main nuclear masses of the thalamus (viewed from the lateral aspect in the lower illustration) colour coded to indicate the areas of cerebral neocortex with which they are interconnected. The lack of colour in the centromedian, intralaminar and reticular nuclei and in areas of the frontal and temporal lobes is not related to the colour code. The reticular nucleus lies lateral to the main mass of the thalamus. Only the anterior pole of the reticular nucleus is shown, its posterior extent being depicted by the heavy interrupted line.

In general, thalamic nuclei both project to and receive fibres from the cerebral cortex (Fig. 21.4). The whole cerebral cortex, including neocortex, paleocortex of the piriform lobe and archicortex of the hippocampal formation, is reciprocally connected with the thalamus. The thalamus is the major route by which subcortical neuronal activity influences the cerebral cortex, and the greatest input to most thalamic nuclei comes from the cerebral cortex.

The projection to the thalamus from the cortex is precisely reciprocal; each cortical area projects in a topographically organized manner to all sites in the thalamus from which it receives an input. Thalamocortical fibres terminate predominantly in layer IV of the cortex. Corticothalamic fibres which reciprocate ‘specific’ thalamocortical pathways arise from modified pyramidal cells of layer VI, whereas those reciprocating ‘non-specific’ inputs arise from typical pyramidal cells of layer V, and may in part be axon collaterals of other cortico-subcortical pathways.

It is customary to consider thalamic nuclei as either ‘specific’ nuclei, which mediate finely-organized and precisely-transmitted sensory information to discrete cortical sensory areas, or as ‘non-specific’ nuclei, which are part of a general arousal system. The specific nuclei are further subdivided into relay nuclei and association nuclei. However, many nuclei classified as specific may also send non-specific projections to widespread cortical areas. Similarly, the division of thalamic nuclei into relay and association groups rests upon the assumption that relay nuclei receive a major subcortical pathway, whereas association nuclei receive their principal non-cortical input from other thalamic nuclei. There is little evidence of significant intrathalamic connectivity, but increasing indications of non-cortical afferent pathways linked to so-called association nuclei: the extensive connectivity between the reticular nucleus and other thalamic nuclei is a notable exception.

ANTERIOR THALAMIC NUCLEI

The anterior group of nuclei are enclosed between the arms of the Y-shaped internal medullary lamina, and underlie the anterior thalamic tubercle (see Figs 19.3, 21.4). Three subdivisions are recognized: the largest is the anteroventral nucleus, the others being the anteromedial and anterodorsal nuclei.

The anterior nuclei are the principal recipients of the mammillo-thalamic tract, which arises from the mammillary nuclei of the hypothalamus. The mammillary nuclei receive fibres from the hippocampal formation via the fornix. The medial mammillary nucleus projects to the ipsilateral anteroventral and anteromedial thalamic nuclei, and the lateral mammillary nucleus projects bilaterally to the anterodorsal nuclei. The nuclei of the anterior group also receive a prominent cholinergic input from the basal forebrain and the brain stem.

The cortical targets of efferent fibres from the anterior nuclei of the thalamus lie largely on the medial surface of the hemisphere. They include the anterior limbic area (in front of and inferior to the corpus callosum), the cingulate gyrus, and the parahippocampal gyrus (including the medial entorhinal cortex and the pre- and para-subiculum). These thalamocortical pathways are reciprocal. There also appear to be minor connections between the anterior nuclei and the dorsolateral prefrontal and posterior areas of neocortex. The anterior thalamic nuclei are believed to be involved in the regulation of alertness and attention and in the acquisition of memory.

MEDIAL THALAMIC NUCLEI

The single component of this thalamic region is the mediodorsal or dorsomedial nucleus, which is particularly large in man. Laterally, it is limited by the internal medullary lamina and intralaminar nuclei. Medially, it abuts the midline parataenial and reuniens (medioventral) nuclei. It is divided into anteromedial magnocellular, and postero-lateral parvocellular, parts.

The smaller magnocellular division receives olfactory input from the piriform and adjacent cortex, the ventral pallidum, and the amygdala. The mediobasal amygdaloid nucleus projects to the dorsal part of the anteromedial magnocellular nucleus, and the lateral nuclei project to the more central and anteroventral regions. The anteromedial magnocellular nucleus projects to the anterior and medial prefrontal cortex, notably to the lateral posterior and central posterior olfactory areas on the orbital surface of the frontal lobe. In addition, fibres pass to the ventromedial cingulate cortex and some to the inferior parietal cortex and anterior insula. These cortical connections are reciprocal.

The larger, posterolateral parvocellular division connects reciprocally with the dorsolateral and dorsomedial prefrontal cortex, the anterior cingulate gyrus and the supplementary motor area. In addition, efferent fibres pass to the posterior parietal cortex.

The mediodorsal nucleus appears to be involved in a wide variety of higher functions. Damage may lead to a decrease in anxiety, tension, aggression or obsessive thinking. There may also be transient amnesia, with confusion developing particularly over the passage of time. Much of the neuropsychology of medial nuclear damage reflects defects in functions similar to those performed by the prefrontal cortex, with which it is closely linked. The effects of ablation of the mediodorsal nuclei parallel, in part, the results of prefrontal lobotomy.

LATERAL THALAMIC NUCLEI

The lateral nuclear complex, lying lateral to the internal medullary lamina, is the largest division of the thalamus. It is divided into dorsal and ventral tiers of nuclei. The lateral dorsal nucleus, lateral posterior nucleus and the pulvinar all lie dorsally. The lateral and medial geniculate nuclei lie inferior to the pulvinar near the posterior pole of the thalamus. The ventral tier nuclei are the ventral anterior, ventral lateral and ventral posterior nuclei.

Medial geniculate nucleus

The medial geniculate nucleus, which is a part of the auditory pathway (see Ch. 37), is located within the medial geniculate body, a rounded elevation situated posteriorly on the ventrolateral surface of the thalamus, and separated from the pulvinar by the superior quadrigeminal brachium. It receives fibres travelling in the inferior quadrigeminal brachium. Three major subnuclei, medial, ventral and dorsal, are recognized within it. The inferior brachium separates the medial (magnocellular) nucleus, which consists of sparse, deeply staining neurones, from the lateral nucleus, which is made up of medium-sized, densely packed and darkly staining cells. The dorsal nucleus overlies the ventral nucleus and expands posteriorly; it is, therefore, sometimes known as the posterior nucleus of the medial geniculate. It contains small- to medium-sized, pale-staining cells, which are less densely packed than those of the lateral nucleus. The ventral nucleus receives fibres from the central nucleus of the ipsilateral inferior colliculus via the inferior quadrigeminal brachium and also from the contralateral inferior colliculus. The nucleus contains a complete tonotopic representation. Low-pitched sounds are represented laterally, and progressively higher-pitched sounds are encountered as the nucleus is traversed from lateral to medial. The dorsal nucleus receives afferents from the pericentral nucleus of the inferior colliculus and from other brain stem nuclei of the auditory pathway. A tonotopic representation has not been described in this subdivision and cells within the dorsal nucleus respond to a broad range of frequencies. The magnocellular medial nucleus receives fibres from the inferior colliculus and from the deep layers of the superior colliculus. Neurones within the magnocellular subdivision may respond to modalities other than sound. However, many cells respond to auditory stimuli, usually to a wider range of frequencies than neurones in the ventral nucleus. Many units show evidence of binaural interaction, with the leading effect arising from stimuli in the contralateral cochlea. The ventral nucleus projects primarily to the primary auditory cortex. The dorsal nucleus projects to auditory areas surrounding the primary auditory cortex. The magnocellular division projects diffusely to auditory areas of the cortex and to adjacent insular and opercular fields.

Lateral geniculate nucleus

The lateral geniculate body, which is part of the visual pathway (see Ch. 40), is a small, ovoid, ventral projection from the posterior thalamus (Fig. 21.5). The superior quadrigeminal brachium enters the posteromedial part of the lateral geniculate body dorsally, lying between the medial geniculate body and the pulvinar.

Fig. 21.5 Coronal section through the brain showing the lateral geniculate nucleus.

(Photograph by Kevin Fitzpatrick on behalf of GKT School of Medicine, London.)

The lateral geniculate nucleus is an inverted, somewhat flattened, U-shaped nucleus and is laminated. Its internal organization is usually described on the basis of six laminae, although seven or eight may be present. The laminae are numbered 1 to 6, from the innermost ventral to the outermost dorsal (Fig. 21.6). Laminae 1 and 2 consist of large cells, the magnocellular layers, whereas layers 4 to 6 have smaller neurones, the parvocellular laminae. The apparent gaps between laminae are called the interlaminar zones. Most ventrally an additional superficial, or S, lamina is recognized.

Fig. 21.6 Coronal sections through the lateral geniculate nucleus illustrating the laminar arrangement of neurones (A) near its central region and (B) near its posterior pole.

The lateral geniculate nucleus receives a major afferent input from the retina. The contralateral nasal hemiretina projects to laminae 1, 4 and 6, whereas the ipsilateral temporal hemiretina projects to laminae 2, 3 and 5. The parvocellular laminae receive axons predominantly of X-type retinal ganglion cells, i.e. more slowly conducting cells with sustained responses to visual stimuli. The faster conducting, rapidly adapting Y-type retinal ganglion cells project mainly to the magnocellular laminae 1 and 2, and give off axonal branches to the superior colliculus. A third type of retinal ganglion cell, the W cells, which have large receptive fields and slow responses, project to both the superior colliculus and the lateral geniculate nucleus, where they terminate particularly in the interlaminar zones and in the S lamina.

The lateral geniculate nucleus is organized in a visuotopic manner and it contains a precise map of the contralateral visual field. The vertical meridian is represented posteriorly, the peripheral anteriorly, the upper field laterally, and the lower field medially. Similar precise, point-to-point, representation is also found in the projection of the lateral geniculate nucleus to the visual cortex. Radially arranged inverted pyramids of neurones in all laminae respond to a single, small area of the contralateral visual field and project to a circumscribed area of cortex. The termination of geniculocortical axons in the visual cortex is considered in detail elsewhere (see Ch. 23).

Aside from retinal afferents, the lateral geniculate nucleus receives a major corticothalamic projection, the axons of which ramify densely in the interlaminar zones. The major part of this projection arises from the primary visual cortex, Brodmann’s area 17, but smaller projections from extrastriate visual areas pass to the magnocellular and S laminae. Other afferents include: fibres from the superficial layer of the superior colliculus (which terminate in the interlaminar zone between laminae 1 and 2, 2 and 3, and around lamina S); noradrenergic fibres from the locus coeruleus; serotoninergic afferents from the midbrain raphe nuclei; and cholinergic fibres from the pontine and mesencephalic reticular formation.

The efferent fibres of the lateral geniculate nucleus pass principally to the primary visual cortex (area 17) in the banks of the calcarine sulcus. It is possible that additional small projections pass to extrastriate visual areas in the occipital lobe, possibly arising primarily in the interlaminar zones.

Lateral dorsal nucleus

The lateral dorsal nucleus is the most anterior of the dorsal tier of lateral nuclei. Its anterior pole lies within a splitting of the internal medullary lamina, and posteriorly it merges with the lateral posterior nucleus. Subcortical afferents to the lateral dorsal nucleus are from the pretectum and superior colliculus. It is connected with the cingulate, retrosplenial and posterior parahippocampal cortices, the presubiculum of the hippocampal formation, and the parietal cortex.

Lateral posterior nucleus

The lateral posterior nucleus, which lies dorsal to the ventral posterior nucleus, receives its subcortical afferents from the superior colliculus. It is reciprocally connected with the superior parietal lobe. Additional connections have been reported with the inferior parietal, cingulate and medial parahippocampal cortex.

Pulvinar

The pulvinar corresponds to the posterior expansion of the thalamus, which overhangs the superior colliculus (Fig. 21.4). It has three major subdivisions, which are the medial, lateral and inferior pulvinar nuclei. The medial pulvinar nucleus is dorsomedial and consists of compact, evenly spaced, neurones. The inferior pulvinar nucleus lies laterally and inferiorly, and is traversed by bundles of axons in the mediolateral plane, an arrangement which confers a fragmented appearance of horizontal cords or sheets of cells separated by fibre bundles. The inferior pulvinar nucleus lies most inferiorly and laterally and is a more homogeneous collection of cells.

The subcortical afferents to the pulvinar are uncertain. Medial and lateral pulvinar nuclei may receive fibres from the superior colliculus. It has been suggested that the inferior pulvinar nucleus receives fibres both from the superior colliculus and directly from the retina and that it contains a complete retinotopic representation.

The cortical targets of efferent fibres from the pulvinar are widespread. In essence, the medial pulvinar nucleus projects to association areas of the parietotemporal cortex, whereas lateral and inferior pulvinar nuclei project to visual areas in the occipital and posterior temporal lobes. Thus, the inferior pulvinar nucleus connects with the striate and extrastriate cortex in the occipital lobe, and with visual association areas in the posterior part of the temporal lobe. The lateral pulvinar nucleus connects with extrastriate areas of the occipital cortex, with posterior parts of the temporal association cortex, and with the parietal cortex. The medial pulvinar nucleus connects with the inferior parietal cortex, with the posterior cingulate gyrus and with the widespread areas of the temporal lobe, including the posterior parahippocampal gyrus, perirhinal and entorhinal cortex. It also has extensive connections with prefrontal and orbitofrontal cortices. Similarly, the lateral pulvinar nucleus may also connect with the rostromedial prefrontal cortex.

Little is known of the functions of the pulvinar. The inferior pulvinar nucleus contains a complete retinotopic representation, and lateral and medial pulvinar nuclei also contain visually responsive cells. However, the latter nucleus, at least, is not purely visual – other modality responses can be recorded, and some cells may be polysensory. Given the complexity of functions of the association areas to which they project, particularly in the temporal lobe (e.g. perception, cognition and memory), it is likely that the role of the pulvinar in modulating these functions is equally complex.

Anteriorly, the major subdivisions of the pulvinar blend into a poorly differentiated region, within which several nuclear components have been recognized, including the anterior or oral pulvinar, the suprageniculate-limitans nucleus and the posterior nuclei. The connectivity of this complex is also not well understood. It is recognized that different components receive subcortical afferents from the spinothalamic tract and the superior and inferior colliculi. Cortical connections centre primarily on the insula and adjacent parts of the parietal operculum posteriorly. Stimulation of this region has been reported to elicit pain, and large lesions may alleviate painful conditions. Similarly, excision of its cortical target in the parietal operculum, or small infarcts in this cortical region, may result in hypoalgesia.

INTRALAMINAR NUCLEI

The term intralaminar nuclei refers to collections of neurones within the internal medullary lamina of the thalamus (Fig. 21.4). Two groups of nuclei are recognized. The anterior (rostral) group is subdivided into central medial, paracentral and central lateral nuclei. The posterior (caudal) intralaminar group consists of the centromedian and parafascicular nuclei. The designations central medial and centromedian are open to confusion; however, they are an accepted part of the terminology of thalamic nuclei in common usage. The centromedian nucleus is much larger, is considerably expanded in man in comparison with other species, and is importantly related to the globus pallidus, deep cerebellar nuclei and motor cortex. Anteriorly, the internal medullary lamina separates the mediodorsal nucleus from the ventral lateral complex. It is occupied by the paracentral nucleus laterally, and the central medial nucleus ventromedially, as the two laminae converge towards the midline. A little more posteriorly, the central lateral nucleus appears dorsally in the lamina as the latter splits to enclose the lateral dorsal nucleus. More posteriorly, at the level of the ventral posterior nucleus, the lamina splits to enclose the ovoid centromedian nucleus. The smaller parafascicular nucleus lies more medially.

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