Thalamus

The thalamus is an ovoid nuclear mass, approximately 4 cm long, that borders the dorsal part of the third ventricle (Figs 15.1–15.3; see also Fig. 1.10). 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 (Fig. 15.4). 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. 15.1 Dorsal half of a brain sectioned in an oblique coronal plane that passes through the cerebral hemispheres, diencephalon, midbrain, pons and medulla oblongata, to show the general disposition of the main structures, some of which are labelled in Figure 15.3. Compare also with Figure 1.10.

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

Fig. 15.2 Dorsal aspect of the caudate nuclei, thalami, pineal gland and tectum, revealed by removal of most of the corpus callosum, the body of the fornix and the tela choroidea.

Fig. 15.3 Coronal section of the brain showing the principal parts of the diencephalon and basal ganglia. Compare with Figure 1.10.

Fig. 15.4 Oblique view of the dorsal aspect of the brain stem and thalamus.

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

The superior (dorsal) surface of the thalamus (see Fig. 15.2) 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 the 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 (see Fig. 5.8). It is usually connected to the contralateral thalamus by an interthalamic adhesion behind the interventricular foramina. 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 thalamus is divided into anterior, medial and lateral nuclear groups by a vertical Y-shaped sheet of white matter, the internal medullary lamina (Figs 15.5, 15.6). In addition, intralaminar nuclei lie embedded within, and surrounded by, the internal medullary lamina. Midline nuclei either abut the ependyma of the lateral walls of the third ventricle medially or lie adjacent to, and to some extent within, the interthalamic adhesion. Reticular nuclei lie lateral to the main nuclear mass, separated from it by the external medullary lamina.

Fig. 15.5 Coronal sections through the diencephalon showing the main nuclear aggregations of nerve cell bodies. 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. 15.6 Main nuclear masses of the thalamus (viewed from the lateral aspect in the lower illustration) have been labelled and colour-coded, and the same colours have been used to indicate the areas of cerebral neocortex interconnected with these nuclei. The lack of colour in the centromedian, intralaminar and reticular nuclei and in restricted areas of the frontal and temporal lobes is not related to the colour code.

In general, thalamic nuclei both project to and receive fibres from the cerebral cortex (see Fig. 15.6). The whole cerebral cortex, not only the neocortex but also the phylogenetically older palaeocortex of the piriform lobe and archicortex of the hippocampal formation, are 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. Corticothalamic fibres that 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 cortical–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 ‘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 on 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 there are increasing indications of non-cortical afferent pathways linked to so-called association nuclei.

Anterior Group of 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 Fig. 15.2, 15.6). Three subdivisions are recognized. The largest is the anteroventral nucleus; the others are the anteromedial and anterodorsal nuclei.

The anterior nuclei are the principal recipients of the mammillothalamic 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 (see Fig. 15.6). 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 the neocortex. The anterior thalamic nuclei are believed to be involved in the regulation of alertness and attention and in the acquisition of memory.

Medial Group of Thalamic Nuclei

The single component of this thalamic region is the mediodorsal or dorsomedial nucleus, which is particularly large in humans. Laterally, it is limited by the internal medullary lamina and intralaminar nuclei (see Figs. 15.5, 15.6). Medially, it abuts the midline parataenial and reuniens (medioventral) nuclei. It can be divided into anteromedial magnocellular and posterolateral parvocellular parts.

The small 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 a few pass 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 over 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 Group of Thalamic Nuclei

The lateral nuclear complex, lying lateral to the internal medullary lamina, is the largest major division of the thalamus (see Fig. 15.6). It is divided into dorsal and ventral tiers of nuclei. The lateral dorsal nucleus, lateral posterior nucleus and 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.

CASE 1 Thalamic Syndrome

A 64-year-old, right-handed hypertensive man notes the onset of numbness on the left side of his body, along with clumsiness of his left hand, improving over several hours. When first seen several days later, he has evidence of a dense left hemisensory disturbance involving pain and temperature appreciation; position sense is intact. Magnetic resonance imaging (MRI) with diffusion imaging shows an acute right thalamic stroke.

Two weeks after the development of this left hemisensory disturbance, he notes the spontaneous onset of severe burning pain in the left upper extremity, left side of the trunk and, to a lesser extent, left lower extremity. At this point, he still exhibits decreased pain and temperature appreciation, as on the initial examination, but there is now a marked overreaction to noxious stimuli (hyperalgesia) and allodynia, especially in the left arm and trunk.

Discussion: Thalamic pain syndrome (Dejerine–Roussy syndrome) is well recognized. The exact pathophysiological mechanism is not known. The ventral lateral thalamus is most commonly involved, but central pain syndromes such as this can occur with lesions outside the thalamus, ranging from the cerebral cortex to the brain stem and spinal cord. Conversely, thalamic strokes by no means always result in a central thalamic pain syndrome. Classic features of the disorder include hyperpathia, hyperalgesia and allodynia.

Medial Geniculate Nucleus

The medial geniculate nucleus, which is a part of the auditory pathway (Ch. 12), 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; therefore, it is 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 do 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 (Ch. 12), is a small ovoid ventral projection from the posterior thalamus (Fig. 15.7). The superior quadrigeminal brachium enters the posteromedial part of the lateral geniculate body dorsally, lying between the medial geniculate body and the pulvinar.