Ventral anterior nucleus

The ventral anterior (VA) nuclear complex lies at the anterior pole of the ventral nuclear group. It is limited anteriorly by the reticular nucleus, posteriorly by the ventral lateral nucleus, and lies between the external and internal medullary laminae. It consists of a principal part (VApc) and a magnocellular part (VAmc). The subcortical connections to this region are largely ipsilateral from the internal segment of the globus pallidus and the pars reticulata of the substantia nigra. The terminal fields from these origins do not overlap. Fibres from the globus pallidus end in VApc. The substantia nigra projects to VAmc. Corticothalamic fibres from premotor cortex (area 6) terminate in VApc and fibres from the frontal eye field (area 8) terminate in VAmc. VA thalamus does not appear to receive fibres directly from the motor cortex. The efferent projections from VA are incompletely known. Some pass to intralaminar thalamic nuclei and others project to widespread regions of the frontal lobe and to the anterior parietal cortex. Their functions are unclear. The VA thalamus appears to play a central role in the transmission of the cortical ‘recruiting response’, a phenomenon in which stimulation of the thalamus can initiate long-lasting, high voltage repetitive negative electrical waves over much of the cerebral cortex.

Ventral lateral nucleus

The ventral lateral (VL) nucleus consists of two major divisions with distinctly different connections and functions. The anterior division, or pars oralis (VLo), receives topographically organized fibres from the internal segment of the ipsilateral globus pallidus. The posterior division, or pars caudalis (VLc), receives topographically organized fibres from the contralateral deep cerebellar nuclei. Additional subcortical projections have been reported from the spinothalamic tract and the vestibular nuclei. Numerous cortical afferents to both VLo and VLc originate from precentral motor cortical areas, including both area 4 and area 6.

The VLo nucleus sends efferent fibres to the supplementary motor cortex on the medial surface of the hemisphere and to the lateral premotor cortex. The VLc nucleus projects efferent fibres to the primary motor cortex where they end in a topographically arranged fashion. The head region of area 4 receives fibres from the medial part of VLc, and the leg region receives fibres from lateral VLc.

Approximately 70% of VL neurones are large relay neurones, and the other 30% are local circuit interneurones. Responses can be recorded in VL thalamus during both passive and active movement of the contralateral body. The topography of its connections, and recordings made within the nucleus, suggest that VLc contains a body representation comparable to that in the ventral posterior nucleus. In patients with tremor, clusters of VL neurones fire in bursts that are synchronous with peripheral tremor: these cells have been designated as ‘tremor cells’. Stereotaxic surgery of the ventral lateral nucleus is sometimes used in the treatment of essential tremor. In the past, thalamotomy was used extensively for the treatment of Parkinson’s disease. The internal segment of the globus pallidus and the subthalamic nucleus are now the preferred neurosurgical targets for Parkinson’s disease (see Ch. 22).

Ventral posterior nucleus

The ventral posterior (VP) nucleus is the principal thalamic relay for the somatosensory pathways. It is thought to consist of two major divisions, the ventral posterolateral (VPl) and ventral posteromedial (VPm) nuclei. The VPl nucleus receives the medial lemniscal and spinothalamic pathways, and the VPm nucleus receives the trigeminothalamic pathway. Connections from the vestibular nuclei and lemniscal fibres terminate along the ventral surface of the VP nucleus.

There is a well-ordered topographic representation of the body in the ventral posterior nucleus. The VPl is organized so that sacral segments are represented laterally and cervical segments medially. The latter abut the face area of representation (trigeminal territory) in VPm. Taste fibres synapse most anteriorly and ventromedially within the ventral posterolateral nucleus.

At a more detailed level, single body regions are represented as curved lamellae of neurones, parallel to the lateral border of the ventral posterior nucleus, such that there is a continuous overlapping progression of adjacent receptive fields from dorsolateral to ventromedial. Considerably less change in location of receptive field on the body is seen when passing anteroposteriorly through the nucleus. While not precisely dermatomal in nature, these curvilinear lamellae of cells probably derive from afferents related to a few adjacent spinal segments. There is considerable distortion of the body map within the nucleus, reflecting the differences in the density of peripheral innervation which occur in different body regions, e.g. many more neurones respond to stimulation of the hand than of the trunk. Within a single lamella, neurones in the anterodorsal part of the nucleus respond to deep stimuli, including movement of joints, tendon stretch, and manipulation of muscles. Most ventrally, neurones once again respond to deep stimuli, particularly tapping. Intervening cells within a single lamella respond only to cutaneous stimuli. This organization has been confirmed by recordings made in the human ventral posterior nucleus.

Single lemniscal axons have an extended anteroposterior terminal zone within the nucleus. Rods of cells running the length of the anteroposterior, dorsoventrally oriented, lamellae respond with closely similar receptive field properties and locations, derived from a small bundle of lemniscal afferents. It appears, therefore, that each lamella contains the complete representation of a single body part, e.g. a finger. Lamellae consist of multiple narrow rods of neurones, oriented anteroposteriorly, each of which receives input from the same small region of the body that is represented within the lamella, and from the same type of receptors. These thalamic ‘rods’ form the basis for both the place- and modality-specific input to columns of cells in the somatic sensory cortex. Spinothalamic tract afferents to the ventral posterolateral nucleus terminate throughout the nucleus. The neurones from which these axons originate appear to be mainly of the ‘wide dynamic range’ class, responding to both low threshold mechanoreceptors and high threshold nociceptors; a smaller proportion of neurones respond exclusively to high-threshold nociceptors. Some neurones respond to temperature changes. There is evidence that spinothalamic tract neurones carrying nociceptive and thermal information terminate in a distinct nuclear area, identified as the posterior part of the ventral medial nucleus (VMpo).

The VP nucleus projects to the primary somatic sensory cortex (SI) of the postcentral gyrus and to the second somatic sensory area (SII) in the parietal operculum. VMpo projects to the insular cortex. Within the primary sensory cortex, the central cutaneous core of the VP nucleus projects solely to area 3b; dorsal and ventral to this, a narrow band of cells projects to both 3b and area 1. The most dorsal and ventral deep stimulus receptive cells project to areas 3a and 2. The whole nucleus projects to the second somatic sensory area.

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.