Specific nuclei

The specific or relay nuclei are reciprocally connected to specific motor or sensory areas of the cerebral cortex. They comprise the nuclei of the ventral tier and the geniculate bodies (nuclei). Their afferent and efferent connections are indicated in Figure 27.1B.

The anterior nucleus receives the mammillothalamic tract and projects to the cingulate cortex. It is involved in a limbic circuit and has a function in relation to memory (Ch. 34).

The ventral anterior nucleus (VA) receives afferents from the globus pallidus, and it projects to the prefrontal cortex.

The anterior part of the ventral lateral nucleus (VL) receives afferents from the globus pallidus and projects to the supplementary motor area. The posterior part of VL is the principal target of the contralateral superior cerebellar peduncle, which originates in the dentate nucleus of the cerebellum; the posterior VL projects to the motor cortex.

The ventral posterior nucleus (VP) receives all of the fibers of the medial, spinal, and trigeminal lemnisci (Figure 27.2). It projects to the somatic sensory cortex (SI). A smaller projection is sent to the second somatic sensory area (SII) at the foot of the postcentral gyrus (see Ch. 29).

Figure 27.2 Coronal section through the thalamus and related structures. LF, lemniscal fibers; LPN, lateral posterior nucleus; MDN, mediodorsal nucleus; RN, reticular nucleus; TCF, thalamocortical fibers; VPN, ventral posterior nucleus.

The VP is somatotopically arranged, as indicated in Figure 27.3. The portion of the nucleus devoted to the face and head is called the ventral posterior medial nucleus (VPM), that for the trunk and limbs the ventral posterior lateral nucleus (VPL). Modality segregation is a feature of both nuclei, with proprioceptive neurons most anterior, tactile neurons in the midregion, and nociceptive neurons at the back. The nociceptive region is sometimes called the posterior nucleus.

Figure 27.3 Somatic sensory map in the ventral posterior thalamic nucleus.

(Redrawn and modified from Ohye 1990, with permission.)

There is no evidence in the VP of an antinociceptive mechanism comparable to that found in the substantia gelatinosa region of the spinal cord and spinal trigeminal nucleus. An unexplained disorder, the thalamic syndrome, may follow a vascular lesion that disconnects the posterior thalamic nucleus from the somatic sensory cortex. In this condition, a period of complete sensory loss may occur on the contralateral side of the body, to be replaced by bouts of severe pain occurring either spontaneously or in response to tactile stimuli. (See also Ch. 35, central poststroke pain.)

The medial geniculate body (medial geniculate nucleus) is the thalamic nucleus of the auditory pathway. It receives the inferior brachium from the inferior colliculus (which carries auditory signals from both ears, Ch. 20), and it projects to the primary auditory cortex in the superior temporal gyrus.

The lateral geniculate body (lateral geniculate nucleus) is the principal thalamic nucleus for vision. It receives retinal inputs from both eyes by way of the optic tract, and it projects to the primary visual cortex in the occipital lobe. The visual pathways are described in Chapter 28.

Association nuclei

The association nuclei are reciprocally connected to the association areas of the cerebral cortex.

The lateral dorsal nucleus has reciprocal connections with the posterior part of the cingulate cortex, which is involved in functions related to memory (Ch. 34).

The mediodorsal nucleus receives inputs from the olfactory and limbic systems and is reciprocally connected with the entire prefrontal cortex. It has functions in relation to cognition (thinking), judgment, and mood.

The lateral posterior nucleus and the pulvinar belong to a single nuclear complex. They receive afferents from the superior colliculus and project to the entire visual association cortex and to the entire parietal association cortex. An ‘extrageniculate visual pathway’ runs from the optic tract to the visual association cortex by way of the superior colliculus and the pulvinar. It has the function of drawing attention to objects of interest in the peripheral field of vision, but it is not itself a source of conscious visual perception.

Non-specific nuclei

The non-specific nuclei are so called because they are not specific to any one sensory modality. They include the intralaminar and reticular nuclei.

The intralaminar nuclei are contained within the internal medullary lamina of white matter. They can be regarded as a rostral continuation of the reticular formation of the midbrain (ascending reticular activating system in Ch. 24). They project widely to the cerebral cortex, as well as to the corpus striatum.

Afferents belonging to the ascending reticular activating system synapse in the intralaminar nuclei, also in the reticular nucleus and in the nucleus of Meynert in the basal forebrain (Ch. 34).

The thalamic reticular nucleus (TRN) is shaped like a shield around the front and lateral side of the thalamus. It is separated from the main thalamus by the external medullary lamina. All of the thalamocortical projections from the specific thalamic nuclei pass through TRN and give collateral branches to it (Figure 27.4). Fusiform neurons within the innermost lamina (VI) of the cerebral cortex project to the thalamic nuclei and also give off collaterals to TRN.

Figure 27.4 Basic synaptic relationships of the thalamic reticular nucleus. (Adapted from Pinault, 2004.) The ‘sensory nucleus’ includes somatic sensory, visual, and auditory thalamic nuclei.

TRN is exclusively made up of inhibitory GABAergic neurons. Most of them project back into the corresponding nucleus and control (modulate) its rate of discharge to the cortex. There is general agreement, based on experimental recordings in rats and primates, that the primary function of TRN is that of saliency, which translates as helping to isolate any novel auditory, visual, or tactile experience from the normal background ‘noise’ of cortical activity in the awake state. The process is known as ‘center-surround’: corticothalamic feedback from lamina VI enhances activity of the stimulated patch of the sensory nucleus (the ‘center’) and simultaneously suppresses ongoing, random activity in the surrounding neurons not directly involved.

Tactile, visual, and auditory sensory modalities may be said to ‘imprint’ the areas of the TRN lattice which they collaterate as they pass through to reach the cerebral cortex. A minority of TRN cells project into other specific nuclei rather than the corresponding one mentioned above. This arrangement could enable TRN to participate in combined processing. This term refers to the simultaneous engagement of more than one sensory modality in a particular sensory task. As an example, an unexpected sound occurring within the lower right visual field, activating a topographically specific patch of the auditory TRN lattice overlying each medial geniculate body, could selectively disinhibit lateral geniculate neurons on the visual pathway from the upper left quadrants of both retinas. In this way, auditory inputs may facilitate selective visual attention to the area of interest.

Oscillation

A remarkable histological feature of TRN neurons is the frequent occurrence of dendritic sheaves. These comprise bundles of dendrites belonging to different neurons, extending in the overall plane of TRN, and linked to one another by dendrodendritic synapses. This arrangement may form the anatomical basis of the phenomenon of oscillation. Oscillation is characterized by spontaneous burst-firing of large groups of TRNs at a rate of 5–15 Hz, usually for a few seconds at a time. The oscillations produce patches of surround inhibition in the underlying thalamocortical neurons, thereby evoking bursts of cortical activity known as sleep spindles, so called because they are detectable by means of electroencephalography at the onset of sleep.

Sleep–wake cycles are described in Chapter 30.

Not represented in Table 27.1 are aminergic afferents passing to the ventral and intralaminar nuclei, from the midbrain raphe (serotonergic) and cerulean nucleus (noradrenergic). The proven value of tricyclic antidepressants in the therapy of chronic pain may be related to drug-induced prolongation of excitatory aminergic effects on thalamocortical neurons.