The thalamus is the largest mass of CNS nuclei and lies at the center of the brain. It consists of two bilateral egg-shaped lobes on opposite sides of the third ventricle. Their upper surfaces comprise the floor of each lateral ventricle, and their lateral surfaces are contiguous with the posterior limb of the internal capsule. The thalamus contains within it several nuclei with very diverse and often independent functions. The thalamic nuclei may be somatosensory, receiving inputs fromsensors of the somatosensory system and the special senses. From these nuclei there are projections to the primary sensory cortex (see next spread). Motor nuclei receive inputs from the cerebellum and the basal ganglia.
Each thalamus has a Y-shaped internal medullary lamina consisting of nerve fibers which are some of the afferent and efferent connections of the thalamic nuclei. The lamina divides each lobe into three main nuclear masses: posteromedial (or mediodorsal), anterior and lateral. Lateral to these nuclear masses is athin, shield-like layer of neurons called the reticular nucleus. The reticular nucleus is the only thalamic nucleus that does not correspond with the cortex. Lying posteriorly (at the back) of the thalamus are the lateral and medial geniculate bodies. For convenience, the thalamic nuclei may be grouped as relay or specific, association and non-specific.
Specific nuclei are those which correspond reciprocally with the sensory and motor areas of the cerebral cortex. The ventral posterior nucleus is the termination site for fibers of the lemniscal system. A somatosensory homunculus has been mapped in the lateral and medial divisions of this nucleus. The head is mapped medially, and the trunk laterally. In both divisions, nociceptive inputs occur towards the back of the homunculus, tactile inputs lie in the middle, and proprioception lies at the front. In other words, there is modality segregation. The ventral anterior nucleus receives inputs from the globus pallidus
The lateral geniculate nucleus receives afferents from the retina, and the medial geniculate nucleus receives afferents from the ear.
The association nuclei are (i) the anterior nucleus, which receives inputs from the mammillothalamic tract and may be involved in memory, (ii) the mediodorsal or posteromedial nucleus, which receives afferents from the limbic and olfactory systems and seems to mediate mood and judgment, and (iii) the pulvinar and lateral posterior nuclei, which are grouped as a single nucleus and receive afferents from the superior colliculus.
The non-specific nuclei include the intralaminar medullary nuclei and the reticular nucleus. The nuclei of the intralaminar medulla (see above) seem to be a rostral projection of the brain stem reticular formation involved in arousal. The reticular nucleus is separated from the other nuclei by the external medullary lamina; it receives collaterals from the thalamocortical fibers as they pass through on their way to the cerebral cortex. The reticular nucleus in turn projects efferent GABAergic inhibitory fibers to the corresponding thalamic nuclei from which it received the collaterals.