Thalamic Nuclei: Projections to Cerebral Cortex

by Sharadsaini on

Thalamic NucleiThere are highly precise point-to-point reciprocal connections between thalamic nuclei and the cerebral cortex. All thalamic nuclei except the reticular nucleus send ipsilateral projections to the cerebral cortex, and all cortical areas receive inputs from the thalamus. Thalamic nuclei that communicate with cortical regions are termed specific nuclei. All the specific nuclei lie in the ventral tier of the lateral nuclear group. The thalamus projects efferents to the cortex in the thalamic peduncles.

The ventral posterior nucleus projects efferents via thalamocortical projections through the posterior limb of the internal  capsule and the corona radiata. which terminate in the primary somatosensory cerebral cortex in the postcentral gyrus. There is a lesser projection to the secondary somatosensory area at the inferior end of the postcentral gyrus. The ventral anterior nucleus projects widely to the frontal cortex, including the supplementary motor area. The ventral  lateral nucleus projects mainly to the motor and premotor areas of the cerebral cortex.

The anterior nuclear group is the most anterior part of the thalamus and is actually part of the limbic system. It receives inputs fromthe mamillary bodies of the hypothalamus via the mamillothalamic tract, and projects principally to the cingulate gyrus, which is seen on the medial surface of the cerebral hemisphere. This nuclear group appears to be associated with emotional status and recent memory.

The ventral lateral nucleus lies caudal to the anterior nucleus. This nucleus projects to the frontal lobe, including the areas of the primary and premotor cortex.

The bilateral lateral geniculate nuclei (also called the lateral geniculate bodies) form small but noticeable swellings or eminences near the posterior pole of the thalamus, just ventral to the pulvinar. These nuclei are the termination site of fibers of the optic tract from the retina, and are thus part of the visual system. Each nucleus projects efferents to the primary visual cortex in the occipital lobe via the retrolenticular portion of the internal capsule, and through the optic radiation.

The medial geniculate nucleus receives fibers carrying auditory information  from the inferior colliculus. The medial geniculate nucleus projects this information to the primary auditory cortex in the temporal lobe via the retrolenticular portion of the internal capsule and the auditory radiation.

The medial (mediodorsal) nuclear group receives inputs from the amygdala, hypothalamus, and from other thalamic nuclei. This nuclear group projects extensively  and reciprocally to the prefrontal cortex and mediates emotion and mood.

The intralaminar nuclei lie in the internal medullary lamina of the thalamus. These nuclei include the centromedian nucleus and parafascicular nucleus. These  nuclei receive afferents from the spinothalamic and trigeminothalamic tracts, and also from the brain stem reticular formation. They send efferents to the basal ganglia, namely the caudate nucleus and the putamen. They also project very extensively to the cerebral cortex. Lesions to these nuclei result in a reduction of the level of consciousness and the perception of pain.

Thalamic Nuclei

Thalamic Nuclei

by Sharadsaini on

Thalamic NucleiThe 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.

Thalamic Nuclei