There 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.