All pathways carrying information from the periphery or the brainstem to the neocortex relay in the nuclei of the posterior thalamus. These nuclei can be divided into two groups on the basis of their structure, connections, and function.
Nonspecific Nuclei. The first group includes the midline (median) and intralaminar nuclei and the medial portion of the ventral anterior nucleus. These nuclei receive ascending input from the mesencephalic reticular formation and from the spinal cord (paleospinothalamic tract), and descending input from the cerebral cortex. They project widely, both to other thalamic nuclei and to the cortex, especially to its frontal regions. These projections are thought to be essential in regulating the general excitability of neurons in the thalamus and cortex.
Another nucleus included in the first group is the reticular nucleus, which overlies the lateral surface of the thalamus. Neurons of this nucleus, which receive input from collaterals of thalamocortical fibers and project back to the thalamus, are thought to constitute a feedback pathway that regulates thalamic excitability.
Specific Nuclei. The second group of nuclei is termed the “specific nuclei” because they project to restricted regions of the cortex (see Plate 2-13). The major specific nuclei and the corresponding cortical regions to which they project are illustrated in matching colors. One set of specific nuclei are the sensory relay nuclei. The ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei receive their input from somatosensory relay neurons via the medial lemniscus, trigeminal lemnisci, and the neospinothalamic tract. They project to the primary (Sm I) and secondary (Sm II) somatosensory cortex. The ventral posterointermediate (VPI) nucleus (not shown) receives input from the vestibular system and projects to the vestibular area in the parietal lobe (see Plate 2-13). The lateral geniculate nucleus receives its input from the optic tract and projects to the primary visual area in the occipital lobe (see Plate 2-13). The principal part of the medial geniculate nucleus receives input from auditory relay nuclei and projects to the primary auditory area in the supratemporal transverse gyrus (see Plate 2-13).
A second set of specific nuclei is involved in the control of motor activity. The ventral lateral (VL) and ventral intermedial (VI) nuclei and the lateral portion of the ventral anterior (VA) nucleus receive input from the cerebellum and basal ganglia, respectively, and project to the precentral motor areas (see Plate 2-13). These areas also receive input from the oral part of the ventral posterolateral nucleus.
The anterior dorsal (AD) (the least prominent of the anterior group of nuclei) and the medial dorsal (MD) nuclei are specifically related to the limbic system, which regulates emotional and autonomic activity (see Plates 2-8 and 2-13). The anterior dorsal nucleus receives input from the hippocampus relayed via the mammillothalamic tract and projects to the cingulate gyrus. The medial dorsal nucleus receives input from the hypothalamus and amygdala and projects to the frontal lobe.
The remaining specific nuclei are related to association areas of the cortex involved in higher integrative mechanisms. They include the lateral dorsal (LD) and lateral posterior (LP) nuclei and the pulvinar complex. The medial, magnocellular part of the medial geniculate nucleus, which receives widespread convergent input from many afferent systems, should probably also be included in this category.
Cortical Connections. In addition to receiving the ascending input described above, all the thalamic nuclei receive descending input from the cerebral cortex, principally from the cortical regions to which they project (see Plate 2-13). These descending projections serve as a two-way feedback system between each cortical area and its thalamic relay nucleus.
Not all the nuclei of the posterior thalamus project to the cerebral cortex. One important nucleus without a cortical projection is the centromedian (CM) nu us, only with the basal ganglia.