thalamus

The large, ovoid mass of gray matter that forms the larger dorsal subdivision of the diencephalon; it is placed medial to the internal capsule and the body and tail of the caudate nucleus. Its medial aspect forms the dorsal half of the lateral wall of the third ventricle; its dorsal surface can be subdivided into a lateral triangle forming the floor of the body (central part) of the lateral ventricle, and a medial triangle covered by the velum interpositum; its tail-like caudal part curves ventralward around the posterolateral aspect of the cerebral peduncle and ends in the lateral geniculate body. The t. is composed of a large number of anatomically and functionally distinct cell groups or nuclei, usually classified as 1) sensory relay nuclei (ventral posterior nucleus, lateral and medial geniculate body) each receiving a modally specific sensory conduction system and in turn projecting each to the corresponding primary sensory area of the cortex; 2) “secondary” relay nuclei (ventral intermediate nucleus and ventral anterior nucleus) receiving fibers from the medial segment of the globus pallidus, the contralateral deep cerebellar nuclei ( i.e., cerebellothalamic fibers), and the pars reticulata of the substantia nigra which project to various regions of the motor cortex; 3) a nucleus associated with the limbic system : the composite anterior nucleus receiving the mamillothalamic tract and projecting to the fornicate gyrus; 4) association nuclei (medial dorsal nucleus, lateral nucleus including the large pulvinar) each projecting to a particular large expanse of association cortex; 5) the midline and intralaminar nuclei or “nonspecific” nuclei (centromedian nucleus, central lateral nucleus, paracentral nucleus, nucleus reuniens). SEE ALSO: dorsal t.. [G. thalamos, a bed, a bedroom]

- dorsal t. the large part of the diencephalon located dorsal to the hypothalamus and excluding the subthalamus and the medial and lateral geniculate bodies (sometime the latter two are collectively called the metathalamus); the dorsal t. includes the major motor and somatosensory relay nuclei, nuclei that project to association areas, and the intralaminar nuclei. SEE ALSO: t..

- ventral t. SYN: subthalamus.

* * *

thal·a·mus 'thal-ə-məs n, pl -mi -.mī, -.mē the largest subdivision of the diencephalon that consists chiefly of an ovoid mass of nuclei in each lateral wall of the third ventricle and serves to relay impulses and esp. sensory impulses to and from the cerebral cortex

* * *

n. (pl. thalami)

one of two egg-shaped masses of grey matter that lie deep in the cerebral hemispheres in each side of the forebrain. The thalami are relay stations for all the sensory messages that enter the brain, before they are transmitted to the cortex. All sensory pathways, except that for the sense of smell, are linked to nuclei within the thalamus, and it is here that the conscious awareness of messages as sensations - temperature, pain, touch, etc. - probably begins.

* * *

thal·a·mus (thalґə-məs) pl. thalґami [L., from Gr. thalamos inner chamber] [TA] a large ovoid mass in the posterior part of the diencephalon forming most of each lateral wall of the third ventricle, composed chiefly of gray substance and associated laminae of white substance. It is divided into anterior, medial, and lateral parts, each part containing groups of nuclei that function as relay centers for sensory impulses and cerebellar and basal ganglia projections to the cerebral cortex. The main groups of thalamic nuclei are the reticular, anterior, median, medial, medullary, intralaminar, ventrolateral, and posterior nuclei. Some authorities consider the subthalamus part of the thalamus and refer to it as the ventral thalamus, calling the posterior part the dorsal thalamus. thalamic adj

Thalamus and adjacent structures in a median sagittal section of the diencephalon and part of the brainstem.