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Medical Term:


Pronunciation: thal'a-mus, -mi

Definition: 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 taillike caudal part curves ventralward around the posterolateral aspect of the cerebral peduncle and ends in the lateral geniculate body. The thalamus 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 and 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 (cerebellothalamic fibers), and the pars reticulata of the substantia nigra that project to various regions of the motor cortex; 3) a nucleus associated with the limbic system, the composite anterior nucleus receiving the mammillothalamic tract and projecting to the fornicate gyrus; 4) association nuclei (medial dorsal nucleus and lateral nucleus including the large pulvinar), each projecting to a particular large expanse of association cortex; or 5) the midline and intralaminar nuclei or “nonspecific” nuclei (centromedian nucleus, central lateral nucleus, paracentral nucleus, and nucleus reuniens).

[G. thalamos, a bed, a bedroom]

See Also: dorsal thalamus

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