Thalamus

The thalamus is a large constellation of nerve cells that form an ovoid mass of grey matter in the base of the brain. It is located between the midbrain (mesencephalon) and the cerebral cortex. In other words, the thalamus is a paired mass of grey matter that lies at the base of each cerebral hemisphere and it is the main part of the diencephalon. As a matter of fact, it makes up four-fifths of the entire diencephalon. Function: in humans, it acts as a relay station for sensory information transmitted from the lower parts of the CNS to the cerebral cortex; it also has somatic motor control.

Each thalamus is approximately egg-shaped with a flattened side toward the third ventricle. Lateral to the thalamus lies a thick sheet of white matter, the internal capsule (capsula interna). The thalamus is a nucleus in which afferent nerve impulses coming from all the sense organs (except the olfactory organs) gather. Each type of sensory pathway has its own specific nuclei (lateral group) in which the impulses are transmitted from one nerve cell to another and are transferred to the appropriate zone of the cerebral cortex.

Associative nonspecific nuclei (medial group) receive stimuli from specific nuclei of the thalamus and from nonspecific structures of the diencephalon, mesencephalon, and medulla oblongata and transmit them to various subcortical and cortical neurons. The thalamus is also involved in primary analysis and synthesis of all stimuli entering the brain from the neuroreceptors. In lower vertebrate animals the thalamus ensures performance of all necessary reflexes; in mammals and man the higher center of integration is the cerebral cortex. Under the thalamus lies the hypothalamus.