Thalamus, metathalamus and epithalamus

The thalamus (thalamus dorsalis; syn.: posterior thalamus, optic tubercle) is a paired formation, having a shape close to ovoid, located on both sides of the third ventricle. In the anterior section, the thalamus narrows and ends in the anterior tubercle (tuberculum anterius thalami). The posterior end is thickened and is called the cushion (pulvinar).

Only two surfaces of the thalamus are free: the medial, facing the third ventricle and forming its lateral wall, and the superior, which takes part in the formation of the bottom of the central part of the lateral ventricle.

The upper surface is separated from the medial surface by the white thin medullary strip of the thalamus (stria medullaris thalamica). The medial surfaces of the posterior thalami, right and left, are connected to each other by the interthalamic fusion (adhesio interthalamica). The lateral surface of the thalamus is adjacent to the internal capsule. Inferiorly and posteriorly, the thalamus borders on the tegmentum of the peduncle of the midbrain.

The thalamus consists of gray matter, in which individual clusters of nerve cells are distinguished - the thalamic nuclei. These clusters are separated by thin layers of white matter. Currently, up to 40 nuclei are distinguished, which perform various functions. The main nuclei of the thalamus are the anterior (nuclei anteriores), medial (nuclei mediates) and posterior (nuclei posteriores). The processes of the nerve cells of the second (conductor) neurons of all sensory conductive pathways (except for the olfactory, gustatory and auditory) come into contact with the nerve cells of the thalamus. In this regard, the thalamus is practically a subcortical sensory center. Some of the processes of the thalamic neurons are directed to the nuclei of the striatum of the corpus striatum of the telencephalon (in this regard, the thalamus is considered a sensory center of the extrapyramidal system), and some - the thalamocortical bundles (fasciculi thalamocorticales) - to the cerebral cortex.

Under the thalamus is the so-called subthalamic region (regio subtalamica - BNA), which continues downwards into the tegmentum of the cerebral peduncle. This is a small area of the brain matter, separated from the thalamus by the hypothalamic groove on the side of the third ventricle. The red nucleus and the black substance of the midbrain continue into the subthalamic region of the midbrain and end there. The subthalamic nucleus (nucleus subthaldmicus, Lewis's body) is located on the side of the black substance.

The metathalamus (the postthalamic region) is represented by the paired lateral and medial geniculate bodies - paired formations. These are oblong-oval bodies that connect to the colliculi of the roof of the midbrain with the help of the handles of the superior and inferior colliculi. The lateral geniculate body (corpus geniculatum laterale) is located near the lower lateral surface of the thalamus, on the side of the pillow. It can be easily found by following the course of the optic tract, the fibers of which are directed to the lateral geniculate body.

Somewhat medially and posteriorly to the lateral geniculate body, under the cushion, is the paired medial geniculate body (corpus geniculatum mediale), on the cells of the nucleus of which the fibers of the lateral (auditory) loop end. The paired lateral geniculate bodies, together with the superior colliculi of the midbrain, are the subcortical centers of vision. The medial geniculate bodies and inferior colliculi of the midbrain form the subcortical centers of hearing.

The epithalamus (epithalamus; suprathalamic region) includes the pineal body, which is connected to the medial surfaces of the right and left thalamus by means of the habenulae. At the points where the habenulae pass into the thalami there are triangular expansions - the habenulae triangles (trigonum habenulae). The anterior sections of the habenulae before entering the pineal body form the commissure of the habenulae (commissura habenularum). In front and below the pineal body there is a bundle of transverse fibers - the epithalamic commissure (commissura epithalamica). Between the epithalamic commissure and the commissure of the habenulae, a shallow blind pocket - the pineal recess - projects into the anterior superior part of the pineal body, into its base.