Thalamus

The thalamus (from Greek thálamos = bedroom, chamber) is a part of the brain. The two thalami are located in the center of the brain, one beneath each cerebral hemisphere and next to the third ventricle. Functionally the thalami can be thought of as relay stations for nerve impulses carrying sensory information into the brain; the thalami receive these sensory inputs as well as inputs from other parts of the brain and determine which of these signals to forward to the cerebral cortex.

Anatomy
The thalami are prominent bulb-shaped regions, typically slightly more than 1 cm in length in humans. They exist on each side of the paleopalliar diencephalon that emerges from the mammalian prosencephalon. The thalamus is largely made of nuclear groups that relate to specific functions in the brain (see List of thalamic nuclei). Thalamic nuclei have subcortical projections, and can be classified as either relay nuclei or association nuclei (see below). Thalamic nuclei also have strong reciprocal connections with the cerebral cortex, forming cortico-thalamo-cortical recurrent loops that are believed may be involved with consciousness.

The two thalami adhere (but do not connect) to one another through the massa intermedia (or interthalamic adhesion).

Function
Traditionally, the thalamus has been thought of as a "relay" that simply forwards signals from auditory, somatic, visceral and visual systems (though not olfactory) to the cerebral cortex. However, newer research suggests that thalamic function may be more complicated. It is currently believed that the thalamus also modulates sensory signals to and from cortex.

Thalamic nuclei can be classified as either "relay nuclei" or "association nuclei" on the basis of the source of their driving inputs, whether they are subcortical or cortical. Relay nuclei receive their driving inputs from subcortical sources including ascending sensory afferents (medial lemniscus for somatosensory information, optic tract for visual information, etc...) and project predominantly to primary sensory cortical areas. On the other hand, association nuclei receive their driving inputs from other cortical areas. (See Sherman and Guillery's "Exploring the Thalamus", 2002)

Alternative ways for subdividing thalamus are also coming into vogue. For example, Ted Jones has recently proposed a Matrix-Core model for the thalamus, which is not subdivided based on nuclei, but rather on chemically-defined populations ofneurons. Specifically, Jones proposes the calbindin immunopositive neurons constitute a "matrix", whereas parvalbumin immunopositive neurons form the "core". In Jones' scheme, the "matrix", which includes much of the intralaminar nuclei, is involved in state functions (arousal, level of attention, mood), whereas the "core" is involved in discriminative sensory-motor functions.

The thalamus plays a major role in regulating arousal, the level of awareness and activity. An animal with a severely damaged or severed thalamus suffers permanent coma.

Pathology
Cerebrovascular accidents (strokes) can cause thalamic syndrome, which results in a burning or aching sensation on one half of a body, often accompanied by mood swings.