Basic anatomy OF THE THALAMUS

The thalamus is believed to both process and relay sensory information selectively to various parts of the cerebral cortex. The two thalami are prominent bulb-shaped masses about 5.7 cm in length, located on each side of the 3rd ventricle, and form the major part of the diencephalon. The thalamus is composed of a complex system of myelinated neurons separated by distinct clusters of neuron cell bodies, which form specific nuclei. Figure 9.1 shows the anatomical arrangement of the thalamus, with clusters of nuclei being divided into three main parts: anterior, medial and lateral, by the intralaminar nucleus. The lateral part is further sub-divided into dorsal and ventral tiers.

Function of the thalamus

Traditionally, the thalamus was considered to act as a relay station within the central nervous system subserving both sensory and motor mechanisms. However, the extent of its afferent and efferent connections with the periphery and the brain and the mass of interconnections between the thalamic nuclei implies that in addition, it has a significant role in controlling and modifying this information. More precisely, the corticothalamic system appears to synchronize the activity of thalamic and cortical neurons.

As a result of its widespread connections, damage to the thalamus may cause a range of presentations.

Function of the thalamic nuclei

The anterior part

This part is strongly connected to the limbic system (cingulate nucleus and hypothalamus) and is concerned with muscle tone related to our emotional state.

The medial part

Particularly the dorsomedial nucleus has reciprocal connections with the pre-frontal cortex in the frontal lobe, the hypothalamus and all other thalamic nuclei. Its role is the integration of all sensory, motor and visceral information entering the thalamus concerning our emotional state.

The medial and anterior parts also receive information from the spinothalamic tract (pain and temperature), which relates to the ‘medial pain system’ and projects to the anterior cingulate cortex. This area is consistently activated by noxious stimuli and has been associated with the affective–emotional aspect of pain.

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