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The dorsal thalamus, usually simply referred to as the thalamus is a subdivision of a brain area called the diencephalon, which also includes the eptithalamus, the ventral thalamus and the subthalamic thalamus.
The thalamus relays sensory impulses from receptors in various parts of the body to the cerebral cortex. A sensory impulse travels from the body surface towards the thalamus, which receives it as a sensation. This sensation is then passed onto the cerebral cortex for interpretation as touch, pain or temperature.
The prevailing opinion among experts is that the thalamus serves as a kind of “gate,” filtering which information from various channels is allowed to be relayed by it for processing. Detailed research has shown that specific thalamic neurons can modulate informational transfer by employing specific discharge modes.
Furthermore, the thalamus is crucial for perception, with 98% of all sensory input being relayed by it. The only sensory information that is not relayed by the thalamus into the cerebral cortex is information related to smell (olfaction).
The thalamus is made up of three distinct groups of cells or nuclei and these include the following:
Aside from olfaction, in all sensory processing, a sensory signal is received by a thalamic nucleus and then directed to the relevant cortical area.
In the visual system, the thalamus receives input from the retina, which is relayed to the brain via the optic nerve. Signals are sent to the lateral geniculate nucleus of the thalamus which then forwards them onto the primary visual cortex (area V1) in the occipital lobe. The thalamus not only relays the information but also processes it, as each of the primary sensory relay areas receives information back from the cerebral cortex (called back projections).
On the other hand, the medial geniculate nuclei relay auditory information between the inferior colliculus of the midbrain and the primary auditory cortex. The thalamus is also important for touch - the ventral posterior nuclei relay information regarding touch and perception of bodily position to the primary somatosensory cortex in the cerebral cortex.
In addition, the thalamic nuclei are strongly and reciprocally linked with the cerebral cortex. These form the thalamo-cortico-thalamic circuits which are thought to regulate consciousness and the thalamus plays a significant role in arousal, wakefulness and alertness. Damage to the thalamus is associated with a significant risk of coma.