Because the thalamus is connected with almost all parts of the central nervous system, including the brainstem and the cerebral cortex, the thalamus influences many different processes in the brain. The thalamus is a part of almost every sensory pathway where it serves as a major sensory relay station. It receives input from various locations in the body, combines and processes this information, and then typically forwards it to the cerebral cortex. The thalamus plays a role in modulating movement through its connections with the basal ganglia, cerebellum, and frontal lobe. It can also influence motivated behaviors (i.e., behaviors in pursuit of a goal, such as seeking food when hungry) via connections with the hypothalamus and the frontal lobe. The thalamus can also alter levels of consciousness by communicating with the reticular formation of the brainstem.
The thalamus is composed of densely packed neuronal cell bodies known as thalamic nuclei. The different thalamic nuclei are involved in the various functions of the thalamus. One of the most important nuclei is the ventral posterolateral (VPL) nucleus, which transmits sensory signals (e.g., vibration, pain, pressure, proprioception, light touch, and temperature) from the
medial lemniscus and the
spinothalamic tract to the
somatosensory cortex, which is a structure located in the
parietal lobe of the brain that is involved in the of processing of various sensory signals. Additionally, there is the ventral posteromedial (VPM) nucleus, which is involved in processing sensations from the face and is also involved in taste sensation (i.e., receives information from the
trigeminal and the
gustatory pathways and projects to the somatosensory cortex). Another
thalamic nucleus is the ventral lateral (VL) nucleus, which relays motor information and can influence movements. The
medial geniculate nucleus of the thalamus projects to the
primary auditory cortex of the
temporal lobe and is involved in hearing. Finally, the
lateral geniculate nucleus receives input from the
retina via the
optic nerve fibers, optic chiasm, and optic tract and then projects to the
visual cortex of the
occipital lobe via the optic radiation.