thalami

The thalami are bilateral, paired nuclei situated deep within the brain, forming the thalamus on either side of the third ventricle. They are large gray-matter structures that act as major relay and processing stations for a wide range of neural pathways, receiving input from the sensory systems, cerebellum, basal ganglia, and limbic structures, and projecting extensively to the cerebral cortex. Most sensory information (except olfaction) passes through the thalamus before reaching the cortex, where it is further processed and integrated.

Anatomy and nuclei: The thalamus consists of several groups of nuclei, commonly described as anterior, medial, lateral, and posterior groups, plus intralaminar and midline nuclei. Key relay nuclei include the ventral posterior complex (VPM and VPL) that transmit somatosensory signals to the primary somatosensory cortex; the lateral geniculate nucleus (LGN) for vision; and the medial geniculate nucleus (MGN) for audition. Motor-related nuclei such as the ventral anterior and ventral lateral relay information from the basal ganglia and cerebellum to the motor cortex. The pulvinar participates in visual attention and multisensory integration, while the intralaminar and mediodorsal nuclei are involved in arousal, attention, and higher cognitive and limbic functions. The anterior nucleus links to limbic circuits via the cingulate and hippocampal networks.

Function and clinical relevance: The thalami coordinate thalamocortical circuits that regulate sensation, movement, perception, and cognition, and contribute to alertness and sleep–wake states. Lesions can cause contralateral sensory loss, thalamic pain syndrome, or thalamic aphasia in dominant-hemisphere injury. Bilateral damage can produce impaired consciousness. Vascular lesions commonly affect perforating branches of the posterior cerebral and posterior communicating arteries.

Developmental and imaging notes: The thalami develop from the diencephalon and are routinely assessed in neuroimaging for stroke, demyelination, tumors, or thalamic syndromes.