Thalamus

The two thalami sit on either side of the third ventricle, above the brainstem and beneath the cerebral hemispheres. Rather than passively forwarding signals, the thalamus filters and modulates them, gating which inputs reach the cortex. It also participates in motor circuits, attention, arousal, and sleep-wake regulation through extensive reciprocal connections with the cortex.

Function is organized by nucleus. The ventral posterolateral (VPL) nucleus relays body sensation from the spinothalamic and dorsal column-medial lemniscus pathways, and the ventral posteromedial (VPM) nucleus relays face sensation from the trigeminal system along with taste; both project to the primary somatosensory cortex. The lateral geniculate nucleus handles vision and projects through the optic radiations to the primary visual cortex, while the medial geniculate nucleus handles hearing and projects to the auditory cortex — remembered as lateral for light, medial for music. The ventral lateral and ventral anterior nuclei receive cerebellar and basal ganglia output and project to motor and premotor cortex, and the anterior and mediodorsal nuclei serve limbic circuits involved in memory and emotion.

Olfaction is the notable exception: smell reaches the olfactory cortex directly without a thalamic relay. Blood supply comes largely from branches of the posterior cerebral artery, and a lesion of the VPL region can produce Dejerine-Roussy syndrome, in which initial contralateral sensory loss is followed by severe, poorly localized burning pain.

USMLE Step 1 tests thalamic nuclei in detail, pairing each with its modality, cortical target, and lesion effect, and connects the ventral nuclei to basal ganglia pathways. The MBLEx covers the thalamus more broadly within nervous system organization, and the NPTE for physical therapist assistants includes it in central nervous system anatomy relevant to sensory and motor deficits after stroke.