Signals of sensory stimuli synapse first on dorsal root ganglia where they then form first-order neurons that travel up the dorsal columns of the spine (fasciculus gracilis or cuneautus) before synapsing again on the medial lemniscus to form second-order neurons. This is called the “dorsal column-medial lemniscus pathway” or DC-ML. Fibers for pain sensation cross within 3 vertebral levels in the spine, while light touch sensation crosses at the level of the medulla. After traveling up to the contralateral medial lemniscus, the fibers synapse again at the ventral posterolateral nucleus of the thalamus (VPL) to form third-order neurons that move into the cortex.
The ventral posteromedial (VPM) thalamic nucleus receives afferent input from the contralateral spinal trigeminal nucleus, providing pain and temperature sensation of the face. The lateral geniculate nucleus (LGN) receives afferent input from the visual pathways, and the medial geniculate nucleus (MGN) receives afferent input from the auditory system. The ventral lateral nucleus (VL) receives input from the deep cerebellar nuclei.