Spinomesencephalic tract

The spinomesencephalic tract (SMT) is one of the three major components of the anterolateral system (ALS) in the central nervous system and serves as a key ascending pathway involved in the transmission of pain sensations. Originating primarily from neurons located in laminae I and IV–VI of the spinal cord's dorsal horn, the spinomesencephalic tract predominantly decussates (crosses the midline) and travels in the anterolateral funiculus to reach the mesencephalon (midbrain). However, some fibers originating from the upper cervical spinal levels may remain ipsilateral.

These fibers convey nociceptive information (pain and temperature) to several midbrain nuclei, including the periaqueductal gray, cuneiform nucleus, red nucleus, superior colliculus (via the spinotectal tract), pretectal nuclei, Edinger–Westphal nucleus, parabrachial nucleus, and the mesencephalic reticular formation. Among these, the periaqueductal gray plays a pivotal role in endogenous pain modulation, and the connections between the spinomesencephalic tract and the periaqueductal gray underscore the tract’s critical function in modulating the body's natural pain control mechanisms.