Anterior spinothalamic tract

The anterior spinothalamic tract is an ascending sensory pathway responsible for transmitting information about light touch and pressure from free nerve endings in the skin and other tissues. First-order neurons, with cell bodies in the dorsal root ganglia, transmit these sensory signals. Their axons enter the spinal cord and travel through the posterolateral tract of Lissauer, ascending or descending a few segments before synapsing with second-order neurons in the posterior gray horns, primarily within the substantia gelatinosa. Substance P is the neurotransmitter involved in this synaptic transmission.

The axons of second-order neurons cross to the contralateral side via the anterior gray and white commissures of the spinal cord, to ascend in the contralateral white column. Out of these, some nerve fibers cross within a single spinal segment and ascend in the lateral funiculus as the lateral spinothalamic tract. Others require several spinal segments to cross the midline and ascend in the anterior funiculus as the anterior spinothalamic tract.

In the medulla oblongata, the anterior spinothalamic tract merges with additional fibers, including the lateral spinothalamic and spinotectal tracts to constitute the spinal lemniscus. The spinal lemniscus (containing the anterior spinothalamic tract) continues through the posterior pons to enter the tegmentum of the mid-brain. They eventually relay the sensory information of light touch and pressure to the VPL (ventral posterior lateral) nucleus of the thalamus.

In the VPL nucleus, second-order neurons synapse with third-order neurons. Axons of these third-order neurons travel via thalamocortical projections through the corona radiata to the primary somatosensory cortex (postcentral gyrus, Brodmann areas 3, 1, and 2). Here, sensory information is spatially represented in an inverted manner (sensory homunculus), with the face and upper limbs projected on the lateral cortex and the lower limbs on the medial surface.