Anterior spinothalamic tract

The anterior spinothalamic tract is an example of an ascending sensory tract. It transmits sensory information of light touch and pressure from free nerve ending receptors in the skin and other tissues. These are first order neurons whose cell bodies reside within the dorsal root ganglion. From here, the axons of first order neurons enter the apex of the posterior gray column of the spinal cord, where they ascend and descend for a few spinal segments. These ascending and descending fibers constitute the posterolateral trat of Lissauer, which synapse with cell bodies of the second order neurons in the posterior gray column, mainly the nucleus of substantia gelatinosa. Here they release a synaptic neurotransmitter, called substance P.

Next, the axons of these second order neurons cross obliquely–in front–via the anterior gray and white commissures of the spinal cord, to ascend in the contralateral white column. 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 ascends along with the lateral spinothalamic and spinotectal tracts to constitute the spinal leminiscus. Next, the spinal leminiscus (containing the anterior spinothalamic tract) passes through the posterior pons to enter the tegmentum of the mid-brain. Afterwards, the second order neurons of the anterior spinothalamic tract relay the light touch and pressure information into the VPL (ventral posterior lateral) nucleus of the thalamus.

The VPL nucleus contains cell bodies of third order neurons which project–as thalamocortical projections–through the corona radiata to reach the post-central gyrus (area 3, 1, 2, a.k.a. the primary sensory area). The sensory representation of the body is ‘inverted’ in the primary sensory cortex (a.k.a. sensory homunculus) with the face and upper limbs being represented on the posterolateral surface of cortex, while the legs being represented on the inner or medial surface.