<P> There are three neurons involved in the pathway: first - order neurons, second - order neurons, and third - order neurons . The first - order neurons reside in dorsal root ganglia and send their axons through the gracile fasciculus and cuneate fasciculus . The first - order axons make contact with second order neurons at the gracile and cuneate nuclei in the lower medulla . The second - order neurons send their axons to the thalamus . The third order neurons arise from thalamus to the postcentral gyrus . </P> <P> The posterior column is composed of gracile fasciculus and cuneate fasciculus . The gracile fasciculus carries input from the lower half of the body and the cuneate fasciculus carries input from the upper half of the body . The gracile fasciculus arise from the fibers more medial than the cuneate fasciculus . </P> <P> When the axons of second - order neurons of the PCML decussate in the medulla, they are called internal arcuate fibers . The crossings of the internal arcuate fibers form the medial lemniscus . </P> <P> The name comes from the two structures that the sensation travels up: the posterior (or dorsal) column of the spinal cord, and the medial lemniscus in the brainstem . </P>

Where do axons of secondary neurons of the anterior spinocerebellar tract decussate