The nerve fibers entering the spinal cord consist of two groups -- the larger fibers subserving the modalities of discriminative touch sensation and those coming from the muscle and joint receptors, and the smaller fibers subserving pain and temperature, as well as light touch sensations.

The larger fibers enter more medially and give off some collaterals locally (including those for the stretch reflex). The main fiber turns upward and ascends in the dorsal area of the white matter, forming the dorsal columns.

The thinly-myelinated and unmyelinated axons enter the dorsal horn of the spinal cord in a more lateral position and form one or more synapses with the nuclear groups in the dorsal horn. Some of them may ascend (or descend) for up to 3 segments in a local pathway, known as the tract of Lissauer. After synapsing in the dorsal horn, the axons of the projection neurons cross in the ventral white commissure and ascend in the anterolateral system (lateral and anterior spinothalamic tracts).