In the dorsal roots, the sensory fibers are first rearranged according to function. Large and heavily myelinated fibers enter the cord just medial to the dorsal horn and divide into ascending and descending branches. The descending fibers and some of the ascending ones enter the gray matter of the dorsal horn within a few segments of their entrance and synapse with nerve cells in the posterior horns as well as with large ventral horn cells that subserve segmental reflexes. Some of the ascending fibers run uninterruptedly in the dorsal columns of the same side of the spinal cord, terminating in the gracile and cuneate nuclei in the upper cervical spinal cord and medulla (Fig. 8-5). The central axons of the primary sensory neurons are joined in the posterior columns by other secondary neurons whose cell bodies lie in the posterior horns of the spinal cord. The fibers in the posterior columns assume a medial position as new fibers from each successively higher root are added laterally, thereby creating somatotopic laminations (see Fig. 7-3).
Of the long ascending posterior column fibers, which are activated by mechanical stimuli of skin and subcutaneous tissues and by movement of joints, only about 25 percent (from the lumbar region) reach the gracile nuclei at the upper cervical cord. The remaining fibers send collaterals to, or terminate in, the dorsal horns of the spinal cord, at least in the cat (Davidoff). An estimated 20 percent of ascending posterior column fibers originate from cells in Rexed layers IV and V of the posterior horns (see Fig. 7-1) and convey impulses from low-threshold mechanoreceptors that are sensitive to hair movement, skin pressure, or noxious stimuli. There are also descending fibers in the posterior columns, including fibers from cells in the dorsal column nuclei.
The posterior columns contain a portion of the fibers for the sense of touch as well as the fibers mediating the senses of pressure, vibration, direction of movement and position of joints, which are also necessary for the integrative sensory experience of stereoesthesia—recognition of surface texture, shape, numbers, and figures written on the skin and two-point discrimination. The nerve cells of the nuclei gracilis and cuneatus and accessory cuneate nuclei give rise to a secondary afferent path, which crosses the midline in the medulla and ascends as the medial lemniscus to the posterior thalamus (Fig. 8-5). However, the fiber pathways in the posterior columns are not the sole mediators of proprioception in the spinal cord (see “Posterior [Dorsal] Column Syndrome” further on).
In addition to the well-defined posterior column pathways, there are cells in the more loosely structured “reticular” part of the dorsal column that receive secondary ascending fibers from the dorsal horns of the spinal cord and from ascending fibers in the posterolateral columns. These dorsal column fibers project to brainstem nuclei, cerebellum, and thalamic nuclei. Many other cells of the dorsal horn nuclei are interneurons, with both excitatory and inhibitory effects on local reflexes or on the primary ascending sensory neurons. The functions of many of the extrathalamic projections of dorsal column cells are unknown (Davidoff).