Point 1










Point 1.  Dorsal Column System

The spinal cord is comprised of an outer zone of white matter and a butterfly-shaped central component of cells and fibers (grey [or gray] matter). The peripherally located white matter consists of three funiculi or columns (funiculus = L., little cord) dorsal, lateral and ventral. I want to focus now on the ascending sensory pathways within the dorsal funiculus, called the dorsal column system. Don't worry about anything else in the diagram below at this time, like abbreviations associated with other pathways!!!

Dorsal Columns at T1All incoming (afferent) information to the spinal cord is conveyed via the dorsal root fibers. Cells in dorsal root ganglia (DRG) possess two processes, one that passes peripherally to pick up information from a sensory receptor and one that passes centrally into the spinal cord. In the case of the dorsal column system, these axons are called alpha-beta fibers.

Receptors and Alpha-Beta FibersAlpha-beta axons are myelinated and measure from 6-12um in diameter. Their peripheral processes possess specialized receptors such as Meissner's corpuscles, Merkel's (tactile) discs and Pacinian corpuscles. Meissner's corpuscles are primarily velocity detectors (movement across the skin) while Merkel's discs are primarily touch pressure receptors (how close the two points of a caliper are). Pacinian corpuscles are velocity detectors and sense vibration. The above three receptors lie in the skin and can account for two point discrimination and vibration. Conscious proprioception (L. proprius = one's own; ceptor = a receiver) is the ability to tell the position of one's limb (is the arm bent or straight??) with the eyes closed. Ruffini corpuscles within joint capsules might convey such information.