others allow large fluxes of sodium that can trigger axonal degeneration. The development of medications that selectively enhance or inhibit the actions of specific subtypes of sodium channels may make it possible to adjust the balance of the channels to preserve normal axon function without silencing them.
The loss of sensory modalities can be as debilitating as the loss of motor function. Although sensory function was not previously a substantial focus of spinal cord injury research, scientists are now making progress in understanding what contributes to the loss of sensation and developing treatments to restore sensory modalities, including touch, temperature, pain, proprioception, and feedback control of movements.
Proprioception is an often overlooked sensation that is critical in coordinating walking and other movements (Box 5-1). Muscles and joints have special sensory neurons designed to signal the CNS about muscle length, the velocity of movements, and the load (or force) being applied. This sensory input is continually used to convey positional sense (awareness of position of the body in space), to trigger spinal reflexes, and to prepare for effective control over movement. Sensory neurons carrying proprioceptive information course from the muscles and joints directly into the spinal cord. There they project to motor neurons in the spinal cord or they course to the brain (through several synapses). The fibers forming the first part of the pathway, from the muscles to the spinal cord, appear to possess recep-
One of the defects of spinal cord injury not often discussed or appreciated is loss of proprioception. As a C5-6 quadriplegic, I have no sense of where my lower limbs are placed and a minimal sense of the positioning of my upper extremities. I can move my arms and legs and can actually walk with braces and someone making sure I don’t fall over because of proprioception/balance. I move my legs particularly only if I can see where they are and where they are going. I literally cannot move my legs without visual sensing of position. I expect that lack of proprioception is an important aspect of motor function and its return after spinal cord injury is an important aspect of regaining function.
—Robert Schimke, Professor Emeritus of Biology, Stanford University