CENTRAL-NERVOUS-SYSTEM PLASTICITY AFTER SPINAL-CORD INJURY IN MAN - INTERLIMB REFLEXES AND THE INFLUENCE OF CUTANEOUS STIMULATION

In persons who have sustained severe injuries to the cervical spinal c ord, electrical stimulation of mixed peripheral nerves in a lower limb can evoke short-latency, bilateral motor responses in muscles of the distal upper limbs; such motor responses have been termed interlimb re flexes. In the present study, we investigated the role that cutaneous stimulation plays in evoking interlimb reflexes. Fifteen subjects with chronic injury (> 1 year) to the cervical spinal cord were investigat ed. Single motor unit activity was recorded from a number of distal up per limb muscles. The lower limb cutaneous area within which stimulati on recruited a given motor unit of the upper limb was defined as that motor unit's 'receptive field'. Activity from a total of 48 single mot or units was analyzed, The majority of motor units responded to light touch, individual hair movement, and thermal (hot and cold) stimulatio n. Excitatory responses were observed bilaterally, although contralate ral responses predominated. Stimulation occasionally resulted in inhib ition of a spontaneously active motor unit. Receptive fields varied a great deal in size, with proximal locations being larger than those en countered in more distal lower limb locations (i.e. the toes). The spi nocervical tract is a possible candidate for mediating some portion of these interlimb reflexes, the origin of which may be due to new growt h (regenerative sprouting) in the spinal cord caudal to a severe injur y.