Central nervous system plasticity after spinal cord injury in man: interlimb reflexes and the influence of cutaneous stimulation

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In persons who have sustained severe injuries to the cervical spinal cord, 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 reflexes. 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 investigated. Single motor unit activity was recorded from a number of distal upper limb muscles. The lower limb cutaneous area within which stimulation 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 motor units was analyzed. The majority of motor units responded to light touch, individual hair movement, and thermal (hot and cold) stimulation. Excitatory responses were observed bilaterally, although contralateral responses predominated. Stimulation occasionally resulted in inhibition of a spontaneously active motor unit. Receptive fields varied a great deal in size, with proximal locations being larger than those encountered in more distal lower limb locations (i.e. the toes). The spinocervical tract is a possible candidate for mediating some portion of these interlimb reflexes, the origin of which may be due to new growth (regenerative sprouting) in the spinal cord caudal to a severe injury.