Interlimb reflexes following cervical spinal cord injury in man.

The reflex interconnection of lower and upper extremity muscles was investigated in subjects with chronic (greater than 1 year post-injury) lesions to the cervical spinal cord. Lower extremity mixed nerves were stimulated with single shocks or with brief trains of high-frequency stimuli of varying intensities. EMG from a number of lower and upper extremity muscles was recorded on magnetic tape for later analysis. In one population of spinal cord injury (SCI) subjects, single stimuli to lower extremity nerves resulted in muscle responses in both ipsi- and contralateral upper extremity muscles. The minimal response latency to a single shock was typically much less in muscles on the ipsilateral side than for contralateral upper extremity muscles. Application of brief trains of stimuli (for example, 2 stimulus pulses at 500 Hz) typically resulted in a large reduction in latency to the contralateral motor response, such that it was now approximately equal to the ipsilateral motor response latency. This decline in response latency was not gradual with increasing afferent input. Instead, the response occurred either early or late, but not at intermediate latencies. Stimuli which were subthreshold for evoking M-waves or H-reflexes were sometimes still adequate to evoke upper extremity motor responses. Once the threshold had been exceeded, the magnitude of the evoked response appeared to be independent of the stimulus magnitude. These reflex interconnections of lower and upper extremities were obtained only from subjects with chronic and motor-complete cervical spinal cord injury. No such interlimb responses were seen in control subjects, or in subjects who had recovered some motor function below the level of their injury, and were now considered to be motor-incomplete quadriplegics.