Low frequency depression of the H wave in normal and spinal man

Abstract

The effects of stimulus repetition rate on the H wave amplitude of intact and spinal cord injury patients were studied. Stimulus repetition rates ranging from 1 shock/10 sec to 10 shock/sec were employed. Each stimulus train consisted of twenty-five repetitive shocks. Summated responses were obtained by use of a computer of average transients and an X-Y plotter. In the intact subject, the H wave displayed a progressive reduction in amplitude at increasing rates of repetitive stimulation. The profile of depression in this frequency-amplitude series was similar to the low frequency depression that occurs in monosynaptic reflex pathways of anesthetized, decapitated and spinal animals when subjected to repetitive stimulation over the same frequency range. A variety of evidence is presented to suggest the same depressive phenomenon is operative in the H wave of man and the monosynaptic reflex of experimental animals. The H wave of spinal cord injury patients displayed virtually no low frequency depression on days of diminished spasticity. On days of heightened spasticity H wave amplitude was low and low frequency depression was evident. These results are thought compatible with the theory that low frequency depression is mediated by interneuronal circuits, including possibly the Renshaw cell system. These circuits would appear hypoactive on days of diminished spasticity and hyperactive on days of heightened spasticity. Evidence from a variety of sources is presented to support this theory.