Recovery cycle of the reflex-evoked muscle potential (H reflex): excitability of spinal motor neurons in the healthy dog.

The recovery cycle for a reflex-evoked muscle potential has been demonstrated in the healthy dog, using the ulnar nerve-interosseous muscle preparation. The amplitude of a test response, at various time intervals (2 to 1,000 ms) after a conditioning response, was compared with the amplitude of the unconditioned response. The resulting ratio was plotted as a function of interstimulus interval. The curve was shown to consist of 3 distinct phases. The 1st phase, called the phase of early responsiveness, lasted from 2 to 20 ms and was characterized by a rapid decrease in amplitude of the test response. The 2nd phase, the phase of unresponsiveness, was characterized by a reduced response, and eventually no response which occurred between 20 and 80 ms. The 3rd phase, the phase of late responsiveness, began with a small test reflex amplitude at 80 ms and increased to 200 ms where it plateaued thereafter. Between 200 and 1,000 ms, there was no significant change in amplitude. Only at 2 ms, where the conditioning and test responses were so close together that they fused, did the mean amplitude of the test response exceed the mean unconditioned response. The reflex-evoked muscle potential recovery curve provides additional proof that the reflex is the equivalent of the H-reflex, and it provides a quantitative measure of spinal cord lower motor neuron excitability in the dog.