An electrophysiological investigation into the monosynaptic H-reflex in the rat

In order to set up a non-invasive, reliable and reproducible model for investigating alpha-motoneuronal activity, we studied the electrophysiological features of a monosynaptic H-reflex in anaesthetised intact rats, anaesthetised and non-anaesthetised rats transected at the level of the obex. Electrical stimulation of the tibial nerve at the ankle elicited an H-reflex, an F-wave and a direct motor (M) response in the plantaris muscles of all preparations. The H-reflex and F-wave exhibited very similar latencies. The H-reflex had a low threshold and a constant latency. Its amplitude increased as a function of stimulus intensity to reach a maximum value but then decreased when the stimulus intensity was further increased. It could follow high rates of stimulation without any change in shape or latency. The F-wave had a lower amplitude which together with its latency varied from one stimulus to the next. It appeared with intensities of stimulation that elicited an almost maximal M-response and did not decrease when the stimulation was increased. It did not appear systematically from one stimulus to the next. The H-reflex, but neither the F-wave nor the direct motor M-response, was depressed both by vibratory stimuli applied on the Achilles' tendon and following nociceptive stimulation of the flexor reflex afferents. This model could be used for assessing any potential direct effect on motoneurones of a physiological or pharmacological conditioning procedure.