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 cu-motoneuronal activity, we studied the electrophysiol ogical features of a monosynaptic H-reflex in anaesthetised intact rat s, anaesthetised and non-anaesthetised rats transected at the level of the obex. Electrical stimulation of the tibial nerve at the ankle eli cited an Ii-reflex, an F-wave and a direct motor (M) response in the p lantaris muscles of all preparations. The H-reflex and F-wave exhibite d very similar latencies. The H-reflex had a low threshold and a const ant latency. Its amplitude increased as a function of stimulus intensi ty to reach a maximum value but then decreased when the stimulus inten sity was further increased. It could follow high rates of stimulation without any change in shape or latency. The F-wave had a lower amplitu de which together with its latency varied from one stimulus to the nex t. It appeared with intensities of stimulation that elicited an almost maximal M-response and did not decrease when the stimulation was incr eased. 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' tend on and following nociceptive stimulation of the flexor reflex afferent s. This model could be used for assessing any potential direct effect on motoneurones of a physiological or pharmacological conditioning pro cedure. (C) 1998 Elsevier Science B.V.