PRESYNAPTIC AND POSTSYNAPTIC MECHANISMS UNDERLYING H-REFLEX CHANGES PRODUCED BY A SELECTIVE VOLUNTARY CONTRACTION

Concurrent recordings of (i) the so[eus H reflex and (ii) the underlyi ng afferent (P-1) and efferent (P-2) neural volleys were performed dur ing a protracted, moderate, isometric, voluntary contraction of the so leus (S) muscle, and the subsequent release period. Besides the expect ed enhancement of the H reflex, muscular contraction caused a signific ant reduction in the corresponding central delay (as extrapolated from variations of P-1-P-2 interval), while the opposite trend occurred du ring the release phase. Control experiments, based on (a) neural block ade below the stimulation site, (b) muscle stretching at the end of th e muscular contraction, (c) changes in amplitude of homonymous and het eronymous S responses, and (d) variations in effectiveness of homonymo us and heteronymous conditioning volleys on the S motoneuronal pool, s howed that both voluntary contraction and the subsequent release perio d are associated with a reduced effectiveness of la afferents, while p ostsynaptic motoneuronal responsiveness is significantly modified only during the actual contraction time. (C) 1998 John Wiley & Sons, Inc.