Modulation of reflex responses during fictive locomotion in the forelimb of the cat.

During cat forelimb fictive locomotion, short-latency reflex pathways were examined by recording nerve discharges and intracellularly from motoneurones. Stimulation of cutaneous afferents, superficial radial nerves, evoked trisynaptic excitation of the elbow flexors, biceps brachii and brachialis, and stimulation of muscle afferents, deep radial nerves, evoked oligosynaptic, i.e. monosynaptic and disynaptic excitation of the flexors. The short-latency excitatory postsynaptic potentials (EPSPs) evoked from both nerves were rhythmically modulated; they were facilitated during the flexion phase and suppressed during the extension phase. Stimulation of high threshold muscle afferents evoked EPSPs with a central delay of ca. 4.2 ms, which were depressed throughout episodes of fictive locomotion. Since the short-latency EPSPs and longer-latency EPSPs in the same motoneurone were differently influenced during fictive locomotion, the effects observed could not be explained by changes occurring at only the motoneuronal level but they probably occurred at the premotoneuronal level. In addition, short-latency cutaneous excitation of the distal muscles, innervated by the median and ulnar nerves, was little modulated during fictive locomotion.