The modulation of presynaptic inhibition in single muscle primary afferents during fictive locomotion in the cat

The aim of this study is to understand the functional organization of presynaptic inhibition in muscle primary afferents during locomotion. Primary afferent depolarization (PAD) associated with presynaptic inhibition was recorded intra-axonally in identified afferents from various hindlimb muscles in L6-L7 spinal segments during fictive locomotion in the decerebrate cat. PADs were evoked by the stimulation of peripheral muscle nerves and were averaged in the different epochs of the fictive step cycle. Fifty-three trials recorded from 39 muscle axons (37 from group I and two from group II) were retained for analysis. The results showed that there was a significant phase-dependent modulation of PAD amplitude (p < 0.05) in a majority of muscle afferents (30 of 39, 77%). However, not all stimulated nerves led to significantly modulated PADs in a given axon (36 of 53 trials, 68%). We also observed that the pattern of modulation (phase for maximum and minimum PAD amplitude and the depth of modulation) varied with each recorded afferent, as well as with each stimulated nerve. We further evaluated the effect of PAD modulation on the phasic transmission of the monosynaptic reflex (MSR) and found that PADs decreased the MSR amplitude in all phases of the fictive step cycle, independent of the PAD pattern in individual group I fibers. We conclude that (1) PAD modulation patterns of all group I fibers contacting motoneurons led to an overall reduction in monosynaptic transmission, and (2) individual PAD patterns could participate in the control of transmission in specific reflex pathways during locomotion.