Synaptic potentials contributing to reflex inhibition in gastrocnemius following tendon electrical stimulation

Objective: To investigate the synaptic potentials underlying reflex inhibition in gastrocnemius following electrical stimulation of the Achilles tendon using single motor unit recordings. Methods: Surface electromyography (SEMG) and single motor unit (SMU) action potentials were recorded from the medial head of left gastrocnemius muscle in eight healthy human subjects. The left Achilles tendon was stimulated electrically while subjects maintained a low contraction level sufficient to record one or two motor units. SMU responses were analysed using peri-stimulus time histogram (PSTH) and peri-stimulus frequencygram (PSF) techniques and compared with SEMG results. Results: A total of 22 SMU experiments were completed. In all trials, a large reflex inhibition (I1) was observed in both SEMG and PSTH followed by a later inhibition (I2). In 50% of trials, SEMG and PSTH showed that both I1 and I2 were followed by excitation (E1 and E2, respectively). PSF analysis showed only a prolonged reduction in SMU discharge rate. This inhibition had a latency of 38 ms and duration of 130 ms, extending into the E1 period in 14 units and more than 200 ms extending into the E2 period in six units. Conclusions: PSF data suggest that tendon electrical stimulation results in a long-lasting inhibition, most likely through the autogenic inhibitory reflex pathway mediated by group I tendon afferents. Significance: These findings emphasise the importance of using both probability (SEMG, PSTH) and frequency (PSF) based techniques in error free estimation of synaptic potentials. (C) 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.