The reflex effects of nonnoxious sural nerve stimulation on human triceps surae motor neurons

1. The effects of low-intensity electrical stimulation of the ipsilateral sural nerve on the reflex response of human triceps surae motor neurons were examined in 169 motor units recorded in 11 adult volunteers: 69 units from soleus (SOL), 48 units from lateral gastrocnemius (LG), and 52 units from medial gastrocnemius (MG). The reflex effects were assessed by the peristimulus time histogram (PSTH) technique, categorized according to onset latencies, and the magnitudes of effects were calculated as percent changes in baseline firing rates. 2. Sural stimulation evoked complex changes in motor-unit firing at onset latencies between 28 and 140 ms. The two most common responses seen in all muscles were a short-latency depression (D1) in firing (mean onset latency = 40 ms) in 42% of all units studied and a secondary enhancement (E2) in firing (mean onset latency = 72 ms) in 43% of all units. In LG, the D1 effect represented a mean decrease in firing of 52% which was statistically different from the changes in MG (42% decrease) and SOL (38% decrease). The magnitudes of E2 effects were similar across muscles with an average of 47% increase in firing. 3. No differences were found in the frequencies of occurrence for the enhancements in firing among the muscles studied. The main difference in reflex responses was the occurrence of an intermediate latency depression (D2) in 27% of the LG units with a mean onset latency of 72 ms. 4. Based on estimates of conduction times for activation of low-threshold cutaneous afferents, the short-latency D1 response likely represents an oligosynaptic spinal reflex with transmission times similar to the Ia reciprocal inhibitory pathway. These findings raise the question as to the possibility of low-threshold cutaneous afferents sharing common interneurons with low-threshold muscle afferent reflexes that have identical onset latencies. The complex reflex effects associated with low-level stimulation of a cutaneous nerve indicate a rich assortment of peripheral responses that may influence a given movement. The predominance of a specific effect is most likely determined by the interaction of this input with other peripheral signals and descending commands specific to a given motor task.