Reflex activation of muscle spindles in human pretibial muscles during standing.

1. Experiments were performed in standing subjects to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the human foot reflexly influence fusimotor neurons innervating pretibial flexor muscles. Recordings were made from 30 identified muscle-spindle afferents, four tendon-organ afferents, and one alpha-motor axon innervating the pretibial flexor muscles. The subjects stood without support or vision on a force platform while trains of electrical stimuli (5 stimuli, 300 Hz) were delivered at nonpainful intensities to the sural nerve or to the posterior tibial nerve at the ankle. 2. Seventeen of the 30 spindle endings had no background discharge, and none was activated by the sural or posterior tibial stimuli. Five silent afferents were given a background discharge by sustained pressure on the relevant tendon, but with two the discharge was dominated by a tremor rhythm obscuring any reflex response to the stimuli. Based on peristimulus time histograms (PSTHs), the sural stimuli then produced increases in discharge of two of the remaining three endings at latencies of 84 and 90 ms. These effects could not be explained by muscle stretch and are presumed to have been fusimotor mediated. 3. When the subjects stood freely without support or vision, 13 muscle-spindle endings had a background discharge, but with three endings tremor developed at the ankle and dominated the spindle discharge. Sural stimuli affected the discharge of five of nine endings unaffected by tremor. With three of these endings, there were changes in discharge that could be explained by muscle stretch.(ABSTRACT TRUNCATED AT 250 WORDS)