Journal of Neurophysiology, Vol 39, Issue 5 1105-1116, Copyright © 1976 by APS
Reflex responses of human soleus muscle to small perturbations
R. B. Stein and P. Bawa
1. A small branch of the nerve to soleus muscle in normal human subjects
was stimulated intramuscularly with a needle electrode while the subjects
were maintaining a steady voluntary contraction. The EMG and force
fluctuations produced by these stimuli were recorded and averaged. 2. In
addition to the M-wave produced directly by stimulating motoneurons and the
resultant twitch contraction, one or more EMG waves were seen with a
latency greater than 100 ms. These later waves produced further
contractions, and when there were several later waves, the EMG and force
fluctuations appeared as a damped oscillation with a frequency between 5.5
and 8 Hz. 3. By varying the angle of the ankle and hence the time course of
the twitch contraction, the timing of the latter waves was shown to be
closely related to the contraction time. Thus, the later waves appeared to
be produced reflexly by the tension fluctuations, rather than directly by
the stimulus. 4. The frequency response function between the tension
fluctuations and the reflux EMG responses was computed. The gain agreed
with that of primary muscle spindle afferents, but the phase data showed
extra lags consistent with a time delay which was too long to be spinal in
origin. This reflex probably involves supraspinal centers.
