The reflex response to sinusoidal stretching of soleus in the decerebrate cat

Abstract

The soleus muscle in the decerebrate cat was stretched sinusoidally through various distances, at various frequencies while tension and electromyographic (emg) activity were recorded. There were 2 patterns of stretch reflex activity: slow stretching led to a large increase in tension in one, whereas tension increased little during slow stretching, but rose steeply during a more rapid stretch in the other. Intermediate states were also seen. Both reflex patterns were abolished when the fusimotor fibers were blocked with Xylocaine. The emg activity was greatest at peak muscle length at low frequencies of stretching; the greatest activity was found during lengthening at higher frequencies. This angular advance of emg activity on length was greatest at 3.3-5 c/s. The emg activity was less in advance of muscle length at the higher frequencies. The angular advance of emg activity was greatest in the preparations that were also the most sensitive to slow stretching. Muscle contraction follows an electrical stimulus with a small delay. The delay between the end of stimulation and the end of muscle relaxation is longer, and may exceed 200 msec in the cat soleus. The timing of the reflex tension during sinusoidal stretching depended on how far the angular advance of the emg combined with the damping properties of muscle offset the time delays in the reflex pathway. Changes in muscle tension generally preceded changes in muscle length. A phase delay of tension was sometimes seen when stretch amplitudes of 1 mm (peak to peak) were used. This only occurred in preparations that also showed little sensitivity to slow stretching. Phase delay of tension usually found with frequencies of approximately 6-8 c/s, corresponds to the clonus frequency for the soleus.