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

1. Soleus muscle in the decerebrate cat was stretched sinusoidally through various distances, at various frequencies while tension and e.m.g. activity were recorded.2. Two patterns of stretch reflex activity were seen. In one, slow stretching led to a large increase in tension, whereas in the other tension increased little during slow stretching, but rose steeply during a more rapid stretch. Intermediate states were also seen.3. Both these reflex patterns were abolished when the fusimotor fibres were blocked with Xylocaine.4. At low frequencies of stretching the e.m.g. activity was greatest at peak muscle length; at higher frequencies the greatest activity was found during lengthening. This angular advance of e.m.g. activity on length was greatest at 3.3-5 c/s. At the higher frequencies the e.m.g. activity was less in advance of muscle length.5. Angular advance of e.m.g. activity was greatest in the preparations that were also the most sensitive to slow stretching.6. 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 cat soleus.7. The timing of the reflex tension during sinusoidal stretching depended on how far the angular advance of the e.m.g. combined with the damping properties of muscle offset the time delays in the reflex pathway.8. Changes in muscle tension generally preceded changes in muscle length. When, however, stretch amplitudes of 1 mm (peak to peak) were used, a phase delay of tension was sometimes seen. This only occurred in preparations that also showed little sensitivity to slow stretching.9. Phase delay of tension was usually found with frequencies of approximately 6-8 c/s, and this corresponds to the clonus frequency for soleus.10. The mechanisms of the two types of reflex behaviour, and the damping properties of the system are discussed.