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1. Nerve impulses were recorded from spindle afferents of the soleus muscle during sinusoidal changes in muscle length at frequencies from 0·03 to 300 c/s. This was done in the decerebrate cat with intact motor outflow and `spontaneous' fusimotor activity. Computer averaging over a number of cycles was used to measure the response, in impulses/sec, at different phases of the cycle.
2. A linear range was described in which the discharges of the endings were approximately sinusoidally modulated, and in which increasing the amplitude of the stretching produced a proportional increase in the response. At 1 c/s the linear range extended up to only about 0·1 mm for primary endings, but was greater than 1 mm for secondary endings.
3. The sensitivity of an ending in its linear range was defined as the amplitude of its response divided by the amplitude of the length change. The sensitivity of both primary and secondary endings increased progressively on increasing the frequency of stretching above 1 c/s. The experimental observations relating the sensitivity to the sinusoidal frequency were fitted over much of the range by a curve given by the vector sum of components proportional to the length and to the velocity of stretching. This curve has two parameters, a sensitivity at low frequencies (S), and a corner frequency (F) at which the length and velocity contributions are equal. The value of F was about 1·5 c/s for both primary and secondary muscle spindle endings. The value of S was very much greater for primary endings (median value 95 impulses/sec/mm) than for secondary endings (median value 7 impulses/sec/mm). The increasing sensitivity of the endings at higher frequencies caused a progressive reduction in the linear range when it was expressed as an amplitude of stretching, but it remained approximately constant when it was expressed as a modulation of the frequency of discharge.
4. The primary endings were also extremely sensitive to maintained changes in length provided they were of sufficiently small amplitude.
5. In the presence of fusimotor activity a high sensitivity of small changes was found over a wide range of muscle lengths. De-efferented endings had a comparable high sensitivity when the muscle was at or beyond physiological full extension, but not when the muscle was shorter.
6. The results are contrasted with those obtained previously using stretches of large amplitude, and the physiological significance of the high sensitivity of primary endings to small stretches is discussed in relation to the reflex control of movement.
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