The effects of fusimotor stimulation during small amplitude stretching on the frequency-response of the primary ending of the mammalian muscle spindle.

Single fusimotor fibres to the de-efferented soleus of the anaesthetized cat were stimulated repetitively while recording the response of single primary afferents to small amplitude sinusoidal stretching at frequencies of 0-5--500 HZ. The response of the ending was determined by averaging its firing for many cycles to construct a 'cycle histogram'. When small enough, the stretching modulated the firing sinusoidally; as the frequency increased the requisite amplitude fell to a fraction of a micron. The amplitude of the modulation (in impulses/sec) divided by be amplitude of stretchin (in mm) gave the sensitivity of the ending for the particular frequency in question. 2. The passive frequency--response curve obtained in the absence of fusimotor stimulation agreed with those obtained before...