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Ventral root filaments were subdivided until they contained a single γ motor fibre to the soleus muscle of the anæsthetized cat. The response of a muscle spindle primary ending to a standard stretch was observed during repetitive stimulation of such single γ fibres. All γ fibres isolated increased the discharge of the ending when the muscle was at a constant length; but some γ fibres (here called dynamic fusimotor fibres) in addition increased the sensitivity of the ending to the dynamic stimulus of stretching; in contrast, others (static fusimotor fibres) tended to decrease the dynamic sensitivity. These two different effects could be produced on the same primary ending by different γ fibres. These differences were independent of the frequency of stimulation, the amplitude of stretch and the velocity of stretching. No other consistent differences were found between the two kinds of fusimotor fibre.
P. B. C. Matthews (Wed,) studied this question.