Position and Velocity Sensitivity of Muscle Spindles in the Cat. IV. Interaction Between Two Fusimotor Fibres Converging on the Same Spindle Ending

Position and velocity responses have been measured during the co‐activation of two indentified fusimotor fibres influencing the same primary or secondary ending in the ankle extensor or flexor muscles of the cat. The effect of double‐fibre activation has been compared with that of stimulating the participating single‐fibres in isolation. During the activation of a dynamic‐static combination complete ‘occlusion’ was usually encountered in the steady discharge at ‘zero’ extension, in the position sensitivity and in the velocity responses to length decrement. Larger responses than any of the two individual actions occurred in the response to dynamic length increment. In the dynamic‐dynamic and in many of the static‐static double‐activations the responses were indistinguishable from those of single‐fibre stimulation at higher stimulus frequency. Other static‐static combinations gave rise to ‘averaging’ of the two single‐fibre effects; this was seen in the position sensitivity and in the velocity responses particularly during static I—static II co‐activations. Possible modes of fusimotor interaction in the spindle are discussed.