Transducer Characteristics of the Muscle Spindle as Revealed by its Receptor Potential

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Abstract
The transducer properties of the sensory endings of the muscle spindle have been studied in isolated spindles of the frog's toe muscle. The depolarization of the sensory endings during dynamic stretch at constant velocity is a rectilinear function of the amount of lengthening of the spindle, for stretches up to about 30 % of its resting length. This suggests that the dynamic phase of the stimulus is transmitted to the endings with little distortion. When the spindle is stretched by more than 30 % of its resting length a saturation phase of constant depolarization is formed. The development of this phase may be explained by the assumption that parallel elastic elements take up the load or that the sensory membrane saturates beyond a given amount of distortion.After release of stretch the depolarization of the endings decreases exponentially with time. The time course of repolarization is considerably slower than the return of the spindle to resting length and relatively independent of the rate of release of stretch. This suggests that the repolarization is governed by the transduction process and related to the electrical proper ties of the sensory membrane.

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