Responses of chelonian muscle spindles to mechanic stretch and fusimotor stimulation

Abstract

The responses of the chelonian [tortoise] muscle spindle to mechanical stretch and fusimotor stimulation were investigated. The passive spindles responded both to the height and to the velocity of ramp and hold stretches and showed a long adaptation time. Only 1 type of sensory ending was observed although quantitatively there were large differences between the spindles. Fusimotor stimulation produced a variety of response patterns, i.e., build up of firing frequency and magnitude of frequency, when different spindles were compared. For some spindles the dynamic and static sensitivity, as measured during ramp and hold stretches, were increased by the motor stimulation. Such increases were not true for all of the spindles. The spindles showed only 1 type of response pattern to sinusoidal stretches during motor stimulation and when it was possible to activate a spindle through different motor nerve branches, the effects on the spindle on stimulating these branches were qualitatively similar. The range of response patterns did not clearly indicate 2 distinct categories which were attributed to different responses to the 2 types of motor endings on the intrafusal spindle fibers. The variation may simply be the result of the differences between individual spindles with respect to number of intrafusal fibers or position of motor innervation.