The responses of frog muscle spindles during stimulation of slow motor axons.

Responses of muscle spindles of the iliofibularis muscle of frog Litoria aurea have been recorded during single shock and repetitive stimulation of single functional motor axons. Repetitive stimulation of axons which innervated slow muscle, and on four occasions, axons which innervated twitch muscle, produced a large increase in the dynamic response of the spindle to a ramp-and-hold stretch. While extrafusal slow muscle did not respond to a single motor volley, some spindles did, especially if at the same time the muscle was being stretched. In an explanation of the effect of muscle stretch on responses of spindles to slow motor volleys it was proposed that stretch acted to reduce the internal motion in muscle fibres produced by a non-uniform distribution of sarcomere lengths. It was proposed that this kind of effect may account for dynamic fusimotor actions in all vertebrate spindles.