The motor innervation of the muscle spindle of the frog is investigated by recording simultaneously the responses of muscle and nerve when individual axons are stimulated.
There is evidence that intrafusal and ordinary muscle fibres receive a common innervation and that, as a result, the discharge from the spindle is intensified during isometric contraction, but is stopped when shortening occurs.
When the muscle is curarized progressively, a state is reached in which all visible contractions are abolished, yet the spindle continues to respond to a motor nerve impulse with a train of afferent spikes. When the block is intensified, all indirect responses to nerve stimulation fail. Closer study of this effect suggests that the spindle response arises from a twitch of the intrafusal muscle fibres, and that the intrafusal motor junctions are more resistant to curarine than the ordinary junctions.
The existence of an accessory motor system in frog muscle, which is supplied by small axons and characterized by a slow non-propagated response, is confirmed. There is no evidence for any specific connexion between the small motor axons and the muscle spindles, although frequently branches of these axons appear to innervate intrafusal as well as other parts of the muscle.