Acute lesion of the intrafusal muscle of muscle spindles: ultrastructural and electrophysiological consequences.

Experiments were performed to functionally separate the neural and muscular elements in muscle spindles. Cat muscle spindles were dissected free in vitro and were transected on both sides of the sensory nerve endings. This resulted in an acute lesion of the intrafusal muscle fibers but spared the sensory endings. Within 45 min, the intrafusal muscle fibers degenerated to the condition in which there were no myofilaments left in the sensory zone and the muscle fiber membrane had begun to disintegrate. During this degeneration period, and afterward, the sensory nerve fibers continued to fire action potentials. However, the postlesion discharge was usually more irregular than normal, and the mean rate of firing was often dramatically increased. In addition, multiplex frequencies of firing were often observed; that is, the interspike intervals were clustered around 2 or more modal values. Computer modeling of 2 action potential initiators competing for access to the same output line suggests that the multiplexing results from at least one of the competitors having a fraction of its spikes aborted. Overall, the results indicate that the nerve endings have an intrinsic capability to function as sensory receptors in the absence of a muscle substrate.