THE FUNCTION OF THE FROG'S SMALL-NERVE MOTOR SYSTEM

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Abstract
Reflex properties of the small-nerve motor system and the mechanical responses it produces were studied in the frog. Depending on the number and frequency of impulses reaching the spinal cord, 2 types of reflexes occur in numerous muscles: (1) large-nerve reflexes, accompanied by propagated impulses and muscle twitches, and (2) small-nerve reflexes, attended by local non-propagated muscle contractions. Each system can be active independently of the other. Small-nerve reflexes arise following weak single or tetanic stimuli to cut central portions of peripheral nerves. With increased stimulus strength or frequency, twitches set up by large-nerve impulses, appear. The small-nerve efferent discharges may follow the rhythm of incoming volleys up to frequencies of 10-12/sec. The 2 reflex types can also be set up by tactile stimuli to the skin. Small-nerve reflexes can be inhibited by stimulation of the same nerves that inhibit the twitches. The differences between the 2 reflex systems seem to be largely quantitative. Tensions set up by small-nerve reflex discharges, recorded largely in the semitendinosus, may attain 10-15% of a maximal single twitch response. Similar tensions are obtained when the small-nerve component in isolated nerve-muscle prepns. is stimulated above a polarization block at frequencies of 20-30/sec. A single small-nerve volley produces no appreciable tension, at most one of a few mgs. in a semitendinosus muscle. If the nerve supply to a muscle containing small-nerve motor fibers is tetanized maximally, the rapid relaxation after stimulation is incomplete. This residual tension varies with stimulation frequency and may attain 10-15% of maximal twitch tension. If the large-nerve component alone is excited the tension returns quickly to the original base line. A residual tension may also persist after cessation of small-nerve excitation alone. A twitch or tetanus causes such residual tensions to collapse. Similar tensions maintained during small-nerve stimulation, on the contrary, are rather enhanced by a twitch or tetanus. The small-nerve system is most effective in developing tension when the initial muscle tension is low; it seems well suited for the maintenance of slow prolonged contractions.

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