FUNCTIONAL ORGANIZATION OF NEUROMUSCULAR SPINDLE IN TOAD

Abstract

The efferent regulation of the muscle spindle was studied in toads by stimulating motor axons and recording sensory discharges and intrafusal activity. Contractile spindle activity was directly observed under transmitted illumination in the extensor longus dig. IV. Intrafusal muscle fibers were impaled with microelectrodes and it was demonstrated that twitch fibers occur within the spindle. Twitch intrafusal fibers are innervated by motor axons which also supply twitch extrafusal fibers. Each intrafusal bundle is supplied by several "large" efferent axons. Branches of these axons, in turn, innervate more than 1 intrafusal bundle. Small-nerve motor fibers(conducting at 2-8 m/sec.) also innervate extra- and intrafusal muscle bundles. Stimulation of the small-nerve efferents activates slow intrafusal fibers and their contraction produces gradual frequency changes of the sensory discharges. Also, one intrafusal bundle may be innervated by large and small motor fibers. In this way each spindle has a double motor control, each producing distinct patterns of frequency changes. The properties of the crustacean, amphibian and mammalian stretch receptors are compared.