Differences in internal structure and nerve terminals of the slow and twitch muscle fibers in the cat superior oblique

Abstract

Abstract Measurements of nerve fiber size in the trochlear nerve indicate that many small nerve fibers are present which could serve as the small nerve motor supply for innervation of the slow muscle fibers in the superior oblique of the cat. Twitch and slow muscle fibers in the superior oblique muscle of the cat have been examined in the electron microscope. In twitch fibers, triads consisting of the transverse tubule and dilated sacs of the sarcoplasmic reticulum occur at the level of junction of A and I bands. In the slow fibers, the sarcoplasmic reticulum is reduced in amount and the transverse tubular system is completely absent. The junctional folds of the postsynaptic membrane of the muscle fiber under the nerve terminal of a twitch fiber are present, although not extensive; in slow fibers, these postjunctional sarcolemmal folds are essentially absent. Some physiological implications of these differences in fine structure of twitch and slow fibers and their respective nerve terminals are discussed.