Structural Identification of Twitch and Slow Striated Muscle Fibers of the Frog

An unusual histological appearance was found by Krfiger (1929) and by Fiirl inger (1930) (see also Krfiger et al., 1933, and Krfiger, 1952) to be characterist ic of a propor t ion of the muscle fibers in those muscles of the frog tha t are able (Sommerkamp, 1928) to give long lasting, slow contractions. This unusual appearance was named "Fe lders t ruk tur" to distinguish it from the usual "F ibr i l l ens t ruktur" associated with twitch fibers (see Fig. 1). These authors made a strong case for believing tha t it is only the Felders t ruktur fibers tha t can give slow contractions, bu t the single a t t empt tha t has been made to check this idea more directly failed to suppor t it (Brecht and Feneis, 1950). Hess (1960), combining light and electron microscopy, showed differences in fine structure and innerva t ion corresponding to Kri iger ' s results, bu t gave no new evidence for supposing tha t the fibers of Felderstruktur type are solely responsible for the slow contractions. Following the pioneer discovery by Tasaki and Mizu tan i (1943) tha t some toad muscles are part ial ly innervated by small d iameter nerves tha t produce only slow contractions, Kuffler and V a u g h a n Will iams (1953) conclusively demon_ strated tha t special nerve and muscle fibers, completely separate from the twitch-producing fibers, are present in cer tain muscles in the frog and are responsible for the slow contractions. 1 Kuffler and V a u g h a n Will iams fur ther showed tha t these slow muscle fibers do not conduct propagated responses, and on this basis and with visual j u d g m e n t of the speed of contract ion we have been able to dist inguish between slow and fast isolated muscle fibers. By examining these isolated fibers by elect ron microscopy, we have confirmed directly the correlat ion between structure and funct ion suggested earlier. We are presenting these results in only a pre l iminary form because of suggestions tha t a third type of fiber is present which may or may not have been included in this series.