The Membrane and Contractile Properties of Extra-Lingual Muscles of the Bullfrog

The membrane and contractile properties of extra-lingual striated muscles, M. hyoglossus and M. genioglossus, of the bullfrog were investigated by histochemical, electrophysiological and mechanographical methods. The obtained results could be summarized as follows : 1) M. hyoglossus, classified by myosin ATPase and succinic dehydrogenase staining methods, was composed of two different types of the fiber, fast and slow fiber types. The former showed large diameter (33.4 μm) than the latter (16.2 μm). The results obtained from AChE staining method also suggest that muscle fibers show two different neuro-muscular junctions as "en plaque" and "en grappe" types, the former observed in fast muscle fiber and the latter in slow muscle fiber. The morphological features of M. genioglossus were nearly the same as those observed in the fast muscle fiber types of M. hyoglossus. 2) The fast muscle fiber of M. hyoglossus and M. genioglossus possessed nearly the same membrane properties at resting and active states. On the other hand, the slow muscle of M. hyoglossus possessed many different properties from those of the fast muscle, i.e. the slow muscle possesses lower membrane potential (-83.1mV vs -92.4mV), larger length constant of the cell (4.24mm vs 1.78mm), larger input resistance (4.03MΩ vs 1.01MΩ), larger time constant of the membrane (42 msec vs 18 msec) and larger membrane resistance (19.3 KΩcm^2 vs 4.06 KΩcm^2) compared with those in the fast muscle. These properties of the muscle could be classified into typical slow muscle fiber which have been described in the other slow muscle fibers of the toad and bullfrog. The fast muscle fiber could but the slow muscle fiber could not evoke the spike by the outward current pulse. 3) Miniature endplate potential (mepp) and endplate potential (epp) could be recorded from both muscle tissues. However, fast and slow muscle fibers showed distinct differences from their features. The fast muscle generated the spike on epp but the slow muscle generated epp alone. The distribution and amplitude of mepp also differed between slow and fast muscle. The obtained results suggest that fast muscle possesses focal innervation "en plaque" and slow muscle diffuse innervation "en grappe". 4) The excitation-contraction coupling mechanism in fast and slow muscle fibers was examined under depolarization induced by excess [K]_0. This mechanism differed between fast and slow muscle fiber, i.e. the latter showed lower threshold to evoke the contraction, thus indicating that epp is enough to evoke the contraction, and the latter higher threshold to evoke the contraction, thus indicating the requirement of spike generation. 5) Contractions induced by either electrical stimulation or excess [K]_0 also differed between fast and slow muscles. The former showed shorter duration of the contraction than the latter, and to complete the tetanus contraction the former required higher frequency of stimulation than the latter. In the latter the tetanus contraction was sustained during the application of excess [K]_0 and in the former it was declined. 6) From the above observations, the functions of M. hyoglossus and M. genioglossus were discussed in relation with the lingual activity.