Slow and Fast Muscle Fibres in the Atlantic Hagfish (Myxine Glutinosa)

The two histologically different types of muscle fibres in the parietal muscle of the hagfish have been studied by means of internal micro‐electrodes and by recording the mechanical response after stimulation of their motor nerves. The one type of fibre gives rapid twitch contractions of about 150 msec duration while those of the other type last more than 500 msec. Accordingly, the muscle fibres are called fast and slow respectively. The resting membrane potentials of the fast fibres were 74.6 mV (S. E. 0.3), that of the slow fibres 46 mV (S. E. 0.5). Indirect stimulation of the fast fibres evoked a characteristic focal end‐plate potential in one end of the fibre. The end‐plate potential gave rise to a conducted action potential with overshoot. The fast fibres appear to be innervated with only one motor axon each. In the slow fibres distributed junction potentials were obtained on indirect stimulation. The time course of the junction potentials varied, partly on account of local spike activity. On repetitive stimulation of slow fibres a plateau depolarization was reached which only moderately exceeded the peak of a single junction potential. The slow fibres are usually innervated with two motor axons reaching the fibre from each end.