Miniature potentials in denervated slow muscle fibres of the frog.

1. Pyriformis and iliofibularis muscles of the frog were studied with micro-electrodes. Both muscles contain a mixture of fast and slow muscle fibres, which can be distinguished by differences in their miniature end-plate potentials (min.e.p.p.s).2. In addition, the membrane time constant of fast fibres is < 20 msec, while that of slow fibres is > 200 msec.3. The characteristic difference in time constant persisted after denervation and allowed the identification of fibre type.4. Denervated slow fibres had min.e.p.p.s of variable time courses and skew amplitude distributions. Their frequency was lower than in normally innervated fibres, and could be increased by hypotonic solutions.5. In analogy with similar observations made previously in fast muscle fibres, it is suggested that after nerve degeneration the Schwann cells of small motor nerve fibres release packages of transmitter which give rise to min.e.p.p.s.