Effects of denervation on resting membrane and action potentials of neonate rat fast and slow twitch muscles.

During the first 20 days postpartum of the rat, a period of rapid muscle growth, resting potentials and maximum rate of rise and overshoot of action potentials recorded from fast and slow twitch muscle fibres all increased, although did not reach adult values. The action potential of fast twitch muscle fibres had a greater maximum rate of rise and overshoot than that of slow twitch muscle fibres of the same age. Innervation had a marked influence upon maintenance of the electrical properties of muscle fibres in the neonate since denervation reduced the value of the resting potential and maximum rate of rise and overshoot of the action potential. Neverthless, some differences between action potentials recorded from the two fibre types remained 4 days after denervation.