ACTION-POTENTIAL GENERATION IN RAT SLOW-TWITCH AND FAST-TWITCH MUSCLES

1. In skeletal muscle fibres, voltage-gated sodium channels are concen trated at the neuromuscular junction. The effect of this accumulation of sodium channels on action potential generation was investigated in rat slow- and fast-twitch muscle fibres. 2. Intracellular microelectro des were used to generate and record action potentials, from an impose d membrane potential of -75 and -90 mV, in junctional and extrajunctio nal regions of the muscle fibre. To identify junctional regions, prepa rations were incubated with 5 x 10(-7) M d-tubocurarine (dTC) to block muscle contraction in response to nerve stimulation whilst allowing e ndplate potentials (EPPs) to be recorded. Injection of rectangular dep olarizing current pulses initiated action potentials at the endplate w ith threshold values several millivolts lower than those generated els ewhere in the fibre. In addition, the maximum rate of rise of the acti on potential was greater at the endplate than in extrajunctional regio ns. 3. In other muscles, neuromuscular transmission was partially bloc ked with dTC (2 x 10(-7) M), such that repetitive nerve stimulation ev oked action potentials and EPPs in the same fibre. The threshold of th ese nerve-evoked action potentials was approximately 50% lower than va lues derived from action potentials generated by current injection. 4. It is concluded that the threshold for action potential generation is significantly lower at the neuromuscular junction than in extrajuncti onal regions of skeletal muscle fibres. Furthermore, nerve-evoked curr ent is more effective at generating an action potential than is inject ed current.