Intra- and extracellular measurements of frog neuromuscular transmission upon stretch of the muscle at different stimulus frequencies.

Flexible intracellular micro-electrodes were used to study the effect of changes in muscle length on the end-plate potential in the isolated m. cutaneus pectoris for different frequencies of stimulation (1/60-5Hz). A 20% step-wise increase in muscle length within the physiological range increases the end-plate potential immediately by about 50% (range 0-120%) at all frequencies tested. At stimulus frequencies lower than 1/5 Hz this increase is sustained during a period of 15 min stretch. At 1 Hz, however, the initial increase in the end-plate potential amplitude on the average declines within a few minutes to a steady-state value about 35% higher than the steady-state end-plate potential before stretch. At 5 Hz, the initial amplitude increase is followed by a decline of about 15 min duration and the final amplitude is not increased in comparison with the pre-stretch amplitude. The amplitude of the compound muscle action potential of the gastrocnemius muscle with intact circulation shows a similar time dependent increase upon stretch at different stimulus frequencies. It is concluded that stretch of a frog muscle gives both an immediate and a sustained increase in transmitter release from the nerve terminals during prolonged stimulation at frequencies up to about 5 Hz. This effect of stretch on transmitter release can improve in vivo neuromuscular impulse transmission.