INTRACELLULAR ACTION POTENTIALS MODIFIED AT MUSCLE END-PLATE BY ADJACENT FIBER ACTIVITY

Abstract

Action potentials were recorded from frog M. sartorius and M. sternocutaneus with microelectrodes internal or external to the active fibers. Spikes obtained by nerve stimulation and recorded external to active cells were characteristically tri-phasic=positive-negative-positive=whether the electrode was in the external medium or impaling a silent cell. Near the neuro-muscular junction initial positivity was reduced and often absent, while the trailing positivity was absent from spikes recorded near fiber ends. Internally recorded spikes were often distorted by small negative-going waves before or after the peak, especially at the neuromuscular junction. Undistorted spikes recorded from cells impaled with a stimulating and a recording electrode were added to the potentials recorded from a silent cell surrounded by active fibers. Resulting distortions imitated the spikes normally recorded in the innervation field of the muscle after nerve stimulation. The characteristic form of the indirectly evoked intracellular action potential seen at the muscle end-plate is attributed to summation of originating spikes in impaled and adjacent fibers.