ACTIVITY-DEPENDENT MODULATION OF THE PRESYNAPTIC POTASSIUM CURRENT INTHE FROG NEUROMUSCULAR-JUNCTION

1. Changes in the electrical properties of frog motor nerve endings ca used by the invasion of an action potential were studied by the perine ural recording technique. Two equal supra maximal stimuli separated by a variable time interval were applied to the nerve trunk. The latency and amplitude of the deflections associated with the nodal Na+ curren t and presynaptic K+ current elicited by the second pulse rr ere compa red with control currents. 2. The deflection associated with the presy naptic K+ current elicited in response to the second stimulus was abse nt at the shortest interstimulus interval and show ed a progressive in crease in its amplitude as the interstimulus interval was lengthened, reaching values greater than control in most terminals. During the sam e period the nodal Na+ current did not change. 3. The experimental res ults were compared with a computer model of the distal axonal segment and its terminal. Response of the model to twin-pulse stimulation mas in marked disagreement with the experimental results unless an inactiv ating K+ channel, with properties derived ad hoc, was incorporated int o the simulation. 4. These results suggest that in the first 6-7 ms af ter a nerve impulse has invaded a frog motor nerve ending, maximal Kconductance remains below the value at rest due to the fast inactivati on of a K+ conductance. Following this, there is a period in which Kconductance is greater than control values although the basis for this is unknown.