Modulation of axonal excitability mediated by surround electric activity: an intra-axonal study.

Intra-axonal recordings were obtained in vitro from frog sciatic nerve axons. Adjusting whole nerve stimulation current to just subthreshold for the impaled axon elicited triphasic threshold changes in that axon. Threshold changes were determined by direct intra-axonal current application. Graded subthreshold depolarizations were present in many axons during the passage of action potentials in surrounding axons. When nerve branches were stimulated and axon recordings were obtained from the main nerve trunk, both branches, the one that activated the impaled axon and the one that did not, elicited threshold changes in the impaled axon. These data indicate on a cellular level that impulse activity in an intact nerve bundle can modulate the excitability of adjacent nonactivated fibers.