Analysis of the Junction Potential of a Small Nerve

Abstract

Fatt and Katz¹ demonstrated with the aid of an intracellular microelectrode that the end-plate potential was produced in the region of the motor end-plate by a sudden and transient reduction in the membrane potential as a result of neuromuscular transmitter action. They further pointed out that the prolonged decline of the end-plate potential was almost exponential and was determined by the electrical properties of the resting muscle membrane. An equivalent circuit of the resting muscle membrane is represented as a condenser in parallel with a battery (membrane potential) with a series resistance (membrane resistance). In the active phase of the end-plate potential, the membrane resistance at the junctional region is suddenly reduced by the transmitter action, that is, the muscle membrane is shortened by a shunting resistance¹. To demonstrate the active process of the end-plate potential, Takeuchi and Takeuchi² and Oomura and Tomita^3,4 recorded a current flowing through the end-plate region of the muscle membrane by stimulating the nerve under conditions where the membrane potential was kept at the resting level, using the voltage-clamp technique^5,6. On the other hand, the small motor nerve fibres which innervate the slow muscle fibres and relate to the tonic function in the maintenance of posture, also produce the junction potential of the small nerve at the junction region by its stimulation⁷. A distinctive feature of this junction potential is that it displays two phases with a terminal phase of hyperpolarization⁷. In the present work an attempt has been made to analyse the mechanism of production of the junction potential of the small nerve, using the voltage-clamp technique.