Conduction of Action Potentials in the Ureter of the Rat

Abstract

The action potential of the rat ureter consists of a fast negative wave and a plateau lasting approximately 1 second. The diphasic potential consists of a fast negative wave, a region of near neutrality and a slow positive wave. When the duration of negativity at the proximal recording electrode is longer than at the distal electrode the slow wave is inverted from positive to negative. The wave form becomes complex as interelectrode distance is increased beyond 1-3 mm. Duration of depolarization is prolonged by atropine, high K saline, to a less extent by acetylcholine and higher quaternary amines. Duration of depolarization is diminished by epinephrine, by low Ca and on repetitive stimulation. Repetitive responses to single shocks and spontaneous rhythmicity result from local treatment with tetrabutyl amine and with acetylcholine, to a slight extent with tetraethyl amine but not with tetramethyl amine. Local rhythmicity induced by acetylcholine is potentiated by eserinization. Responses of the ureter to slightly suprathreshold stimuli are conducted at an average of 3.4 cm/second. At several times threshold an abrupt drop in latency of response occurs which corresponds to velocity of 30 cm/second. Conduction is blocked reversibly by nicotine applied locally at high concentrations (10-3-10-4). Hexamethonium blocks at 10-3, 933F blocks at 10-3.5. Procaine at toxic concentrations blocks; tetracaine blocks conduction at 10-4.5-10-4. After tetracaine block, acetyl- choline can still initiate rhythmic discharge. Locally applied d-tubocurarine (10-4) and decamethonium but not Mytolon block conduction. At high amplification small "prespikes" are seen ahead of the muscle potentials. These "prespikes" remain after the muscle response is blocked by d-tubocurarine or nicotine but both responses are eliminated by tetracaine. The "prespikes" are conducted at a velocity approximately 10 times that of the muscle impulses. It is concluded that they are nerve impulses with some function other than activation of the muscle. Strands of non-myelinated nerve fibers with associated internuncial cells but no ganglion cells were observed histologically. Records with a point lead at varying distances from the ureter indicate short electric dipoles of the muscle response. Injury potentials rarely exceed 2 mv. Conduction in a ganglionic net, in parallel nerve fibers and in a protoplasmic continuum are contraindicated; some type of ephaptic conduction system is favored.