The electrical threshold of A-V nodal cells of isolated rabbit heart preparations was estimated by techniques which permitted stimulation and recording through the same micropipette. Cells within the node were significantly less excitable than adjacent atrial and ventricular tissues; thresholds exceeded 5 X 10~~° amp in some instances. Recovery of excitability was delayed well beyond repolarization; in cells of the midnodal region, "diastolic" threshold was not attained until 0.2 to 0.5 seconds

more »

... after restoration of resting membrane potential. A cumulative effect of frequency upon excitability (fatigue) was demonstrated. At high driving rates, the lag in recovery was further delayed, and the late diastolic threshold was increased. Responses to single-cell stimulation indicate that summation may be an important feature of propagation within the node. ADDITIONAL KEY WORDS fatigue of A-V transmission atrioventricular conduction summation in A-V node Schematic diagram of circuits used for intraceUular stimulation and recording. ME = microelectrode; P = potentiometer; C = calibrator; A 2 = amplifier for recording transmembrane action potential; Aj = current monitor.