And then came the microelectrode

It is difficult to fully appreciate the contributions made by the microelectrode to the understanding of cardiac electrophysiology without some idea of the prior state of the field. What was known about cardiac electrophysiology had been derived largely from studies of the electrocardiogram and cardiac electrogram and by the application of stimuli, usually electrical, to the heart. The voltage–time course of the atrial and ventricular action potentials had been revealed by the recording of monophasic potentials through suction electrodes or other means of registering injury potentials. The various phases of refractoriness had been defined by stimulation but this information usually was not related to the monophasic action potential. Evidence of the basic mechanisms underlying the observed phenomena was largely lacking. The major contributions to understanding the nature and mechanisms for arrhythmias and conduction disturbances resulted from precise measurements of the electrocardiogram and careful deductive reasoning. Early studies on in vitro preparations 1 had demonstrated reentrant excitation and identified an inexcitable barrier, unidirectional block and slow conduction as the necessary substrate for this arrhythmia. These abnormalities were attributed to a vaguely described depression of excitability. Some ectopic rhythms were ascribed to abnormal impulse generation and several theories as to mechanism were presented but largely without experimental justification. All in all one knew what was happening in the heart but without an understanding of why. The ability to record the transmembrane potentials of cardiac muscle brought about a rapid and immense increase in understanding of the basis for normal and abnormal cardiac electrical activity. Intracellular electrodes had been employed to study giant nerve fibers 2 and permitted the brilliant experiments and deductions by Hodgkin, Huxley and Katz 3,4 among others. These experiments provided for the first time an understanding of the conductance changes and ionic currents causing the depolarization and repolarization shown … * Address for correspondence: 600 Gropetree Drive, Apt. 6 EM, Key Biscane, FL 33149, USA