Abstract
Cable properties of sheep cardiac Purkinje fibres were studied under resting and paced conditions. Standard micro-electrode techniques were used to apply intracellular current pulses and record the resultant voltage changes at various distances from the current input. In a parallel set of experiments, fibre dimensions were measured after freezing and serial sectioning. Fibres selected on the basis of a cylindrical appearance had approximately uniform cross-sectional diameters which varied +/- 12% along their length. Electrotonic potentials recorded at rest and in diastole (under conditions that minimized diastolic depolarization) adhered quite closely to the behaviour expected for a unidimensional cable provided voltages were recorded greater than or equal to one fibre diameter from the current source. The unidimensional space constant, input resistance, and membrane time constant were significantly larger during quiescence than in diastole. These differences were accounted for by a 90% increase in membrane resistance at rest. There was no significant change in internal longitudinal resistance nor membrane capacitance associated with activity. The voltage distribution close to the current input (i.e. within one fibre diameter) strongly deviated from the theoretical three-dimensional voltage decay expected for a homogeneous cylinder. This finding suggests that the transverse resistance to current flow is much greater than the longitudinal resistance. The anisotropic behaviour within the cardiac Purkinje fibre may explain several previous observations: (i) the lack of a relationship between conduction velocity and fibre diameter; and (ii) the much shorter liminal length for excitation in Purkinje fibres than for point-stimulated squid axons.