The action potential in cardiac cells is composed of four main components: an upstroke (phase 0), early partial repolarization (phase 1), a plateau (phase 2), and final repolarization (phase 3). However, in the slow-response action potential, the upstroke is less steep, phase 1 is absent, the plateau is shorter and less flat, and the transition from the plateau to final repolarization is less distinct.

By blocking fast sodium channels with tetrodotoxin, slow responses can be generated in fast-response fibers under specific conditions. This can be observed in Purkinje fiber action potentials, where the control tracing shows a prominent notch separating the upstroke from the plateau. As the concentration of tetrodotoxin increases, there is a graded blockade of the fast sodium channels, resulting in progressively less prominent upstrokes and notches. In the final tracing, the notch disappears and the upstroke becomes very gradual, resembling a typical slow response.

Certain cells in the heart, such as those in the SA and AV nodes, exhibit slow-response action potentials. In these cells, depolarization is mainly achieved through influx of Ca++ via L-type calcium channels instead of Na+ influx through fast sodium channels. Repolarization in these fibers occurs through inactivation of the calcium channels and increased K+ conductance through the iK1 and iK channels.

Propagation of an action potential along a cardiac muscle fiber occurs through local circuit currents, similar to nerve and skeletal muscle fibers. When the wave of depolarization reaches the end of a cell, the impulse is conducted to adjacent cells through gap junctions. Impulses are more easily transmitted along the length of the cell (isotropic) than laterally from cell to cell (anisotropic) due to preferential localization of gap junctions at the ends of the cell. Gap junctions have low electrical resistance, allowing ionic current to pass from one cell to another, similar to cytoplasm. This flow of charge enables intercellular propagation of the impulse.

Conduction characteristics differ between fast-response and slow-response fibers. In fast-response fibers, activation of fast sodium channels occurs when the transmembrane potential suddenly changes from a resting value of approximately -90 mV to the threshold value of approximately -65 mV. The inward Na+ current rapidly depolarizes the cell at that site, causing it to become part of the depolarized zone and displacing the border. This process is repeated, continuously moving the border down the fiber as a wave of depolarization.
Fig. 16.1A ) consist of four principal components: an upstroke (phase 0), an early partial repolarization (phase 1), a plateau (phase 2), and a final repolarization (phase 3). However, in the slow-response action potential (see
Fig. 16.1B ), the upstroke is much less steep, early repolarization (phase 1) is absent, the plateau is less prolonged and not as flat, and the transition from the plateau to the final repolarization is less distinct.
Blocking fast sodium channels with tetrodotoxin in a fast-response fiber can generate slow responses under appropriate conditions. The Purkinje fiber action potentials shown in
Fig. 16.11 clearly exhibit the two response types. In the control tracing (A), the typical fast-response action potential displays a prominent notch, as a result of ito, that separates the upstroke from the plateau. Progressively higher concentrations of tetrodotoxin produce a graded blockade of the fast sodium channels, as demonstrated in tracings B to E. The upstroke and notch are progressively less prominent in tracings B to D. In tracing E, the notch has disappeared, and the upstroke is very gradual; this action potential resembles a typical slow response.
•Fig. 16.11 Effect of Tetrodotoxin, Which Blocks Fast Sodium Channels, on the Action Potentials (Tracings A to E) Recorded in a Purkinje Fiber. Theconcentrationsoftetrodotoxinwere0mol/LintracingA,3×10−8mol/LintracingB,3×10−7mol/LintracingC,and3×10−6mol/LintracingsDandE;EwasrecordedlaterthanD.(RedrawnfromCarmelietE,VereeckeJ.Pflügers Arch. 1969;313:300.)100 mV 1 sec A B C D E
Certain cells in the heart, notably those in the SA and AV nodes, exhibit slow-response action potentials. In these cells, depolarization is achieved mainly by influx of Ca++ through L-type calcium channels instead of influx of Na+ through fast sodium channels. Repolarization is accomplished in these fibers by inactivation of the calcium channels and by the increased K+ conductance through the iK1 and iK channels (see
Fig. 16.3).
An action potential traveling along a cardiac muscle fiber is propagated by local circuit currents, much as it is in nerve and skeletal muscle fibers (see ). When the wave of depolarization reaches the end of the cell, the impulse is conducted to adjacent cells through gap junctions (see
Chapter 2).
Impulses pass more readily along the length of the cell (isotropic) than laterally from cell to cell (anisotropic) because gap junctions are preferentially located at the ends of the cell. Gap junctions are rather nonselective in their permeability by ions and have a low electrical resistance that allows ionic current to pass from one cell to another. The electrical resistance of gap junctions is similar to that of cytoplasm. The flow of charge from cell to cell follows the principles of local circuit currents and therefore allows intercellular propagation of the impulse.
Conduction of the Fast Response
In fast-and slow-response fibers, the characteristics of conduction differ. In fast-response fibers, fast sodium channels are activated when the transmembrane potential •Fig. 16.12 Theroleoflocalcurrentsinthepropagationofawaveofexcitationdownacardiacfiber.
of one region of the fiber suddenly changes from a resting value of approximately −90 mV to the threshold value of approximately −65 mV. The inward Na+ current then rapidly depolarizes the cell at that site. This portion of the fiber subsequently becomes part of the depolarized zone, and the border is displaced accordingly. The same process then begins at the new border. This process is repeated again and again, and the border moves continuously down the fiber as a wave of depolarization (
Fig. 16.12