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The duration of the action potential decreases as the cycle length diminishes. This relationship between action potential duration and cycle length is mediated by changes in gK, which involve two types of potassium channels: those that conduct the delayed rectifier K+ currents (iKr and iKs) and those that conduct the transient outward K+ current (ito).

The iK current is activated at near zero Vm, but it activates slowly, remains activated for hundreds of milliseconds, and inactivates very slowly. As a result, as the basic cycle length decreases, each action potential tends to occur earlier in the inactivation period of the iK current initiated by the preceding action potential. Consequently, the shorter the basic cycle length, the greater the outward K+ current during phase 2 and the shorter the action potential duration.

The ito current also affects the relationship between cycle length and action potential duration. The ito current is activated at near zero potential, and its magnitude varies inversely with cardiac frequency. Therefore, as cycle length decreases, the consequent increase in the outward K+ current shortens the plateau.

Excitation of the heart occurs in an orderly manner through the heart's conduction system. The SA node, which is the pacemaker of the heart, initiates the spread of action potentials throughout the atria. This excitation reaches the AV node, where conduction is slowed to allow atrial contraction and adequate ventricular filling. Excitation then rapidly spreads throughout the ventricles via the Purkinje fibers, ensuring coordinated contraction of the ventricular myocytes.

Cardiac function, including heart rate and contraction strength, is controlled by the autonomic nervous system. However, intact innervation is not necessary for cardiac function. Even in a patient with a denervated, transplanted heart, adaptation to stressful situations can still occur. This ability lies in certain intrinsic properties of cardiac tissue, particularly its automaticity.

Automaticity and rhythmicity, the ability to initiate its own beat and the regularity of pacemaking activity, respectively, allow a perfused heart to beat even when it is completely removed from the body. The vertebrate heartbeat is myogenic in origin. When the coronary vasculature of an excised heart is artificially perfused, rhythmic cardiac contractions can persist for many hours. Cells in the atria and ventricles can initiate beats, with these cells mainly residing in nodal tissues or specialized conducting fibers of the heart.

The SA node is the region of the mammalian heart that typically generates impulses at the highest frequency and serves as the main cardiac pacemaker. Detailed mapping of electrical potentials on the surface of the right atrium has revealed two or three sites of automaticity, located 1 or 2 cm from the SA node itself, that function along with the SA node as an atrial pacemaker complex. Sometimes, all these loci initiate impulses simultaneously, while other times, the site of earliest excitation shifts depending on factors such as the level of autonomic neural activity.

In humans, the SA node is positioned posteriorly in the groove at the junction between the superior vena cava and the right atrium. It is approximately 8 mm long and 2 mm thick. The sinus node artery runs lengthwise through the center of the node. The SA node consists of two principal cell types: small, round cells with few organelles and myofibrils, and slender, elongated cells that resemble "ordinary" atrial myocardial cells. The round cells are likely the pacemaker cells, while the slender, elongated cells are responsible for conducting the impulses within the node and to the nodal margins.

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Fig. 16.16 . Note that as cycle length diminishes, the duration of the action potential decreases. This direct correlation between action potential duration and cycle length is mediated by changes in gK that involve at least two types of potassium channels: namely, those that conduct the delayed rectifier K+ currents (iKr and iKs) and those that conduct the transient outward K+ current (ito).
The iK current is activated at values of Vm near zero, but the current activates slowly, remains activated for hundreds of milliseconds, and also inactivates very slowly. As a consequence, as the basic cycle length diminishes, each action potential tends to occur earlier in the inactivation period of the iK current initiated by the preceding action potential. Therefore, the shorter the basic cycle length, the greater the outward K+ current during phase 2 and hence the shorter the action potential duration.
The ito current also influences the relationship between cycle length and action potential duration. The ito current is also activated at near zero potential, and its magnitude varies inversely with cardiac frequency. Therefore, as cycle length decreases, the consequent increase in the outward K+ current shortens the plateau.
Natural Excitation of the Heart and the Electrocardiogram
Excitation of the heart normally occurs in an ordered manner, which allows effective pumping of blood. This ordered excitation occurs via the heart’s conduction system (
Fig. 16.17 ). The SA node is the pacemaker of the heart and initiates the spread of action potentials throughout the atria. This spread of excitation reaches the AV node, where conduction is slowed so that atrial contraction can occur and the ventricles can be adequately filled. Excitation then spreads rapidly throughout the ventricles via the Purkinje fibers so that the ventricular myocytes contract in a coordinated manner. The properties of each component of the heart’s conduction system are described in the next sections.
•Fig. 16.17 Thecardiacconductionsystem.
CHAPTER 16 Elements of Cardiac Function
The autonomic nervous system controls various aspects of cardiac function, such as the heart rate and contraction strength. However, cardiac function does not require intact innervation. Indeed, a patient with a cardiac transplant, whose heart is completely denervated, may still adapt well to stressful situations. The ability of a denervated, transplanted heart to adapt to changing conditions lies in certain intrinsic properties of cardiac tissue, especially its automaticity.
The properties of automaticity (the ability to initiate its own beat) and rhythmicity (the regularity of pacemaking activity) allow a perfused heart to beat even when it is completely removed from the body. The vertebrate heartbeat is myogenic in origin. If the coronary vasculature of an excised heart is artificially perfused with blood or an oxygenated electrolyte solution, rhythmic cardiac contractions may persist for many hours. Some cells in the atria and ventricles can initiate beats; such cells reside mainly in nodal tissues or specialized conducting fibers of the heart.
As noted, the region of the mammalian heart that ordinarily generates impulses at the greatest frequency is the SA node; it is the main cardiac pacemaker. Detailed mapping of the electrical potentials on the surface of the right atrium has revealed that two or three sites of automaticity, located 1 or 2 cm from the SA node itself, serve along with the SA node as an atrial pacemaker complex. At times, all these loci initiate impulses simultaneously. At other times, the site of earliest excitation shifts from locus to locus, depending on certain conditions, such as the level of autonomic neural activity.
In humans, the SA node is approximately 8 mm long and 2 mm thick, and it lies posteriorly in the groove at the junction between the superior vena cava and the right atrium. The sinus node artery runs lengthwise through the center of the node. The SA node contains two principal cell types: (1) small, round cells that have few organelles and myofibrils and (2) slender, elongated cells that are intermediate in appearance between the round and “ordinary” atrial myocardial cells. The round cells are probably the pacemaker cells; the slender, elongated cells probably conduct the impulses within the node and to the nodal margins.