Excitation Originates Within The Heart Muscle Cells

Cardiac muscle, like skeletal muscle, is a striated muscle and much of the mechanism of contraction is similar between the two muscle types. The electrophy-siology of the two muscles differs dramatically, however. In skeletal muscle an action potential in the synaptic terminal of a motor neuron coming from the central nervous system releases a transmitter substance, acetylcholine, which triggers a very brief action potential on the muscle cell, causing it to contract. Skeletal muscle contracts in an all-or-none fashion and the force generated, for any given length, will be the same for every twitch (for a review, see Chapter 7). The action potential is so short in skeletal muscle that a single action potential generates an insignificant amount of force. Usable force can only be achieved by stimulating the fiber repeatedly with a train of neural discharges (temporal summation). Individual motor units can be stimulated within a muscle as a further means of control. Cardiac muscle differs in several important respects. First, the cardiac action potential is not initiated by neural activity. Instead, specialized cardiac muscle tissue in the heart itself initiates the action potential, which then spreads directly from muscle cell to muscle cell. Neural influences have only a modulatory effect on the heart rate. Second, the duration of the cardiac action potential is quite long. As a result, the full force of cardiac contraction results from a single action potential. The force of contraction is not the same for every beat of the heart and can be modulated by the cardiac nerves. Finally, all cells in the heart contract together as a unit in a coordinated fashion with every beat.

This chapter examines the electrical properties of the cardiac muscle cells. We begin by looking at the different types of cells in the heart. The properties and ionic mechanism of cardiac action potentials, the processes of excitation-contraction coupling, and the regulation of the heart rate will be explained.

Two broad types of cells are found within the heart: (1) contractile cells and (2) conductile cells. Contractile cells are the cells of the working myocardium and constitute the bulk of the muscle cells that make up the atria and the ventricles. An action potential in any one of these cells leads to a mechanical contraction of that cell. Furthermore, an action potential in one cardiac muscle cell will stimulate neighboring cells to undergo an action potential, such that activation of any single cell will be propagated over the whole heart. Conductile cells are specialized muscle cells that are involved with the initiation or propagation of action potentials but have little mechanical capability. The principal conduc-tile cells are indicated in Fig. 1. Of critical importance is the sinoatrial (SA) node. The SA node (sometimes called the sinus node) lies in the right atrium near the vena cava. SA nodal cells generate spontaneous action

Purkinje Fibers '

FIGURE 1 Structure of the conduction system of the heart. Structures colored blue are those responsible for generating and propagating the wave of excitation that leads to contraction of the heart. (Modified from Katz AM. Physiology of the heart. New York: Raven Press, 1992.)

Purkinje Fibers '

FIGURE 1 Structure of the conduction system of the heart. Structures colored blue are those responsible for generating and propagating the wave of excitation that leads to contraction of the heart. (Modified from Katz AM. Physiology of the heart. New York: Raven Press, 1992.)

potentials and act as the normal pacemaker for the heart. Because the SA node is located in the atria, action potentials will first be propagated over the atria, making them the first structures in the heart to contract.

Action potentials spreading across the atria eventually reach another conducting structure known as the atrioventricular (AV) node. The AV node is located in the interatrial septum between the ostium of the coronary sinus and the septal leaflet of the tricuspid valve. The AV node serves two important functions. One is to relay the wave of depolarization from the atria to the ventricles. A sheet of connective tissue associated with the atrioventricular valves separates the atria from the ventricles, and the AV node is the only conductive link between the atria and the ventricles. The second function is to delay the spread of excitation from the atria to the ventricles. The AV node cells are specialized to conduct very slowly. This delay permits the atria to eject their blood and therefore fill the ventricles before the latter begin to contract.

The fibers of the AV node give rise to fibers of the AV bundle (common bundle or bundle of His), which in turn divides into two major branches, the left bundle branch and the right bundle branch. These branches then divide into an extensive network of Purkinje fibers. The Purkinje fibers are conductile cells that conduct action potentials very rapidly. They are interwoven among the contractile cells of the ventricles and serve to quickly spread the wave of excitation throughout the ventricles. Because they conduct so rapidly, all muscle cells of the ventricles appear to contract in unison. It is important to emphasize that all of these conductile structures (e.g., SA node, AV node, Purkinje network) are not nervous tissue but, rather, specialized cardiac muscle cells.

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