Clinical Note

Any cell in the heart is in theory capable of a spontaneous action potential. The SA node happens to have the fastest intrinsic rate and, hence, is the pacemaker. If the AV node is injured by disease, conduction through it can be lost and ventricular cells will be isolated from the pacemaker in the atrium. In that case the cells with the next highest rhythm, usually in the bundle of His, will keep the ventricles beating but at a much reduced rate. In this condition, called complete heart block, the atria and ventricles beat independently of one another. While the appearance of a ventricular pacemaker keeps the heart beating, the reduced ventricular rate is usually too slow to maintain a suitable cardiac output and the patients have severe symptoms such as fatigue and even loss of consciousness.

(Recall from Chapter 5 that the greater the diameter of a nerve axon, the greater the propagation velocity of nerve action potentials.) A Purkinje fiber has a particularly high density of gap junctions between its cells, and they will conduct action potentials at about 4 m/sec over the ventricle. Atrial and ventricular muscle cells on the other hand conduct at only 1 m/sec.

Cells of the SA and AV nodes, like the Purkinje fibers, also have a reduced quantity of contractile proteins. As discussed later, nodal cells lack fast sodium channels and that reduces the rate at which they depolarize. They also are much smaller than either the contractile or Purkinje cells. Both of these properties cause them to have a low conduction velocity. AV nodal cells also have a reduced density of gap junctions that even further depresses their conduction velocity to only about 0.05 m/sec. It is the low propagation velocity of the AV nodal cells that provides the delay between atrial and ventricular contraction.

The ultrastructural features of a typical cardiac muscle cell are illustrated in Fig. 3. At first glance, the ultrastructure appears much like a skeletal muscle cell (see Chapter 7). Common features include characteristic A, I, and Z bands, a T-tubule system, and sarcoplasmic reticulum (SR). Some subtle differences exist, however, between cardiac muscle and the skeletal muscle ultrastructure. One difference lies in the T tubules (which stands for "transverse" tubules because they are transversely oriented to the long axis of the cell). They are centered on the Z band with only one tubule per sarcomere. Mammalian skeletal muscle is modified to have two T tubules per sarcomere, which reduces the distance over which calcium must diffuse to reach the sliding filaments. This results in an extremely fast twitch that can provide an important survival benefit. Cardiac muscle does not require such a fast activation time and, hence, a single T tubule per sarcomere is sufficient.

The SR in cardiac contractile cells consists of two types of structures: (1) the sarcotubular network, making up the bulk of the SR, is in proximity to the contractile machinery, and (2) the subsarcolemma cisternae, the region at which the SR abuts the T tubules. The subsarcolemma cisternae are equivalent to the terminal cisternae of skeletal muscle cells.

Finally, cardiac myocytes contain a large quantity of mitochondria reflecting the aerobic nature of cardiac muscle metabolism. Most of the metabolic energy of the heart comes from oxidative metabolism of fatty acids and lactate, with glucose accounting for only a small fraction of the energy source. Although the heart can derive some energy by anaerobic glycolysis of glucose, it is not enough to keep up with the high energy demands of a beating heart. As a result, interruption of the heart's oxygen supply will cause a cessation of mechanical activity within less than 1 min and irreversible injury of the cells will begin within 20 min if the oxygen supply is not restored.

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