Excitation Of The Muscle Cell

All muscle cells have resting membrane potentials in the range of —70 to —90 mV. As in nerve, this potential is due to the presence of ionic concentration gradients (with K+ being greater intracellularly and Na+ greater extracellularly) and to the resting membrane being much more permeable to K+ than to Na+. Muscle cells are excitable due to the presence of voltage-dependent ion channels in their cell membranes. However, the type of channels present, the manner in which channels are activated, and the way in which their activation leads to E-C coupling vary among muscle types.

Skeletal Muscle

Skeletal muscle closely resembles nerve in that the voltage-dependent channel is a sodium channel. Once the sarcolemma is depolarized to the threshold for opening of sodium channels, a positive feedback opening of channels occurs and the membrane potential reverses toward the sodium equilibrium potential. Opening of the sodium channel is time dependent; the channel quickly inactivates and the membrane repolari-zes. The entire action potential lasts about 1 msec. Skeletal muscles do not exhibit spontaneous action potentials because there are no inherent mechanisms to depolarize the cells to threshold. Also, there are no connections between individual skeletal muscle cells to allow for conduction of activity from one cell to another. Each cell depends on innervation by a motor nerve. These nerves release acetylcholine (ACh) at their junctions with the muscle cell (neuromuscular junction). The ACh binds to specific receptors on the muscle cell membrane to bring about an increase in permeability of that portion of the cell membrane to Na+ and K+. This results in depolarization of adjacent areas of the membrane to threshold, at which point an action potential ensues (see Chapter 6).

Skeletal muscle cells are large, multinucleated cells that result from the embryonic fusion of many myoblasts. Thus, sarcomeres and SR in the center of the muscle are many microns away from the cell surface. This would make it impossible for sarcolemmal action potentials to have any effect on these structures, if it were not for the fact that the sarcolemma invaginates to make contact with each and every terminal cisterna. If a muscle cell is sectioned properly, a sarcolemmal invagination, a transverse (T) tubule, can be seen running between two terminal cisternae at right angles to the SR. This complex of tubules is called a triad (Fig. 6). Although there is intimate contact at the triad, the T tubule and the SR retain their individual membranes so that lumenal separation is maintained. The T tubules allow action potentials to penetrate into the interior of the cell and to influence the terminal cisternae of the SR. The action potential does not propagate into the SR, but it does bring about the release of calcium. Because the action potential spreads rapidly over the entire sarco-lemma, including the T tubules, the entire muscle can be activated almost simultaneously.

Cardiac Muscle

The events in cardiac muscle differ from those in skeletal muscle; in addition, there are differences among the various types of cardiac muscle cells. As discussed in

T Tubules

T Tubules

Terminal cisternae Longitudinal SR

FIGURE 6 Transverse (T) tubules in skeletal muscle. The sarco-lemma invaginates to reach the interior of the muscle cell such that it contacts the terminal cisternae of the SR. Thus, events initiated by the membrane action potential are brought into close proximity to the SR to release CA2+ by mechanisms that are not understood. The rapid, almost simultaneous propagation of the action potential down the T tubules allows for almost simultaneous initiation of the contractile event.

Terminal cisternae Longitudinal SR

FIGURE 6 Transverse (T) tubules in skeletal muscle. The sarco-lemma invaginates to reach the interior of the muscle cell such that it contacts the terminal cisternae of the SR. Thus, events initiated by the membrane action potential are brought into close proximity to the SR to release CA2+ by mechanisms that are not understood. The rapid, almost simultaneous propagation of the action potential down the T tubules allows for almost simultaneous initiation of the contractile event.

Chapter 11, cardiac muscle cells in the sinoatrial and atrioventricular nodes exhibit action potentials spontaneously and act as pacemakers, whereas other muscle cells are specialized to conduct action potentials throughout the ventricles. These cells are not discussed in this chapter because their importance is not related to their contractile activity. The cells described here are the ventricular myocardial cells, which contract to pump blood through the pulmonary and systemic circuits. In these cells, the resting membrane potential has the same basis as that in nerve and skeletal muscle. However, when the membrane is depolarized to threshold, two types of voltage-dependent channels open—a sodium channel that opens and closes within a few milliseconds (similar to what is found in skeletal muscle) and a calcium channel that opens more slowly and can stay open for several hundred milliseconds. It is this second channel that accounts for calcium entry from the extracellular space. Like the sodium channel, opening of the calcium channel is also time dependent. It eventually closes to allow for repolarization of the cell.

Myocardial cells normally do not exhibit spontaneous action potentials; however, they are connected to one another via low-resistance membrane contact points. If the myocardial cells are connected to pacemaker cells and conducting cells, the action potentials initiated in the pacemaker cells will rapidly propagate throughout the myocardial cells. Conduction from cell to cell is by way of electrotonic spread and does not require a chemical transmitter such as ACh.

Cardiac muscle cells are smaller than skeletal muscle cells; however, they do have an extensive T tubule system. In fact, in cardiac muscle, the T tubules are larger. They not only invaginate to run perpendicular to the SR, but they also turn and run parallel alongside the SR. Perhaps it is these structural characteristics that allow the T tubules not only to participate in the release of calcium from the terminal cisternae but also to facilitate the entry of significant amounts of calcium during the action potential. As in skeletal muscle, T tubules allow for spread of excitation into the interior of the cells so that all portions are excited almost simultaneously (see Chapter 11 for further discussion of these points).

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