Coordination of Contraction Across Cardiac Muscle Fibers

In order for the contraction of a heart chamber to be able to propel the expulsion of fluid, all the individual muscle fibers making up the walls of that chamber must contract together. It is this unified contraction that constricts the cavity of the chamber and drives out the blood into the blood vessels of the circulation. In skeletal muscles, an action potential in one muscle fiber is confined to that fiber and does not influence neighboring fibers; therefore, contraction is restricted to the particular fiber undergoing an action potential. In cardiac muscle, however, the situation is quite different. When an action potential is generated in a cardiac muscle fiber, it causes action potentials in the neighboring fibers, which in turn set up action potentials in their neighbors, and so on. Thus, the excitation spreads rapidly out through all the muscle fibers of the chamber. This insures that all the fibers contract together.

What characteristic of cardiac muscle fibers allows the action potential to spread from one fiber to another? The answer can be seen by looking at the microscopic structure of the cells of cardiac muscle, shown schematically in Figure 12-3. At the ends of each cardiac cell, the plasma membranes of neighboring cells come into close contact at specialized structures called intercalated disks. The contact at this point is sufficiently close that electrical current flowing inside one fiber can cross directly into the interior of the next fiber; in electrical terms, it is as though the neighboring cells form one larger cell. Recall from Chapter 6 that an electrical current flowing along the interior of a fiber has at each point two paths to choose from: across the plasma membrane or continuing along the interior of the fiber. The amount of current taking each path at a particular point depends on the relative resistances of the two paths; the higher the resistance, the smaller the amount of current taking that path. Normally, at the point where one cell ends and the next begins, there is little opportunity for current to flow from one cell to the other because the current would have to flow out across one cell membrane and in across the other in order to do so; this is a high resistance path because current must cross two membranes. At the specialized structure of the intercalated disk, however, the resistance to current flow across the two membranes is low, so that the path to the interior of the neighboring cell is favored. This means that depolarizing current injected into one cell during the occurrence of an action potential can spread directly into neighboring cells, setting up an action potential in those cells as well. The low resistance path from one cell to another is through membrane structures called gap junctions. These structures consist of arrays of small pores directly connecting the interiors of the joined cells. The pores are formed by pairs of protein molecules, one in each cell, that attach to each other and bridge the small extracellular gap between the two cell membranes (Figure 12-3). The pores at the center of each of these gap junction channels are

PL- V Lt 'v

^WPP

■■■■MB ^JB^HH

Gap in me

Cardiac Cell Current Flow Directional

Figure 12-3 Electrical current can flow from one cardiac muscle cell to another through specialized membrane junctions located in a region of contact called the intercalated disk. The current flows through pores formed by pairs of gap junction channels that bridge the extracellular space at the intercalated disk.

nel ell 2

Figure 12-3 Electrical current can flow from one cardiac muscle cell to another through specialized membrane junctions located in a region of contact called the intercalated disk. The current flows through pores formed by pairs of gap junction channels that bridge the extracellular space at the intercalated disk.

Figure 12-4 An experiment in which the membrane potentials of two cells are measured simultaneously with intracellular microelectrodes. (a) A depolarizing current is injected into cell 1. (b) If the cells are not electrically coupled, the depolarization occurs only in the cell in which the current was injected. (c) If the cells are electrically coupled via gap junctions, a depolarization occurs in cell 2, as well as in cell 1.

aligned, permitting small molecules like ions to pass directly from one cell to the other.

When electrical current can pass from one cell to another, as in cardiac muscle, those cells are said to be electrically coupled. Figure 12-4 illustrates an electrophysiological experiment to demonstrate this behavior. When current is injected into a cell, no response occurs in a neighboring cell if the cells are not electrically coupled. If the two cells are coupled via gap junctions, a response to the injected current occurs in both cells because the ions carrying the current inside the cell can pass directly through the gap junction. If the depolarization is large enough, an action potential will be triggered in both cells at the same time.

0 0

Responses

  • morven kelly
    What are action potentials that spread from one muscle fiber to another through structures called?
    6 years ago

Post a comment