Cardiac Muscle Electrical Activity

Sarcomeres are linked end to end into assemblies known as myofibrils, which run the length of the long axis of the cardiac cell and are also placed side to side to fill most of the internal volume of the cell. The nucleus of a mature cardiac cell is found on the periphery of the cell along with the sarcoplasmic reticulum. The sarcoplasmic reticulum is a vesicular structure that acts as an internal calcium-store and is an analog of the endo-plasmic reticulum of other cells.

Cardiac cells are connected and communicate with one another by junctions of two types. First, intercalated disks form strong mechanical bonds between myocytes that allow force to be transmitted across the myocardium. These structures are formed by the protein-protein associations at the membrane surface of the neighboring cells. Second, gap junctions form electrical connections between cardiac cells.

Sarcomere Length (mm)

Fig. 7. Length-tension relationship. Isometric force is, in part, determined by the overlap of the thick and thin filaments. At short sarcomere lengths, the contractile proteins are too crowded to work optimally. At long sarcomere lengths, there is a progressive reduction in the overlap of the thick and thin filaments. Optimal overlap of the thick and thin filaments results in maximal isometric force output.

Sarcomere Length (mm)

Fig. 7. Length-tension relationship. Isometric force is, in part, determined by the overlap of the thick and thin filaments. At short sarcomere lengths, the contractile proteins are too crowded to work optimally. At long sarcomere lengths, there is a progressive reduction in the overlap of the thick and thin filaments. Optimal overlap of the thick and thin filaments results in maximal isometric force output.

Proteins known as connexins form six-member hemi-chan-nels called connexons on the surface of cardiac cells; connexons on the surface of adjacent cardiac cells dock with one another to form gap junctions. The opening of gap junctions provides for direct electrical and chemical communication between the cytoplasmic space of the adjoining cells. Most importantly, activation signals are passed through gap junctions from cell to cell in cardiac tissue. The electrical communication, provided by gap junctions, facilitates the seemingly simultaneous coordinated contractions of cardiac muscle (Fig. 10, p. 118). For more details, refer to Chapter 9. Note that intercalated discs connect ends of muscle cells to one another and enable frequent branching of myocardial cells at points of anastomosis; also note the separation of fiber paths by capillaries. With this arrangement of fibers, lateral shifting and interdigitation of cells is facilitated. However, longitudinal shifting of cells relative to one another is practically impossible (Fig. 11, p. 118).

The electrical activity of cardiac muscle cells is fundamental to normal function and takes advantage of the properties of the cell membrane to pass charged species selectively from inside to outside and vice versa. Most cells build a charge gradient through the action of ion pumps and ion-selective channels. The charge difference across a membrane is known as the resting membrane potential of a cell (Fig. 9). The energy related to discharging this potential is commonly coupled with cellular functions. In excitable cells, transient changes in the electrical potential (action potentials) are used to either communicate or do work. Importantly, in the myocyte, it is required to initiate the process known as excitation-contraction-coupling.

The extracellular fluid has an ionic composition similar to that of blood serum and has millimolar free calcium (Table 1). The intracellular concentration of calcium is higher, but at rest much of it is bound to proteins or sequestered in organelles (mitochondria, sarcoplasmic reticulum). Hence, free myoplas-mic concentrations are very low (~10_7 mM) (Fig. 12).

ATP-dependent ion pumps, channel proteins, and ion exchange proteins are all required to maintain the difference in

Fig. 8. A schematic illustration of the shape changes in ventricular fiber arrangement during the cardiac cycle is shown.
Fig. 9. Ion channels and pumps work to maintain a polarization of ions across the cell membrane. See text for details.

ion concentrations. This separation of charged species across a resistive barrier (in this case, the cell membrane) generates an electrical potential Eion. For individual ions, the value of this potential can be calculated using the Nernst equation:

ion zF [inside]

Fig. 10. Gap junctions. (Above) Surface of the cell membrane with a plaque of connexons. (Below) Gap junction formation by the docking of connexons on adjacent cell membranes.

Fig. 11. Geometry of cardiac muscle cells. Cardiac cells often branch and connect to adjacent myocytes. At the intercalated disks between myocytes, gap junctions allow for cell-cell communication, and des-mosomes provide mechanical support. See text for further details.

tions of ion species on both sides of the membrane as well as their relative permeabilities. To determine the overall membrane potential Em, the Goldman-Hodgkin-Katz equation takes into account the equilibrium potentials for individual ions and the permeability (conductance) of the membrane for each species such that

Fig. 11. Geometry of cardiac muscle cells. Cardiac cells often branch and connect to adjacent myocytes. At the intercalated disks between myocytes, gap junctions allow for cell-cell communication, and des-mosomes provide mechanical support. See text for further details.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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