In order to understand how the heart regulates itself, there are some characteristic features of cardiac muscle cells to consider more closely.
All resting muscle cells, including cardiac muscle cells, are polarized, i.e. the interior of the cells are negative with respect to the exterior. Each cell is surrounded by a membrane being permeable to particular ions, especially Na*, K*.
In the resting cell, there is an equilibrium between the concentration of these ions within the cell and outside of the cell, but the distribution of the ions is unequal, there being a high extracellular concentration of Na* and a high intracel-lular concentration of K*. Since there are more extracellular Na+ ions than intracellular K* ions, the interior of the cell will be negative with respect to the exterior, creating the membrane potential.
If a microelectrode is inserted into the cell, this resting membrane potential can be measured as negative, approx. -90 mV.
When the cell receives a stimulus, e.g. if an electrical current is applied to to its surface, the membrane potential changes and becomes less negative, triggering an influx of Na* ions into the cell.
If only a very weak current is applied, it will result merely in a transient, moderate reduction of the membrane potential. A stronger stimulus, however, will cause the number of Na+ ions pouring into the cell to become so great that the resting membrane potential breaks down: the cell is activated and contracts. The formerly negative membrane potential rises abruptly to zero and beyound to an overshoot of approximately +30 mV. For cardiac cells this depolarisation occurs at approx. -50 mV, called the threshhold potential.
After the rapid cell depolarization and contraction, Na* ions are pumped out of the cell again, causing at first a rapid phase of repolarisation towards zero potential, then a plateau phase with the potential maintained at or near zero, and finally a rapid repolarisation phase to restore the resting membrane potential.
Tiie entire process of potential changes from the beginning of depolarization to the end of repolarisation is called the action potential of the cell.
Depolarization in cardiac muscle cells is a rather short process that lasts only a few milliseconds, whereas repolarisation lasts for about 500-600 ms.
The cardiac muscle cell can only depolarize when the membrane potential is more negative than the threshold potential, i.e. it is not possible to depolarize the cell in the repolarization period, the cell is said to be in the refractory period. This indicates that impulses can only spread to non-excited cells.
In ordinary muscle, the cells lie parallel and the depolarization of one cell does not influence its neighbouring cells. Myocardial cells, however, are joined end-to-end by structures to form an interrelated network. Therefore, they are able not only to contract but also to initiate a depolarisation of their neigbour cells and propagate the contraction to each other. In this way all cells are stimulated, providing coordinated contraction of the heart and effective pumping.