1.5.1 Cellular physiology
The normal activation process within the heart and within each individual myocyte is dependent on specialised cellular physiology. Cardiac tissue has four important characteristics. These properties are present to differing degrees in different cells within the heart.
- Conductivity: Cardiac cells are joined in a functional syncytium that allows excitation to pass from cell to cell. Specialised cardiac cells have fast or slow conducting properties which govern the rate of spread of the electrical impulse through them.
- Excitability: Like nervous tissue, individual cardiac cells exhibit the all-or-none phenomenon, i.e. once a threshold is reached they are completely depolarised by an action potential.
- Automaticity: Specialised cardiac cells possess automaticity. This means that they will spontaneously depolarise due to the membrane potential grabecoming less negative until the threshold potential is reached. The rate at which this occurs depends on the type of cardiac tissue and on autonomic nervous control. It is also affected by disease, electrolyte conand by drug therapy.
- Refractoriness: This property ensures that all cardiac cells have a period after activation during which no level of further stimulus will cause an action potential. It is an important feature of myocardial cells because it prevents tetanic spasm of cardiac muscle. The duration of the refractory period also depends on the type of cardiac tissue, autonomic nervous control, disease states and drug therapy.
These electrophysiological features result from specific properties of the cardiac cell membrane. The membranes contain specialised pumps and channels which selectively concentrate specific ions within the cell and/or remove them from it. The most notable feature is a very high concentration of potassium and low level of sodium within the resting cell. The different concentrations of ions and proeither side of the membrane result in a potential difference in electrical charge across the cell membrane. The intracellular potential of a resting myocell is -90 mV in comparison with the extracellular fluid. Depolarisation results in a reversal of this potential difference, and is then followed by repoThe process has been divided into five phases, 0-4 (Figure 1.10). During each phase, specific channels and pumps are responsible for movement of different ions, each of which affect the membrane potential. The relative contributions of the different currents which result from this ion movement vary in different tissues within the conduction pathways. This determines the different properties of each specialised cell, and the effects of changes in extracellular ion concentrations, disease and drugs on the cell.