All of the above functions are dependent upon the opening and closing of transmembrane ion channels for sodium, potassium, and calcium in cardiac tissue.5 The opening and closing of these ion channels depends upon both the voltage across cell membrane, and time. Channels that are open will close spontaneously with time. Simplistically, voltage dependent channels can exist in at least three states. These are:
1. Resting and ready to be opened by an appropriate voltage change across the cell membrane in which they are sited.
2. Open and in a conducting state for their particular ion.
3. Inactive and in a nonconducting state from where they convert back to the resting state with time, or they can be locked in this state at particular membrane voltages.
The major ion channel families in cardiac tissue include those for sodium, calcium, and potassium ions, and possibly chloride and stretch channels. There is only one major type of sodium channel in cardiac tissue, one major type of calcium channel, and many types of potassium channels. The genes for most of these channels have been identified, as have their proteomes, as well as the intimate details of their anatomical and functional mechanisms.5
The presence and functioning of these channels varies with cardiac tissue type to provide the basic electrical behavior characteristic of different cardiac tissues that include SA and AV nodes, various types of atrial tissue, His/ Purkinje tissue (arising at the ventricular end of the AV node and spread throughout the ventricles providing a highspeed conduction pathway to the ventricles), and ventricular tissue that anatomically and electrophysiologically exists in at least three types (endocardial, mid-myocardial, and epicardial). Each type of tissue has channels that, when open, provide:
1. Depolarizing currents (Na+ and/or Ca2 + dependent currents) that occur in all tissues.
2. Pacing currents normally found in the SA and AV nodes, and His/Purkinje tissue.
3. Repolarizing currents (K+ dependent currents) which vary widely with different species and tissues, but provide the mechanism whereby the action potential returns to a stable resting state in most tissue.
4. Miscellaneous other ionic currents that are of lesser importance and are 'gated' by either voltage, hormones or neurotransmitters.
Some of the characteristic action potentials (and their underlying ionic currents) found in the various types of heart tissue are shown in Figure 2 that includes both pacing and nonpacing cells. In pacing cells the membrane potential is not stable and the constant decay is due to the pacemaker current. The currents that participate in the generation of such potentials are also illustrated.
Ventricular action potential
Atrial action potential
SA nodal action potential
IKr IKs IKACh
Figure 2 The characteristic shapes of various cardiac action potentials, and the ionic currents that underlie the genesis of such potentials. The figure shows representative action potentials seen in (left to right) ventricular, atrial, and nodal cardiac cells. An action potential for ventricular Purkinje cells is superimposed on the ventricular potential. In the case of the action potentials, calibration is in millivolts (mV) on the vertical axis, and time in milliseconds (ms) on the horizontal axis. The lower part of the figure shows the different currents responsible for the generation of the different action potentials. The time axis for these currents is the same as for potential, whereas that for currents is not to scale. A downward current indicates an inward current and vice versa.
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