Unformatted text preview: Pacemaker Activity - 99% of cardiac muscle cells are contractile and do not initiate their own action potentials. The remaining 1% of the cells are autorhythmic and intrinsically initiate their own action potentials at a regular frequency. This process is referred to as pacemaker activity and is controlled by the generation of pacemaker potentials. Pacemaker Potential - an oscillation of the membrane potential which causes the cell to reach threshold and generate an action potential at a regular interval (Figure 9-7). It is controlled by 3 separate changes in membrane permeability. A transient decrease in K+ permeability (due to inactivation) coupled to a constant permeability to Na+ causes the membrane potential to depolarize. This depolarization causes an increase in permeability to Ca++ (T-channel) that leads to further depolarization of the membrane potential and causes the cell to reach threshold. The cell generates an action potential when a second increase in permeability to Ca++ occurs (L-channel). The depolarization resulting from the action potential causes an increase in K+ permeability and the membrane potential repolarizes. When the cell repolarizes, the K+ permeability again decreases (due to inactivation) and the process starts over again (Figure 9-7). Nodes - specialized cardiac muscle cells capable of pacemaker activity are grouped together to form nodes (Figure 9-8,9-10). ...
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- Fall '08