of the action potential is the phase of rapid depolarization. During this phase the cell is stimulated and the cell membrane becomes more permeable to sodium ions. Fast sodium channels open and sodium rushes into the cell.
In phase 1
, known as the period of initial repolarization, there is a brief very rapid attempt to return to resting membrane potential. During this time sodium influx ends in fast channels and potassium begins to flow out of the cell.
is called the plateau phase. During this phase there is an influx of calcium and sodium through the slow channels (though this sometimes also occurs during phases 0 and 1). This influx is responsible for the refractory period during which the depolarized state is maintained. Calcium also plays a role in maintenance of normal heart muscle contractility.
is known as the phase of rapid repolarization. Membrane potential is driven to the negative level due to the increased loss of intracellular potassium.
, the quiescent period, is the time when the cell returns to the resting membrane potential level (-90 mV). During this period, normal distribution of sodium and potassium are restored.
The sodium potassium pump plays a role during these latter phases. ATP is required to fuel this pump and an adequate level of serum magnesium insures that the pump performs properly. Repolarization will be prolonged if the pump malfunctions, thus prolonging the QT interval on the ECG. This may lead to a potentially lethal arrhythmia, torsade de pointes, a polymorphic ventricular tachycardia.
The heart can pump unless an electrical stimulus occurs first. Generation and transmission of electrical impulses depend on the automaticity, excitability, conductivity and contractility of cardiac cells.
- Automaticity refers to a cell's ability to initiate an impulse. Pacemaker cells possess this ability.
- Excitability results from ion shifts across the cell membrane and indicates how well a cell responds to an electrical stimulus.
- Conductivity is the ability of a cell to transmit an electrical impulse to another cardiac cell. Contractility refers to how well the cell contracts after receiving a stimulus.
As impulses are transmitted cardiac cells undergo cycles of depolarization and repolarization.
- Polarized - cardiac cells are at rest meaning - no electrical activity takes place
- Resting potential cell membranes separate different concentrations of ions such as sodium and potassium and create a more negative charge inside the cell.
- Cell depolarization (action potential) - a stimulus causes the ions to cross the cell membrane.
- Repolarization electrical charges within the cell reverse and return to normal. The cell attempts to return to its resting state.
- Cycles of depolarization-repolarization This cycle consists of 5 phases 0-4.
0 cell receives an impulse from a neighboring cell and is depolarized
1 - Early rapid repolarization (resting phase)
2 - Plateau phase period of slow repolarization
3 - Rapid repolarization phase During the last half of this phase the cell is in the relative refractory period, a very strong stimulus can depolarize it.
During phases 1,2 and the beginning of phase 3 the cell is in its absolute refractory period. No stimulus can excite the cell.
4 Resting phase of the action potential. By the end of phase 4 the cell is ready for another stimulus.
Once depolarization and repolarization occur the resulting electrical impulse travels through the heart along a pathway called the conduction system Impulses travel. SA node Located in the upper right corner of the right atrium. Pacemaker of the heart. 60-100 times a minute.
Internodal tracts and Bachmann's bundle to the AV node
Responsible for (delaying the impulses that reach it.) The nodal tissue itself has no pacemaker ability, but the tissue around it (junctional tissue) has pacemaker ability. 40-60 times a minute. This delay allows the ventricles to complete their filling phase as the atria contract. Also allows the cardiac muscle to contract to it's fullest for peak cardiac output.
Bundle of HIS: the Bundle branches Resumes the rapid conduction of the impulse through the ventricles. The bundle divides into the right and left branches. Purkenjie Fibers : Network of nervous tissue that extends through the ventricles. Can serve as a pacemaker at a rate of 20-40 times a minute.