Cardiac pacemaker
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The
Sometimes a secondary pacemaker sets the pace, if the SA node is damaged or if the
Control
Primary pacemaker
The
The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells. Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells. This starts the depolarization and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes.
Cells in the SA node spontaneously depolarize, ultimately resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers via the autonomic nervous system, so that the average resting heart rate in adult humans is about 70 beats per minute.
Secondary (AV junction and Bundle of His)
Impulses from the sinus node reach the atrioventricular node which acts as the secondary pacemaker. The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.
Further down the electrical conducting system of the heart is the
The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the
Generation of action potentials
There are 3 main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase 1 or 2, just phases 0, 3, and 4.
Phase 4 - Pacemaker potential
The key to the rhythmic firing of pacemaker cells is that, unlike other neurons in the body, these cells will slowly depolarize by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials.
In all other cells, the resting potential (-60mV to -70mV) is caused by a continuous outflow or "leak" of potassium ions through ion channel proteins in the membrane that surrounds the cells. However, in pacemaker cells, this potassium permeability (efflux) decreases as time goes on, causing a slow depolarization. In addition, there is a slow, continuous inward flow of sodium, called the "funny" or pacemaker current. These two relative ion concentration changes slowly depolarize (make more positive) the inside membrane potential (voltage) of the cell, giving these cells their pacemaker potential. When the membrane potential gets depolarized to about -40mV it has reached threshold (cells enter phase 0), allowing an action potential to be generated.
Phase 0 - Upstroke
Though much faster than the depolarization of phase 4, the upstroke in a pacemaker cell is slow compared to that in an axon.
The SA and AV node do not have fast sodium channels like neurons, and the depolarization is mainly caused by a slow influx of calcium ions. (The funny current also increases). Calcium enters the cell via voltage-sensitive calcium channels that open when the threshold is reached. This calcium influx produces the rising phase of the action potential, which results in the reversal of membrane potential to a peak of about +10mV. It is important to note that intracellular calcium causes muscular contraction in contractile cells, and is the effector ion. In heart pacemaker cells, phase 0 depends on the activation of L-type calcium channels instead of the activation of voltage-gated fast sodium channels, which are responsible for initiating action potentials in contractile (non-pacemaker) cells. For this reason, the pacemaker action potential rising phase slope is more gradual than that of the contractile cell (image 2).
Phase 3 - Repolarization
The reversal of membrane potential triggers the opening of potassium leak channels, resulting in the rapid loss of potassium ions from the inside of the cell, causing repolarization (Vm gets more negative). The calcium channels are also inactivated soon after they open. In addition, as sodium channels become inactivated, sodium permeability into the cell is decreased. These ion concentration changes slowly repolarize the cell to resting membrane potential (-60mV). Another important note at this phase is that ionic pumps restore ion concentrations to pre-action potential status. The
Clinical significance
Damage to the SA node
If the SA node does not function, or the impulse generated in the
Ectopic pacemaker
An
Artificial pacemakers
An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or just pacemaker is an implanted medical device that generates electrical impulses delivered by electrodes to the chambers of the heart either the upper atria, or lower ventricles to cause the targeted chambers to contract and pump blood. By doing so, the artificial pacemaker takes over from the primary SA node pacemaker to regulate the function of the heart's electrical conduction system.
References
- )
- ISBN 0-13-193480-5.
- ^ Junctional Rhythm at eMedicine