Lesson 3 - Pacemaker and purkinje action potentials Flashcards
Describe the role of the SA node in cardiac contraction.
The SA-node is the natural cardiac pacemaker and is composed of specialized myocytes that have the property of automaticity. The SA-node’s rapid pace of depolarization controls the overall rate and rhythm of the heart.
What are the phases of pacemaker cell’s action potential?
- Phase 4: Spontaneous Depolarization
- Phase 0: Depolarization (or Upstroke)
- Phase 3: Repolarization
Describe Phase 4 of the pacemaker action potential.
- Compared to the non-pacemaker action potential, the SA-nodal action potential has no true resting membrane potential and instead depolarizes slowly. - - The action potential in
pacemaker cells has a resting potential of -60 mV. At this phase, there is a continuous
outward leak of potassium ions and slow inward flow of sodium ions, called the funny
current. At the same time, calcium ions also flow inward. This continuous inward flow of
sodium and calcium ions makes the cell more positive.
Describe Phase 0 of the pacemaker action potential.
When the SA-nodal action potential reaches the threshold potential of -40 mV, the T-type
calcium channels open and release more calcium ions. The increased inflow of calcium ions
further increases the action potential to -30 mV. At this stage, the T-type calcium channels close and the L-type calcium channels open. More calcium ions are released, depolarizing the cell to 0 mV. The inward movement of calcium ions intracellularly is slower than that of the non-pacemaker action potential, which is why the slope of phase 0 is slower.
Describe Phase 3 of the pacemaker action potential.
Upon reaching 0 mV, the calcium channels are inactivated and the influx of calcium ions is
interrupted. Potassium channels open and allow potassium ions to leave the cell. The efflux
of potassium ions causes the cell to repolarize slowly. As the downward slope of
repolarization reaches -60 mV, the funny current is reinitiated and the cycle is repeated.
Describe AV-Nodal action potential.
With a few minor variations in phase 0, the AV-nodal and SA-nodal action potentials are comparable.
In order to provide enough time for ventricular filling, the AV-node’s action potential is delayed. Impulses move slowly across the AV-node because of the twisting nature of the cellular pathways and the nodal cell diameter. The AV-node acts as the intrinsic pacemaker at a reduced rate in the case that the SA-node fails.
Describe the Purkinje action potential.
The Purkinje action potential resembles that of the cardiac myocytes. The Purkinje fibers
transmit action potentials through the heart muscle, starting from the apex and then to the left and right ventricles. This allows the heart to contract as a whole. Depolarization of the
Purkinje fibers is rapid due to larger cell diameter. The ventricular cells are last to be
depolarized due to shorter duration of the action potential, however it is the first to be
repolarized.
Describe cardiac rate control in pacemaker cells.
Under normal circumstances, the dominant pacemaker of the heart is the SA-node, which
has an inherent rate of 60-100 bpm. In cases when the SA-node malfunctions, other specialized cells are capable of generating an impulse for depolarization. These specialized cells are the AV-node, which has an inherent rate of 40-60 bpm, and His-Purkinje system, which has an inherent rate of 20-40 bpm. This fallback mechanism is important for survival because it maintains cardiac output (volume of blood pumped in 1 minute) in cases of
inappropriate function of the SA-node and AV-node.
What is the normal activation sequence in the pacemaker conduction system?
- SA Node (60-100 bpm)
- Atrial myocardium
- AV Node (40-60 bpm)
- Bundle of His (20-40 bpm)
- Bundle Branches (20-40 bpm)
- Purkinje network (20-40 bpm)
- Ventricular myocardium
