CARDIAC PACEMAKER CELLS
www.propofology.com
Dr. David Lyness
@Gas_Craic
SPONTANEOUS ELECTRICAL DEPOLARISATION, GENERATING THE INTRINSIC ELECTRICITY OF THE HEART
MAGNESIUM
Mg acts as a physiologic Calcium Channel inhibitor by slowing slow
L-Type Calcium channel during PHASE 2. This reduces further Ca release by the Sarcoplasmic Reticulum which leads to reduced automaticity, contractility and conductivity through cardiac tissue, including the AVN.
- The heart has auto-rhythmicity: a denervated heart (cardiac transplant patient) continues to contract spontaneously.
- Pacemaker cells do not have a stable resting action potential.
- Spontaneous depolarization of the pacemaker potential = auto-rhythmicity.
- The pacemaker potential = a decrease in membrane permeability to potassium, a slow inward current because of calcium influx via
T-type (transient) calcium channels, and an increased sodium current because of sodium– calcium exchange.
- Once the threshold potential is reached, L-type calcium channels open, calcium ions enter the cell, and depolarization occurs.
- In contrast to the cardiac myocyte action potential, there is no inward movement of sodium ions during depolarisation.
- Repolarization (phase 3) occurs because of an increase in potassium permeability.
- At the SA node, potassium permeability can be further enhanced by vagal stimulation.
- This has the effect of hyperpolarizing the cell and reducing the rate of firing.
- Sympathetic stimulation has the opposite effect.
- The SA node has the highest rate of spontaneous depolarization and therefore suppresses the other pacemakers.
- In the denervated heart, the SA node discharges at a rate of approximately 100 times/min.
- Vagal tone leads to a lower heart rate in healthy subjects at rest.
- From the SA node, impulses spread throughout the atria to the AV node at a rate of 1 m/s.
- The AV node is the only means of electrical connection between the atria and the ventricles.
- Conduction here is slow (approximately 0.05 m s21) = AV node will only transmit a maximum of 220 impulses min, so protecting
- In the denervated heart, the SA node discharges at a rate of approximately 100 times/min.
- Vagal tone leads to a lower heart rate in healthy subjects at rest.
- From the SA node, impulses spread throughout the atria to the AV node at a rate of 1 m/s.
- The AV node is the only means of electrical connection between the atria and the ventricles.
- Conduction here is slow (approximately 0.05 m s21) = AV node will only transmit a maximum of 220 impulses min, so protecting
the ventricles from high rates of atrial depolarization.
- Depolarisation spreads AVN —> Bundle of His — > right and left bundle branches.
- The left bundle itself divides into anterior and posterior divisions.
- Then, impulses travel through the ventricular muscle via network of Purkinje fibres, at a velocity of 1–4 m/s.
- The conducting system is arranged so that the apices of the ventricles contract before the bases, propelling blood out of the chambers.
- Depolarisation spreads AVN —> Bundle of His — > right and left bundle branches.
- The left bundle itself divides into anterior and posterior divisions.
- Then, impulses travel through the ventricular muscle via network of Purkinje fibres, at a velocity of 1–4 m/s.
- The conducting system is arranged so that the apices of the ventricles contract before the bases, propelling blood out of the chambers.
- The pacemaker potential is seen in cells in the SA and AV nodes. - Different from myocytes because phases 1 and 2 are absent.
LOADING AWESOME
0 = DEPOLARISATION - Voltage Gated Potassium Channels Open
3 = REPOLARISATION - Potassium Efflux
4 = PACEMAKER POTENTIAL - slow influx of Na and increase in calcium conductance
NO SODIUM INFLUX DURING DEPOLARISATION
- Cardiac myocytes can be divided into work cells and pacemaker cells.
- Work cells have a large stable resting membrane potential and display a prolonged action potential with a plateau phase.
- Pacemaker cells have smaller unstable resting potentials and spontaneously depolarize, generating the intrinsic electrical activity of the heart.
- Pacemaker cells are found in the SA and AV nodes.
- The cells of the bundle of His and some Purkinje cells are also capable of spontaneous firing.
LOADING AWESOME