Pacemaker Cell Action Potentials + Cardiac Conduction System

Cardiac Conduction System

Three cell populations in the heart are capable of spontaneously generating action potentials and setting the pace of the heart; makes up about 1% of the total number of cardiac muscle cells

Pacemaker Cell Action Potentials

Result from a reversal in membrane potential from about −60 mV to about +10 mV inside the plasma membrane; occurs because of unequal concentrations of calcium, sodium, and potassium ions moving through voltage-gated channels

Pacemaker Potential

Occurs slowly because of the lack of voltage-gated sodium ion channels in the sarcolemma of many pacemaker cells; Oscillates and occurs in a cycle with the last event triggering the first because of the HCN channels which open when the membrane hyperpolarizes, which begins a slow depolarization

Slow Initial Depolarization Phase

Begin in a hyperpolarized state of about −60 mV; hyperpolarized membrane triggers voltage-gated HCN channels to begin to open; more sodium ions leak in than potassium ions leak out

Full Depolarization Phase

When membrane reaches a threshold value of −40 mV, voltage-gated calcium ion channels open, calcium ions enter the cell which causes the membrane to fully depolarize

Repolarization Phase

Calcium ion channels are time-gated for closing so after about 100–150 msec they begin to close; voltage-gated potassium ion channels begin to open; potassium ions exit the cell, and the membrane begins to repolarize

Minimum Potential Phase

Potassium ion channels remain open until the membrane reaches its minimum potential; membrane is hyperpolarized, which triggers potassium ion channels to begin closing; the HCN channels begin to open, and the cycle begins again

Sinoatrial (SA) Node

Located in the right atrium inferior and lateral to the opening of the superior vena cava; cells have the fastest intrinsic rate of depolarization of about 60–70 or more times per minute; influenced by the sympathetic and parasympathetic nervous systems

Atrioventricular (AV) Node

Cluster of pacemaker cells located posterior and medial to the tricuspid valve; cells have a slower intrinsic rate of depolarization of about 40–50 times per minute

Purkinje Fiber System

Slowest group of pacemaker cells that depolarize about 20 times per minute; Action potentials rely on different ion channels, called Atypical Pacemakers; Include: Atrioventricular Bundle (AV Bundle)—Penetrates heart’s fibrous skeleton in inferior interatrial septum and superior interventricular septum; Right and Left Bundle Branches—Course along right and left sides of interventricular septum, respectively; Terminal Branches—Penetrate the ventricles and contact the contractile cardiac muscle cells

Pacing the Heart: Sinus Rhythm

Each population of pacemaker cells can pace the heart, but the one that depolarizes the fastest sets the rate; SA Node is the normal pacemaker of the entire heart; Electrical rhythms generated and maintained by the SA node are known as Sinus Rhythms; AV node and Purkinje fiber system only conduct action potentials generated by the SA node unless the SA node ceases to function; the AV node can successfully pace the heart, but slowly

Ectopic Pacemaker

Group of other cells attempt to pace the heart resulting in irregular rhythms

Conduction Pathway through the Heart

(1) SA node generates an action potential, which spreads to atrial cells and the AV node—Requires about 0.03 second; (2) After the AV node delay, the action potential is conducted to the AV bundle and then to the right and left bundle branches; (3) The action potential spreads from the bundle branches along the Purkinje fibers to the contractile cells of the ventricles

AV Node Delay

Conduction slows as the result of the low number of gap junctions between AV nodal cells, and the presence of the nonconducting fibrous skeleton that surround the AV node, known as the AV node delay; lasts about 0.13 second; allows time for the ventricles to fill with blood

Purkinje Fiber Conduction

Purkinje fibers conduct action potentials faster than any other cell in the myocardium (150 times faster than cells of the AV node). This speed is due to differences in their ion channels and the large number of gap junctions in their membranes; lasts about 0.06 second; Total action potential duration is about 0.22 second