Myocardial Autorhythmicity and Contractile Myocytes

AP for Skeletal and Cardiac Myocytes

• cardiac pacemaker cell has longer duration than skeletal muscle

• cardiac contractile muscle has even longer duration

• shape of cardiac contractile muscle graph could change depending on heart rate

• membrane potential of the pacemaker cell does not remain constant at resting state after repolarization

Autorhythmic Cells

• spontaneously generate action potentials

• depolarization of the autorhythmic cells then spread rapidly to adjacent contractile cells through gap junctions

Autorhythmicity

• refers to the combination of both the automaticity and rhythmicity properties

• auto is to initiate own pace making activity

• ryth is to maintain the regularity of pace making activity

• membrane slowly depolarizes and drifts toward threshold between action potentials

• pace maker cells exhibit the slow response in their action potentials

• 3 phases of the permeability changes for various ions are observed in the AP of the pacemaker cells

Phase 4 Autorhythmicity

• gradual depolarization due to slow ionic influx creating funny current (I_f)

• I_f is initiated by the opening of Na channels causing slow influx of Na into pacemaker cells through HCN channels which open during the end of repolarization

• There are opening of transient Ca channels during the last half of the pacemaker potential that give rise to Ca influx

• decays of K efflux due to closing of K channels towards end of repolarization

• onset of pacemaker current

• the net result is the resting membrane potential becomes progressively more positive for the pacemaker cells

Phase 0 Autorhythmicity

• opening of the long-lasting voltage gated channels for Ca after reaching threshold (rapid influx of Ca)

• depolarization occurs once the threshold potential is reached

• no fast Na channel is involved, and the slope of the upstroke is not as steep as that of the AP for the contractile myocytes

Phase 3 Autorhythmicity

• starts with the gradual closing of voltage gated Ca channels (Ca influx stops)

• voltage gated K channels are now activated (rapid efflux)

• repolarization occurs

• once the membrane potential reaches the maximum diastolic potential, phase 4 will start again

Rate of Diastolic Depolarization

• can influence cardiac rhythmicity

• at phase 4

• steeper slope will reach threshold quicker

• norep. increases rate of diastolic depolarization which increases slope of phase 4 which increases heart rate

Maximum Diastolic Potential

• can influence cardiac rhythmicity

• induces hyperpolarization by K efflux

• MDP becomes more negative, it needs a longer time to reach threshold potential, so heart rate decreases

• Ach also decreases the slop of phase 4, further decreasing heart rate

Threshold Potential

• can influence cardiac rhythmicity

• quinidine

• an antimalarial and antipyretic drug

• shifts threshold voltage towards zero

• takes longer time to reach the threshold potential, so decreased heart rate

Phase 0 Re/depolarization

• rapid depolarization phase

• occurs when the pacemaker potential spreads to the contractile myocytes and reaches the threshold of the contractile myocytes

• triggers influx of Na through Na channels

Phase 1 Re/depolarization

• early repolarization stage

• rapid inactivation of the Na channels

• together with the activation of the transient outward K current

• brief efflux of K

Phase 2 Re/depolarization

• plateau phase

• so you can push blood out with adequate time

• balance between K efflux and Ca influx

• K efflux: through K channels known as delayed rectifier K channels

• Ca influx: through long lasting Ca channels

Phase 3 Re/depolarization

• repolarization

• starts with inactivation of the L-type Ca channels

• with the continuation on the efflux of K

• inside the cell membrane becomes progressively more negative

• cell is unexcitable during phases 0,1,2, and part of 3

Phase 4 Re/depolarization

• restoration of ionic concentrations

• concentration of Na and K return to their resting state by Na-K pumps

• with 2 K entering and 3 Na leaving

• Ca by both the Na-Ca exchangers and ATP driven Ca pumps