Heart Physiology: Cardiac Action Potentials

Cardiac Action Potentials

• The cardiac action potential is a brief change in voltage (membrane potential) across the cell membrane of heart cells.
• It is caused by the movement of charged atoms (ions) between the inside and outside of the cell, through proteins called ion channels.
• Cardiac action potentials differ from action potentials found in other types of electrically excitable cells, such as nerves.
• Unlike the action potential in skeletal muscle cells, the cardiac action potential is not initiated by nervous activity.
• Instead, it arises from a group of specialized cells known as pacemaker cells, which have automatic action potential generation capability.
• In healthy hearts, these cells form the cardiac pacemaker and are found in the sinoatrial node in the right atrium.

Phases of Cardiac Muscle Action Potential (Fast Response)

• Phase 0 (Depolarization): Fast sodium channels open, allowing sodium to rapidly flow into the cell and depolarize it.
• Phase 1 (Initial Repolarization): Fast sodium channels close, and potassium ions leave the cell through open potassium channels.
• Phase 2 (Plateau): Calcium ions influx and potassium efflux, causing the action potential to plateau.
• Phase 3 (Rapid Repolarization): Calcium channels close, and potassium channels open, permitting potassium ions to exit the cell rapidly.
• Phase 4 (Resting Membrane Potential): The cell membrane potential returns to its resting level, averaging about -80 to -90 millivolts.

Sinoatrial Node Action Potentials (Slow Response)

• Cells within the sinoatrial node have no true resting potential but generate regular, spontaneous action potentials.
• The depolarizing current is carried into the cell primarily by relatively slow Ca++ currents instead of by fast Na+ currents.
• The action potentials are slower due to the lack of fast Na+ channels and currents operating in SA nodal cells.

Control of the Heart by the Autonomic Nervous System

• The sympathetic nervous system increases heart rate (positive chronotropy) by decreasing the time to produce an action potential in the SA node.
• The parasympathetic nervous system decreases heart rate (negative chronotropy) by increasing the time taken to produce an action potential in the SA node.

Velocity of Signal Conduction in Cardiac Muscle

• The velocity of conduction of the excitatory action potential signal along both atrial and ventricular muscle fibers is about 0.3 to 0.5 m/sec.
• The velocity of conduction in the specialized heart conductive system—in the Purkinje fibers—is as high as 4 m/sec in most parts of the system.
• The velocity of action potential in the AV node is about 0.02-0.05 m/sec.
• Three important factors influencing the speed of action potential conduction are:
  • The number of gap junctions
  • The size or diameter of the fibers
  • The amplitude of the action potential (potential difference between the peak of depolarization and the resting potential of the cell membrane)

This quiz covers the cardiac action potential, a brief change in voltage across the cell membrane of heart cells caused by the movement of ions through ion channels.