Ch 14b Cardiovascular Physiology: Electrical Conduction and Cardiac Action Potential

Cardiovascular Physiology

Electrical Conduction in the Heart

The heart's rhythmic contractions are coordinated by a specialized electrical conduction system. This system ensures that the heart beats in a synchronized manner, allowing efficient pumping of blood throughout the body.

• Sinoatrial (SA) node: The primary pacemaker of the heart, located in the right atrium. It sets the pace of the heartbeat at approximately 70 beats per minute (bpm).

• Atrioventricular (AV) node: Located at the junction between the atria and ventricles, it has a slower intrinsic rate (about 50 bpm) and can act as a pacemaker if the SA node fails.

• Purkinje fibers: Specialized conducting fibers in the ventricles with an intrinsic rate of 25–40 bpm. They can also serve as pacemakers under certain conditions.

• Internodal pathway: Conducts impulses from the SA node to the AV node, routing the direction of electrical signals so the heart contracts from apex to base.

• AV node delay: The AV node slows conduction, allowing the atria to contract and fill the ventricles before ventricular contraction begins.

• Bundle of His and bundle branches: Purkinje fibers transmit electric signals down the atrioventricular bundle (bundle of His) to the left and right bundle branches, ensuring coordinated ventricular contraction.

Example: If the SA node is damaged, the AV node or Purkinje fibers can take over as pacemaker, but the heart rate will be slower.

Cellular Basis of Electrical Conduction

Electrical signals in the heart are generated and propagated by specialized myocardial cells. These include autorhythmic cells and contractile cells, which communicate via gap junctions in intercalated disks.

• Autorhythmic cells: Generate spontaneous action potentials that initiate each heartbeat.

• Contractile cells: Respond to electrical signals by contracting, enabling the heart to pump blood.

• Intercalated disks: Specialized connections between cardiac muscle cells containing gap junctions that allow rapid transmission of electrical impulses.

Example: The coordinated contraction of the heart is possible due to the rapid spread of electrical current through gap junctions.

Cardiac Action Potential

The cardiac action potential is the electrical event that triggers contraction in cardiac muscle cells. It differs from the action potential in skeletal muscle and neurons due to its unique phases and ion channel involvement.

• Phases of cardiac action potential:

  1. Depolarization: Rapid influx of Na+ ions.

  2. Plateau phase: Sustained by Ca2+ influx, which prolongs the action potential and prevents tetanus.

  3. Repolarization: Efflux of K+ ions restores the resting membrane potential.

• Autorhythmic cell action potential: Characterized by spontaneous depolarization due to slow Na+ influx (funny current) and Ca2+ channels.

• Contractile cell action potential: Has a distinct plateau phase due to Ca2+ entry.

Equation:

$\text{Action Potential Duration} = \text{Depolarization Phase} + \text{Plateau Phase} + \text{Repolarization Phase}$

Example: The plateau phase in cardiac muscle prevents summation and tetanus, ensuring rhythmic contractions.

Summary Table: Key Components of Cardiac Electrical Conduction

Component	Location	Function	Intrinsic Rate (bpm)
SA Node	Right atrium	Primary pacemaker	~70
AV Node	Junction of atria and ventricles	Secondary pacemaker, delays impulse	~50
Purkinje Fibers	Ventricular walls	Conduct impulses rapidly to ventricles	25–40
Bundle of His	Interventricular septum	Conducts impulses to bundle branches	N/A

Additional info: The notes are based on lecture slides from "Human Physiology: An Integrated Approach" by Dee Unglaub Silverthorn, focusing on the electrical conduction system and cardiac action potential, which are central to understanding cardiovascular physiology.