Excitation & Conduction in the Heart

Excitation & Conduction

• The heart has a specialized excitatory and conductive system composed of specialized cardiac muscles
• This system's function is to generate and conduct rhythmical electrical impulses that initiate heart muscle contraction
• The system controls conduction of impulses from one part of the heart to another
• This coordinated action ensures excitation throughout all heart muscle sections (atria and ventricles)

Learning Objectives

• Students should be able to describe the ion currents in different phases of pacemaker cell and ventricular muscle cell action potentials.
• Students should know the factors contributing to the longer duration of cardiac action potentials.
• They should be able to outline the normal sequence of depolarization in the heart.
• Students need to understand the role of specialized cells in conduction. The effects of conduction failures in different areas should be explained.
• The function of the AV node as the only electrical pathway between atria and ventricles should be described. The functional significance of a slow conduction rate through the AV node should be analyzed.
• The role of the SA node as the cardiac pacemaker in normal situations should be understood.
• The relationship between action potentials and mechanical activity in cardiac and skeletal muscle fibers, as well as why tetanic contractions don't happen in a healthy heart, should be explained.
• Students need to understand how the sympathetic and parasympathetic nervous systems influence cardiac function. The dominant autonomic nervous system branches at rest and during exercise should be identified.

Specialized Excitatory & Conductive System

• The sinus node (sinoatrial node) is the primary pacemaker
• It generates normal rhythmical impulses
• Located in the upper wall of the right atrium behind the superior vena cava opening
• Internodal pathways carry impulses from the SA node to the AV node (interconnecting fibers)
• The AV node delays impulses from atria before passing them to ventricles- crucial for efficient heart function.
• Located in the posterior wall of the right atrium
• The A-V bundle conducts impulses from atria to ventricles, insulated from the atria by fibrous tissue
• Right & Left bundle branches of Purkinje fibers conduct impulses to all parts of the ventricles. This ensures their rapid activation

Functional relevance of slow conduction through AVN

• The AV node contains fewer gap junctions between cells compared to other structures in the cardiac conduction system.
• This decreased gap junction presence leads to slower impulse transmission.
• The delay allows atria to completely empty their blood into the ventricles before ventricular contraction begins.
• Reduced conduction velocity in the AV node results in reduced ventricular filling, stroke volume, & cardiac output

Action Potential of cardiac muscles

• Depolarization: rapid influx of Na+ ions leads to a rise in membrane potential
• Plateau phase: sustained depolarization due to opening of slow Ca++ channels and delayed K+ closing. Maintains the contraction.
• Repolarization: efflux of K+ ions restores the resting membrane potential.
• The action potential of cardiac muscles lasts longer than that of skeletal muscles to ensure prolonged contraction, essential for cardiac function

Causes of plateau

• Slow, prolonged opening of the slow Na+/Ca++ channels.
• Decreased permeability of K+ channels. Results in increased Ca++ influx maintaining the plateau
• The return of the MP is delayed
• This maintains the plateau for a longer time.

Advantages of the long plateau

• Prolonged ventricular contraction in comparison to skeletal muscle (15 times longer)
• Ca++ influx during contraction, making cardiac muscle contractions stronger
• Delay in repolarization results in an absolute refractory period (necessary to prevent tetanic contractions)

Con...

• Prolonged refractory period in cardiac muscle, shielding the heart from rapid re-excitation.
• So heart muscle cannot exhibit tetanus during high-frequency stimulation. Implies the necessity of relaxations for efficient filling.
• Allows sufficient ventricular relaxation time for the ventricles to fill with blood efficiently.

Cardiac Pacemaker (S-A node)

• The SA node is the heart's primary pacemaker (70-80 beats/min under normal circumstances).
• The SA node initiates the heart's rhythmic electrical impulses, initiating atrial and ventricular contraction and the heartbeat.
• Other latent pacemakers like the AV node and the Purkinje fibers (15 to 40 beats/min) can act as backup pacemakers if the SA node fails

Impulse transmission through ventricles

• Special Purkinje fibers carry impulses from the AV node to the ventricles
• Ventricular muscle fibers are large.
• Extremely high level of permeability of gap junctions in these fibers.
• This enables rapid impulse transmission to facilitate synchronized ventricular contraction.
• The impulse through the bundle branches in the ventricular septum, and Purkinje fiber terminals is 0.03 seconds

Con...

• Transmission from endocardial surface to epicardial surface takes 0.03 seconds
• The total duration from the initial bundle branches to the end of ventricular muscle fibers is about 0.06 seconds.
• Rapid conduction of the action potential throughout the ventricles ensures efficient contraction and blood ejection.

Velocity of transmission

• In the specialized fibers(Purkinje fibers) the Velocity of transmission: ranges from 1.5 - to 4.0 m/sec.

Influence of vagal (parasympathetic) stimulation and sympathetic stimulation

• Vagus nerve stimulation (parasympathetic) decreases the conduction rate (and heart rate) by increasing K+ efflux, leading to hyperpolarization of the heart's pacemaker cells.
• Sympathetic stimulation increases the conduction rate (and heart rate) as it increases the rate of depolarization in pacemaker cells due to increased Na+ & Ca++ influx into cells.

Significance of the Heart's long plateau

• Prolonged duration of the cardiac action potential is essential for a sustained cardiac muscle contraction, preventing tetanus-like sustained contractions.
• It allows time for the ventricular filling before ventricular contraction begins.

Relationship between Action Potential (AP) & mechanical activity in skeletal and cardiac muscle

• In skeletal muscle, the refractory period is short. Thus, it can sum APs and contractions leading to tetanus if repeatedly stimulated.
• In cardiac muscle, the refractory period is longer; hence, summation of APs and contractions isn't possible and thus tetanus can't occur. This prevents uncontrolled contractions and ensures smooth, coordinated pumping action.

Consequences of conduction failure

• In specialized cells of the conduction pathway, if there's dysfunction, it can lead to an irregular heartbeat.
• A faster pacemaker elsewhere in the heart takes over if the SA node malfunctions.
• In a condition called AV block, the upper chambers (atria) beat at a normal rate while the lower chambers (ventricles) beat less frequently, or not at all – in a less-coordinated manner.

Con...

• Blockage of impulse transmission from SA node to other components of the heart leads to the establishment of a new pacemaker in the AV node, or penetrating portions of the AV bundle.
• Failure to transmit impulses in the conduction system can result in an abnormal heart rhythm (arrhythmia). This means the heart doesn't beat in a coordinated or predictable way – it beats too slowly, too quickly, or in an irregular rhythm.
• Ventricular escape occurs if the primary pacemaker fails, and Purkinje fibers become the new pacemaker at a slower rate.

MCQ

• The slowest discharge rate occurs in the AV nodal fibers, followed by Purkinje fibers, and the fastest in the sinus nodal fibers.
• The resting membrane potential of ventricular muscle fibers is between -75 mV and -85 mV