CB005 updated Cardiac Action Potentials

Cardiac Action Potentials Overview

Cardiac action potentials are essential for the electrical activity of heart muscle, coordinating the heartbeat through a series of depolarization and repolarization events. Different types of cardiac muscle fibers exist, with varying properties tailored for specific functions in the heart.

Objectives

• Recognize different types of cardiac fibers: Understanding the distinction between atrial, ventricular, and pacemaker fibers.

• Differentiate between fast and slow response action potentials and their ionic basis: Emphasizing the role of sodium, calcium, and potassium ions in action potential phases.

• Identify cardiac properties and major components such as primary/secondary pacemakers: Recognizing the roles of the SA node, AV node, and conduction pathways.

• Explain the genesis of heartbeat and the relationship between action potentials and muscle contractions: Linking electrical events to mechanical contractions.

• Describe excitation-contraction coupling and effects of drugs or mutations on cardiac contraction: Discussing how calcium dynamics influence contraction strength.

Structure of Cardiac Muscle

Cardiac muscle is striated and composed of branched, interconnecting fibers that form a functional syncytium. The cells, known as cardiomyocytes, are attached end-to-end through specialized structures called intercalated discs, which have three key components:

• Desmosomes: Provide mechanical stability and prevent fiber separation during contraction.

• Tight Junctions: Facilitate seamless connections between fibers, contributing to the mechanical syncytium.

• Gap Junctions: Enable low-resistance channels for rapid electrical communication across cells, allowing for synchronized heart contractions.

Syncytium Concept

The heart functions as a functional syncytium, where depolarization of one cell leads to the activation of neighboring cells, effectively causing the entire muscle region to contract as a single unit through an all-or-none principle.

Functional Divisions of Cardiac Tissue

1. Nodal (Pacemaker) Tissue: Responsible for initiating excitation waves. The SA node is the primary pacemaker, followed by the AV node as a secondary node, and the Purkinje fibers as a tertiary pacemaker.

2. Junctional (Conductive) Tissue: Facilitates the rapid conduction of electrical impulses throughout the heart, including the His Bundle and Purkinje fibers.

3. Contractile Tissue: Comprises the atrial and ventricular myocardium, responsible for the pumping action of the heart in response to electrical stimulation.

Properties of Cardiac Muscle

• Autorhythmicity: Cardiac muscle exhibits the ability to generate spontaneous action potentials without external stimuli.

• Excitability: Cardiac muscle cells can respond to various stimuli, generating action potentials that initiate contraction.

• Conductivity: This property allows the rapid spread of electrical impulses through the myocardium, ensuring coordinated contractions.

• Contractility: Cardiac muscle can contract forcefully in response to electrical stimulation, which is vital for effective blood circulation.

Importance of Spontaneous Depolarizations

Spontaneous depolarizations are critical for heart rhythm generation, primarily initiated by the SA node, which sets the pace for cardiac activity. These depolarizations lead to action potentials that propagate through cardiac tissue.

Slow Response Action Potential (Nodal Tissue)

• Pacemaker Potential: Spontaneous depolarization occurs up to -40 mV, primarily due to the influx of sodium (Na+) and calcium (Ca++) ions via hyperpolarization-activated cyclic nucleotide-gated channels (If channels).

• Depolarization Phase: Characterized by continued Ca++ influx, leading to the threshold potential and subsequent generation of action potentials.

• Repolarization Phase: Driven by outward movement of potassium (K+) ions, restoring the resting membrane potential.

Fast Response Action Potential (Atrial & Ventricular Fibers)

• Resting Membrane Potential (RMP): Stabilized at approximately -90 mV due to a balance of ion concentrations across the membrane.

• Depolarization: Rapid due to Na+ influx through voltage-gated sodium channels, which occurs very quickly, allowing for swift conduction of impulses.

• Repolarization: Exhibits a triphasic pattern involving rapid K+ efflux, completing the return to RMP after a plateau phase facilitated by Ca++ influx.

Summary of Phases of Fast Response AP

1. Phase 0: Rapid depolarization due to Na+ influx.

2. Phase 1: Small repolarization from K+ efflux.

3. Phase 2: Plateau phase, where Ca++ influx balances K+ efflux.

4. Phase 3: Repolarization back to RMP.

5. Phase 4: Maintenance of stable RMP, with specific mechanisms for pacemaker tissue to remain dynamic.

Sinus Rhythm and Pacemakers

• Primary Pacemaker: SA node, generating approximately 100 impulses/min, modulated by autonomic influences such as vagal tone.

• Secondary Pacemaker: AV node with an intrinsic rate of 40-60 impulses/min, acting as a backup regulatory mechanism.

• Tertiary Pacemaker: Purkinje fibers, which can generate 25-40 impulses/min in the absence of higher node activity.

Action Potential Morphology in Cardiac Tissues

Different cardiac muscle tissues exhibit unique action potential waveforms, correlating with their specific roles in the heart's electrical activation sequence, illustrating the electrical activity during heartbeat cycles.

Regulation of Heart Rate and Contraction

Heart rate is regulated by the frequency of action potentials generated in the SA node. Alterations can lead to conditions such as tachycardia (elevated heart rate) or bradycardia (depressed heart rate). Contraction strength is directly influenced by intracellular calcium levels, which are modulated by action potential frequency.

Refractory Periods

• Absolute Refractory Period (ARP): Following depolarization, it's impossible to generate a new action potential, protecting the heart from tetanic contractions.

• Relative Refractory Period (RRP): New action potentials can be initiated, but only with sufficiently strong stimuli, allowing recovery of excitability.

Clinical Significance on Drug Action

Antiarrhythmic and antihypertensive medications impact cardiac function by modulating action potential characteristics—affecting conduction velocity, refractory periods, and contractility, which is essential in treating various cardiac conditions.

ECG and Cardiac Action Potential Correlation

The electrocardiogram (ECG) provides critical insights into cardiac electrical activity, with the P wave corresponding to atrial depolarization, the QRS complex representing ventricular depolarization, and the T wave reflecting ventricular repolarization.

References

• Klabunde, R.E. "Cardiovascular Physiology Concepts".

• Pocock, G. & Richards, C. "Human Physiology".

• Guyton, A.C. & Hall, J.E. "Textbook of Medical Physiology".