Cardiac Action Potentials & Conduction Pathway

Cardiac Action Potentials & Conduction Pathway

• Cardiac action potentials are generated intrinsically, without external nerve input (myogenic). Skeletal and smooth muscles require nervous input.
• This is in contrast to skeletal muscle, which requires motor nerve input, and smooth muscle, which requires autonomic nervous system input.
• The sino-atrial node (SAN) initiates the heart's electrical activity, regulating heartbeat rate and rhythm.
• Pacemaker potentials are specialized action potentials fired by the SAN; their firing rate directly correlates to heart rate. These action potentials occur at 1 per second, creating a heart rate of 60 beats per minute.
• SAN cells aren't contractile
• The electrical activity is conducted through specialized pathways.

Intro to Medicine Module Sessions

• The sessions overview the cardiovascular system (CVS).
• Blood pressure regulation and treatment of hypertension are included.
• An introduction to electrocardiography (ECG) is part of the curriculum.
• Related topics like anatomy, clinical skills, muscle contraction, pharmacology, and autonomic nervous system content are referenced as relevant.

Learning Objectives (LOBs)

• Students will describe the ionic mechanisms driving pacemaker potentials in the sinoatrial node (SAN) and the action potentials in atria and ventricles.
• The electrical conduction pathways from the SAN to ventricular myocytes will be outlined, correlating with the electrocardiogram (ECG) pattern.
• Students need to describe electro-contractile coupling in the heart, including the role of action potential in contraction and the role of the plateau/refractory phase.

Lecture Outline

• Part 1: Covers the pacemaker potential at the sinoatrial node, the electrical conduction pathway that forms the basis of the electrocardiogram (ECG) pattern.
• Part 2: Focuses on atrial/ventricular action potentials, and electrical activity coupled to contraction.

Heartbeat Initiation

• Heartbeat originates at the sinoatrial node (SAN).
• SAN pacemaker potentials fire at a rate of 1 per second, resulting in a heart rate of 60 beats per minute.

How SAN Produces Pacemaker Potentials

• SAN's resting membrane potential is unstable (non-equilibrium).
• Phase 0: Voltage-gated Ca2+ channels open, causing Ca2+ influx—depolarization.
• Phase 3: Voltage-gated potassium channels open, causing efflux—repolarization.
• Phase 4: Hyperpolarization-activated Na+ channels (If channels) contribute to the slow depolarization leading to the next action potential

SAN Electrical Activity Conduction Through the Heart

• Electrical activity initiated at the SAN, across the atria, to the AV node, then to the ventricles.
• The AV node slows the electrical conduction to allow the atria to finish contracting before the ventricles contract.
• Conduction is fast from the bundle of His to the Purkinje fibers.
• Electrical activity concludes in the ventricles. Intracalated discs allow for fast conduction between cells.

Conductance Pathway & ECG Relationship

• The electrocardiogram (ECG) represents the electrical conduction throughout the heart.
• The ECG doesn't directly show contraction.
• P wave = atrial depolarization, QRS wave = ventricular depolarization, T wave = ventricular repolarization
• Heart rate and rhythm are profoundly impacted by the pacemaker potential frequency and the conduction pathway.

Electrical-Contraction Coupling Through the Heart

• Electrical activity generated in the SA node spreads through the atria to stimulate contraction.
• The AV node slows the conduction before stimulating the ventricles (allowing the atria to finish contracting and fill the ventricles).
• Excitation conducts rapidly through the bundle of His into the ventricles via the Purkinje fibers, then contraction occurs sequentially — starting at the apex and spreading to the base of the heart.

Atrial/Ventricular Action Potentials

• Atrial/ventricular action potentials originate from the SA node.
• These cells have a stable resting membrane potential.
• Phase 0: Voltage-gated Na+ channels open—Na+ influx—depolarization—rapid phase
• Phase 1: Repolarization- brief initial repolarization.
• Phase 2 (Plateau Phase): Voltage-gated Ca2+ channels (VGCCs) open—sustained Ca2+ influx.
• Phase 3: Phase 3 - repolarization- efflux of K+.
• Phase 4: Resting membrane potential is reestablished at -90mV.
• Plateau phase prevents early excitability.
• Tissue is unexcitable (refractory) during the plateau phase.

How Atrial/Ventricular Action Potentials Couple to Contraction

• Increased cytosolic Ca2+ levels trigger contraction.
• Ca2+ influx through voltage-gated Ca2+ channels causes a release of intracellular Ca2+.
• Calcium-induced calcium release (CICR) also triggers the release of stored Ca2+.
• The released Ca2+ binds to contractile proteins, initiating contraction.

Example SAQs

• Students will be asked to compare and contrast the action potentials in the SAN and ventricles.
• They'll analyze the role of the AV node.
• Outline the steps coupling the ventricular action potential to contraction.
• Explain the function of the ventricular action potential plateau phase.

Explore the intricacies of cardiac action potentials and the conduction pathways within the cardiovascular system. This quiz covers the roles of the sino-atrial node, pacemaker potentials, and their significance in regulating heart rate. Get ready to test your knowledge on essential concepts in cardiology.