Heart IIIb

Lecture III: The Heart

Introduction

• Instructor: Dr. Christopher Potter

• Published by: Pearson Education, Inc., 2018

• Focus: Understanding the action potentials within heart cells, highlighting the differences between cardiac pacemaker cells and contractile cells.

Page 1: Overview

• Introduces the heart and its significance, setting the stage for detailed discussions related to action potentials in cardiac physiology.

Page 2: Action Potential in Neurons

Membrane Potential Dynamics

• Membrane Potential: It changes in response to stimuli, moving from resting potential (-70 mV) through depolarization to action potential (+50 mV).

• Phases:

  • Depolarization: Rapid rise in potential.

  • Repolarization: Return to resting potential.

  • Refractory Periods: Following the action potential, preventing further stimulation during this phase.

Page 3: Generation of Action Potential

Phases of Action Potential

1. Graded Depolarization: The membrane potential reaches the threshold (-60 mV).

2. Rapid Depolarization: Sodium channels open, causing sodium influx.

3. Repolarization Process: Inactivation of sodium channels and activation of potassium channels returning the cell to resting state.

4. Refractory Periods:

   • Absolute Refractory Period: No response to stimulus.

   • Relative Refractory Period: Increased stimulus needed for response.

Page 4: Pacemaker Potential in SA Node

Key Functions of SA Node Cells

1. Pacemaker Potential: Slow depolarization initiated by HCN channels; this is essential for heart rhythm.

2. Depolarization Phase: Triggered by the influx of calcium ions.

3. Repolarization Phase: Resumed by potassium efflux, resetting the cycle for the next action potential.

• Duration: SA node action potentials span approximately 800 ms and there’s no true resting potential.

Page 5: Ion Channels in SA Node

Key Ion Channels

• Major channels include:

  • HCN Channels: Responsible for pacemaker potential.

  • Calcium Channels: Crucial for further depolarization.

  • Potassium Channels: Involved in repolarization.

• The "Funny Current" arises from the sodium influx through HCN channels, integral to maintaining heart rhythm.

Page 6: Cardiac Conduction System Cells

Types of Cardiac Cells

1. Conducting Cells: Relay impulses but do not contract.

2. Pacemaker (Nodal) Cells: Auto-rhythmic, including cells in the SA and AV nodes.

3. Contractile Cells: Do not initiate impulses themselves; rely on conducting cells.

Page 7: Action Potential in Contractile Cells

Characteristics of Contractile Cell Action Potentials

• Resting Membrane Potential: ~-90 mV.

• Dynamic: Involves ion channels of Na+, Ca2+, and K+; however, lacks HCN channels.

• Stimuli: Induced by depolarization from neighboring cells around them.

Page 8: Duration of Action Potential

Contrast with Skeletal Muscle

• AP Duration: 250-300 msec in contractile cells, significantly longer than in skeletal muscle (preventing tetany).

• Phases of AP: Rapid depolarization, plateau, and repolarization.

Page 9: Details of Rapid Depolarization

Importance of Ion Channels

• Na+ Channels: Key for rapid depolarization; both absolute and relative refractory periods are critical for preventing abnormal heart rhythms.

Page 10: Plateau Phase

Mechanism of Plateaus

• Competition between potassium efflux and calcium influx leads to a plateau in the membrane potential, stabilizing it before repolarization begins.

Page 11: Repolarization Phase

Return to Resting State

• Following the plateau, K+ efflux predominates while Ca2+ channels close.

Page 12: Action Potential and Contraction

Relationship Between AP and Mechanical Activity

• The action potential triggers contraction in muscle fibers; calcium ions are fundamental for muscle contraction.

• Calcium Sources: A portion is supplied by the plateau phase, which triggers further release from the SR.

Page 13: Electrical Coupling of Cells

How Cardiac Cells Synchronize

• Contractile cells are interconnected via gap junctions, ensuring coordinated contraction across the heart muscle.

Page 14: Electrocardiogram (ECG)

ECG Overview

• Represents the summation of electrical activity among various cardiac cells, including pacemaker and contractile action potentials.

Page 15: Educational Resource

• Video link to further illustrate differences between nodal and contractile action potentials.

Pages 16-28: ECG Interpretation and Cardiac Arrhythmias

Components and Function of ECGs

• Components: P wave, QRS complex, T wave - correspond to stages of atrial and ventricular depolarization and repolarization.

• Arrhythmias: Explains various conditions like PACs, PVCs, Atrial Fibrillation, and Ventricular Tachycardia, detailing their impact on heart rhythm and function.

ECG Analysis Chart

• Detailed descriptions match arrhythmias with their ECG signature, facilitating assessment and recognition of abnormal heart rhythms.