Ch20 Heart Conduction

Conducting System Heartbeat

• Definition: A single cardiac contraction where all heart chambers contract in series.

  • Order of Contraction:

    • First: Atria

    • Then: Ventricles

Types of Cardiac Muscle Cells

• Autorhythmic Cells (Pacemaker):

  • Function: Control and coordinate heartbeat.

• Contractile Cells:

  • Function: Produce contractions that propel blood.

The Conducting System

• Structure: Made up of specialized cardiac muscle cells that initiate and distribute electrical impulses stimulating contraction.

• Autorhythmicity: Cardiac muscle tissue can contract without neural or hormonal stimulation.

Components of Conducting System

• Pacemaker Cells Found In:

  • Sinoatrial (SA) Node

  • Atrioventricular (AV) Node

• Conducting Cells Found In:

  • Internodal pathways of atria

  • Atrioventricular (AV) bundle

  • Bundle branches and Purkinje fibers of ventricles.

Pacemaker Potential

• Definition: Gradual depolarization of pacemaker cells.

• Characteristics:

  • No stable resting membrane potential.

  • Rates of spontaneous depolarization:

    • SA Node: 60–100 action potentials/min

    • AV Node: 40–60 action potentials/min

• The SA node establishes sinus rhythm (heart rhythm).

Impulse Conduction through the Heart

1. SA Node Activity and Atrial Activation Begin

2. Stimulus Spreads Across Atria to AV Node

3. Impulse Delay: 100 msec at AV node to allow for atrial contraction.

Impulse Pathway

• Impulse travels from AV bundle:

  • To left and right bundle branches in interventricular septum.

  • To Purkinje fibers and papillary muscles via moderator band.

• Ventricular Contraction Begins after the impulse reaches Purkinje fibers, completing atrial contraction.

ECG Tracings

• Overview: Record electrical events in the heart.

• Useful in diagnosing damage through abnormal patterns.

• Components:

  • P Wave: Atrial depolarization.

  • QRS Complex: Ventricles begin contracting shortly after R wave.

  • T Wave: Ventricular repolarization.

Heart Rhythm Disturbances

Abnormal Heart Rhythms:

• Bradycardia: Slow heart rate.

• Tachycardia: Fast heart rate.

• Ectopic Pacemaker: Abnormal cells generating high rates of action potentials.

Electrocardiogram Details

• Electrodes are placed at specific body surface locations to record ECG.

• Abnormal patterns can indicate heart issues.

Cardiac Contractile Cells

• Structure: Interconnect with intercalated discs (desmosomes and gap junctions).

• Features:

  • Small size, single central nucleus, branching interconnections.

• Action Potential Phases:

  • Rapid depolarization (fast sodium channels)

  • Plateau (slow calcium channels)

  • Repolarization (slow potassium channels).

Energy for Contraction

• Source: Aerobic energy from metabolism of fatty acids and glucose.

• Oxygen is delivered through circulation; myocytes store oxygen in myoglobin.

The Cardiac Cycle

• Definition: Encompasses activities from the start of one heartbeat to the next.

Systole and Diastole

• Phases per chamber:

  • Atrial Systole

  • Atrial Diastole

  • Ventricular Systole

  • Ventricular Diastole

• Blood Pressure Dynamics: Rises during systole and falls during diastole; blood flows from high to low pressure.

Factors Affecting Cardiac Output

• Formula: CO = HR × SV

  • CO = Cardiac Output (mL/min)

  • HR = Heart Rate (beats/min)

  • SV = Stroke Volume (mL/beat)

• Stroke Volume: SV = EDV - ESV (End-diastolic volume - End-systolic volume).

Autonomic Innervation and Heart Rate

• Controlled by the cardiac plexus:

  • Parasympathetic: Vagus nerves;

  • Sympathetic: Cardioacceleratory center in the medulla oblongata.

• Factors influencing heart rate:

  • Circulating hormones (e.g., epinephrine, norepinephrine).

  • Bainbridge Reflex: Increases heart rate due to increased venous return.

Stroke Volume Control

• Affected by EDV and ESV:

  • Preload: Degree of ventricular stretching during diastole.

  • Contractility: Force produced during contraction at a given preload.

  • Afterload: Resistance against which the ventricle must eject blood.