Cardiac Conduction, Output, and Regulation

The Electrical Activity of Cardiac Muscle Cells

In this lesson, you will learn about the structural and electrical properties of cardiac muscle cells.
Specific Topics Covered:

1. Structure of Cardiac Muscle
2. Membrane Potentials and Ion Movement in Cardiomyocytes
      - 2a. Cardiac Conducting Cells
      - 2b. Cardiac Contractile Cells
      - 2c. Calcium Ions

Introduction to Muscle Tissue

• Three Types of Muscle Tissue:
     - Skeletal Muscle:
       - Location: Forms muscles of the muscular system.
       - Function: Voluntary movement; protects internal organs.
     - Cardiac Muscle:
       - Location: Forms the heart.
       - Function: Involuntary movement.
     - Smooth Muscle:
       - Location: Found in many internal organs.
       - Function: Moves materials such as blood, hormones, and secretions.
• Commonality: All muscle cells are known as myocytes, capable of contraction when activated by electrical signals.

1. Structure of Cardiac Muscle

• Cardiomyocytes Characteristics:
     - Size and Shape: Shorter than skeletal muscle cells with smaller diameters.
     - Striations: Features alternating dark A bands and light I bands arranged in sarcomeres, similar to skeletal muscle.
     - Transverse (T) Tubules:
       - Function: Penetrate the sarcolemma into the cell, allowing electrical impulses to reach the cell interior.
       - Location: T tubules are located at Z discs (compared to junctions of A and I bands in skeletal muscle), leading to fewer T tubules.
     - Sarcoplasmic Reticulum: Stores few calcium ions, requiring calcium influx from outside cells, resulting in slower contraction onset.
     - Energy Supply: High number of mitochondria for energy production during contractions.
     - Nucleus: Typically a single, central nucleus; some cells may contain two or more nuclei.
     - Branching: Cardiac muscle cells branch freely.
• Intercalated Discs:
     - Description: Junctions between adjoining cardiomyocytes.
     - Components: Consist of desmosomes and gap junctions facilitating synchronized contractions.
     - Function: Allow ion passage and electrical activation spread through gap junctions.
     - Significance: Strong connections due to contractile forces exerted during contractions.

Additional Notes on Cardiac Muscle Structure

• Aerobic Respiration:
     - Main Metabolites: Primarily utilizes lipids and carbohydrates.
     - Storage: Contains myoglobin, lipids, and glycogen in its cytoplasm.
• Repair Limitation:
     - Damaged cardiac muscle cells have limited ability to repair via mitosis.
     - Evidence shows potential presence of stem cells in the heart.
     - Heart contractions require long refractory periods to prevent tetany; vital for life since sustained contraction impedes blood circulation.

2. Membrane Potentials and Ion Movement in Cardiomyocytes

2a. Cardiac Conducting Cells

• Types of Cardiac Cells:
     - Myocardial Contractile Cells: Form 99% of atrial and ventricular cells; responsible for contractions.
     - Myocardial Conducting Cells: Form the conduction system (1% of cells), have fewer myofibrils.
• Action Potentials:
     - Differences from skeletal muscle; conducted via sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) ions.
     - Resting Potential: Conducting cells lack a stable resting potential.
• Spontaneous Depolarization:
     - Na⁺ influx causes membrane potential to rise from -60 mV to -40 mV, termed pacemaker potential.
     - Ca²⁺ channels open for rapid depolarization to +15 mV; K⁺ channels open during repolarization back to -60 mV.

2b. Cardiac Contractile Cells

• Distinct Action Potential Phases:
     - Stable Resting Phase: -80 mV (atria) and -90 mV (ventricles).
     - Phases of Action Potential:
       - Phase 0: Na⁺ influx; +30 mV.
       - Phase 1: Sodium channels close.
       - Phase 2: Plateau phase (Ca²⁺ influx).
       - Phase 3: K⁺ efflux initiates repolarization.
       - Phase 4: Resting phase; cycle resumes.
     - Duration: Full action potential lasts 250-300 ms.
     - Refractory Periods:
       - Absolute refractory period: 200 ms
       - Relative refractory period: 50 ms; crucial for effective contraction without tetany.

2c. Calcium Ions

• Critical Roles:
     - Influx causes the prolonged plateau phase, critical for effective function.
     - Combine with troponin in muscle contraction similar to skeletal muscle.
     - Calcium Sources: 20% from plateau phase, remainder from sarcoplasmic reticulum.
  

Conclusion of Cardiac Electrical Activity

• Reviewed structural roles of cardiomyocytes in synchronizing contractions, how membrane potentials and ion movements function in conducting and contractile cells, and significance of ion channels.

The Conduction System of the Heart

Overview of the Conduction System

• The heart can initiate an electrical impulse rapidly and continuously, known as autorhythmicity.
• Heart rate is modulated by the endocrine and nervous systems.
• Components:
     - Sinoatrial (SA) Node
     - Atrioventricular (AV) Node
     - Atrioventricular Bundle (Bundle of His)
     - Bundle Branches
     - Purkinje Fibers

1a. Sinoatrial (SA) Node

• Location: Right atrium, near the superior vena cava.
• Function: Pacemaker of the heart; highest rate of depolarization.
• Impulse Duration: Takes around 50 ms to reach AV node through internodal pathways.
• Internodal Pathways: Three bands to conduct impulse, including Bachmann’s bundle from right to left atrium.

1b. Atrioventricular (AV) Node

• Location: Inferior portion of the right atrium.
• Function: Slows impulse transmission (approximately 100 ms delay) to allow ventricles time to fill after atrial contraction.

1c. Atrioventricular Bundle and Purkinje Fibers

• Pathway: Bundle of His moves down the interventricular septum; divides into left and right bundle branches.
• Function of Purkinje Fibers: Quick conduction to ventricle muscle cells, allowing simultaneous contraction.

1d. Rates of Conduction System Firing

• SA Node: 80-100 bpm without nervous input.
• AV Node: 40-60 bpm if SA node fails.
• Bundle & Purkinje Fibers: 15-30 bpm each, which can be inadequate for sustaining life.

Electrocardiogram (ECG)

Overview of ECG

• Definition: Tracing of electrical activity of the heart recorded through surface electrodes.
• Leads: Standard ECG uses 3, 5, or 12 leads; more leads provide detailed information.
• Calibration: Normally set at 10 mV/mm vertically and 25 mm/s horizontally.

Key Components of ECG

• Waveforms: P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).
• Intervals: PR (time delay at AV node), ST (ventricular pressure plateau), QT (total depolarization and repolarization).

ECG Interpretation

• Normal Heart Sounds: S1 (closing AV valves) and S2 (closing semilunar valves).
• Murmurs: Abnormal sounds due to turbulent blood flow.
• Common Abnormalities: Heart blocks, arrhythmias, and fibrillation.

Cardiac Cycle

Overview of Cardiac Cycle

• Definition: Sequence of contraction (systole) and relaxation (diastole) of the heart.
• Function: Efficient blood pumping through regulated coordination of heart chambers.

Phases of the Cardiac Cycle

1. Atrial Systole and Diastole

• Atrial Systole: Contraction from superior to inferior chambers. Contributes remaining volume of blood to ventricles.

2. Ventricular Systole

• Total Duration: 270 ms, divided into two phases:
     - Phase 1: Isovolumetric contraction, no blood ejection.
     - Phase 2: Ventricular ejection, blood pumped out to pulmonary trunk/aorta.

3. Ventricular Diastole

• Duration: ~430 ms and divided into two phases:
     - Early phase: Isovolumetric and closing of semilunar valves.
     - Late phase: Closure of AV valves, blood flow from atria into ventricles.

Heart Sounds

• Normal Sounds: S1 and S2 associated with valve closure.
• S3 and S4: Rare and associated with specific conditions or heart structure.

Cardiac Output

Key Concept of Cardiac Output

• Definition: Amount of blood pumped by each ventricle per minute.
• Formula: CO = HR × SV.
     - Normal adult resting values: CO around 5.25 L/min.

Influencing Factors

• Heart Rate and Stroke Volume: Both change with exercise, fitness level, health.
• Maximum Output: Changes significantly during maximum exercise; up to 19.5 L/min.

Stroke Volume Influence Factors

1. Preload

• Definition: Load on ventricles before contraction. Increased venous return raises EDV.

2. Contractility

• Definition: Strength of heart contraction; influenced by health status.

3. Afterload

• Definition: Resistance the ventricles must overcome to pump blood.

Cardiac Regulation

Cardiovascular Centers

• Location: Cardiac centers in medulla initiate neural control of heart rate.
• Sympathetic vs Parasympathetic Stimulation: Increases vs decreases heart activity, respectively.

Influencing Factors on Heart Rate

• Hormones: Adrenaline, thyroid hormones impact rate.
• Neural Factors: Chemoreceptors, baroreceptors, and limbic response affect regulation.

Summary of Factors Affecting Heart Rate

• Variability due to individual health, fitness, and external influences such as drugs and environmental changes.