Cardiac Muscle Physiology

Metabolism and Energy Use in Cardiac Muscle

Cardiac muscle is highly specialized for continuous, rhythmic contraction and relies almost exclusively on aerobic respiration for energy production.

• Aerobic Respiration: Cardiac muscle contains more mitochondria than skeletal muscle, reflecting its greater dependence on oxygen.

• Fuel Adaptability: Cardiac muscle can utilize glucose, fatty acids, and even lactic acid produced by skeletal muscle activity.

• Oxygen Supply: The primary risk of inadequate blood supply to the myocardium is lack of oxygen, not lack of nutrients.

• Comparison to Skeletal Muscle: Skeletal muscle can contract anaerobically for short periods and restore reserves using excess postexercise oxygen consumption (EPOC).

Additional info: Cardiac muscle's reliance on aerobic metabolism makes it highly sensitive to ischemia (lack of blood flow).

Cardiac Electrophysiology

Action Potentials in Pacemaker Cells

Pacemaker cells are autorhythmic cardiac cells that spontaneously depolarize, setting the rhythm of the heart.

• Pacemaker Potential: Due to special ion channel properties, pacemaker cells have an unstable resting membrane potential that slowly depolarizes toward threshold.

• Depolarization: At threshold (about -40 mV), Ca2+ channels open, allowing Ca2+ influx, which causes the rising phase of the action potential.

• Repolarization: Ca2+ channels inactivate and K+ channels open, allowing K+ efflux and returning the membrane potential to its resting value.

Key Equations:

• Resting membrane potential:

• Threshold for pacemaker cells:

Example: The sinoatrial (SA) node contains typical pacemaker cells that set the heart's rhythm.

Action Potentials in Contractile Cardiac Muscle Cells

Contractile cardiac muscle cells are responsible for the heart's pumping action and have a distinct action potential with a plateau phase.

• Depolarization: Fast voltage-gated Na+ channels open, causing rapid Na+ influx and membrane potential reversal.

• Plateau Phase: Slow Ca2+ channels open, allowing Ca2+ influx and prolonging depolarization.

• Repolarization: Ca2+ channels inactivate, K+ channels open, and K+ efflux restores resting potential.

Key Equation:

• Action potential duration (cardiac):

Comparison Table:

Additional info: The plateau phase ensures efficient blood ejection and prevents tetanic contractions.

The Intrinsic Cardiac Conduction System

Components and Pathway

The heart's intrinsic conduction system consists of specialized noncontractile cells that initiate and distribute electrical impulses, ensuring coordinated contraction.

• Sinoatrial (SA) Node: Located in the right atrial wall; sets the pace (sinus rhythm) at ~75 beats/minute.

• Atrioventricular (AV) Node: Located in the interatrial septum; delays impulse by ~0.1 s to allow atrial contraction.

• AV Bundle (Bundle of His): Only electrical connection between atria and ventricles; located in the interventricular septum.

• Right and Left Bundle Branches: Conduct impulses through the interventricular septum toward the apex.

• Subendocardial Conducting Network (Purkinje Fibers): Penetrate the apex and ventricular walls, ensuring rapid and coordinated ventricular contraction.

Sequence of Excitation:

1. SA node generates impulse.

2. Impulse spreads through atria via gap junctions and internodal pathway to AV node.

3. Impulse delayed at AV node.

4. Impulse travels through AV bundle.

5. Impulse splits into right and left bundle branches.

6. Impulse reaches Purkinje fibers, causing ventricular contraction from apex upward.

Example: The coordinated contraction ensures efficient blood ejection from the ventricles.

Pacemaker Hierarchy and Rates

Different components of the conduction system have intrinsic rates of depolarization.

Additional info: If the SA node fails, slower pacemakers can take over, but at reduced rates.

Extrinsic Innervation of the Heart

Autonomic Nervous System Control

Although the heart can beat independently, the autonomic nervous system modulates heart rate and force.

• Sympathetic Nervous System: Increases heart rate and contractility via the cardioacceleratory center in the medulla oblongata.

• Parasympathetic Nervous System: Decreases heart rate via the cardioinhibitory center and vagus nerve.

• Cardiac Plexus: Network of nerves innervating the heart, including both sympathetic and parasympathetic fibers.

Example: During exercise, sympathetic stimulation increases heart rate and force of contraction.

Clinical Considerations: Arrhythmias and Pacemakers

Arrhythmias and Fibrillation

Defects in the conduction system can lead to abnormal heart rhythms.

• Arrhythmias: Irregular heart rhythms due to conduction defects.

• Fibrillation: Rapid, irregular contractions; ventricles become ineffective as pumps.

• Defibrillation: Electrical shock to restore normal rhythm.

• Implantable Cardioverter Defibrillators (ICDs): Devices that monitor and correct abnormal rhythms.

Example: A defective SA node may result in an ectopic focus or AV node pacing (junctional rhythm).

Electrocardiography (ECG)

Principles of ECG

An electrocardiogram (ECG) records the electrical activity of the heart and is a composite of all action potentials generated by nodal and contractile cells.

• Electrodes: Placed at various body sites to measure voltage differences.

• 12-lead ECG: Standard clinical setup for comprehensive heart activity analysis.

• Waves and Intervals: Each wave and interval represents specific electrical events in the heart.

Example: Abnormalities in ECG tracings can indicate arrhythmias, conduction blocks, or myocardial infarction.

Summary Table: Cardiac Muscle vs. Skeletal Muscle

Additional info: The long refractory period in cardiac muscle is essential for proper heart function and prevention of tetanic contractions.