Electricity and the Heart

Introduction to Cardiac Electricity

The heart's ability to contract and pump blood is driven by innate biologic electricity. This electrical activity is essential for normal cardiac function and can be recorded and analyzed using an electrocardiogram (EKG/ECG). Abnormalities in the heart's electrical activity are often indicative of disease.

• Biologic electricity refers to the movement of ions across cell membranes, generating electrical potentials.

• EKG (Electrocardiogram) is a diagnostic tool that records the heart's electrical activity over time.

• Many cardiac diseases are diagnosed by detecting abnormal electrical patterns on the EKG.

• Cardiac cells are electrically polarized at rest, meaning there is a difference in charge across their membranes.

• Membrane pumps (such as the sodium-potassium pump) maintain this polarity by actively transporting ions.

Membrane Pumps and Polarization

Membrane pumps are specialized proteins that help maintain the electrical polarity of cardiac cells. The sodium-potassium pump is a key example, moving sodium ions out and potassium ions into the cell, creating a negative charge inside relative to the outside.

• Polarization is the state of having a negative charge inside the cell compared to the outside.

• Membrane pumps require energy (ATP) to function.

• Maintaining polarization is essential for the cell's ability to generate action potentials.

• Example: The sodium-potassium pump exchanges 3 Na+ ions out for 2 K+ ions in, maintaining the resting membrane potential.

Depolarization and Repolarization

Depolarization

Depolarization is the fundamental electrical event in the heart, characterized by the loss of internal negativity in cardiac cells. This process is propagated from cell to cell, creating a wave of electrical activity that can be detected by electrodes placed on the body.

• Depolarization occurs when sodium ions rush into the cell, making the inside more positive.

• This wave of depolarization triggers muscle contraction.

• EKG electrodes detect these electrical changes as characteristic waves.

Repolarization

Repolarization is the process by which cardiac cells return to their resting state, restoring internal negativity. This is essential for the cells to be ready for the next cycle of depolarization.

• Repolarization involves potassium ions leaving the cell, restoring the negative charge inside.

• There is a brief refractory period during which the cell cannot be re-excited.

Propagation of Electrical Signals

The electrical signal travels through cardiac tissue in a coordinated manner, ensuring efficient contraction of the heart chambers.

• Depolarization spreads from cell to cell via gap junctions.

• Repolarization follows, allowing the heart to relax.

• Additional info: The speed and direction of propagation are critical for normal heart rhythm.

Types of Cardiac Cells

Overview of Cardiac Cell Types

The heart contains three main types of cells involved in its electrical activity: pacemaker cells, electrical conducting cells, and myocardial cells.

• Pacemaker cells: Initiate and regulate the heartbeat.

• Electrical conducting cells: Rapidly transmit electrical signals throughout the heart.

• Myocardial cells: Contract to pump blood.

Pacemaker Cells

Pacemaker cells are specialized for initiating the heart's electrical impulses. They are small (5–10 μm long) and can depolarize spontaneously, generating regular action potentials without external stimulation.

• Action potential: One complete cycle of depolarization and repolarization.

• Pacemaker cells do not have a true resting potential; they continuously cycle through depolarization and repolarization.

• Most pacemaker cells are located in the sinoatrial (SA) node in the upper right atrium.

• The SA node typically fires 60–100 times per minute, setting the heart's rhythm.

• Additional info: Other pacemaker regions include the atrioventricular (AV) node and Purkinje fibers, which can take over if the SA node fails.

Action Potential in Pacemaker Cells

The action potential in pacemaker cells is characterized by a gradual depolarization (prepotential), rapid depolarization, and repolarization. This cycle is responsible for the rhythmic firing of the heart.

• Maximum voltage is reached at peak depolarization.

• Resting potential is less negative than in other cardiac cells, allowing spontaneous depolarization.

• Example: The SA node's action potential can be represented graphically, showing phases of depolarization and repolarization.

Key Equations

• Resting membrane potential (typical cardiac cell):

• Action potential phases (simplified):

Summary Table: Cardiac Cell Types