The Heart – Electrical Activity, Pacemaker Cells, and the Cardiac Cycle

The Heart: Electrical Activity and Pacemaker Cell Action Potentials

Introduction to Cardiac Electrophysiology

The heart's rhythmic contractions are controlled by specialized electrical signals generated and propagated by cardiac muscle cells. Understanding the action potentials in these cells is essential for comprehending normal heart function and arrhythmias.

Recording Cardiac Electrical Activity: The Electrocardiogram (ECG)

• Electrocardiogram (ECG or EKG): A non-invasive test that records the electrical activity of the heart using skin electrodes.

• Clinical Application: Used to diagnose arrhythmias, conduction abnormalities, heart enlargement, and myocardial damage.

• Procedure: Electrodes are placed on the skin at specific locations to detect electrical changes during the cardiac cycle.

• Example: A standard 12-lead ECG provides a comprehensive view of the heart's electrical activity from multiple angles.

Electrical Action Potentials in Cardiac Cells

General Principles of Action Potentials

• Action Potential: A rapid change in membrane potential that propagates along the cell membrane, essential for nerve and muscle function.

• Phases: Includes depolarization (Na+ influx), repolarization (K+ efflux), and sometimes hyperpolarization.

• Resting Membrane Potential: The baseline electrical charge across the cell membrane, typically negative inside relative to outside.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Example: In neurons, the action potential is initiated when the membrane potential reaches approximately -55 mV.

Pacemaker Cell Action Potentials

Pacemaker cells in the heart, primarily located in the sinoatrial (SA) node, generate spontaneous action potentials that set the heart rate.

• Unstable Resting Membrane Potential: Pacemaker cells do not have a true resting potential; instead, their membrane potential slowly drifts toward threshold due to continuous cation (mainly Na+) leakage.

• Pacemaker Potential: The gradual depolarization caused by the influx of more cations than are leaving the cell, bringing the membrane potential close to threshold.

• Depolarization: Once threshold is reached, voltage-gated Ca2+ channels open, allowing Ca2+ influx and rapid depolarization.

• Repolarization: Voltage-gated Ca2+ channels close, and voltage-gated K+ channels open, allowing K+ to exit the cell, restoring negativity.

• Cycle: After repolarization, cation leakage channels reopen, and the cycle repeats, generating rhythmic action potentials.

• Example: The SA node typically initiates 60–100 action potentials per minute in a healthy adult.

Key Ion Channels in Pacemaker Cells

• HCN Channels ("Funny" Channels): Allow slow Na+ influx during pacemaker potential.

• Voltage-Gated Ca2+ Channels: Open at threshold, causing rapid depolarization.

• Voltage-Gated K+ Channels: Open during repolarization, allowing K+ efflux.

Summary Table: Pacemaker Cell Action Potential Phases

Phase	Main Ion Movement	Key Channels	Result
Pacemaker Potential	Na+ influx	HCN ("funny") channels	Slow depolarization toward threshold
Depolarization	Ca2+ influx	Voltage-gated Ca2+ channels	Rapid depolarization
Repolarization	K+ efflux	Voltage-gated K+ channels	Return to negative membrane potential

Comparison: Pacemaker vs. Neuronal Action Potentials

• Pacemaker Cells: Spontaneous, slow depolarization due to Na+ leak; Ca2+ influx for depolarization.

• Neurons: Stable resting potential; rapid depolarization due to Na+ influx.

• Repolarization: Both use K+ efflux to repolarize the membrane.

Key Equations

• Ohm's Law (for membrane potential):

$V = IR$

• Nernst Equation (for equilibrium potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Cardiac Output:

$CO = SV \times HR$

where $CO$ is cardiac output, $SV$ is stroke volume, and $HR$ is heart rate.

Clinical Relevance

• Arrhythmias: Abnormalities in pacemaker cell function can lead to irregular heart rhythms.

• Artificial Pacemakers: Devices that replace or support the function of the SA node in patients with conduction disorders.

• ECG Interpretation: Changes in the shape, timing, or sequence of ECG waves can indicate specific cardiac pathologies.

Additional info: The provided images and notes are consistent with standard college-level Anatomy & Physiology content on cardiac electrophysiology and the cardiac cycle. The explanations have been expanded for clarity and completeness.