Learning Objective: By the end of this session, the learner will be able to describe the purpose and phases of an action potential, compare action potential characteristics in neurons, skeletal muscle, and cardiac muscle, and explain how rapid depolarization and repolarization enable signal conduction and contraction in excitable tissues.

Introduction to the Action Potential

An action potential (AP) is a brief, dramatic change in membrane potential characterized by a rapid depolarization followed by repolarization back toward the resting state. This electrical event allows excitable tissues to perform their essential functions:

• Neurons: rapid transmission of electrical signals across long distances
• Skeletal muscle: initiation of muscle contraction
• Cardiac muscle: synchronized electrical activation and rhythmic contraction

Although the fundamental mechanism—ion flux through voltage-gated channels—is similar across tissues, action potentials vary significantly in shape, amplitude, and duration depending on the cell type and channel properties.

Comparing Action Potentials in Excitable Cells

Action potentials differ greatly among neurons, skeletal muscle, and cardiac muscle cells. These differences arise from variations in:

• Types of voltage-gated channels
• Timing of channel opening/closing
• Relative contribution of Na⁺, K⁺, and Ca²⁺ currents
• Presence or absence of plateaus (e.g., cardiac muscle)
• Duration (milliseconds vs. hundreds of milliseconds)

Activity

Key Distinguishing Features

• Neuron AP:
  Brief (≈ 2–3 ms), rapid depolarization via voltage-gated Na⁺ channels, followed by repolarization via K⁺ channels.
• Skeletal muscle AP:
  Similar to neurons but slightly longer (≈ 5 ms); essential for excitation–contraction coupling.
• Cardiac ventricular AP:
  Much longer duration (≈ 200–300 ms) due to a plateau phase maintained by Ca²⁺ influx, crucial for preventing tetany.

Activity