Ace the CVS Pharmacy Exam 2026 - Your Prescription for Success!

The action potential in muscle and nerve fibers is primarily caused by the opening of fast sodium channels. When a stimulus reaches a neuron or muscle cell, these fast sodium channels open rapidly, allowing sodium ions to flow into the cell. This influx of sodium ions depolarizes the cell membrane, causing the inside of the cell to become more positive relative to the outside. This depolarization is the initial phase of the action potential and is crucial for the conduction of electrical signals along nerves and for muscle contraction.

Fast sodium channels are characterized by their quick opening and inactivation, which facilitates the rapid upstroke of the action potential. The subsequent phases of the action potential involve other channels, such as potassium channels, but the initial trigger is primarily due to the rapid influx of sodium through these fast sodium channels. Understanding this mechanism is key in both physiology and pharmacology, as it plays an essential role in how signals are transmitted throughout the nervous system and how muscle contractions are initiated.