Action Potential

Action Potential

A resting sarcolemma is polarized, meaning there is a potential difference (voltage) across the plasma ­membrane, whereas the inside portion is negative compared with its outer membrane face. Action potentials occur because of a specific sequence of electrical changes. When they begin, they happen all along the sarcolemma surfaces. This requires three steps: generation of an end plate potential, depolarization, and repolarization:

■■ End plate potential generation: As ACh molecules bind to its receptors at the neuromuscular junction, chemically (ligand) gated ion channels open. This allows sodium and potassium ions to pass. The driving force for sodium ions is greater than for potassium ions. Therefore, more sodium diffuses in that potassium diffuses out. There is a transient change in membrane potential, as the inner sarcolemma depolarizes (becomes less negative). The local, initial depolarization is known as an end plate potential.

■■ Depolarization (action potential generation and propagation): The end plate potential begins an action potential by moving to adjacent membrane­ areas. It opens voltage-gated sodium channels, so that sodium ions can enter along its electrochemical­ gradient. When the membrane voltage known as the threshold is reached, an action potential is initiated (generated). The action potential propagates along the length of the sarcolemma in every direction from the neuromuscular junction. The local depolarization wave of the action potential spreads to nearby areas of the sarcolemma. Voltage-gated sodium channels are opened. Sodium ions then diffuse into the cell along its electrochemical gradient.

■■ Repolarization (the sarcolemma is restored to its polarized state): Like the depolarization wave, the repolarization wave result from membrane permeability changes. Sodium channels close while voltage- gated potassium channels open. The potassium ion concentration is much higher inside cells than in extracellular fluid. Therefore, potassium diffuses quickly out of muscle fibers to restore negatively charged conditions inside (FIGURE 9-5). During repolarization, a muscle fiber begins a refractory period, because the cell cannot be stimulated again until repolarization is complete.