What effect does the end-plate potential have on the muscle fiber?

• A. Global potential across the muscle fiber.
• B. A local depolarization at the motor endplate that can trigger an action potential in the muscle fiber.
• C. Hyperpolarization of the muscle membrane.
• D. A plateau phase in skeletal muscle.

Answer: (B)

The end-plate potential is a local depolarizing signal produced when acetylcholine released from the motor neuron binds to nicotinic receptors at the muscle’s endplate. This opens ligand-gated cation channels, mainly letting in Na+ (with some K+ out), so the small region of the membrane at the endplate becomes less negative. If this depolarization is large enough to reach the threshold of nearby voltage-gated Na+ channels, an action potential is generated that spreads across the muscle fiber and triggers contraction. It’s a local, graded potential, not a global one, and it is depolarizing rather than hyperpolarizing. There isn’t a plateau phase in skeletal muscle like in cardiac muscle.