Final answer:
The release of acetylcholine at the motor endplate does not always cause the muscle fiber to fire and propagate the potential along its length. The end-plate potential must reach a threshold and trigger an action potential for the muscle fiber to contract and propagate the potential.
Step-by-step explanation:
The postsynaptic potential that occurs in the motor endplate in response to release of acetylcholine by the terminal button does not always cause the muscle fiber to fire and propagate the potential along its length. The release of acetylcholine causes a depolarization of the muscle fiber, known as an end-plate potential. However, for the muscle fiber to fire and propagate the potential, the end-plate potential must reach a threshold and trigger an action potential. This action potential then travels along the length of the muscle fiber, leading to muscle contraction.