What major ion movement leads to the end plate potential in muscle fibers?

The end plate potential in muscle fibers is primarily caused by the influx of sodium ions. When an action potential travels along a motor neuron and reaches the neuromuscular junction, it triggers the release of the neurotransmitter acetylcholine from the synaptic vesicles into the synaptic cleft. Acetylcholine then binds to nicotinic receptors on the muscle fiber's sarcolemma.

This binding causes sodium channels to open, allowing sodium ions to flow into the muscle fiber. The rapid influx of sodium ions depolarizes the membrane potential of the muscle fiber, leading to the end plate potential. If this depolarization is sufficient, it can trigger an action potential that propagates along the muscle fiber, ultimately leading to muscle contraction. The role of sodium ions in this process is crucial, as their movement is what initiates the electrical signal within the muscle fiber, making them the major contributors to the end plate potential.