Once a motor neuron has fired all the muscle fibers in contract?

Once an action potential reaches the motor end plate, it triggers the release of acetylcholine. This neurotransmitter then diffuses across the synaptic gap and binds to receptors on the muscle fiber membrane. This binding triggers a conformational change in the receptor that allows sodium ions to flow into the muscle fiber. This depolarization propagates along the muscle fiber membrane and causes the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin, which causes a change in the conformation of the troponin-tropomyosin complex. This allows the myosin head groups to bind to the actin filaments and initiate muscle contraction.

The number of muscle fibers that are recruited in a single contraction is determined by the strength of the stimulus. A weak stimulus will only excite a few motor units, while a strong stimulus will excite more motor units. The more motor units that are excited, the stronger the contraction will be.