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Everything starts by the signal of the brain, this signal is called as action potential, this signal travel all over a cell which is called motor neuron. The neuromuscular junction is the name of the place where the motor neuron reaches a muscle cell. When the signal reaches the neuromuscular junction, a chemical message is released by the motor neuron. The message is called as acetylcholine, this one joins with the receptors in the outside of the muscle fiber. That starts a chemical reaction within the muscle.
A molecular process with various steps inside the muscle fiber starts when the acetylcholine joins to the receptors in the muscle fiber membrane. The proteins inside muscle fibers are organized into long chains that can interact with each other, reorganizing to shorten and relax. When acetylcholine reaches receptors on the membranes of muscle fibers, membrane channels open and the process that contracts a relaxed muscle fibers begins as how we demonstrated on the next page: