Identify the steps involved in the generation and propagation of an action potential. Why is it an all or nothing response? -Muscle fatigue is the complete physiological inability to contract. However, fatigue does not normally set in during general exercise. In this case, your muscles are often tired from the exercise. What do you think could be some of the causes of this general tiredness? 1 -Acetylcholine is released by the motor neuron at the neuromuscular junction and binds to receptors on the muscle cell’s membrane. Where else in the body is acetylcholine an important neurotransmitter?