How a Muscle Contracts

A motor neuron, a nerve ending that is attached to a muscle fiber in
Petzold’s huge bicep, sends a message to contract. This message goes to the neuromuscular junction, which is the connection between the actual nerve fiber and muscle fiber. As the message to contract reaches the terminals a neurotransmitter, a chemical messenger for the body, is released into the gap between the terminal end, and the sarcolemma and motor end plate called the synaptic cleft. The particular neurotransmitter is called acetylcholine. Without acetylcholine muscles would not be able to contract. The acetylcholine then combines with receptors to stimulate movement, or a muscle impulse. As a result, calcium ions flood into to the sarcoplasmic reticulum. This forms connections between the actin and myosin filaments. A cross bridge is attached to a myosin filament and when the actin filament slides opposite of the myosin, the cross bridge forms new links. All of this can be accomplished only if there is an energy source. The energy source for movement of a muscle contraction called ATP. ATP is stored in cells and gives the energy for muscles to contract for only a short amount of time. When this happens, the ATP becomes ADP, and the ADP needs to pick up another phosphorus molecule to become ATP again and keep energy flowing to the muscles. A small, fast, and involuntary muscle contraction is called a twitch which follows all the same steps. Once the neurons stop sending messages the muscle can finally relax to where muscle activity decreases, ending a full muscle contraction.