Transcription
Say you're bending down to tie your shoe. To do even this simple task, your body goes through a sequence of steps.
These steps begin with your brain sending a signal to a motor neuron to stimulate a muscle contraction. Then, the synaptic vesicles and the synaptic end bulb release acetylcholine. The acetylcholine crosses the neuromuscular junction in the synaptic cleft.
The acetylcholine then binds to receptors that open Na+ channels. An inflow of sodium ions generates a muscle action potential. Acetylcholinesterase destroys the acetylcholine so that another muscle contraction action potential cannot occur unless more ACH is released.
Then, Ca2+ channels open, allowing the calcium ions to flood into the sarcoplasm, also known as the cytoplasm of the muscle fiber cell. Calcium ions bind to troponin on the thin filaments, exposing binding sites for myosin. Myosin then binds to actin, causing contraction of the muscle fiber.
Then, the calcium channels close. The calcium ion active transport pumps use a lot of ATP to pump the calcium ions back out of the muscle fiber cell. Without calcium to keep troponin out of the action, it slides back into position and blocks the binding of actin and myosin. Then your step is complete and the muscle relaxes.