The junction between nerve and muscle

The neuromuscular junction is the exact spot where a nerve talks to a muscle. It is a synapse, that is a contact point for passing signals along, but a specialized one: it connects the motor neuron, the nerve cell that commands movement, with the skeletal muscle fiber.

The electrical signal traveling down the nerve is called an action potential. When it reaches the end of the nerve it opens some channels and calcium comes in. That calcium sticks to a protein called synaptotagmin, and that makes small sacs, the vesicles, release what they hold: acetylcholine, the chemical messenger of this junction.

Acetylcholine crosses over and opens its receptors on the motor end plate, the area of the muscle membrane set up to receive it. Sodium comes in, and a single push of that kind is enough for the muscle to fire its own electrical signal.

And here is the detail that keeps things from turning to chaos: an enzyme in the synaptic cleft, acetylcholinesterase, destroys the acetylcholine immediately. It is a message that crosses a gap and that someone erases on arrival, so the muscle gets a brief push and not a continuous one.