What role does acetylcholine have at the neuromuscular junction.

The neuromuscular junction is the meeting point between a nerve and a muscle, specifically a motor neurone and a muscle fibre. It is the way in which the brain communicates with the muscles of the body to induce movement.When an action potential in a nerve reaches an neuromuscular junction (NMJ) it causes depolarisation of the end of the neurone, this results in voltage gated calcium channels opening. Calcium can now enter the neurone and this causes vesicles (which are little sacs in the cell that contain the chemical acetylcholine) to travel to the very end of the neurone and release their contents (acetylcholine). Think of acetylcholine as the messenger molecule, this is the molecule that the nerve uses to 'speak' to the neuromuscular junction. Acetylcholine travels in between the nerve and muscle, this space is called the synaptic cleft, and binds to receptors on the muscle fibre. These receptors are called nicotinic acetylcholine receptors. The binding of acetylcholine to these receptors causes sodium to enter through these receptors (known as ligand gated ion channels) and this causes depolarisation of the muscle fibre. It is important to note that two acetylcholine molecules must bind to a nicotinic acetylcholine receptor to activate it. Finally, an enzyme called acetylcholinesterase hydrolyses (breaks down) the acetylcholine that is left in the synaptic cleft. This is an important function as it stops the acetylcholine always activating the muscle, if it wasn't there your muscles would be constantly contracting.