Acetylcholine
![Acetylcholine pathway.](/Images/godic/202412/14/Acetylcholine_Pathway0553.png")
![Acetylcholine processing in a synapse. After release acetylcholine is broken down by the enzyme acetylcholinesterase.](/Images/godic/202412/14/Acetylcholine_synapse0553.png")
![Muscles contract when they receive signals from motor neurons. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows: (1) The action potential reaches the axon terminal. (2) Calcium ions flow into the axon terminal. (3) Acetylcholine is released into the synaptic cleft. (4) Acetylcholine binds to postsynaptic receptors. (5) This binding causes ion channels to open and allows sodium ions to flow into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which induces muscle contraction. Labels: A: Motor neuron axon B: Axon terminal C. Synaptic cleft D. Muscle cell](/Images/godic/202412/14/The_Muscle_Contraction_Process0553.png")
![Components and connections of the parasympathetic nervous system.](/Images/godic/202412/14/1503_Connections_of_the_Parasympathetic_Nervous_System0553.jpg")
Acetylcholine is an organic chemical that functions in the brain and body of many types of animals, including humans, as a neurotransmitter—a chemical released by nerve cells to send signals to other cells. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Parts in the body that use or are affected by acetylcholine are referred to as cholinergic. Substances that interfere with acetylcholine activity are called anticholinergics.