Acetylcholine is the chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate. Acetylcholine (ACh) is an abundant neurotransmitter in the human body. It is found in both the central nervous system (CNS) and the peripheral nervous system (PNS). The name acetylcholine is derived from its structure. It is a chemical compound made up of acetic acid and choline. Cholinergic synapses are those in which transmission is mediated by acetylcholine. It serves a number of critical functions, many of which can be impaired by diseases or drugs that influence the function of this neurotransmitter. Acetylcholine can be found in all motor neurons, where it stimulates muscles to contract. From the movements of the stomach and heart to the blink of an eye, all of the body's movements involve the actions of this important neurotransmitter.It is also found in many brain neurons and plays an important role in mental processes, such as memory and cognition. Acetylcholine has numerous functions in the body. 

Acetylcholine was the first neurotransmitter to be identified. It was discovered by Henry Hallett Dale in 1914, and its existence was later confirmed by Otto Loewi. Both individuals were awarded the Nobel Prize in Physiology or Medicine in 1936 for their discovery. In the PNS, acetylcholine is a major part of the somatic nervous system. Within this system, it plays an excitatory role leading to the voluntary activation of muscles. Within the autonomic system, acetylcholine controls a number of functions by acting on neurons in the ​sympathetic and parasympathetic systems. It is also involved in the contraction of smooth muscles and dilation of blood vessels, and it can promote increased body secretions and a slower heart rate. Because acetylcholine plays an important role in muscle actions, drugs that influence this neurotransmitter can cause various degrees of movement disruption and even paralysis. 

Acetylcholine also acts at various sites within the CNS, where it can function as a neurotransmitter and as a neuromodulator. It plays a role in motivation, arousal, attention, learning, and memory, and is also involved in promoting REM sleep. Disrupted levels of acetylcholine may be associated with Alzheimer's disease. Drugs and substances that interrupt acetylcholine function can have negative effects on the body and can even lead to death. Examples of such substances include some types of pesticides and nerve gasses.