Neurotransmitters are chemicals released by neurons, and enable our body and brain to communicate. Not only was acetylcholine the first identified neurotransmitter, it is the body's most universal. To its presence we owe every laugh, sadness, remembered birthday, eye-blink and handshake. Acetylcholine arouses the craving for chocolate, and afterward rewards us with pleasure. Without this restless chemical, our hearts would be unable to relax.
A useful example of how acetylcholine works can be found by comparing neurons in our brain, muscle tissue, organs and glands to baseball players. Visualize them throwing baseballs—neurotransmitters—across synaptic gaps. “Catchers,” the neuron's dendrites, are activated by the incoming signal, in this case acetylcholine. The chemical then sparks skeletal muscles into an appropriate response, such as raising a mitted hand to catch a ball. Other neurons have receptors for different neurotransmitters like dopamine, a regulator of movement and emotion, and endorphin to manage stress and pain.
To carry the game analogy further, like baseballs acetylcholine moves in a flash and doesn't linger. An enzyme, cholinesterase, quickly breaks it down in order to prevent over-contraction of the muscle. Acetylcholine plays a different role in the central nervous system, where it regulates the cholinergic processes, activates or inhibits memory, engages and releases attention, controls emotional processing, the endocrine system and sleep cycles.
Evidence exists showing that acetylcholine is important to REM sleep, or rapid eye movement, the last stage of fundamental sleep cycles. It also is associated with skeletal paralysis, popularly known as sleep paralysis, where a person is conscious of being fully awake but unable to move. The phenomenon usually occurs among narcoleptics, and rarely in around 40% of healthy people.
Acetylcholine also regulates the gastrointestinal system, triggering peristalsis in the stomach as well as sequencing circular muscles during digestive contractions. In urinary functions, the neurotransmitter modulates bladder capacity and pressure of voluntary voiding. In the respiratory system, acetylcholine activates glandular secretions by way of the autonomic, or parasympathetic, nerve impulses. Given the multitude of functions, it's easy to understand the devastating effects of a neuromuscular disease such as myasthenia gravis, which sends the autoimmune system's antibodies to destroy acetylcholine receptors in skeletal muscles, thus corroding smooth neurotransmission and effective muscular function.
Arthur James Ewins, an English chemist, first identified acetylcholine in 1913, although its significance to the central nervous system and optimal bodily functions wasn't confirmed until 1921, when German physiologist Otto Loewi showed that acetylcholine is activated by stimulation of the vagus nerve, resulting in the slowing of heart muscles. Later experimentation by others revealed that the chemical also was working as a transmitter at numerous synapses, and even in such invertebrates as simple corals, jellyfish, and most insects. The great insight and work of Otto Loewi and his colleague, physiologist Sir Henry Dale, earned both the 1936 Nobel Prize for Physiology or Medicine.