The first neurotransmitter ever identified was acetylcholine, a chemical transmitter in both the central and parasympathetic nervous system in humans and many other organisms. It was discovered by Henry Hallett Dale and Otto Loewi in 1914; this discovery earned them the Nobel Prize in Physiology or Medicine in 1936.
Chemically, acetylcholine is an ester of acetic acid and choline with the very long chemical formula CH3COOCH2CH2N+(CH3)3. It is a very effective deliverer of sodium ions, which stimulate muscle contractions and excites nerves. An increase in acetylcholine causes a decreased heart rate and increased production of saliva, as well as readying the muscles for work. In high doses, it can cause convulsions and tremors. In deficient levels, it can contribute to motor dysfunction.
In most circumstances, acetylcholine is removed quickly after acting by the enzyme acetylcholinesterase. Nerve agents often act by inhibiting this enzyme, resulting in a continued stimulation of the muscles, glands, and central nervous system. Controlling these same nerve agents may prove to be cures, however, in diseases like Alzheimer's Disease; sufferers show a marked shortage of acetylcholine.
Botulin is another toxin that acts on acetylcholine by suppressing its release (which is how botox injections work – the muscles have no stimulation). Nicotine and muscarine also act on acetylcholine by increasing receptor activity. Atropine and scopolamine block these receptors, gaining them the title of anticholinergic agents. And myasthenia gravis, a disease characterized by muscular weakness and fatigue, is caused by an immune response against acetylcholine receptors.
Acetylcholine is released by several mechanisms, including:
* pre- and post-ganglionic parasympathetic neurons
* preganglionic sympathetic neurons
* somatic neurons (controlling sensation and muscle contraction)
* some CNS neurons
Direct Use of Acetylcholine in Medicine
Acetylcholine can have some interesting effects when administered directly, or when blocked, hindered, or mimicked. Cataract surgery sometimes requires the use of acetylcholine to constrict the pupil rapidly.
Cholinesterase inhibitors delay the degradation of acetylcholine, and are used to reverse muscle relaxants and sometimes to treat Alzheimer's disease. They also have a place in cardiopulmonary resuscitation.
Web Resources On Acetylcholine
Alzheimer’s, Memory, and Acetylcholine
Acetylcholine and hallucinations
Book Resources On Acetylcholine
Acetylcholine in the Cerebral Cortex by L. Descarries, et al
CNS Neurotransmitters and Neuromodulators: Acetylcholine by T. W. Stone