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Acetylcholine

Explanations > Brains > Acetylcholine

Composition | Function | Dysfunction | See also

Acetylcholine (ACh) is a very common neurotransmitter, being found in central, peripheral, autonomic and somatic nervous systems.

The adjective acetylcholinergic means 'related to acetylcholine'. Thus acetylcholinergic receptors accept acetylcholine as triggers.

Composition

Chemically, acetylecholine is an ester of acetate (acetic acid) and choline. Choline is a natural amine and an essential nutrient in the Vitamin B group. It is create in certain neurons by the enzyme choline acetyltransferase from the choline and acetyl-CoA. CoA is created by mitochondria and is common throughout the body.

Destruction

Acetylcholine is broken down by the enzyme acetylcholinesterase (AChE) which is common in the synaptic cleft where it converts actylcholine into the inactive metabolites choline and acetate. AChE is very efficient, breaking up over 5000 ACh molecules per second.

About 50% of choline is absorbed back into the terminal button via reuptake where it is re-converted into ACh (along with new choline provided by axoplasmic transport.

Function

In the Peripheral Nervous System, acetylecholine acts to stimulate muscle movement. Acetylcholine receptors on the muscles accept acetylcholine and cause skeletal muscles to contract. Interestingly, they cause heart muscles to relax.

In the Central Nervous System, it has a range of effects including arousal and reward, as well as learning and short-term memory (using synaptic plasticity, the ability to change neuron connection strength).

As a neuromodulator, it can exist in the cerebrospinal fluid and regulate a range of neurons directly as opposed to through a single synaptic connection.

There are two main types of acetylcholine receptor(AChR):

• nicotinic acetylcholine receptors (nAChR) are ionotropic and pass sodium, potassium and chlorine ions.
• muscarinic acetylcholine receptors (mAChR) are metabotropic and have a longer-term effect.

Dysfunction

Alzheimer's disease is associated with a shortage of acetylcholine in the brain. Some treatments act to inhibit acetylcholinesterase.

Drugs

Botulinum toxin prevents the release of ACh and is highly toxic (a teaspoonful could kill everyone in the world).

Black widow spider venom stimulates ACh release and is fatal only for the weak, such as the elderly and young children.

AChE inhibitors are used where diseases attack ACh.

nAChR are also stimulated by nicotine. Muscle nAChR are blocked by curare whilst neuronal nAChR are blocked by hexamethonium.

mAChR are also stimulated by muscarine. They are blocked by atrophine.

See also