Acetylcholine

Acetylcholine (ACh) is an ester of acetic acid and choline. It acts as a neurotransmitter in the peripheral and central nervous system. In the periphery, it transmits the signal on the neuromuscular plates and is a mediator of the parasympathetic autonomic system.

Centrally, it acts as a neurotransmitter and neuromodulator. Here it has a role in maintaining consciousness, attention and the formation of memory. ACh acts on two types of receptors - on the one hand, on ionotropic nicotinic receptors and, on the other hand, on metabotropic muscarinic receptors.

Chemical properties, synthesis
ACh is synthesized in cholinergic neurons by the enzyme choline acetyltransferase. This enzyme binds the acetic acid residue to choline via an ester bond. Acetyl-CoA and choline serve as substrates.

Choline is synthesized through the sequential methylation of ethanolamine, which is formed by decarboxylation of serine. S-adenosylmethionine (active methionine, SAM) serves as a donor of methyl groups. The resulting acetylcholine has the properties of a strongly polar quaternary ammonium compound - regardless of pH, it has a positive charge, so it does not penetrate into cells or, for example, through the blood-brain barrier.

Degradation of ACh takes place hydrolytically, extracellularly on the membrane of the synaptic cleft with the help of the enzyme acetylcholine esterase - inactive metabolites choline and acetate are formed

Neuromuscular plate
Acetylcholine acts on the neuromuscular plate, which is a special type of chemical synapse between a motoneuron and a skeletal muscle motor unit. After being washed into the synaptic cleft, it binds to nicotinic receptors (nAChR). There is a change in the membrane potential of the muscle fiber and an intracellular washout of Ca 2+ ions from the sarcoplasmic reticulum, which are essential for muscle contraction. For more details, see the article Neuromuscular disc.

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