Light micrograph (LM) of Acetylcholine crystals. Acetylcholine was the first neurotransmitter to be identified. It is a chemical transmitter in the central nervous system (CNS) as well as in the parasympathetic nervous system in many organisms including humans. It is the only transmitter found at the synapses between motor neurons and skeletal muscles. When it binds to acetylcholine receptors of muscle fibres, it stimulates those fibres to contract. Acetylcholine is also used in the brain, where it tends to cause excitatory actions. The glands that receive impulses from the parasympathetic part of the autonomic nervous system are also stimulated in the same way. This is why an increase in acetylcholine causes a decreased heart rate and increased production of saliva. Magnification: x40 when shortest axis printed at 25 millimetres.
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