"Caption: 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 fibers, it stimulates those fibers 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. Botulin acts by suppressing the release of acetylcholine. Nicotine acts by increasing the activity of certain acetylcholine receptors, as does muscarine. Conversely, atropine and scopolamine act by blocking these receptors. Atropine and scopolamine are anticholinergic agents. The disease myasthenia gravis, characterized by muscle weakness and fatigue, occurs when the body inappropriately produces antibodies against acetylcholine receptors, and thus inhibits proper acetylcholine signal transmission.
*(Magnifications are based on a 35mm slide image of 24mm in the narrow dimension.)