The nervous system communicates with the body by sending electrical impulses from the central nervous system (CNS) through long nerves cells to the peripheral nervous system (PNS), muscles and glands. For this to work, neighbouring nerve cells must be able to convey these electrical signals between them. When an electrical nerve impulse reaches a junction with another nerve cell (synapse), or with a muscle cell (neuromuscular junction), it must transmit its signal by chemical means. The influx of ions at a synaptic or neuromuscular junction stimulates the release of vesicles containing neurotransmitters, which diffuse across the gap and bind to receptors on the adjoining nerve or muscle cell, thereby continuing the response.
There are various types of neurotransmitters that elicit different, and sometime opposing, types of responses. One of the first neurotransmitters to be discovered was acetylcholine, which is produced by the enzyme choline acetyltransferase, using acetyl coenzyme A (acetyl CoA) and dietary choline as substrates. Acetylcholine release can be either excitatory (promoting a signal), or inhibitory depending upon the type of receptor on the adjoining cell. Once acetylcholine has activated its receptor to transmit its signal, it needs to be broken down to prepare the synapse for the arrival of the next signal; this is accomplished by the enzyme acetylcholinesterase, which removes acetylcholine from the synapse by breaking it down into inactive fragments. Nerve gases such as sarin that are used in warfare, as well as organophosphate insecticides such as parathion, achieve their effects through the inhibition of acetylcholinesterase, thereby allowing acetylcholine to remain active and in contact with its receptor. Antidotes such as atropine are used to block acetylcholine receptors, thereby nullifying the effects of excessive acetylcholine.
So how does acetylcholine work? By binding to a receptor on an adjoining cell, acetylcholine can causes the activation of the receptor, which can then have different downstream effects, depending upon which acetylcholine receptor is activated. These effects lead to a cascade of events that transmit the signal inside the cell, resulting in the continuation of a nerve impulse, the movement of muscle, or many other responses in a variety of different tissues. For example, the sight, smell and taste of food can cause the vagus nerve to release acetylcholine, which binds to receptors on parietal cells, causing an influx of calcium ions that activate intracellular phosphokinase enzymes; this in turn results in the activation of a proton pump to expel hydrogen ions, which can then combine with chloride ions to form the hydrochloric acid that is required for the digestion of food in the stomach.