Connections of parasympathetic division of the autonomic nervous system
Chemical signaling in the autonomic nervous system
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic , meaning that acetylcholine (ACh) is released, or adrenergic , meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.
The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor . Both receptor types bind to ACh and cause changes in the target cell. The nicotinic receptor is a ligand-gated cation channel and the muscarinic receptor is a G protein–coupled receptor . The receptors are named for, and differentiated by, other molecules that bind to them. Whereas nicotine will bind to the nicotinic receptor, and muscarine will bind to the muscarinic receptor, there is no cross-reactivity between the receptors. The situation is similar to locks and keys. Imagine two locks—one for a classroom and the other for an office—that are opened by two separate keys. The classroom key will not open the office door and the office key will not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the Biology Department that opens both the classroom and the office doors. This is similar to ACh that binds to both types of receptors. The molecules that define these receptors are not crucial—they are simply tools for researchers to use in the laboratory. These molecules are exogenous , meaning that they are made outside of the human body, so a researcher can use them without any confounding endogenous results (results caused by the molecules produced in the body).
The adrenergic system also has two types of receptors, named the alpha (α)-adrenergic receptor and beta (β)-adrenergic receptor . Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G protein–coupled receptors. There are three types of α-adrenergic receptors, termed α 1 , α 2 , and α 3 , and there are two types of β-adrenergic receptors, termed β 1 and β 2 . An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine . The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH 3 ) in epinephrine. The prefix “nor-” actually refers to this chemical difference, in which a methyl group is missing.