Muscarinic receptors are structures found in the membranes of some cells which are responsive to muscarine, among several other chemical compounds. These structures play an important role in the function of the parasympathetic nervous system, which includes glandular tissue, heart muscle, and smooth muscle tissues. One way in which muscarinic receptors function is in the regulation of heart rate, in concert with several other processes in the body.
These structures are part of a larger family of proteins known as acetylcholine receptors or cholinergic receptors, because they respond to acetylcholine. The other main type of cholinergic receptor is the nicotinic receptor. Like other proteins found in the cell membrane, muscarinic receptors are sensitized to a number of different chemical compounds which can trigger various responses. These responses can also be created artificially with the use of pharmaceuticals which act either as muscarinic receptor antagonists or muscarinic receptor agonists, depending on the desired effect of the medication.
Muscarinic acetylcholine receptors have a number of isoforms which can be found in different parts of the body. These isoforms have been identified by researchers studying different kinds of tissue in the body with the goal of learning more about how the body functions. All of the isoforms act by triggering the opening of ion channels with a cascading reaction, unlike nicotinic receptors, which open ion channels directly to allow impulses to travel freely. One could think of muscarinic receptors as the door buzzers located in apartments; when someone rings from downstairs, the person in the apartment can trip the buzzer to allow a guest in, rather than a nicotinic receptor, which opens the door directly.
Muscarine stimulates muscarinic receptors, while atropine depresses them. Understanding which compounds affect these proteins and how these compounds work is important for pharmaceutical researchers who would like to develop products capable of targeting muscarinic receptors. Atropine, for example, is used to dilate the pupils of the eye for eye exams.
In addition to being stimulated or depressed by pharmaceuticals and substances generated by the body as part of the body's complex regulatory system, these receptors can also react to parts of the human diet or to toxins which people ingest. These toxins can be produced by organisms such as plants, fungi, and bacteria, and when they enter the body, they trigger the activity of muscarinic receptors, sometimes generating highly unpleasant symptoms. For example, compounds such as belladonna and jimsonweed contain atropine, explaining the effects experienced by people who consume these compounds, intentionally or otherwise.