Final answer:
The nicotinic receptor and the muscarinic receptor are two classes of receptors in the cholinergic system. They bind to acetylcholine and cause changes in the target cell. Nicotine binds to the nicotinic receptor, muscarine binds to the muscarinic receptor, and acetylcholine can bind to both types of receptors.
Step-by-step explanation:
The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to acetylcholine (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. It's similar to locks and keys - each receptor type has its own specific key. However, acetylcholine can bind to both types of receptors, acting like a master key that can open multiple locks.
The molecules that define these receptors, such as nicotine and muscarine, are simply tools for researchers to use in the laboratory. They are exogenous, meaning they are made outside of the human body, allowing researchers to use them without any confounding results caused by endogenous molecules produced in the body.