Final answer:
The cholinergic system has two main types of receptors: nicotinic and muscarinic receptors, both of which bind to acetylcholine. Nicotinic receptors are ligand-gated cation channels activated by nicotine, while muscarinic receptors are G protein-coupled receptors activated by muscarine. These receptors are specific to their ligands and do not show cross-reactivity.
Step-by-step explanation:
The cholinergic system includes two main types of receptors, nicotinic receptors and muscarinic receptors, each binding to the neurotransmitter acetylcholine (ACh). Nicotinic receptors are found in the neuromuscular junction (NMJ) and other synapses within the nervous system. They are ligand-gated cation channels, which means they open to allow ions to pass through the membrane when ACh binds to them. Substances such as nicotine can also activate these receptors similarly to ACh.
In contrast, muscarinic receptors are G protein-coupled receptors that are activated by ACh and are found in various locations throughout the nervous system, including the autonomic nervous system and in the brain. Compounds such as muscarine from certain mushrooms can bind to muscarinic receptors. However, there is no cross-reactivity: nicotine does not bind to muscarinic receptors, and muscarine does not bind to nicotinic receptors.
This specificity is much like a lock and key mechanism, with ACh acting as a 'master key' that can bind to both nicotinic and muscarinic receptors, causing changes in the target cell. The use of specific molecules to define these receptors, such as nicotine and muscarine, is mostly for research purposes to study the cholinergic system without interference from endogenous substances.