Excitatory nicotinic acetylcholine (ACh) receptors are frequently found in the membranes of postsynaptic neurons, especially in the central nervous system. They are also found in the membranes of skeletal muscle fibers. The term nicotinic is applied to these ACh receptors because they can be stimulated by the drug nicotine. A second type of acetylcholine receptor, which can be stimulated by the drug muscarine, is found in the heart, smooth muscles, glands, and brain. They differ in structure and mechanism of action and will not be discussed here.
Nicotinic receptors are composed of five (5) polypeptide subunits, which together form a water-filled ion channel in the lipid bilayer. Normally, the gates are closed and ions are prevented from passing through the channel. Two of the receptor's subunits have binding sites for ACh molecules. When both binding sites are occupied by ACh, the gated subunits change shape (= conformation) and the channel opens. Nicotinic ACh receptors are also known as chemical-gated or ligand-gated channels. The term ligand is used because it refers to one molecule binding to another (larger) molecule. While it is open, sodium (Na+) and potassium (K+) ions diffuse through the channel. Na+ moves in and K+ moves out. The exchange of ions causes the postsynaptic neuron to depolarize.
