Acetylcholine receptors are transmembrane proteins that bind to acetylcholine and are found in the nervous system. There are two types: nicotinic and muscarinic. Nicotinic receptors are ion channels that respond to nicotine and allow cations to flow through the pore, causing depolarization. Muscarinic receptors are G protein-coupled receptors involved in the parasympathetic nervous system and recovery mechanisms in neurons. They are found in five forms throughout the body, each with different functions.
An acetylcholine receptor is a transmembrane receptor protein that binds to the neurotransmitter acetylcholine, although it can also bind to other ligands with lower affinity. It is found in several forms throughout the central and peripheral nervous systems of the body, most typically in the cell membranes of neurons. It plays an important role in the autonomic nervous system. The two main types of acetylcholine receptors are nicotinic and muscarinic.
Nicotinic acetylcholine receptors are named because they respond strongly to nicotine upon binding. This type of acetylcholine receptor is a ligand-gated ion channel, with a central pore surrounded by five protein subunits that vary according to the function and location of the receptor in the body. It is a non-specific channel, which means that different types of similarly sized positively charged ions can pass through at the same time. When acetylcholine binds, the channel opens, allowing cations to flow freely through the pore. In cells, this usually occurs as sodium enters and potassium leaves, but calcium ions can also pass through the channel.
These channels are found in the nervous system and neuromuscular junctions. In typical neurons, the nicotinic acetylcholine receptor can open and depolarize the membrane potential of the cell with the movement of sodium and potassium ions. This is known as an excitatory postsynaptic potential (EPSP) which can cause a neuron to fire creating an action potential. In muscles, the release of calcium through receptor channels can cause them to contract. In this process, calcium ions reveal the binding sites between actin and myosin, allowing them to bind and contract the muscle.
A muscarinic acetylcholine receptor gets its name from its binding affinity for muscarine, a chemical found in mushrooms, relative to nicotine. These receptors differ from nicotinic acetylcholine receptors because they are G protein-coupled receptors (GPCRs) rather than ion channels and because they are involved in the body’s parasympathetic nervous system. They are often involved in recovery mechanisms in neurons and inhibitory mechanisms throughout the body.
Muscarinic receptors are found in five main forms labeled M1 to M5. Examples of the first, M1, are found in postganglionic neurons, where they mediate action potential recovery or slow initial membrane depolarization. The second type, M2, is found in the heart, where it lowers the heart rate and force of atrial contraction to counter stimulation by the sympathetic nervous system. The third, M3, is found in smooth muscle and blood vessel walls throughout the body and causes slow constriction in these areas. The fourth, M4, is a general signal blocker found throughout the central nervous system, while the location of the fifth, M5, is unknown.
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