Nicotinic acetylcholine receptors are specialized proteins in nerve cells that respond to the neurotransmitter acetylcholine (ACh). They occur in many places in the body, especially at the junctions where nerve cells contact muscles, sending the electrical signals that tell the muscle cells to contract. Skeletal muscle movement relies on the nicotinic acetylcholine receptors to receive its feedback from the brain and spinal cord. If diseases impair the receptor function, loss of voluntary muscular control may result in paralysis.
Acetylcholine is the neurotransmitter by which nerves cause muscle cells to contract, and thus it is crucial for all voluntary movement of the limbs and any of the body parts controlled by skeletal muscle. Like many other chemical messengers in living systems, ACh binds to special proteins called receptors embedded within the membrane of the muscle cells, causing a chemical reaction that results in muscular contraction. There are two types of ACh receptors, also called cholinergic receptors: nicotinic and muscarinic, each named for the chemical — other than acetylcholine — that binds to the receptor and activates it.
In the central nervous system, nicotinic acetylcholine receptors moderate the chemical pathways that assist the brain's arousal pathways, which are responsible for waking, alertness, and attention. ACh receptors within the brain often inhibit the activation of neuronal groups in these pathways. In the peripheral nervous system, nicotinic ACh receptors are located on the nerves that form neuromuscular junctions with skeletal muscle. When the signal to move a muscle in the leg, for instance, is sent by the brain's cerebral cortex, nicotinic ACh receptors on the nerves of the appropriate leg muscle will convey this information to the muscle.
Nicotinic acetylcholine receptors were named after the observation that the chemical nicotine found in tobacco products triggers the same reaction in these receptors as does acetylcholine. Nicotine causes the opening of the receptors' sodium channels, starting the chemical cascade that will result in a muscular contraction. This is why tobacco use and nicotine withdrawal have muscular effects, including the twitches associated with quitting cigarette smoking. The muscarinic acetylcholine receptors, which cause smooth muscles to contract, respond to the chemical muscarine but not to nicotine.
Diseases that afflict the nicotinic ACh receptors often result in paralysis and the loss of normal muscle tone. Some paralyzing neurotoxins also bind to the nicotinic acetylcholine receptors, but instead of activating them, these toxins completely block the receptors from signaling the muscle. This leaves the nerve unable to induce muscular contraction. Similarly, the autoimmune disease myasthenia gravis destroys the nicotinic acetylcholine receptors by producing antibodies against them and making them targets of the immune system, which causes serious muscular weakness.