Nicotinic acetylcholine receptors, or nAChRs, are neuron receptor proteins that respond to the neurotransmitter acetylcholine. Nicotonic receptors also respond to drugs, including the nicotinic receptor agonist nicotine. They are found in the central nervous system of humans, and also play two important roles in the peripheral nervous system: (1) they transmit outgoing signals from the presynaptic to the postsynaptic cells within the sympathetic and parasympathetic nervous system, and (2) they are the receptors found on skeletal muscle that receive acetylcholine released to signal for muscular contraction. Nicotinic receptors are also found in other creatures. In insects, the cholinergic system is limited to the central nervous system.