Nicotine is a pharmacologic substance that binds to nicotinic receptors of the autonomic nervous system. Nicotinic receptors are located on sympathetic and parasympathetic ganglia, neuromuscular junctions, and the central nervous system.
The neuronal nicotinic receptors (NN) are structurally different from the neuromuscular junction nicotinic receptors (NM), although they are both cholinergic ion channel receptors. Nicotine affects the neuronal nicotinic receptors, but in toxic doses will cause muscle weakness and paralysis due to the effects on nicotinic receptors of the neuromuscular junction.
Nicotine stimulates receptors, but in toxic doses desensitizes them causing blockade of transmission. As well, the net effect of stimulation of autonomic ganglia depends on whether the parasympathetic or sympathetic nervous system is dominant for a particular organ. Stimulation of autonomic ganglia causes the release of catecholamines from the adrenal medulla, producing a mild increase in heart rate and blood pressure. The central nervous system (CNS) stimulation will increase alertness and sometimes cause tremors. Vomiting is seen with larger doses due to a direct effect on the chemoreceptor trigger zone in the brain stem, since nicotine crosses the blood-brain barrier.