Nicotine is a chemical compound that is present in tobacco. When tobacco is smoked, nicotine is absorbed through the wall lining of the small air sacs in the lungs. When sniffed or chewed, it is absorbed through the mucous membranes of the nose or mouth. Nicotine can also be absorbed through the skin.
Regardless of how nicotine is absorbed, it enters the bloodstream where it circulates throughout the body and travels to the brain where it crosses the blood-brain barrier. Once in the brain, it binds to and activates receptors called the cholinergic receptors.
These cholinergic receptors are present in the brain as well as in other areas such as the muscles, heart, adrenal glands and other vital organs. Normally, these receptors are activated by the neurotransmitter acetylcholine which is produced at nerve endings in the brain and in the nerves of the peripheral nervous system.
The actions of acetylcholine help to maintain healthy respiration, heart function, muscle movement and cognitive functions such as memory.
Since nicotine has a similar structure to acetylcholine, it can activate the cholinergic receptors. However, unlike acetylcholine, nicotine enters the brain and disrupts its normal functioning. Regular smoking leads to a change in the number of cholinergic receptors and to changes in their sensitivity to nicotine. This can lead to the development of nicotine tolerance.
Once this happens, the affected person needs to use nicotine regularly to maintain normal brain function. If the level of nicotine falls, the smoker may experience unpleasant withdrawal symptoms that lead to them "topping up" their nicotine levels by smoking again. Because of its highly addictive properties, smoking is considered by the American Heart Association to be one of the hardest addictions to break.
Nicotine is an alkaloid that is found in certain plants. It makes up 0.6 to 3.0% of the dry weight of tobacco. Nicotine is found in tobacco plants (Nicotiana tabacum) where it is synthesized in the roots and accumulates in the leaves. It is an oily liquid that is miscible with water in its base form. Nitrogenous forms of nicotine form salts with acids that are soluble in water.