Nicotine is a chemical compound that is present in tobacco.
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Absorption
When tobacco is smoked, nicotine is absorbed through the wall lining of the small air sacs in the lungs. Comparatively, when tobacco products are sniffed or chewed, nicotine is instead absorbed through the mucous membranes of the nose or mouth. Nicotine can also be absorbed through the skin.
Regardless of how nicotine is absorbed, once this chemical enters the bloodstream, it circulates throughout the body and travels to the brain, where it crosses the blood-brain barrier. Once in the brain, nicotine binds to and activates receptors called the cholinergic receptors.
Effects
Cholinergic receptors are primarily present in the brain, but can also be found 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 both nerve endings in the brain as well as nerves of the peripheral nervous system (PNS).
The actions of acetylcholine on cholinergic receptors 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 both the number of cholinergic receptors and as well as the sensitivity of these receptors to nicotine. As a result, nicotine tolerance can develop.
Once an individual begins to develop tolerance to nicotine, 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, the American Heart Association has declared the smoking of tobacco products to be one of the hardest addictions to break.
2-Minute Neuroscience: Nicotine
Chemistry
Nicotine is an alkaloid that is found in certain plants, such as the Nicotiana tabacum plant that is used to produce tobacco products.
In tobacco, nicotine makes up between 0.6 and 3.0% of the total dry weight. Within the tobacco plant, nicotine is synthesized in the roots and accumulates in the leaves. This chemical 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.
References
Further Reading
Switching from smoking to vaping improves respiratory health