Tobacco—What it Is and What it Does - Physical Properties Of Nicotine

Tobacco is a plant native to the Western Hemisphere. It contains nicotine, a drug classified as a stimulant, although it has some depressive effects as well. Nicotine is a poisonous alkaloid that is the major psychoactive (mood-altering) ingredient in tobacco. (Alkaloids are carbon- and nitrogen-containing compounds that are found in some families of plants. They have both poisonous and medicinal properties.)

Nicotine's effects on the body are complex. It affects the brain and central nervous system, as well as the hypothalamus and pituitary glands of the endocrine (hormone) system. Nicotine easily crosses the blood-brain barrier and accumulates in the brain—faster than caffeine or heroin, but slower than Valium (a sedative medicine used to treat anxiety). In the brain, nicotine imitates the actions of the hormone epinephrine (adrenaline) and the neurotransmitter acetylcholine, both of which heighten awareness. Nicotine also triggers the release of dopamine, which enhances feelings of pleasure, and endorphins, "the brain's natural opiates," which have a calming effect.

As noted earlier, nicotine acts as both a stimulant and a depressant. By stimulating certain nerve cells in the spinal cord, nicotine relaxes the nerves and slows some reactions, such as knee-jerk reflex. Small amounts of nicotine stimulate some nerve cells, but these cells are depressed by large amounts. In addition, nicotine stimulates the brain cortex (the outer layer of the brain) and affects the functions of the heart and lungs.