The body has three basic varieties of message-bearing chemicals. Cytokines stir up or suppress the mechanisms of the immune system to deal with invading bacteria and viruses. Hormones transmit messages to start or stop activity in organs or tissues, controlling blood pressure, the production of red blood cells, the amount of sugar in the blood, and regulating the salt and water balance in the body, among many other activities. Finally, neurotransmitters carry messages through the spaces between nerve cells, or between nerve and muscle cells. All these chemicals allow the various organs to communicate with one another and help the body as a whole to function.
Neurotransmitters help messages travel along nerves. These messages may be reports of sensations, orders to muscles, informationabout various parts of the body, or orders to produce hormones. Whatever the message, information passes through a nerve cell as an electrical impulse. Neurotransmitters carry the message between nerve cells. The transmitting ends of the cell store as many as 10,000 tiny chemical molecules. There are about 50 different varieties, depending on the message being sent. Thousands of these neurotransmitters squirt out across the space between nerve cells. The surface of the next cell is covered with molecules called receptor sites. Each receptor site accepts only one variety of neurotransmitter, just as a lock receives a key. Depending on which receptor is “unlocked,” a particular message is passed along.
A similar lock-and-key system, the blood-brain barrier, is supposed to protect the brain from strange chemicals in the body. The problem is that nicotine gets past this defense. Its molecular structure mimics one of the body's natural neurotransmitters. Once it's in the bloodstream, nicotine travels to the brain, binds to receptor sites there, and sets off a chain of reactions, the basic nicotine cycle.
The nicotine stimulates production of the hormone epinephrine, a form of adrenaline, which usually acts to put the body on alert in lifeor-death situations. The hormone causes a surge of insulin, releasing sugar into the blood and creating a burst of energy. As the nicotine level in the body decreases, so does the sugar. The smoker feels depressed and tired-and lights another cigarette. That's the basic nicotine cycle, but much more goes on in the body. Epinephrine revs up other body functions as preparation for great exertion. Both breathing and heartbeat become faster, putting stress on the lungs and heart. Blood vessels constrict, making blood pressure rise. Most importantly, nicotine activates the parts of the brain involved with pleasure. It releases the “feelgood” neurotransmitter, dopamine, the same reaction that occurs when addictive drugs like cocaine or heroin enter the body.
The effects are different. Heroin is considered a sedative. It slows down the body and brain. Cocaine is a stimulant. It speeds things up. Depending on how much nicotine a person takes in and his or her mood at the time, the drug can act either as a stimulant or a sedative. That is why smokers may describe their first cigarette of the day as “waking them up.” It also explains why a cigarette after a meal “relaxes” them.
Nicotine differs from cocaine and heroin in one important way. Most drugs, including marijuana, ecstasy, and inhalants, have an intoxicating effect. People who use such drugs become clumsy, confused, and disoriented. Nicotine does not have these effects. In 1996, scientists at Brookhaven Laboratory, a major nuclear research facility, used body-scanning technology to discover that smokers have low levels of an enzyme called monoamine oxidase B (MAO B) in their brains. Enzymes trigger chemical reactions in the body, such as digesting food or breaking down body chemicals. MAO B is called the “killjoy chemical” because it breaks down dopamine, turning off feelings of pleasure in the brain.
Something in cigarette smoke stops, or inhibits, MAO B. Physicians use MAO inhibitors to treat people suffering from depression. It seems that smoking not only delivers nicotine to the body but also prolongs the pleasurable effects of nicotine and may even have a long-term effect on a smoker's mood.
The Brookhaven researchers made a further discovery in 2003. Comparing full-body scans of smokers and nonsmokers, they also found low levels of MAO B in the heart, liver, and kidneys of smokers. MAO B breaks down chemicals that cause high blood pressure. Since surges in blood pressure can cause crippling and even fatal strokes, this discovery points to a new health concern for smokers. Along with other recent findings, these discoveries show that there is still much more to learn about the effects smoking-and nicotine-have on the body.
Besides releasing dopamine to create feelings of pleasure, are there any positive aspects connected with nicotine? Smokers find that nicotine helps them concentrate in noisy environments. It also helps control their appetite, especially the desire for sweets. Smokers often worry that if they quit, they will gain weight. Perhaps the most important effect of nicotine on the brain is the way that it helps people handle stress. When smokers try to quit, stress becomes a big problem. Not only must they face their everyday problems but also the additional strain of breaking a powerful habit. Still worse, their usual way of dealing with difficulties-lighting up a cigarette-is now a problem in itself.
It is general knowledge that it is difficult to give up an addictive substance. Less is known about tolerance, the process in which a body becomes used to an addictive substance. Users have to take in ever-larger doses of the substance to feel its effects, which is why smokers increase their habits from single cigarettes to a pack a day, or even more.
Nicotine has a fairly short life span in the body. Half the amount taken in with a cigarette is gone within 40 minutes. What happens then? Many smokers light up their next cigarette.
