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Figure

This photograph, published by the American Medical Association in 1944, shows a man using a prosthetic hand to light a cigarette—indicating that even the medical community then accepted smoking as a routine part of life. Today, physicians feel quite differently, as the Surgeon General's warning on cigarette packages and advertisements indicates.

channel within the receptor opens, allowing positive ions to enter the neuron (fig. 10C). When a certain number of positive ions enter, the neuron is stimulated to release (by exocytosis) the neurotransmitter dopamine from its other end. The dopamine provides the pleasurable feelings associated with smoking. Addiction stems from two sources, researchers hypothesize — seeking the good feelings of sending off all that dopamine and avoiding painful withdrawal symptoms.

Binding nicotinic receptors isn't the only effect of nicotine on the brain. When a smoker increases the number of cigarettes smoked, the number of nicotinic receptors on the brain cells increases. This happens because of the way that the nicotine binding impairs the recycling of receptor proteins by endocytosis, so receptors are produced faster than they are taken apart. How

Cigarette ever, after a period of steady nicotine exposure, many of the receptors malfunction and no longer admit the positive ions that trigger the nerve impulse. This may be why as time goes on it takes more nicotine to produce the same effects.

Many questions remain concerning the biological effects of tobacco smoking. Why don't lab animals experience withdrawal? Why do people who have successfully stopped smoking often start again six months later, even though withdrawal eases within two weeks of quitting? Why do some people become addicted easily, yet others smoke only a few cigarettes a day and can stop anytime? While scientists try to answer these questions, society must deal with questions of rights and responsibilities that cigarette smoking causes. ■

Nicotine

Ion channel

Outside nerve cell

Membrane lipid bilayer

Inside nerve cell

Nicotine

Ion channel

Outside nerve cell

Membrane lipid bilayer

Inside nerve cell

a protein subunit

ß protein subunit

Receptor a protein subunit

ß protein subunit

Receptor

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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