Nicotine Actions and the Physiology of Smoking

Nicotine is a naturally occurring compound that is classified as a liquid alkaloid. An alkaloid is an organic compound made up of carbon, hydrogen, nitrogen, and sometimes oxygen (nicotine lacks oxygen). Another example of an alkaloid is caffeine. Both caffeine and nicotine are similar in that they provide stimulation upon ingestion, a factor that encourages their habitual use.

 

Nicotine can gain entry to the bloodstream by several routes. Tobacco can also be applied as a wad to the inside of the mouth, enabling nicotine to diffuse across the mucous membranes of the gums to the bloodstream. As a component of a cigarette, nicotine can be inhaled into the lungs. There, it encounters the alveoli, diffuses across the alveolar membrane, and enters the bloodstream. Once in the bloodstream nicotine travels to the brain and then is delivered throughout the body.

The physiological effects of nicotine—both the good feelings and the irritability upon nicotine deprival--are due to its effects on the brain. Nicotine reaches the brain within 10-15 seconds of entering the bloodstream. Initially, it causes the rapid release of adrenaline, the hormone that elicits a "fight or flight" response. Adrenaline causes the heart rate and blood pressure to increase and breathing to become rapid and shallow. Also glucose is released into the blood and, at the same time, the release of insulin can be blocked. Because insulin stimulates cells to take up excess glucose, the net effect is to increase the concentration of glucose in the blood. This increased sugar and the elevation of the metabolic rate--the rate at which nutrients are converted to energy--can act to produce a weight loss in smokers. However, this healthy effect is more than compensated for by the damage to the body from smoking. Also, evidence is conclusive now that long-term intake of nicotine increases the level of the so-called "bad" cholesterol, LDL, that damages arteries.

The target of nicotine in the brain are neurons. Neurons are the fundamental cells of the nervous system. Neural impulses move from neuron to neuron, even though the neurons may not physically contact one another. Chemicals called neurotransmitters provide the bridge. One such neurotransmitter is acetylcholine. Nicotine acts by occupying a space on a neuron that would otherwise be a binding site for acetylcholine. The binding of nicotine also causes the neuron to "fire" (i.e., to create an action potential that travels down the neuron). In contrast to the tightly regulated quantities of acetylcholine in the brain, nicotine levels are not regulated. Thus, upon exposure, brain activity increases in a haphazard fashion throughout the brain.

The global stimulation of brain activity produces an alert feeling. Also, the "reward center" of the brain is stimulated, producing a pleasant and happy feeling (for example, the hormone endorphin is released in greater quantity, producing the euphoric "runner's high") that encourages the repeating of the behavior that invoked the response. The need to take in nicotine can become addictive.

In response to the continued presence of nicotine, the neurons increase the number of neuroreceptors for nicotine. When nicotine use is abruptly stopped, these physiological adaptations remain. As a result, the body cannot function in the same way it did in the presence of the drug. For about a month, until the neuroreceptors readjust, feelings of irritability, anxiety, depression and craving for nicotine will be present. Indeed, of the millions of people who try and stop smoking each year, only 10% are successful.

 

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نوشته شده در Mon 8 Dec 2008ساعت 7:0 PM توسط سیندرلا| |