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What are the physiological effects of nicotine?

Now that we have a general outline of the neuroanatomy of acetylcholine, and about where nicotine receptors are and the general impact they have on the body, we are more informed about nicotine's physiological effects (Questions 7 and 8). Obviously, this is very complicated, as there are many receptor subtypes. Different locations in the body utilize different subtypes.

First, let's focus on the sympathetic nervous system locations: Those include (1) the adrenal medulla, considered essentially to be its own sympathetic ganglion, which releases the hormone adrenalin (along with other hormones); (2) the neuromuscular junction, which causes the skeletal muscles to activate; and finally, (3) the sweat glands on the skin. Thus, nicotine plays a role in releasing adrenalin, activating skeletal muscle, and keeping the body temperature regulated when it exerts itself. While skeletal muscles respond to nicotine, its direct impact is relatively insignificant at the doses normally ingested in the form of cigarette smoke. However, muscle twitching can occur in an overdose. Nicotine's impact on muscles is not why it is used nor is it related to its addictive potential. Here we can see how nicotine is associated with energy and action.

Nicotine also plays a role in the parasympathetic nervous system, principally through its actions on the autonomic ganglion, which stimulates the parasympathetic nervous system and, as a result, its terminal actions on various organ systems involved in rest and restoration. These effects include activation of the gut, slowing of the heart, relaxation of the blood vessels, and stimulation of the sex organs. Acetylcholine is responsible for arousal and erection via the parasympathetic nervous system, while epinephrine is responsible for orgasm and ejaculation via the sympathetic nervous system, both of which respond to nicotine. In early history, nicotine was thought to be an aphrodisiac, and this explains why. Table 1 outlines the various responses that occur in the major organs receiving input from the autonomic nervous system.

What are the psychological effects of nicotine?

In the brain, nicotine acts on the nicotinic acetylcholine receptors, which in turn act as neuromodulators that affect the actions of many other neurotransmitters. These include: acetylcholine, norepinephrine, epinephrine, dopamine, serotonin, and endorphins. The psychological effects of these various neurotransmitters include improved attention, concentration, learning and memory, in addition to enhanced pleasure, diminished pain response, and decreased anxiety.

Thus, nicotine offers a seeming paradox that cannot be duplicated by any other drug available. This paradox is a relaxed alertness, which only further adds to its appeal as a drug. It appears that smokers also can modify one effect over the other by changing the way they inhale. Studies have shown that smokers who wish to achieve a stimulating effect take short quick puffs, which produce a low level of blood nicotine thereby stimulating nerve transmission.

Table 1 Responses of Major organs to Autonomic Nerve impulses

organ

sympathetic stimulation

(preganglionic nicotinic acetylcholine, postganglionic adrenergic)

Parasympathetic stimulation

(preganglionic nicotinic, postganglionic muscarinic acetylcholine)

Heart

Increased heart rate

Decreased heart rate

Increased force of contraction

Decreased force of contraction

Increased conduction velocity

Decreased conduction velocity

Arteries

Constriction Dilation

Dilation

Veins

Constriction Dilation

Lungs

Bronchial muscle relaxation

Bronchial muscle contraction

Increased bronchial gland secretions

Gastrointestinal

tract

Decreased motility

Contraction of sphincters

Increased motility Relaxation of sphincters

Liver

Glycogen breakdown Glucose synthesis Lipid breakdown

Glycogen synthesis

Kidney

Renin secretion

Bladder

Relaxation

Contraction

Contraction of sphincter

Relaxation of sphincter

Uterus

Contraction of pregnant uterus

Relaxation of pregnant and non-pregnant uterus

Eye

Dilates pupil

Constricts pupil Increased secretions

Salivary glands

Viscous salivary secretions

Watery salivary secretions

Additionally, it appears that at low doses, nicotine modulates the actions of norepinephrine and dopamine in the brain. This further enhances attention and concentration in a manner similar to traditional stimulants. At higher doses, nicotine also modulates the effects of serotonin opiate activity, which produces a relaxed, calming effect. (Questions 32 and 83 have more information about the effects of nicotine.)

 
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