HUMAN BIOLOGY

260 Chapter 13

morphine, an analgesic (painkiller), is derived from the

seed pods of the opium poppy. Like its cousin heroin, it

blocks pain signals by binding with certain receptors on

central nervous system neurons. heroin, morphine, and

synthetic versions of it such as OxyContin produce euphoria.

abusive use of such drugs has caused thousands of deaths.

marijuana is a hallucinogen. In low doses it slows motor

activity and causes mild euphoria. It can also cause visual

hallucinations. Like alcohol, it skews the performance of

complex tasks, such as driving. so-called “herbal incense”—

including the drugs marketed as spice and K2—have

similar but stronger and even more addictive effects than

marijuana does.

the body eventually may develop drug tolerance,

meaning that it takes larger or more frequent doses to

produce the same effect. tolerance reflects physical drug

dependence. the liver produces enzymes that detoxify

drugs in the blood. tolerance develops when the level of

those enzymes rises in response to the ongoing presence

of the drug in the bloodstream. In effect, a drug user

must increase his or her intake to stay ahead of the liver’s

growing ability (up to a point) to break down the drug.

In psychological drug dependence, or habituation, a user

begins to crave the feelings asso ciated with a particular

drug. Without a steady supply of it the person can’t “feel

good” or function normally. table 13.3 lists warning signs

of potentially serious drug dependence. habituation and

tolerance both are evidence of addiction.

When different psychoactive drugs are used together,

they can interact dangerously. For example, alcohol and

barbiturates (such as seconal and nembutal) both depress

the central nervous system. used at the same time, they

can depress respiratory centers in the brain enough to

cause death.

psychoactive substances bind to neuron receptors in the

brain. the neurons then send or receive altered messages.

such drugs typically affect parts of the brain that govern

consciousness and behavior. some also alter heart rate,

respiration, sensory processing, and muscle coordination.

many affect a pleasure center in the hypothalamus and

artificially boost the sense of pleasure we associate with

eating, sex, or other activities. several major recreational

drugs are legal and used regularly by people around the

globe (Figure 13.26).

two common legal psychoactive drugs—caffeine and

nicotine—are stimulants. studies suggest that regularly

consuming modest amounts of caffeine (such as a daily cup

or two of tea or coffee) probably doesn’t harm health. heavy

caffeine consumption (including some popular “energy

drinks”) is associated with negative side effects, including

sleeplessness, abnormal heart rhythms, and elevated

blood pressure. nicotine directly stimulates certain sensory

receptors. It also increases the heart rate and blood pressure.

Illegal stimulants include cocaine and amphetamines such

as ecstasy (mdma) and “bath salts.” at first amphetamines

cause a flood of the neurotransmitters norepinephrine

and dopamine, which stimulate the brain’s pleasure center.

Over time, how ever, the brain slows its production of those

substances and depends more on the amphetamine. Chronic

users may become psychotic, depressed, and malnourished.

they may also develop heart problems. Cocaine stimulates

the pleasure center by blocking the reabsorption of

dopamine and other neurotransmitters. It also weakens

the cardiovascular and immune systems.

alcohol is a depressant, even though it produces a high at

first. drinking only an ounce or two diminishes judgment and

can lead to disorientation and uncoordinated movements.

Blood alcohol concentration (BAC) measures the percentage

of alcohol in the blood. In most states, someone with a BaC

of 0.08 per milliliter is considered legally drunk. When the

BaC reaches 0.15 to 0.4, a drinker is visibly intoxicated and

can’t function normally. a BaC greater than 0.4 can kill.

1. Tolerance—it takes increasing amounts of the drug to produce
   the same effect.


2. Habituation—it takes continued drug use over time to maintain
   self-perception of functioning normally.


3. Inability to stop or curtail use of the drug, even if there is
   persistent desire to do so.


4. Concealment—not wanting others to know of the drug use.


5. Extreme or dangerous behavior to get and use a drug, as by
   stealing, asking more than one doctor for prescriptions, or
   jeopardizing employment by drug use at work.


6. Deteriorating professional and personal relationships.


7. Anger and defensive behavior when someone suggests there
   may be a problem.


8. Preferring drug use over previous activities.

Table 13.3 Warning Signs of Drug Addiction*

Figure 13.26 alcohol, caffeine, and nicotine are

“mind-altering” drugs in common use.

the brain on “Mind-altering” drugs

13.13

FOCUS ON HEALTH

*Having three or more of these signs may be cause for concern.

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