Although a neuron has a single axon, axons may branch many times. Thus, impulses leaving a neuron of a neuronal pool may exhibit divergence (di-ver'jens) by reaching several other neurons. For example, one neuron may stimulate two others; each of these, in turn, may stimulate several others, and so forth. Such a pattern of diverging axons can amplify an impulse—that is, spread it to increasing numbers of neurons within the pool (fig. 10.20b).

As a result of divergence, an impulse originating from a single neuron in the central nervous system may be amplified so that enough impulses reach the motor units within a skeletal muscle to cause forceful contraction. Similarly, an impulse originating from a sensory receptor may diverge and reach several different regions of the central nervous system, where the resulting impulses can be processed and acted upon.

Figure 10.20

(a) Axons of neurons 1 and 2 converge to the cell body of neuron 3.

Figure 10.20

(a) Axons of neurons 1 and 2 converge to the cell body of neuron 3.

The nervous system enables us to experience the world and to think and feel emotion. This organ system is also very sensitive to outside influences. Clinical Application 10.5 discusses one way that an outside influence can affect the nervous system—drug addiction.

O Define neuropeptide.

What is a neuronal pool?

Define facilitation. What is convergence?

What is the relationship between divergence and amplification?

Drug Addiction

Drug abuse and addiction are ancient as well as contemporary problems. A 3,500-year-old Egyptian document decries that society's reliance on opium. In the 1600s, a smokable form of opium enslaved many Chinese, and the Japanese and Europeans discovered the addictive nature of nicotine. During the American Civil War, morphine was a widely used painkiller; cocaine was introduced a short time later to relieve veterans addicted to morphine. Today, abuse of drugs intended for medical use continues. LSD was originally used in psychotherapy but was abused in the 1960s as a hallucinogen. PCP was an anesthetic before being abused in the 1980s.

Why do people become addicted to certain drugs? Answers lie in the complex interactions of neurons, drugs, and individual behaviors.

The Role of receptors

Eating hot fudge sundaes is highly enjoyable, but we usually don't feel driven to consume them repeatedly. Why do certain drugs compel a person to repeatedly use them, even when knowing that doing so can be dangerous — the definition of addiction? The biology of neurotransmission helps to explain how we, and other animals, become addicted to certain drugs.

Understanding how neurotrans-mitters fit receptors can explain the actions of certain drugs. When a drug alters the activity of a neurotransmitter on a postsynaptic neuron, it either halts or enhances synaptic transmission. A drug that binds to a receptor, blocking a neurotransmitter from binding there, is called an antagonist. A drug that activates the receptor, triggering an action potential, or that helps a neurotransmitter to bind, is called an agonist. The effect of a drug depends upon whether it is an antag onist or an agonist; on the particular behaviors the affected neurotransmitter normally regulates; and in which parts of the brain drugs affect neurotransmitters and their binding to receptors. Many addictive substances bind to receptors for the neurotransmitter dopamine, in a brain region called the nucleus accumbens.

With repeated use of an addictive substance, the number of receptors it targets can decline. This means that the person must use more of the drug to feel the same effect. Neural pathways that use the neurotransmitter norepi-nephrine control arousal, dreaming, and mood. Amphetamine enhances norepi-nephrine activity, thereby heightening alertness and mood. Amphetamine's structure is so similar to that of norepi-nephrine that it binds to norepinephrine receptors and triggers the same changes in the postsynaptic membrane.

Cocaine has a complex mechanism of action, both blocking reuptake of norepinephrine and binding to molecules that transport dopamine to postsynaptic cells. Cocaine's rapid and short-lived "high" reflects its short stay in the brain—its uptake takes just four to six minutes, and within twenty minutes the drug loses half its activity.

GABA is an inhibitory neurotrans-mitter used in a third of the brain's synapses. The drug valium causes re-

laxation and inhibits seizures and anxiety by helping GABA bind to receptors on postsynaptic neurons. Valium is therefore a GABA agonist.

Nicotine Addiction

Many medical professionals agree that cigarette smoking is highly addictive (figs. 10A and 10B). According to the Diagnostic & Statistical Manual of Mental Disorders, a person addicted to tobacco

1. must smoke more to attain the same effects (tolerance) over time;

2. experiences withdrawal symptoms when smoking stops, including weight gain, difficulty concentrating, insomnia, restlessness, anxiety, depression, slowed metabolism, and lowered heart rate;

3. smokes more often and for longer than intended;

4. spends considerable time obtaining cigarettes;

5. devotes less time to other activities;

6. continues to smoke despite knowing it is unhealthy;

7. wants to stop, but cannot easily do so.

Nicotine causes addiction, and the addiction supplies enough of the other chemicals in cigarette smoke to destroy health. The site of nicotine's activity is the neuron.

An activated form of nicotine binds protein receptors, called nico-tinic receptors, that are parts of cell membranes of certain brain neurons. These receptors normally receive the neurotransmitter acetylcholine. When sufficient nicotine binds, a (continued)

Shier-Butler-Lewis: I III. Integration and I 10. Nervous System I: I I © The McGraw-Hill

Human Anatomy and Coordination Basic Structure and Companies, 2001

Physiology, Ninth Edition Function cal application

cal application

Drug Addiction (continued)

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  • reginard
    What is the relationship between divergence and amplification?
    4 years ago

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