Drug Actions and Drug Effects
1. A Brief Overview of Neural Physiology
It is useful to know something about how drugs act upon the human nervous system even in a course on the sociology of drug and alcohol problems. In a sense, the psychophysiological actions of drugs are the "raw material" from which drug effects are created and shaped by psychological, interactional, and cultural processes. Most psychoactive drugs that are used recreationally are pharmacologically "messy," and act upon the central nervous system (CNS) in a variety of ways. For instance, it is difficult to determine the exact mechanisms that account for the pleasurable effects of alcohol because this substance has such a complex and widespread impact on the CNS.
Given this lack of specificity in the psychophysiological action of alcohol, it is not surprising that the effects or states of intoxication that people experience when they drink vary widely across cultures. In a broad ranging review of anthropological evidence on alcohol use in dozens of societies, MacAndrew and Edgerton (1969) found that typical forms of "drunken comportment" range from displays of aggression and violence in some societies to peaceful and introspective states of intoxication in other societies. Even in the same society, users who have learned different expectations or norms about the effects of a given substance may experience quite different drug-induced states. Also, the setting or situations within which drugs are used can profoundly influence the nature of effects that users experience. Thus, sociologists and anthropologists take sharp exception to the popular notion that drug effects are pharmacologically determined by drug actions.
Nevertheless, a brief look at the way drugs affect the functioning of nerve cells helps in understanding the actions and potential hazards of different drugs. For example, the acute toxic effects of alcohol are primarily due to its action as a CNS depressant—it slows down and may even stop basic physiological processes such as breathing. Other substances, such as methamphetamine, stimulate or intensify neural processes, which may have undesirable side effects in some users. To understand these problematic consequences of drug action, we need to examine how nerve cells normally transmit information throughout the CNS.
As shown in the diagram at the right, nerve cells in the brain and elsewhere in the CNS are composed of two basic parts: (1) a cell body, which contains the cell nucleus and branch-like structures known as dendrites and (2) an arm or extension known as the axon, whose ends are called terminals. The terminals contain microscopic sacs, the vesicles, which serve as storage containers for naturally-produced chemicals known as neurotransmitters. Another important feature of the terminal is that it is separated from the dendrites of other nerve cells by the synapse (or synaptic cleft)—a space less than 1/10,000 of an inch across. The synaptic cleft is the "main arena" for the action of most psychoactive drugs.
Neurotransmitters, such as serotonin and dopamine, are "chemical messengers" that send signals across the synapse from one nerve cell to another. When a nerve cell is activated and receives a message, an electrical impulse travels down the cell axon. In response to this impulse, the vesicles release a small quantity of a neurotransmitter into the synapse. Some of this neurotransmitter from the sending cell fits and bonds with particular receptor sites on the dendrites of receiving cells, more-or-less like a key fitting into a lock. For many types of neurotransmitter, including serotonin and dopamine, this "excites" the receiving nerve cell, which produces an eletrical impulse that then travels down its axon to repeat the process of communication with other cells. However, some other neurotransmitters operate by inhibiting this electrical impulse. We will focus on the former type of neurotransmitter here.
Immediately after a neurotransmitter is released into the synapse, the sending nerve cell retrieves some of the chemical back into its terminal vesicles through a process known as reuptake. This process ends the chemical excitation of the receiving cell by removing the neurotransmitter from the synapse, and it allows "recycling" of the neurotransmitter for future use. Also, some of the neurotransmitter remaining in the synapse is broken down and neutralized by other chemicals. Thus, these processes normally function to terminate the flow of information from one nerve cell to another. Next, we examine how psychoactive substances such as the anti-depressant, Prozac (fluoxetine), and the stimulant, cocaine, alter these processes and produce certain drug actions.
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