Chapter 4

Chapter 4: The Nervous System

The goal of this chapter is to introduce basic concepts and terminology that will help you understand the effects of psychoactive drugs on the brain and on behavior.  Humans must maintain homeostasis, meaning their internal environment must be within certain limits for factors such as temperature, acidity, and water and sodium content.  Hormones and neurotransmitters are types of chemical messengers in the body that help maintain homeostasis.

Glial cells and neurons are two components of the nervous system.  Glia provide structure, eliminate waste, and create the blood-brain barrier.  Neurons analyze and transmit information in a process involving an electrical signal.  Neurotransmitters act over brief time periods and very small distances because they are released into the synapse between neurons and are then rapidly cleared from the synapse.

Three nervous systems in the body are the central nervous system, the somatic nervous system, and the autonomic nervous system.  The central nervous system consists of the brain and spinal cord, which integrate learning and memory and coordinate activity.  The somatic nervous system interacts with the external environment and carries sensory and movement information.

The autonomic nervous system monitors and controls the body's internal environment and involuntary functions.  The two branches of the autonomic nervous system, the sympathetic and parasympathetic branches, act in opposition. Many psychoactive drugs have autonomic influences on heart rate, blood pressure, and other body systems and structures.

Key neurotransmitters associated with the actions of psychoactive drugs include dopamine, acetylcholine, norepinephrine, serotonin, GABA, glutamate, and endorphins.  Specific neurotransmitters and psychoactive drugs are associated with specific chemical pathways.  For example, the mesolimbic dopamine system is thought to be a critical pathway for the dependence produced by many drugs.

The life cycle of a typical neurotransmitter chemical involves:

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1.    Uptake of precursors

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2.     Synthesis of the transmitter

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3.     Storage in vesicles

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4.     Release into the synapse

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5.     Interaction with the receptor

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6.     Reuptake into the releasing neuron

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7.     Metabolism by enzymes

Psychoactive drugs act on chemical pathways either by altering the availability of a neurotransmitter at a synapse or by directly interacting with a neurotransmitter receptor.

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