Due to their complexity, the mechanisms by which various drugs act in the CNS have not always been clearly understood. In recent decades, however, dramatic advances have been made in the methodology of CNS pharmacology. It is now possible to study the action of a drug on individual neurons and even single receptors within synapses. The information obtained from such studies is the basis for several major developments in studies of the CNS. First, it is clear that nearly all drugs with CNS effects act on specific receptors that modulate synaptic transmission. While a few agents such as general anesthetics and alcohol may have nonspecific actions on membranes (although these exceptions are not * The author thanks Dr. Roger A. Nicoll for his contributions to previouseditions.
fully accepted), even these non–receptor-mediated actions result in demonstrable alterations in synaptic transmission.
Second, drugs are among the most valuable tools for studying CNS function, from understanding the mechanism of convulsions to the laying down of long-term memory. Both agonists that mimic natural transmitters (and in many cases are more selective than the endogenous substances) and antagonists are extremely useful in such studies. Third, unraveling the actions of drugs with known clinical efficacy has led to some of the most fruitful hypotheses regarding the mechanisms of disease. For example, information about the action of antipsychotic drugs on dopamine receptors has provided the basis for important hypotheses regarding the pathophysiology of schizophrenia. Studies of the effects of a variety of agonists and antagonists on γ-aminobutyric acid (GABA) receptors have resulted in new concepts pertaining to the pathophysiology of several diseases, including anxiety and epilepsy.
A full appreciation of the effects of a drug on the CNS requires an understanding of the multiple levels of brain organization, from genes to circuits to behavior. This chapter provides an introduction to the functional organization of the CNS and its synaptic transmitters as a basis for understanding the actions of the drugs described in the following chapters.
The CNS is composed of the brain and spinal cord and is responsible for integrating sensory information and generating motor output and other behaviors needed to successfully interact with the environment and enhance species survival. The human brain contains about 100 billion interconnected neurons surrounded by various supporting glial cells. Throughout the CNS, neurons are either clustered into groups called nuclei or are present in layered structures such as the cerebellum or hippocampus. Connections among neurons both within and between these clusters form the circuitry that regulates information flow through the CNS.
Neurons are electrically excitable cells that process and transmit information via an electrochemical process. There are many types of neurons in the CNS, and they are classified in multiple ways: by function, by location, and by the neurotransmitter they release. The typical neuron, however, possesses a cell body (or soma) and specialized processes called dendrites and axons (Figure 21–1). Dendrites, which form highly branched complex dendritic “trees,” receive and integrate the input from other neurons and conduct this information to the cell body. The axon carries the output signal of a neuron from the cell body, sometimes over long distances. Neurons may have hundreds of dendrites but generally have only one axon, although axons may branch distally to contact multiple targets. The axon terminal makes contact with other neurons at specialized junctions called synapses where neurotransmitter chemicals are released that interact with receptors on other neurons.
In addition to neurons, there are a large number of nonneuronal support cells, called glia, that perform a variety of essential functions in the CNS. Astrocytes are the most abundant cell in the brain and play homeostatic support roles, including providing metabolic nutrients to neurons and maintaining extracellular ion concentrations. In addition, astrocyte processes are closely associated with neuronal synapses where they are involved in the removal and recycling of neurotransmitters after release and play increasingly appreciated roles in regulating neurotransmission (see below).