THE REMARKABLE RANGE OF HUMAN behavior depends on a sophisticated array of sensory receptors connected to the brain, a highly flexible neural organ that selects from among the stream of sensory signals those events in the environment and in the internal milieu of the body that are important for the individual. The brain actively organizes sensory information for perception, action, decision-making, aesthetic appreciation, and future reference—that is to say, memory. It also ignores and discards information judiciously, one hopes, and reports to other brains about some of these operations and their psychological manifestations. All this is accomplished by interconnected nerve cells.
Individual nerve cells, or neurons, are the basic signaling units of the brain. The human brain contains a huge number of these cells, on the order of 86 billion neurons, that can be classified into at least a thousand different types. Yet this great variety of neurons is less of a factor in the complexity of human behavior than is their organization into anatomical circuits with precise functions. Indeed, one key organizational principle of the brain is that nerve cells with similar properties can produce different actions because of the way they are interconnected.
Because relatively few principles of organization of the nervous system give rise to considerable functional complexity, it is possible to learn a great deal about how the nervous system produces behavior by focusing on five basic features of the nervous system:
The structural components of individual nerve cells;
The mechanisms by which neurons produce signals within themselves and between each other;
The patterns of connection between nerve cells and between nerve cells and their targets (muscle and gland effectors);
The relationship of different patterns of interconnection to different types of behavior; and
How neurons and their connections are modified by experience.
The parts of this book are organized around these five major topics. In this chapter, we introduce these topics in turn in an overview of the neural control of behavior. We first consider the structure and function of neurons and the glial cells that surround and support them. We then examine how individual cells organize and transmit signals and how signaling between a few interconnected nerve cells produces a simple behavior, the knee-jerk reflex. We then extend these ideas to more complex behaviors, mediated by more complex and malleable circuits.
The Nervous System Has Two Classes of Cells
There are two main classes of cells in the nervous system: nerve cells, or neurons, and glial cells, or glia.
Nerve Cells Are the Signaling Units of the Nervous System
A typical neuron has four morphologically defined regions: (1) the cell body, (2) dendrites, (3) an axon, and (4) presynaptic terminals (Figure 3–1). As we shall see, each region has a distinct role in generating signals ...