2: Nerve Cells, Neural Circuitry, and Behavior

Introduction

• The Nervous System Has Two Classes of Cells

  • Nerve Cells Are the Signaling Units of the Nervous System

  • Glial Cells Support Nerve Cells

• Each Nerve Cell Is Part of a Circuit That Has One or More Specific Behavioral Functions

• Signaling Is Organized in the Same Way in All Nerve Cells

  • The Input Component Produces Graded Local Signals

  • The Trigger Zone Makes the Decision to Generate an Action Potential

  • The Conductive Component Propagates an All-or-None Action Potential

  • The Output Component Releases Neurotransmitter

  • The Transformation of the Neural Signal from Sensory to Motor Is Illustrated by the Stretch-Reflex Pathway

• Nerve Cells Differ Most at the Molecular Level

• Neural Network Models Simulate the Brain's Parallel Processing of Information

• Neural Connections Can Be Modified by Experience

The remarkable range of human behavior depends on a sophisticated array of sensory receptors connected to a highly flexible neural organ —the brain —that selects from among the stream of sensory signals those events in the environment that are important to the individual. In other words, the brain actively organizes perception, some of which is stored in memory for future reference, and some of which is transformed into immediate behavioral responses. All this is accomplished by interconnected nerve cells.

Individual nerve cells or neurons are the basic units of the brain. The human brain contains a huge number of these cells, on the order of 10¹¹ neurons, that can be classified into at least a thousand different types. Yet the complexity of human behavior depends less on the variety of neurons than on their organization into anatomical circuits with precise functions. One key organizational principle of the brain, therefore, is that nerve cells with similar properties can produce different actions because of the way they are interconnected.

Because relatively few principles of organization give rise to considerable 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:

1. The structural components of individual nerve cells;

2. The mechanisms by which neurons produce signals within and between nerve cells;

3. The patterns of connections between nerve cells and between nerve cells and their targets: muscles and gland effectors;

4. The relationship of different patterns of interconnection to different types of behavior; and

5. How neurons and their connections are modified by experience

The various parts of this book are organized around these five major topics. In this chapter we provide an overview of the neural control of behavior by introducing these topics together. 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. Finally, we consider how changes in signaling by specific cells can modify behavior.