Principles of Neural Science, Fourth Edition / Edition 4

Chapter 1: The Brain and Behavior

The Last Frontier Of The biological sciences-their ultimate challenge-is to understand the biological basis of consciousness and the mental processes by which we perceive, act, learn, and remember. In the last two decades a remarkable unity has emerged within biology The ability to sequence genes and infer the amino acid sequences from the proteins they encode has revealed unanticipated similarities between proteins in the nervous system and those encountered elsewhere in the body. As a result, it has become possible to establish a general plan for the function of cells, a plan that provides a common conceptual framework for all of cell biology, including cellular neurobiology. The next and even more challenging step in this unifying process within biology, which we outline in this book, will be the unification of the study of behaviorthe science of the mind-and neural science, the science of the brain. This last step will allow us to achieve a unified scientific approach to the study of behavior.

Such a comprehensive approach depends on the view that all behavior is the result of brain function. What we commonly call the mind is a set of operations carried out by the brain. The actions of the brain underlie not only relatively simple motor behaviors such as walking or eating, but all the complex cognitive actions that we believe are quintessentially human, such as thinking, speaking, and creating works of art. As a corollary, all the behavioral disorders that characterize psychiatric illness-disorders of affect (feeling) and cognition (thought)-are disturbances of brain function.

The task of neural science is to explain behavior in termsof the activities of the brain. How does the brain marshal its millions of individual nerve cells to produce behavior, and how are these cells influenced by the environment, which includes the actions of other people? The progress of neural science in explaining human behavior is a major theme of this book.

Like all science, neural science must continually confront certain fundamental questions. Are particular mental processes localized to specific regions of the brain, or does the mind represent a collective and emergent property of the whole brain? If specific mental processes can be localized to discrete brain regions, what is the relationship between the anatomy and physiology of one region and its specific function in perception, thought, or movement? Are such relationships more likely to be revealed by examining the region as a whole or by studying its individual nerve cells? In this chapter we consider to what degree mental functions are located in specific regions of the brain and to what degree such local mental processes can be understood in terms of the properties of specific nerve cells and their interconnections.

To answer these questions, we look at how modern neural science approaches one of the most elaborate cognitive behaviors-language. In doing so we necessarily focus on the cerebral cortex, the part of the brain concerned with the most evolved human behaviors. Here we see how the brain is organized into regions or brain compartments, each made up of large groups of neurons, and how highly complex behaviors can be traced to specific regions of the brain and understood in terms of the functioning of groups of neurons. In the next chapter we consider how these neural circuits function at the cellular level, using a simple reflex behavior to examine the way sensory signals are transformed into motor acts.

Two Opposing Views Have Been Advanced on the Relationship Between Brain and Behavior Our current views about nerve cells, the brain, and behavior have emerged over the last century from a convergence of five experimental traditions: anatomy, embryology, physiology, pharmacology, and psychology.

Before the invention of the compound microscope in the eighteenth century, nervous tissue was thought to function like a gland-an idea that goes back to the Greek physician Galen, who proposed that nerves convey fluid secreted by the brain and spinal cord to the body's periphery The microscope revealed the true structure of the cells of nervous tissue. Even so, nervous tissue did not become the subject of a special science until the late 1800s, when the first detailed descriptions of nerve cells were undertaken by Camillo Golgi and Santiago Ram6n y Cajal.

Golgi developed a way of staining neurons with silver salts that revealed their entire structure under the microscope. He could see clearly that neurons had cell bodies and two major types of projections or processes: branching dendrites at one end and a long cable-like axon at the other. Using Golgi's technique, Ram6n y Cajal was able to stain individual cells, thus showing that nervous tissue is not one continuous web but a network of discrete cells. In the course of this work, Ram6n y Cajal developed some of the key concepts and much of the early evidence for the neuron doctrine-the principle that individual neurons are the elementary signaling elements of the nervous system.

Additional experimental support for the neuron doctrine was provided in the 1920s by the American embryologist Ross Harrison, who demonstrated that the two major projections of the nerve cell-the dendrites and the axon-grow out from the cell body and that they do so even in tissue culture in which each neuron is isolated from other neurons. Harrison also confirmed Ram6n y Cajal's suggestion that the tip of the axon gives rise to an expansion called the growth cone, which leads the developing axon to its target (whether to other nerve cells or to muscles).

Physiological investigation of the nervous system began in the late 1700s when the Italian physician and physicist Luigi Galvani discovered that living excitable muscle and nerve cells produce electricity. Modern electrophysiology grew out of work in the nineteenth century by three German physiologists-Emil DuBois-Reymond, Johannes Mtiller, and Hermann von Helmholtz-who were able to show that the electrical activity of one nerve cell affects the activity of an adjacent cell in predictable ways.

Pharmacology made its first impact on our understanding of the nervous system and behavior at the end of the nineteenth century, when Claude Bernard in France, Paul Ehrlich in Germany, and John Langley in England demonstrated that drugs do not interact with cells arbitrarily, but rather bind to specific receptors typically located in the membrane on the cell surface. This discovery became the basis of the all-important study of the chemical basis of communication between nerve cells.

The psychological investigation of behavior dates back to the beginnings of Western science, to classical Greek philosophy. Many issues central to the modern investigation of behavior, particularly in the area of perception, were subsequently reformulated in the seventeenth century first by Ren6 Descartes and then by John Locke, of whom we shall learn more later. In the midnineteenth century Charles Darwin set the stage for the study of animals as models of human actions and behavior by publishing his observations on the continuity of species in evolution. This new approach gave rise to ethology, the study of animal behavior in the natural environment, and later to experimental psychology, the study of human and animal behavior under controlled conditions.

In fact, by as early as the end of the eighteenth century the first attempts had been made to bring together biological and psychological concepts in the study of behavior. Franz Joseph Gall, a German physician and neuroanatomist, proposed three radical new ideas. First, he advocated that all behavior emanated from the brain. Second, he argued that particular regions of the cerebral cortex controlled specific functions. Gall asserted that the cerebral cortex did not act as a single organ but was divided into at least 35 organs (others were added later), each corresponding to a specific mental faculty. Even the most abstract of human behaviors, such as generosity, secretiveness, and religiosity were assigned their spot in the brain. Third, Gall proposed that the center for each mental function grew with use, much as a muscle bulks up with exercise...