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CHAPTER 2
How Is the Brain Organized? An Overview of Brain Structure The Functional Organization Brain Terminology of the Brain The Brain’s Surface Features Principle 1: The Sequence of Brain Processing The Brain’s Internal Features Is “In Integrate Out” Microscopic Inspection: Cells and Fibers Principle 2: Sensory and Motor Divisions Exist Focus on Disorders: Meningitis and Throughout the Nervous System Encephalitis Principle 3: The Brain’s Circuits Are Crossed Focus on Disorders: Stroke Principle 4: The Brain Is Both Symmetrical and Asymmetrical Principle 5: The Nervous System Works A Closer Look at Neuroanatomy Through Excitation and Inhibition The Cranial Nervous System Principle 6: The Central Nervous System Has The Spinal Nervous System Multiple Levels of Function The Internal Nervous System Principle 7: Brain Systems Are Organized Both Focus on Disorders: Magendie, Bell, and Bell’s Hierarchically and in Parallel Palsy Principle 8: Functions in the Brain Are Both Localized and Distributed
A. Klehr / Stone Images Micrograph: Carolina Biological Supply Co. / Phototake 36 p
hen buying a new car, people first inspect the In many ways, examining a brain for the first time is outside carefully, admiring the flawless finish similar to looking under the hood of a car. We have a vague W and perhaps even kicking the tires. Then they sense of what the brain does but no sense of how the parts open the hood and examine the engine, the part of the car that we see accomplish these tasks. We may not even be responsible for most of its behavior—and misbehavior. able to identify many of the parts. In fact, at first glance the This means gazing at a maze of tubes, wires, boxes, and outside of a brain may look more like a mass of folded fluid reservoirs. All most of us can do is gaze, because tubes divided down the middle than like a structure with what we see simply makes no sense, except in the most many interconnected pieces. See what you can make of the general way. We know that the engine burns gasoline to human brain in Figure 2-1. Can you say anything about make the car move and somehow generates electricity to how it works? At least a car engine has parts with regular run the radio and lights. But this tells us nothing about shapes that are recognizably similar in different engines. what all the engine’s many parts do. What we need is in- This is not true of mammals’ brains, as shown in Figure 2-2. formation about how such a system works. When we compare the brain of a cat with that of a human, for example, we see that there is an enormous difference not just in overall size, but in the relative sizes of parts and in structure. In fact, some parts present in one are totally ab- sent in the other. What is it that all these parts do that makes one animal stalk mice and another read textbooks? To make matters worse, even for trained research sci- entists, the arrangement of the brain’s parts does not just seem random, it really is haphazard. The challenge that we face in learning about the brain is to identify some regulari- ties in its organization and to establish a set of principles that can help us understand how the nervous system works. After decades of investigation, we now have a good idea of how the nervous system functions, at least in a gen- eral way. That knowledge is the subject of this chapter. But before we turn our attention to the operation manual for the brain and the rest of the nervous system, let us examine what the brain is designed to do. Knowing the brain’s func- tions will make it easier to grasp the rules of how it works. Perhaps the simplest statement of the brain’s functions is that it produces behavior, as seen in Chapter 1. There is more to this statement than is immediately apparent, how- ever. In order for the brain to produce behavior, it must have information about the world, such as information about the objects around us—their size, shape, move- ment, and so forth. Without such information, the brain Figure 2-1 cannot know how to orient and direct the body to produce View of the human brain when the skull is opened. The gyri (bumps) and sulci (cracks) of the cerebral hemispheres are visible, an appropriate response. This is especially true when the but their appearence gives little information about their function. response needed is some complex behavior, such as