How Is The Brain Organized? - NDSU
CHAPTER2How Is the BrainOrganized?An Overview of Brain StructureBrain TerminologyThe Brain’s Surface FeaturesThe Brain’s Internal FeaturesMicroscopic Inspection: Cells and FibersFocus on Disorders: Meningitis andEncephalitisFocus on Disorders: StrokeA Closer Look at NeuroanatomyThe Cranial Nervous SystemThe Spinal Nervous SystemThe Internal Nervous SystemFocus on Disorders: Magendie, Bell, and Bell’sPalsyThe Functional Organizationof the BrainPrinciple 1: The Sequence of Brain ProcessingIs “InIntegrateOut”Principle 2: Sensory and Motor Divisions ExistThroughout the Nervous SystemPrinciple 3: The Brain’s Circuits Are CrossedPrinciple 4: The Brain Is Both Symmetrical andAsymmetricalPrinciple 5: The Nervous System WorksThrough Excitation and InhibitionPrinciple 6: The Central Nervous System HasMultiple Levels of FunctionPrinciple 7: Brain Systems Are Organized BothHierarchically and in ParallelPrinciple 8: Functions in the Brain Are BothLocalized and DistributedA. Klehr / Stone ImagesMicrograph: Carolina Biological Supply Co. / Phototake36
hen buying a new car, people first inspect theIn many ways, examining a brain for the first time isoutside carefully, admiring the flawless finishsimilar to looking under the hood of a car. We have a vagueand perhaps even kicking the tires. Then theysense of what the brain does but no sense of how the partsopen the hood and examine the engine, the part of the carthat we see accomplish these tasks. We may not even beresponsible for most of its behavior — and misbehavior.able to identify many of the parts. In fact, at first glance theThis means gazing at a maze of tubes, wires, boxes, andoutside of a brain may look more like a mass of foldedfluid reservoirs. All most of us can do is gaze, becausetubes divided down the middle than like a structure withwhat we see simply makes no sense, except in the mostmany interconnected pieces. See what you can make of thegeneral way. We know that the engine burns gasoline tohuman brain in Figure 2-1. Can you say anything aboutmake the car move and somehow generates electricity tohow it works? At least a car engine has parts with regularrun the radio and lights. But this tells us nothing aboutshapes 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,Wfor example, we see that there is an enormous differencenot just in overall size, but in the relative sizes of parts andin structure. In fact, some parts present in one are totally absent in the other. What is it that all these parts do that makesone animal stalk mice and another read textbooks?To make matters worse, even for trained research scientists, the arrangement of the brain’s parts does not justseem random, it really is haphazard. The challenge that weface in learning about the brain is to identify some regularities in its organization and to establish a set of principlesthat can help us understand how the nervous systemworks. After decades of investigation, we now have a goodidea of how the nervous system functions, at least in a general way. That knowledge is the subject of this chapter. Butbefore we turn our attention to the operation manual forthe brain and the rest of the nervous system, let us examinewhat the brain is designed to do. Knowing the brain’s functions will make it easier to grasp the rules of how it works.Perhaps the simplest statement of the brain’s functionsis that it produces behavior, as seen in Chapter 1. There ismore to this statement than is immediately apparent, however. In order for the brain to produce behavior, it musthave information about the world, such as informationabout the objects around us — their size, shape, movement, and so forth. Without such information, the brainFigure 2-1View of the human brain when the skull is opened. The gyri(bumps) and sulci (cracks) of the cerebral hemispheres are visible,but their appearence gives little information about their function.cannot know how to orient and direct the body to producean appropriate response. This is especially true when theresponse needed is some complex behavior, such as 37
38 CHAPTER ainstemCatCerebrumCerebellumBrainstemOlfactory ure 2-2BrainstemThis view of the brain’s primary purpose may seem ab-Inspection of the outside features of the brains of a cat, rat, monkey,and human shows them to differ dramatically in size and in generalappearance. The rat brain is smooth, whereas the other brains havefurrows in the cerebral cortex. The pattern of furrows differsconsiderably in the human, the monkey, and the cat. The cat brainand, to some extent, the monkey brain have long folds that appearto run much of the length of the brain, whereas the human brainhas a more diffuse pattern. The cerebellum is wrinkled in all speciesand is located above the brainstem. The brainstem is the route bywhich information enters and exits the brain. The olfactory bulb,which controls the perception of smells, is relatively larger in cats andrats but is not visible in monkeys and humans, because it is small andlies under the brain.stract to you, but it is central to understanding how thePhotos courtesy of Wally Welker, University of Wisconsin ComparativeMammalian Brain Collection.no such thing as sound. Rather, there is only the movementbrain functions. Consider the task of answering a telephone.The brain directs the body to pick up the receiver when thenervous system responds to vibrating molecules of air bycreating the subjective experience of a ring. We perceivethis sound and react to it as if it actually existed, when infact the sound is merely a fabrication of the brain. That fabrication is produced by a chain reaction that takes placewhen vibrating air molecules hit the eardrum. In the absence of the nervous system, especially the brain, there isof air molecules.The subjective nature of the experiences that the braincreates can be better understood by comparing the realitiescatching a ball. To perform complex behaviors, the ner-of two different kinds of animals. You are probably awarevous system has organs designed to receive informationthat dogs perceive sounds that humans do not. This differ-from the world and convert this information into biologi-ence in perception does not mean that a dog’s nervous sys-cal activity that produces subjective experiences of reality.tem is better than ours or that our hearing is poorer. Rather,The brain thus produces what we believe is reality in ordera dog brain simply creates a different world from that offor us to move. These subjective experiences of reality areour brain. Neither subjective experience is “right.” The dif-essential to carrying out any complex task.ference in experience is merely due to two different sys-
HOW IS THE BRAIN ORGANIZED?tems for processing physical stimuli. The same differences We can now identify the brain’s three primary functions:exist in visual perceptions. Dogs see very little color,whereas our world is rich with color because our brains1. to produce behavior;create a different reality from that of a dog’s brain. Such dif-2. to create a sensory reality; andferences in subjective realities exist for good reason: they3. to create knowledge that integrates information fromallow different animals to exploit different features of theirdifferent times and sensory domains and to use thatenvironments. Dogs use their hearing to detect the move-knowledge to guide behavior.ments of mice in the grass, whereas early humans probablyused color for such tasks as identifying ripe fruit in trees.Each of the brain’s three functions requires specific ma-Evolution, then, equipped each species with a view of thechinery. The brain must have systems to create the sensoryworld that would help it survive.world, systems to produce behavior, and systems to inte-These examples show how a brain’s sensory experi-grate the two.ences help guide an organism’s behavior. For this link be-In this chapter, we consider the basic structures andtween sensory processing and behavior to be made, thefunctions of those systems. First, we identify the compo-brain must also have a system for accumulating, integrating,nents of the nervous system. Then we look at what thoseand using knowledge. Whenever the brain collects sensorycomponents do. Finally, we look at how the parts work to-information, it is essentially creating knowledge about thegether and at some general principles of brain function.world, knowledge that can be used to produce more effec-Many of the ideas introduced in this chapter are devel-tive behaviors. The knowledge currently being created inoped throughout the rest of the book, so you may want toone sensory domain can be compared both with pastreturn to this chapter often to reconsider the basic princi-knowledge and with knowledge gathered in other domains.ples as new topics are introduced.AN OVERVIEW OF BRAIN STRUCTUREThe place to start our overview of the brain’s structure is to “open the hood” by opening the skull and looking at the brain snug in its home. Figure 2-1 shows a brainviewed from this perspective. The features that you see are part of what is called thebrain’s “gross anatomy,” not because they are ugly, but because they constitute a broadoverview. Zooming in on the brain’s microscopic cells and fibers is largely reserved forChapter 3, although this section ends with a brief introduction of some terms usedfor these tiny structures. Those terms are just a few of a great many new terms thatyou will encounter in this book, which is why we deal with brain terminology in general before moving on to a look at the brain itself. Because many of the words in thischapter will seem foreign to you, they will be accompanied by a pronunciation guideat their first appearance.Brain TerminologyThere are hundreds, even thousands of brain regions, making the task of masteringbrain terminology seem daunting. To make matters worse, many structures have several names, and many terms are often used interchangeably. This peculiar nomenclature arose because research on brain and behavior has spanned several centuries.When the first anatomists began to examine the brain with the primitive tools of theirtime, they made many erroneous assumptions about how the brain works, and thenames that they chose for brain regions are often manifestations of those errors. ForLink to an index listing the roots ofneuroanatomical terms atwww.worthpublishers.com/kolb/chapter2.39
CHAPTER 2Figure 2-3(A)Meaning “above,”sometimes referredto as superiorMeaning “middle”iorPosterMedialMeaning “front,”sometimes referredto as frontal or rostralMeaning “tail,”sometimes referredto as caudalLaterarAnteriolMeaning “side”VentralAnatomical terms are used to describeanatomical locations. (A) Anatomicaldirections relative to the head and brain.Because a human is upright, the termsposterior and caudal (both meaning“tail”) refer to a slightly differentorientation for the human headcompared with the head of a fourlegged animal. (B) Anatomical directionsrelative to the body.Dorsal Meaning “below” or“belly,” sometimesreferred to as ntralPosteriorVentral40Figure 2-4An afferent nerve carries informationinto the brain, and an efferent nervetakes information out of the brain andcontrols movement of a muscle.This afferent nervecarries informationfrom sensoryreceptors in skinto the brain.This efferent nervecarries informationfrom the brain tothe neuronscontrolling legmuscle, causinga response.Sensoryendingsinstance, they named one region of the brain the gyrus fornicatus because theythought it had a role in sexual function. In fact, most of this region has nothing to dowith sexual function. Another area was named the red nucleus because it appears reddish in fresh tissue. This name denotes nothing of the area’s potential functions,which turn out to be the control of limb movements.As time went on, the assumptions and tools of brain research changed, but thenaming continued to be haphazard and inconsistent. Early investigators named structures after themselves or objects or ideas. They used different languages, especiallyLatin, Greek, and English. More recently, investigators have often used numbers orletters, but even this system lacks coherence because the numbers may be Arabic orRoman numerals and are often used in combination with letters, which may be eitherGreek or Latin. When we look at current brain terminology, then, we see a mixture ofall these naming systems.Despite this sometimes confusing variety, many names do include informationabout a structure’s location in the brain. Table 2-1 summarizes these location-relatedterms, and Figure 2-3 shows how they relate to body locations. Structures found onthe top of the brain or on the top of some structure within the brain are dorsal. Struc-
HOW IS THE BRAIN ORGANIZED? 41tures located toward the bottom of the brain or one ofTable 2-1 Orientation Terms for the Brainits parts are ventral. Structures found toward the midTermMeaning with respect to the nervous systemdle of the brain are medial, whereas those located toAnteriorLocated near or toward the front or the headward the side are lateral. Structures located toward thefront of the brain are anterior, whereas those locatedCaudalLocated near or toward the tailtoward the back of the brain are posterior. SometimesDorsalOn or toward the back or, in reference to brain nuclei, locatedthe terms rostral and caudal are used instead of anteaboverior and posterior, respectively. And, occasionally, theFrontal“Of the front“ or, in reference to brain sections, a viewingterms superior and inferior are used to refer to strucorientation from the fronttures that are located dorsally or ventrally (these termsInferiorLocated belowdo not label structures according to their importance).LateralToward the side of the bodyIt is also common to combine terms. For example, aMedialToward the middle; sometimes written as mesialstructure may be described as dorsolateral, whichmeans that it is located “up and to the side.”PosteriorLocated near or toward the tailYou should also learn two terms that describe theRostral”Toward the beak”; located toward the frontdirection of information flowing to and from cells inSagittalParallel to the length (from front to back) of the skull; used inthe brain. Afferent refers to information coming intoreference to a planethe brain or a part of the brain, whereas efferent refersSuperiorLocated aboveto information leaving the brain or one of its parts,VentralOn or toward the belly or side of the animal in which the bellymeaning that efferent refers to brain signals that trigislocated or, in reference to brain nuclei, located belowger some response (Figure 2-4). These words are verysimilar, but there is an easy way to keep them straight.The letter “a” in afferent comes alphabetically before the “e” in efferent, and sensory information must come into the brain before an outward-flowing signal can trigger a response. Therefore, afferent means “incoming” and efferent means “outgoing.”On the CD, visit the module on theCentral Nervous System to better visualize the various planes of the brain.The Brain’s Surface FeaturesReturning to the brain in the open skull, you are now ready to examine its structuresmore closely. The first thing to notice is that the brain is covered by a tough materialknown as the meninges [men in jeez (the accented syllable is in boldface type)], whichis a three-layered structure, as illustrated in Figure 2-5. The outer layer is known as thedura mater (from Latin, meaning “hard mother”). It is a tough double layer of fibroustissue enclosing the brain in a kind of loose sack. The middle layer is the arachnoidlayer (from Greek, meaning “like a spider’s web”). It is a very thin sheet of delicateFigure 2-5SkullDuramaterArachnoidlayerMeningesPia materBrainSubarachnoid space(filled with CSF)The brain is covered by thick coveringsknown as the meninges and is cushionedby a fluid known as the cerebrospinalfluid (CSF).
42 CHAPTER 2Figure 2-6In these views of the human brain (fromthe top, bottom, side, and middle), thelocations of the frontal, parietal,occipital, and temporal lobes of thecerebral hemispheres are shown, as arethe cerebellum and the three major sulci(the central sulcus, lateral fissure, andlongitudinal fissure) of the cerebralhemispheres.Dorsal viewCentral sulcusParietallobeFrontallobePhotos courtesy of Yakolev ntral viewTemporal lobeCerebellumFrontallobeBrainstemCranial nervesLateral viewCentral llobeMedial viewOccipitallobeCentral lobeBrainstemCerebellum
HOW IS THE BRAIN ORGANIZED?connective tissue that follows the brain’s contours. The inner layer is the pia mater (fromLatin, meaning “soft mother”). It is a moderately tough membrane of connectivetissue fibers that cling to the surface of the brain. Between the arachnoid and piamater is a fluid, known as cerebrospinal fluid (CSF), which is a colorless solution ofsodium chloride and other salts. It provides a cushion so that the brain can move orexpand slightly without pressing on the skull. (Meningitis is an infection of themeninges. Its symptoms are described in “Meningitis and Encephalitis” on page 46.)If we remove the meninges, we can now remove the brain from the skull and examine its various parts. As we look at the brain from the top or the side, it appears tohave two major parts, each wrinkly in appearance. The larger part is the cerebrum[sa ree brum], which consists of two cerebral hemispheres, the left and the right, andthe smaller part is the cerebellum [sair a bell um]. Both the cerebrum and the cerebellum are visible in the brains shown in Figure 2-2. Each of these structures is wrinkled in large-brained animals because its outer surface is made of a relatively thinsheet of tissue, the cortex, that has been pushed together to make it fit into the skull.To see why the cortex is wrinkled, force a piece of writing paper, 81 2 by 11 inches, intoa cup. The only way is to crinkle the paper up into a ball. Essentially the same crinklingup has been done to the cortex of the cerebrum and the cerebellum. Like a crinkledpiece of paper, much of the cortex is invisible from the surface. All we can see fromthe surface are bumps and cracks. The bumps are known as gyri [jye rye; singular:gyrus (jye russ)], whereas the cracks are known as sulci [sul sigh; singular: sulcus(sul kus)]. Some of the sulci are very deep and so are often called fissures. The twobest-known fissures are the longitudinal fissure and the lateral fissure, both of whichare shown in Figure 2-6, along with the central sulcus.If we now look at the bottom of the brain, we see something completely different.The cerebrum is still the wrinkled part, but now there is also a whitish structure downthe middle with little tubes attached. This middle structure is known as the brainstem, and the little tubes are cranial nerves that run to and from the head.One final gross feature is obvious: the brain appears to be covered in blood vessels. As in other parts of the body, the brain receives blood through arteries and sendsit back through veins to the kidneys and lungs for cleaning and oxygenation. The arteries come up the neck and then wrap around the outside of the brainstem, cerebrum, and cerebellum, finally piercing the brain’s surface to get to its inner regions.Figure 2-7 shows the three major arteries that feed blood to the cerebrum — namely,the anterior, middle, and posterior cerebral arteries. Because the brain is very sensitiveto loss of blood, a blockage or break in
appearance. The rat brain is smooth, whereas the other brains have furrows in the cerebral cortex. The pattern of furrows differs considerably in the human, the monkey, and the cat. The cat brain and, to some extent, the monkey brain have long folds that appear to run much of the length of the brain, whereas the human brain has a more diffuse .
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