fundamentals of cognitive neuroscience€¦ · surveys, in chapter 7, basic information about the...

P A R T 3FUNDAMENTALS OFCOGNITIVENEUROSCIENCE

At this point, you should be developing a sense of the kinds of issues that can ariseat the neurolaw intersection, why those issues arise, the contexts in which theyarise, and how they can play out. You should also be beginning to see some ofthe large patterns in the complex relationships of law, brains, and behavior.And you should be developing an appreciation not only of how and why attorneysmay fight over neuroscience in the courtroom, but also how judges will be guidedin their attempt to referee.

Part III provides the basic background and essential tools you need to understandthe fundamental biology and technology of brain sciences. This will not be as intim-idating as you might think. Our goal as authors is not that readers should becomeentry-level neuroscientists, which would be entirely unrealistic (and also entirely unne-cessary). Our goal is to help readers to develop informed ways of understanding andthinking about the intersections of law and neuroscience. For aspiring attorneys, forexample, this should provide the same basic, starting familiarity as one gets in otherlaw school courses, upon which further experience in practice later builds.

For these reasons, Part III provides a tour, from the ground floor up, of howthe brain is built and how it operates, as well as the techniques we use to study andtreat it. Moreover, in one of this book’s most important chapters, we detail thelimitations of neuroscience that should temper overzealous interpretations ofevidence — by oneself or opposing counsel.

P A R T S U M M A R Y

This Part:

� Surveys, in Chapter 7, basic information about the structure and function ofthe brain.� Introduces, in Chapter 8, the technologies for monitoring and manipulating

the human brain.� Identifies, in Chapter 9, particular limits and cautions in interpreting brain

imaging evidence. Those seeking further information on how to read andunderstand a brain imaging study will find a user-friendly example, withexplanatory annotations, in the Appendix.

193

C H A P T E R7Brain Structure andBrain Function

We saw that an exact knowledge of the structure of the brain was of supremeinterest. . . . To know the brain, we said, is equivalent to ascertaining thematerial course of thought and will.

— Santiago Ramon y Cajaly

Your brain is a network of neurons, bursting with electrical activity flowingthrough your synapses. Makes you think, doesn’t it?

— Anonymous

C H A P T E R S U M M A R Y

This chapter:� Introduces the basic vocabulary needed to describe and explain the parts of

the brain most salient for legal contexts.� Introduces basic concepts of brain function.� Introduces the types of cells in the nervous system, and summarizes how

neurons signal to each other.

I N T R O D U C T I O N

How do humans maintain such exquisite control of the body? How do we walk,talk, ride bicycles, learn legal systems, conceive theorems, and compose music?How do we decide to shoot someone with a gun? Understanding brain structureand function can help us answer questions like these. Neuroscience research hasaccumulated many layers of detail and complexity, but this chapter will focus onlyon the most basic fundamentals necessary to deal with the legal topics explored inthis coursebook. These fundamentals will provide the foundation on which a law-yer can build a more specialized knowledge base to succeed in neurolaw. Evenwithout a background in neuroscience, biology, or psychology, reading thischapter provides a solid foundation on which to understand and discuss thecases and reading material that comprise the remainder of the coursebook. Wewill focus on basic terminology and concepts while avoiding overwhelming layers

y Santiago Ramon y Cajal, Recollections of My Life (1937). Ramon y Cajal won the Nobel Prize inPhysiology or Medicine in 1906 for his descriptions of the fine structure of the nervous system.

195

of detail. Those wishing to read more about particular brain structures are encour-aged to consult the additional resources listed at the end of this chapter and

provided on the coursebook web site. Those usingthe electronic version of this book can click on hyper-links to view additional related material, such as colorimages and videos.

Most students without a science background enterthis course with some trepidation — worrying that theywill be out of their depth. If you feel this, you are ingood company. You will be introduced to new termi-nology and basic facts about the brain. We assure youthat you can learn what you need to know to be suc-cessful in practicing neurolaw, to appreciate both thepromise and the limitations of neuroscience in the

legal arena, and to debate the ethical implications of scientific advances. Youwill exit knowing vastly more than the average lawyer about the relationshipbetween brains and behaviors. And you will also learn what sources to consult,and what questions to research, when issues arise in your future practice.

As a starting point, you should recognize that you already know more aboutbrain structure than you think you do. You already know that your brain is in yourhead, and like your head it has a top and bottom, an inside and an outside. Youalready know that your brain has different lobes or parts. And you likely alreadyappreciate that different parts of the brain have different functions.

You also probably know that your brain has two sides (called ‘‘hemispheres’’),and that the two sides have different specialized functions. You probably alreadyknow (perhaps from TV commercials hawking mood-altering drugs) that yourbrain function depends on specific chemical processes. You may also know thatyour brain function also depends on electrical processes. And after reading thefirst few chapters in this coursebook you probably now know that brains are builtfrom a very large number of cells called neurons.

Given all that you already know, all we need to do now is fill in some details.This chapter will begin with a description of the structure of the human brain, start-ing with large scale structure followed by smaller scales of organization. This will befollowed by a description of brain function. But as you begin examining the ratherimpersonal diagrams and photographs of a human brain,1 keep in mind that thedisembodied brain in the photograph was once a living, breathing person who, likeHamlet’s poor Yorick, was ‘‘a fellow of infinite jest, of most excellent fancy’’ withdesires, wishes, plans, and fears — who was distressed or amazed at the end. Theorganic dynamics of your brain enable your learning and locution as a student aswell as the excitement or anxiousness you may be experiencing right now.

Just as when you began learning law, some things will seem familiar, and somewill be foreign. And, again as with law, some of what’s foreign will arrive in Latin orGreek. The good news is that the meanings underlying the foreign-sounding terms

Test Your Knowledge. We encourage

you to take advantage of the online

resources made available on the course-

book web site. On the web site, links

allow for: viewing 3-D images of the

brain; watching videos to see brain sci-

ence in action; and reading in greater

detail about how each part of the brain

works.

1. Images of humans’ and other species’ brains are readily found on the internet. To introducethe variety of brains in different species including human, we invite you to look at this site:http://www.brainmuseum.org/. An atlas of the human brain with many clearly diagrammed figurescan be found at www.thehumanbrain.info/.

196 Chapter 7. Brain Structure and Brain Function

are generally far less intimidating than they sound. Early anatomists named thingsin the brain by what they looked like — their location and shape, their color andtexture — using descriptions just like you would use, such as ‘‘front,’’ ‘‘big,’’‘‘dark,’’ and ‘‘dense.’’ Later, other anatomists used numbers to identify differentareas of the brain, much like zip codes, so people could refer to the same locationswithout having to point and poke with their fingers.

You should also understand that the terminology of the nervous system has ahistory that makes some terms peculiar but resistant to change. Just as it can take awhile to become acquainted with street names in a new city, brain terminologytakes a little getting used to. But just as you can learn to navigate a new city quickly,with some exposure, instruction, and effort, the basic layout of the brain will rap-idly become familiar. To aid in this process, new terms-of-art will be announced initalics, and every italicized word or term will be found in the glossary at the end ofthe book.

The chapter is composed of several sections. Section A will introduce the termsused to describe location in the brain. Then the functional and anatomical sub-systems of the brain will be described in Section B. The cells that comprise thebrain, neurons and glia, will then be introduced in Section C. Section D puts brainfunction into its evolutionary context. Section E will survey the organization of thehuman cerebral cortex (cerebral refers to brain and cortex refers to outer covering)with basic explanations for the function of each lobe. The diverse functions of thebrain structures buried beneath the cerebral cortex will then be summarized inSection F. Section G briefly overviews the function of blood supply to the brain. Itis important to understand how critical the blood supply is for normal brainfunction and also to understand what functional brain imaging measures. SectionH explains in more detail how neurons signal to one another, and Section I thenexplains how mind-altering drugs affect brain function to change thought andmood. Finally, Section J synthesizes the material in this chapter by describinghow the brain controls and monitors the actions it produces, relating to actusreus and mens rea.

A. ORIENTATION AND LOCATION IN THE BRAIN

To be proficient in speaking about the human brain, one must be able to describethe region to which one is referring. Anatomists use special terms for describingspatial relations in the human brain. The necessity for these terms arises from thefact that our brains are three-dimensional objects that are often viewed from manydifferent angles. To a neurosurgeon operating on an upright subject, down and tothe left will mean something substantially different than it would to anyone viewingbrain images taken from a body lying face up. Specialized terms exist to removeambiguity in brain anatomy, just as North or South have a more permanentmeaning than Left or Right.

Geographers use two perpendicular axes — longitude (the north-south axis)and latitude (the east-west axis) — to describe locations on the surface of theEarth. For the most part, there’s no need to refer to specific locations deep inthe interior. Not so with the brain. Anatomists use particular coordinate systems

197A. Orientation and Location in the Brain

when referring to the human body and the brain. And these systems have threeelements, rather than two, because we need to be able to discuss interior features,not just surface ones.

Specifically, we use three perpendicular axes to describe positions in the threedimensions of the brain. As shown in Figure 7.1, the three axes are (1) the anterior-posterior axis, (2) the dorsal-ventral axis, and (3) the medial-lateral axis.

The anterior-posterior axis runs from front to back. Two other terms are alsoused: rostral and caudal. Rostral refers to the nose, so it is synonymous with anterior.Caudal refers to the tail, so it is synonymous with posterior. Thus, the front of thehead (the forehead) is the anterior (rostral) pole and the rear of the head isthe posterior (caudal) pole. The forehead is anterior (or rostral) to the ears,and the back of the head is posterior (or caudal) to the ears.

The dorsal-ventral axis runs from top to bottom. Two other terms may also beused, superior and inferior. Thus, the top of the head is dorsal (superior) to the chin.

Figure 7.1 Orientation and position in the human brain. The top left panel illustratesthe orientations of the major planes of each section and labels the terms for describingposition in the front-to-back and the top-to-bottom axes. The bottom left panel illus-trates a simplified sagittal section through the center midline of the brain. The verticalline in the coronal section below indicates the location of this section. Location in thefront-to-back and top-to-bottom axes is labeled. The orientation of the dorsal-ventralaxis changes from approximately horizontal in the spinal cord to vertical in the brain.The top right panel illustrates a simplified coronal section through the brain at thelevel indicated by the vertical arrow in the sagittal section above. Location in the top-to-bottom and left-right axes is labeled. Note that neurologists label left-right as if thebrain section is in the patient facing you. The bottom right panel illustrates a simplifiedhorizontal section through the brain at the level indicated by the horizontal line in thesagittal section above. Location in the front-to-back and left-right axes is labeled.


The nose is inferior to the eyes. The dorsal-ventral axis is also used to describe thebody of other animals. For example, we refer to the dorsal fin of fish. Similarly, thetop of a fish’s head is dorsal to the bottom. As you know, the human head isrotated 90 degrees from the axis of the body because we stand up, so the top ofthe head points toward the sky while our back points toward the horizon. Never-theless, the dorsal-ventral axis can be applied to both the brain and the spinal cord.

The medial-lateral axis runs left to right, but it also is referenced to the naturalsymmetry of the human body. Medial refers to the middle, and lateral refers to theside. The ears are lateral relative to the nose. The center of the head is medial, andthe side of the head is lateral.

The three perpendicular axes define three anatomical planes (Figure 7.1).The coronal plane is a given slice through the brain, perpendicular to a linedrawn from anterior to posterior (or parallel to a line drawn from ear to ear).Locations in the coronal plane are identified as dorsal-ventral (more or lesstop/bottom) and medial-lateral (more or less middle/side), as well as left-right.

Before going further, you need to know something important about ‘‘left’’ and‘‘right.’’ When you look at the coronal section in Figure 7.1, you would likelydescribe the left side of the brain as being on the left side of the image. You shouldbe aware that radiologists have a convention that reverses this. That is, for a radi-ologist the right side of the brain is on the left side of the image and vice versa —because radiologists prefer to see the image as it would appear in a patient who isfacing them, or as if they are looking at them from the bottom. This is importantbecause when you look at coronal or horizontal sections in publications, you needto know whether the authors are using the radiological convention. In manycurrent scientific brain-imaging studies the radiological convention is no longerused. And it can make all the difference in consequences for the brain whethersome malformation or damage is on the left or right side.

The sagittal plane is a slice through the brain perpendicular to a medial-lateralline. Locations in the sagittal plane are identified as dorsal-ventral and anterior-posterior. The midsagittal plane is right down the center, the most medial part ofthe brain.

The horizontal plane is a slice through the brain, perpendicular to a line drawnfrom dorsal to ventral. Locations in the horizontal plane are identified as anterior-posterior (or rostral-caudal) and medial-lateral. (As with the coronal plane, whenyou look at horizontal sections in publications, you need to know whether theauthors are using the radiological convention.)

If you really want to master this brain terminology, you should practice draw-ing Figure 7.1 from memory with all the labels. With a few attempts, they willbecome as familiar as mens rea or pro se. If you don’t want to memorize this,then at least mark these pages for reference when you encounter these terms inthe chapters to come.

B. SUBSYSTEMS OF THE NERVOUS SYSTEM

Although the brain is a single ‘‘organ’’ of the body, it consists of a stunninglycomplex and beautiful collection of structures and circuits that are ultimately

199B. Subsystems of the Nervous System

connected to all the body’s other organs like sensory receptors, muscles in thelimbs and the heart, and glands to guide and control behavior. The brain hasbeen compared to a computer (though even this comparison may fail to capturethe totality of the brain’s complexities). Like a computer, the brain has inputs andoutputs and it transforms and stores ‘‘information.’’2 As you know, computers arebuilt from diverse electrical components connected in complex circuits. An elec-trical circuit consists of the paths of wires connecting different components to oneanother to perform the designed function; if the wires become disconnected orhappen to be connected in the wrong way, then the device will not work asdesigned. Brain circuits consist of cells called neurons and glia. Unlike electricaldevices in which wires are separate from the device components, neurons areboth the processing units and the connections, while glia support the functionof the neurons. The brain consists of numerous complex circuits that work inconcert to accomplish the various functions.

We now provide an overview of the major subsystems of the human brain. Wewill describe both functional (Figure 7.2) and anatomical subdivisions (Figure7.3). However, keep in mind that while subdividing is useful for exposition, thesubsystems function in a highly integrated way. The brain enables us to perceivethe world outside and within our body, to plan actions and to control our behavior.These functions require coordinated processes in the cerebral cortex, the thala-mus, basal ganglia, cerebellum, brainstem (consisting of midbrain, pons andmedulla), and spinal cord.

The brain responds to sensory input from outside in the world and from insidethe body. From the outside, the brain is sensitive to distant stimuli conveyedthrough light, sound, and smell. The brain is also sensitive to more local stimulithat contact the body through taste, and touch. From the inside, the brain is sen-sitive to inputs from sensors in the muscles, joints, and gut so that it responds tostimuli within the body such as stomach or bladder distension.

The brain has two major streams of output. The brain controls the heart, thegut, and various glands in the body; it is necessary for the basic functions of sus-taining life. The brain also controls the various muscles that move the face, thelimbs, and the rest of the body. This system is typically under voluntary control andproduces the range of activities we do like walking and talking, reading andwriting. It should be clear that our brains are embedded in the stream of eventsthat we experience. Thus the actions produced by the brain through the body willresult in changes in our senses of the external and internal environment. Forexample, if you shift gaze from this page to another location in your environment,you will see something else. Similarly, if you run as fast as you can for a few min-utes, the rates of your respiration and heartbeat will increase, and your sweatglands will help cool your body. Your brain senses if you are hungry or thirstyand motivates actions to satisfy those drives. Your brain also senses if you arebored or tired and can control itself to sustain attention for at least another

2. Metaphors are useful but should be recognized for what they are. The brain is not actually acomputer. Scientists in each era of history analogize the brain to the most advanced technology of thetime. Before the computer, the brain was analogized to a hydraulic system, then a land-line telephoneexchange.


page or two. Your brain not only controls your body — it also, to some extent,controls itself.

Circuits in the cerebral cortex and associated structures analyze the signals con-veyed by the sensory system. The brain generates body movements to respond tosensory stimulation, internal states, and goals through two major systems that neu-roscientists refer to as the motor system. The first controls the muscles that move thelimbs, the face, and the eyes. This system is typically under voluntary control andproduces the range of activities we do like walking and talking, reading andwriting.

The second controls internal organs like the glands, gut, and heart. Thismotor system is also referred to as the autonomic or visceral system and is furtherdivided into sympathetic and parasympathetic subsystems. It is an oversimplification,but we can say that the sympathetic subsystem organizes fight-flight responses, andthe parasympathetic subsystem organizes rest-and-digest maintenance. In otherwords, the sympathetic nervous system can divert blood flow from the gastrointes-tinal system to the skeletal muscles and lungs, increasing heart rate and dilatingthe pupils. In contrast, the parasympathetic nervous system can divert blood flowto the gastrointestinal tract, slowing the heart rate.

How do all of these systems communicate with one another so effectively? Theanswer, in a word, is neurons. The centrality of the neuron — also known as a‘‘nerve cell’’ — is why the field of brain science is called neuroscience, and it’swhy we label the human communication network the ‘‘nervous system.’’ The

Perception, Planning, Control

Cerebral cortex, Thalamus, Basal ganglia, Cerebellum,

Brainstem, Spinal cord

InputMusclesthat movebody, limbsand face

EyesEarsSkinNoseMouth

MusclesJointsGut

Actions

Internal Environment

ExternalEnvironment

Autonomicmuscles inheart, gutand glands

Output

Figure 7.2 Functional Subdivisions of Human Brain. Details in Text.

201B. Subsystems of the Nervous System

nervous system refers to a system of nerve cells, communicating with one anotherto produce human perception, emotion, thought, and action.

Neuroscientists divide the nervous system into two subsystems: the central ner-vous system (CNS) consisting of the brain and the spinal cord and the peripheralnervous system (PNS) consisting of everything else — that is, the nerves that com-municate with sensory receptors, muscles, internal body organs, and glands. Theautonomic nervous system can be classified as part of the PNS. Not being protectedby the skull or spinal column, the nerves in the PNS are more exposed to potentialfor injury, so they can regrow if crushed or cut. In contrast, nerves in the CNS willnot regrow. This is why significant damage to the spinal cord or brain commonlyresults in permanent disabilities.

The CNS is so important to life that nature has endowed it with more protec-tion than just the bones of the skull and spine. Beneath the encasing bones, theCNS is protected by another collection of structures called the meninges. Themeninges are composed of three layers. The outer layer is the dura mater, aleather-like tissue inside the bone. The inner layer is the pia mater, a delicatefilm that seals the brain in cerebrospinal fluid. The middle layer is the arachnoidmater, a web-like substance connecting pia mater to dura mater. While themeninges are essential for protecting the brain, they can also lead to braindamage; the cyst that developed in Weinstein’s brain arose from excessive growthof the arachnoid mater.

Neuroscientists recognize three broad subdivisions of the CNS: the forebrain,brainstem, and spinal cord. The forebrain consists of the cerebral cortex and the thal-amus. The brainstem consists of the midbrain, pons, cerebellum and medulla oblongata.The spinal cord exits the skull and extends nearly to the end of the spinal column.Take a moment to review Figures 7.2 and 7.3. As you will see, this is not as com-plicated as its description in prose might suggest.

Spinal cord

Cerebellum

Forebrain

BrainstemMidbrainPonsMedulla

CerebrumThalamus }

}

Figure 7.3 Structural Subdivisions of the Central Nervous System. Details in Text.


C. CELLS OF THE NERVOUS SYSTEM

The CNS is comprised of two major types of cells: neurons and glia. Brain functionarises from neurons communicating with each other, as well as with glands andmuscles throughout the body. What neurons communicate (their outputs, so tospeak) depends on the pattern of influences (inputs) they receive from other neu-rons. These processes and interactions are facilitated and supported in variousways by glia (which is derived from the Greek word for ‘‘glue’’).

1. Neurons

Neurons come in many shapes and sizes, but, as illustrated in Figure 7.4 they allhave three main parts: (1) the cell body, (2) the axon, and (3) the dendrites. The cellbody (also called the soma) encloses the nucleus in which genes regulate proteinproduction and other organelles that organize and sustain the chemical reactionsthat keep the cell alive and performing its functions.

A single axon exiting the cell body is the output end of the neuron, enabling itto communicate with other neurons or glands or muscles. This communicationcan happen because neurons have the special property of excitability. That is, theyrespond to a stimulus (like a pinch or an electric shock) by generating a nerveimpulse. Another term for nerve impulse is action potential. Nerve impulses arerapid on-off signals that are sparked at the cell body and are transmitted to the

Nucleus

Cytoplasm

Dendrites

Myelin sheathAxon

Axon terminals

Nodes of RanvierSoma

Mitochondrion

Axon

Neurotransmitters

Vesicle

Cleft

Receptor siteDendrite

Glia cell

Synapse

Figure 7.4 The General Structure of a Neuron. Details in Text.

203C. Cells of the Nervous System

multiple ends of the axon. This means that nerve impulses from a single neuronare amplified as the axon branches and excites hundreds or even thousands ofother neurons or muscle fibers or gland cells.

The speed at which nerve impulses travel along axons is not immeasurably fast;it is about 20 to 50 meters per second, depending on the size of the axon. In fact,nerve impulse transmission is slow enough that the difference in communicationtime between the hand and brain versus the foot and brain can be measured reli-ably. Nerve impulses are transmitted more rapidly if axons are wrapped in a myelinsheath interrupted by periodic nodes.

Axons come in many sizes and shapes. Some axons are microscopically short,reaching only to the dendrites of nearby neurons (a distance of less than 100micrometers or 0.004 inches). Other axons are very long, extending from onelocation in the brain to the spinal cord or all the way from the spinal cord tofar-distant muscles in the feet or fingers. When they reach their targets, they pro-duce branches to contact with specific neurons, muscle fibers or gland cells.

Neurons receive inputs from axons on their dendrites — resembling thebranches of trees. Dendrites are the input end of the neuron, receiving signalsfrom other neurons and translating these into electrical signals that are transmit-ted through a main trunk into the cell body. The signals from multiple inputneurons are integrated by the dendrites, and a neuron signals the completionof that integration by producing a nerve impulse that is transmitted along theaxon to its terminals making contact with other neurons (or muscles or glands).Different kinds of neurons have diverse shapes and sizes of dendrites throughwhich they accomplish their specialized functions.

Axons contact neurons, muscles and glands at specialized sites called synapses.At each synapse there is a tiny gap (known as the synaptic cleft) that separates theaxon of a transmitting neuron from the dendrite of the receiving neuron. Theaxon of the first neuron influences the dendrite of the second neuron by releasinga neurotransmitter that floats like springtime pollen in this gap. Neurotransmittersare small molecules that are stored in the terminal of the transmitting neuron insmall spheres called vesicles. Neurotransmission occurs when the contents of thesevesicles are released into the synaptic cleft; this is caused by the arrival of a nerveimpulse at the axon terminal. The neurotransmitter molecules diffuse across thecleft until they encounter receptors that are embedded in the receiving neuron’sdendrites. The binding of the neurotransmitter to the receptor (like a key in alock) triggers other events within the receiving neuron that constitute the commu-nication. The neurotransmitter molecules do not remain in the synaptic cleft verylong. Several processes remove them; these include being taken back into the axonterminal to use again and breakdown by other chemical processes in the synapse.

The human brain has roughly 85 billion neurons, which connect to oneanother in complex and intricate ways. A typical neuron has a ‘‘social circle’’ ofdirect connections with between 1,000-10,000 other neurons. By responding selec-tively to the inputs they receive, neurons can be said to interpret the pattern ofinputs — similar to the way that a computer processes information through anested series of ‘‘if-then’’ rules.

Different neurons with different neurotransmitters convey different kinds ofinfluence. The specific nature of these influences will be described further below.Before proceeding, though, let’s make an important point. Many authors and


scientists write about neurons ‘‘processing information’’ or ‘‘signaling.’’ Theseterms are natural consequences of adopting the metaphor that the brain is likea computer. In fact, scientists have a very specialized, quantitative definition of‘‘information’’ that is used for devices like cell phones and computers, and thisquantity can be used to describe neuron processes. However, it is crucial to remem-ber that describing the brain as a computer is only a metaphor; it is not the finalscientific theory of brain function. The reason this is important is that the meta-phor can encourage an insidious form of dualism in which statements about infor-mation processing by neurons presume some kind of homunculus (or ghost in themachine) that ‘‘reads’’ that information. There is no homunculus; it’s only neu-rons through and through.

With billions of neurons, each connected to thousands of other neurons, yourbrain has hundreds of trillions of synapses. And while you are born with (by andlarge) all of the neurons you will ever have, your synapses do not remain static overtime. From the moment you were born your brain was engaged in synapticplasticity — a process whereby the synapses that are strengthened through use sur-vive and those that are weakened through lack of use are pruned. Which synapsessurvive depends crucially on the interaction of genes and the environment inwhich a child is raised. Insights about this early synaptic plasticity have played arole in policy proposals to provide educational and social services in the birth tothree-year age period.

The final structure of the brain is a record of its evolutionary and developmen-tal history. The cell bodies of neurons are not randomly spread throughout theCNS. Instead, they are organized into dense collections that bring their dendritesinto close proximity with one another to receive inputs from axons of particularneurons. Where neuron cell bodies are densely packed with their surroundingdendrites and associated glia cells appears like gray matter to anatomists, andwhere myelinated axons are densely packed appears as white matter. The cerebralcortex is one example of a dense collection of neurons, or gray matter, surround-ing the white matter formed by the axons running to and from the cerebral cortex.In the interior of the brain are other collections of neuron cell bodies or graymatter that we will describe below.

2. Glia

Glial cells (from the Greek ‘‘glue’’) support and enhance the function of neurons.Neuroscientists distinguish different kinds of glial cells. But for this course it isenough to appreciate that one kind of glia forms the myelin sheath aroundaxons to enhance the speed of nerve impulse transmission, while another kindof glia facilitates synaptic transmission by absorbing neurotransmitters and con-necting neurons to blood vessels.

While glial cells provide essential functions in the brain, they are also suscep-tible to uncontrolled division resulting in cancerous growths, called gliomas. Glio-mas are the most common type of brain tumor and are incredibly lethal. One sideeffect of a growth resulting from a glioma can be behavioral effects due to impinge-ment on certain brain structures.

205C. Cells of the Nervous System

D. WHERE BRAINS COME FROM

More could be said about how brains came to be on this planet than will be here.But two particular ideas should be kept in mind while considering the question.First, brains exist because certain kinds of organisms (mammals, say, as distinctfrom flowers) have a body plan that — absent a nervous system that coordinatesbodily functions and associates perceptions with actions — would lie immobile andquickly die. At the most fundamental level, brains enable sensitivity to and mobilityin the complex and challenging environments in which they must navigate to findfood and mates and avoid predators. In short, brains enable behavior.

Second, inquiries about the brain, like inquiries about all other features oforganisms, including their behaviors, are divisible into separate ‘‘why’’ and‘‘how’’ questions. These are often conflated by the unwary. Biologists oftenrefer to the ‘‘why’’ questions as being about the evolutionary history of features(how they came over generations to exist as they do) while referring to ‘‘how’’questions as being about detailed mechanisms (what are the physical and chemicalpathways by which such a thing exists?). Biologists typically label the ‘‘why’’ ques-tions as inquiries into the ‘‘ultimate causes’’ of a feature, in contrast to the ‘‘how’’inquiries into the so-called ‘‘proximate causes.’’3

To illustrate, suppose you were interested in the phenomenon of male birds(in species like robins) singing in the spring. From the perspective of ultimate orevolutionary causation, those birds sing in the spring because it was more effectivein translating energy into successful mating opportunities (by advertising health,strength, interest, and location) than many of the alternatives, such as singingexclusively in the low-food depths of winter, or responding to the coming of springonly with stubborn silence, burrowing, one-legged hopping, or the like. That is, theultimate cause of singing behavior is derived from the fact that the remote ances-tors of today’s singing males — through their singing — claimed territory, attractedmates, and left more offspring than did contemporaries not predisposed to sing.To the extent that the ability to sing and the urge to respond to certain environ-mental cues with singing were influenced by genetically heritable predispositions,the proportion of male robins in successive generations that sang inevitablyincreased over time until we now observe the trait to be typical of males of thespecies. In contrast, from the perspective of proximate or mechanistic causation,those birds sing because the lengthening of the day triggers hormonal changesthat in turn prompt the bird’s body to contract muscles in ways that pass airover vocal cords shaped in ways that result in air vibrations perceived as a song.Both kinds of causes operate simultaneously — whether you are talking about birdbrains or human brains.

So, why and how do our brains function the way they do? To even begin toanswer this question, we must frame it within an evolutionary history extendingover millions of years, as well as a more recent social history over tens or hundredsof thousands of years. We must also appreciate the fact that our brain shares muchin common with the brains of other primates. Diverse primate species separated

3. The law’s term-of-art ‘‘proximate cause’’ — though similar in its content — has a quite distinctorigin and implication.


from common ancestors by tens of millions of years have very different sizes ofbrains that nonetheless share common principles of structure and function.

While bearing certain similarities with the brains of other species, the primatebrain is nonetheless structurally distinct in numerous fundamental ways fromother mammal brains such as those of carnivores (cats and dogs) and rodents(rats and mice). As a result of the similarity among primate brains, nonhumanprimates exhibit many ‘‘human’’ traits such as cooperation, deception, aversionto inequity, and even some psychological quirks often thought to be uniquelyhuman, such as the endowment effect (a propensity to value something justreceived at more than one would have ‘‘paid’’ to acquire it an instant ago).Because of these commonalities, scientists gain important insights into thehuman brain from studies of non-human primates (among other species). And,this, coupled with the rise of modern technologies for investigating both non-human and human brains, has afforded remarkable advances in ourunderstanding of the why’s and how’s of brain structures and functions.

E. ORGANIZATION OF THE HUMAN CEREBRAL CORTEX

We now jump up from the microscopic realm to a scale that you may find morecomfortable: the larger features of the brain landscape. After a brief overview forgeneral orientation, we will consider each of the four main lobes of the cerebralcortex.

1. Overview

The cerebral cortex is necessary for memory, attention, awareness, language,thought, and consciousness. The importance of the cerebral cortex for the lawshould be clear when you appreciate that it is necessary for reasoning, problemsolving, planning, and impulse control, not to mention basic sensory processing(like reading this text). Thus, as you can imagine, many of the legal cases involvingneuroscience are concerned with activity somewhere in the cerebral cortex. Thecerebral cortex is a relatively thin layer of neurons on the surface of the brain,spanning typically around 3 millimeters (roughly a doctor’s tongue depressor).

In humans, like other large-brained animals, the cerebral cortex is denselyfolded such that nearly two-thirds of the cortex is submerged into the groovesthat give the brain its characteristically wrinkled appearance. The grooves arecalled sulci (singular is sulcus), while the exposed surface parts of the ripplingfolds of cortex are referred to as gyri (singular is gyrus). This folding and crumplingis necessary to fit the approximately 325 square inches (that’s a large pizza) ofcerebral cortex into the confines of the skull.

The cerebral cortex has two apparently symmetrical hemispheres: left andright. Each hemisphere is divided into four lobes that are defined by the mostprominent sulci and are also associated with different functional properties thatwill be reviewed later in the chapter.

The four lobes of the cerebral cortex are the frontal, parietal, temporal, andoccipital (Figure 7.5). The names of the four lobes derive from the names of thedifferent bones of the skull that overlay them. They can all be understood, and

207E. Organization of the Human Cerebral Cortex

easily distinguished, by learning the location of three key features: (a) the centralsulcus; (b) the lateral sulcus; and (c) the preoccipital notch.

The frontal and parietal lobes fall on opposite sides of the central sulcus, whichis a major sulcus (indeed one of the longest, straightest, and most visible grooves)that runs across the medial-lateral axis in the center of the cortex. If you remembersome of the orientation terminology from earlier in the chapter, you’ll see that thefrontal lobe is anterior of the sulcus and the parietal lobe is posterior to the sulcus.The temporal lobe is separated from the frontal lobe by the lateral sulcus, one ofthe most prominent structures in the human brain. The lateral sulcus also partiallyseparates the temporal lobe from the parietal lobe. Finally, the occipital lobe,which is located at the posterior extreme of the cerebral cortex, is separatedfrom the parietal lobe by the parieto-occipital sulcus and from the temporal lobeby the preoccipital notch (which, unlike the central and lateral sulci, is more visiblefrom the rear of the brain than it is from the side).

As mentioned earlier, the cerebral cortex is composed of two nearly identicalhemispheres. The longitudinal fissure separates the two hemispheres. And the twohemispheres are connected by bundles of axons called commissures, the largestbeing the corpus callosum, consisting of 250 million axons that interconnect thetwo hemispheres. With certain exceptions, the processing of sensory stimuli andthe control of movement take place in the opposite side of the brain. For example,movements of the left hand are initiated in the right hemisphere (and vice versa),and the processing of visual information from the right visual field occurs in the

Frontal lobe

Parietal lobe

Temporal lobe

Occipitallobe

Inferiorpreoccipital

notch

Parieto-occipitalsulcus

Centralsulcus

Lateral sulcus

Figure 7.5 Locations of and Boundaries between the Four Lobes of the CerebralCortex. Other Structures are not Labeled.


left hemisphere (and vice versa). In humans the two hemispheres are specializedfor particular functions. In general, the left hemisphere is responsible for language,for example, and the right hemisphere is responsible for navigating in space.

We will now describe in a little more detail the organization and function ofdifferent parts of the cerebral cortex. We appreciate the complexity of this infor-mation for the novice, but we remind you that if your brain weren’t this complex,you couldn’t be in law school.

2. Frontal lobe

In humans, the frontal lobe is the largest of all, supporting our unique ability tobehave so flexibly, to navigate complex social relationships, to communicate withsymbols through speech, art and music, to contemplate mindboggling options,and to plan further into the future than any other species on the planet. Thefrontal lobe is organized into four major gyri: precentral, superior frontal, middlefrontal, and inferior frontal (see Figure 7.6). Based on function, the frontal lobecan be divided into three broad areas: the primary motor cortex, the secondarymotor cortex, and the prefrontal cortex.

Superiorfrontal sulcus

Precentralgyrus

Postcentralgyrus

Occipital gyri

Inferiorfrontal sulcus

Orbitofrontalcortex

Superiorfrontal gyrus

Middlefrontal gyrus

Inferiorfrontal gyrus Superior

temporal gyrus

Middletemporal gyrus Inferior

temporal gyrus

Sylvianfissure

Superiortemporalsulcus

Inferiortemporalsulcus

Angulargyrus

Supramarginalgyrus

Precentralsulcus

Centralsulcus Postcentral

sulcus

Superior parietal lobule

Intra-parietalsulcus

Figure 7.6 Major Gyri and Sulci of the Human Cerebral Cortex.


The primary motor area occupies the caudal end (remember that means: towardthe rear) of the frontal lobes, nestled in the central sulcus. The primary motor areais necessary for producing organized, coordinated movements of the body such asspeech, or reaching, grasping, and manipulating. The axons of certain neurons inprimary motor cortex extend to the brainstem and spinal cord to communicatewith (innervate) the neurons that in turn communicate with the muscles of the faceand limbs. The pattern of this communication is organized such that there is a‘‘map’’ of the body in primary motor cortex, with places there corresponding toplaces in and around the body.

Two secondary motor areas are anterior to the primary motor cortex. One iscalled the supplementary motor area and the other is known as the premotor area. Theseareas are responsible for higher level planning of movements in coordination withevents in the world and ongoing movements that may be occurring in an extendedsequence (as, for example, when you type a paper).

Anterior to these secondary motor areas lies the prefrontal cortex (PFC), the cor-tical regions that will be encountered most frequently in this coursebook. The PFChas been subdivided into dorsolateral, ventrolateral, ventromedial, and orbitofron-tal regions.

The PFC is regarded as the ‘‘CEO’’ of the brain, being connected with effec-tively every other area of the cerebral cortex and everystructure beneath the cerebral cortex that will bedescribed below. The PFC is responsible for the high-est level control of behavior.

The first indication that the PFC played this rolewas the result of a freak accident in 1848 involving aman named Phineas Gage. An unintended explosionpropelled an iron rod through Gage’s head, enteringbelow his left eye and exiting the top of his skull. Theentire left frontal lobe was destroyed, though theright hemisphere remained intact. Descriptionsof Gage’s behavior following the accident indicatedthat he suffered from an inability to exerciseany form of behavioral restraint. He was describedthis way:

The equilibrium or balance, so to speak, between hisintellectual faculties and animal propensities, seemsto have been destroyed. He is fitful, irreverent,indulging at times in the grossest profanity (whichwas not previously his custom), manifesting but littledeference for his fellows, impatient of restraint oradvice when it conflicts with his desires, at times per-tinaciously obstinate, yet capricious and vacillating,devising many plans of future operations, which areno sooner arranged than they are abandoned inturn for others appearing more feasible. . . . Inthis regard his mind was radically changed, so decid-edly that his friends and acquaintances said he was‘‘no longer Gage.’’

Navigating the Prefrontal Cortex The

prefrontal cortex (PFC) is a region of

the brain you’ll encounter many times

in this coursebook. You will see discus-

sion of the following four subregions of

the PFC: the dorsolateral, ventrolateral,

ventromedial, and orbitofrontal regions.

Thus, it’s worth taking a few minutes

now to study these different parts of

the PFC. Imagine you have to explain in

a brief where your client’s PFC injury was

in detail. Armed with what you learned

earlier in this chapter, you should be

able to translate the four PFC subregions

into rough locations in the brain. For

example, dorsolateral is a combination

of ‘‘dorsal’’ (top) and ‘‘lateral’’ (side) — in

the same way that the compass direction

‘‘Northwest’’ is a combination of ‘‘North’’

and ‘‘West’’ — such that the dorsolateral

region of the PFC is roughly the upper

side area of the PFC. Using the same

approach, take a moment to describe

where each of the other regions are.

Which is just above the eyes?


John Martyn Harlow, Recovery from the Passage of an Iron Bar Through the Head, 2 Pub.Mass. Med. Soc. 327, 339-40 (1868).

Extensive research has confirmed the impression, illustrated by this case, thatthe PFC is critical for the ability to predict outcomes, delay gratification, comparemultiple options, assess risk, adapt to changing rules and goals, and redirect atten-tion based on new information. However, each of these functions is accomplishedby different parts of the prefrontal cortex, so let us consider them in turn.

Dorsolateral prefrontal cortex (commonly abbreviated DLPFC) has been associ-ated with attentional control, decision-making, the integration of information,high-level control of behavior, working memory of items during planning anddeliberation, and inductive reasoning. The DLPFC is critical for our ability tothink abstractly so as to anticipate and simulate events that might not necessarilybe true or events that might not be occurring in the present. An example of thiswould be predicting the outcome of future events based on hypothetical situations.

Ventrolateral PFC (VLPFC) occupies the most lateral and ventral portion of thePFC. This region of cortex in the left hemisphere includes an area known as Broca’sarea that is responsible for generating speech.4 Other parts of this region contrib-ute to regulating movements in response to arbitrary cues. Ventromedial PFC(VMPFC) and orbitofrontal PFC (OPFC) have been linked to motivation,decision-making and judgment.

3. Parietal lobe

From where does the frontal lobe receive the information that it evaluates? Onerich source of information is the parietal lobe. The parietal lobe is strongly linkedto the frontal lobe, providing sensory guidance for the actions produced by thefrontal lobe. That guidance integrates information across the senses into a unifiedframework to guide action in space and time.

Primary somatosensory cortex occupies the rostral end of the parietal lobe,nestled in the central sulcus, adjacent to primary motor cortex. This corticalarea is responsible for our sense of touch, pain, temperature, and limb position.Different nerve pathways beginning at different receptors convey these differentsensory modalities with different speeds. This is why when a person hits his thumbwith a hammer, he feels the sensation of the pressure of the object against histhumb before he feels the pain.

Beyond the sense of touch, the parietal lobe is involved in cognitive processesand has been known as association cortex. Areas in association cortex are neitherspecifically motor nor exclusively sensory in function. Neuroscientists surmise thatassociation cortex is where converging inputs catalyze everything from coordinatedcomplex body movements to creativity to social cognition. The organizing of con-vergent information for different tasks and abilities is accomplished in a variety ofsubregions. Some of these regions are involved in visual orienting and attention toguide actions through our spatial environment. Accordingly, damage to these

4. Broca’s area and Wernicke’s area (see below) are almost always in the left hemisphere of right-handed individuals, especially males. If you are left-handed, then it is possible that language productionand comprehension depends more on the right hemisphere.


regions renders the patient unable to notice objects in the environment eventhough they can see them just fine. Other regions are involved in speech compre-hension. Accordingly, damage to a region located in the supramarginal gyrus alsoknown as Wernicke’s area renders patients incapable of speech comprehension. Yetanother region, known as the temporoparietal junction (TPJ), is involved in whatpsychologists call ‘‘theory of mind.’’ Accordingly, damage to the TPJ renderspatients challenged to interpret the thoughts, intentions, and desires of others.

4. Temporal lobe

Another important source of inputs to the frontal lobe is the temporal lobe. Ingeneral, the temporal lobe provides information about the identity of objects bysupporting memory and recognition.

The primary auditory processing cortex is located in the temporal lobe nearthe posterior extreme of the lateral fissure. As is the case in the primary somato-sensory processing area, the processing of auditory input does not end with theprimary auditory cortex. As previously mentioned, certain vision processing forguiding movements in space occurs in the parietal lobes of the brain. Complemen-tary vision processing for recognizing objects like faces occurs in the temporallobes. The recognition of all of the complex objects in our lives requires learning,and the temporal lobes also contain structures that are essential for forming andretrieving memories.

5. Occipital lobe

The occipital lobe is the smallest of the lobes in the human cerebral cortex. It isthe visual processing center of the mammalian brain. The primary visual cortex islocated at the posterior pole of the occipital lobe and is surrounded by a variety ofother areas that perform more complex analyses of the visual image. Thus, damageto the posterior pole results in profound loss of vision, while damage to morerostral regions results in more subtle problems with visual perception. Forexample, damage restricted to a small region that is specialized for perceptionof visual motion renders patients incapable of seeing motion even though theycan see the color, shape, and location of objects.

The scientific study of perception investigates basic questions about how sen-sitive we are to the presence of stimuli in the different sensory modalities (vision,hearing, touch, smell, taste) and about how well we can discriminate differentstimuli. In considering the function of the parts of the brain that accomplish sen-sation, it is important to appreciate that they can be fooled. Psychologists havediscovered many illusions that illustrate just how much of our perception of theworld is constructed by the brain.5

It is easy to see why perception is legally relevant. What a witness hears, or whatyou hear when a client is explaining her case to you, are central to legal practice.The limits of perception are also highly relevant in many legal contexts. Forexample, at what distance can one reliably discriminate the faces of different

5. Many websites display visual illusions. We like this one: http://www.michaelbach.de/ot/.


individuals? Perception varies with experience. For example, the ability to discrim-inate among objects (like cars) varies with the amount of experience one has view-ing and considering such objects.

Another essential fact about perception is that we are bombarded by manymore stimuli than we can respond to. Therefore, perception is selective. Inshort, we perceive those few objects to which we pay attention.

F. ORGANIZATION OF SUBCORTICAL SYSTEMS

We noted earlier that the cerebral cortex is a relatively thin layer of neurons foldedover the surface of the brain. When the early anatomists peeled away the cerebralcortex and underlying white matter, they discovered many other collections ofneuron cell bodies arranged in various ways from the forebrain to the brainstem.They are known as ‘‘subcortical’’ structures, because, if you were to drill into some-one’s head from the skull, you would find these other structures ‘‘beneath’’ thecortex.

This section surveys the major subcortical structures. Subcortical structures canbe organized into functional subdivisions or into anatomical subdivisions. We willdo both by describing two functional systems (the limbic system and the basal gan-glia) and these anatomical subdivisions (thalamus, hypothalamus plus pituitary gland,brainstem including cerebellum, pons, medulla oblongata, and the spinal cord). Thissection of the chapter probes a deeper level of detail than the previous sections,but the detail is provided so that when you encounter these structures later in thebook, you can refer back to this section to be reminded of their location andfunction.

1. Limbic System

Researchers formulated the term limbic system in the early 1950s to organize theirunderstanding of the function of a diverse group of structures. The word ‘‘limbic’’was derived from the Latin word for ‘‘border’’ and was originally used to labelstructures at the border between the cerebral hemispheres and subcortical struc-tures. Some scientists believe that the term is obsolete, and newer, more accurateclassifications have been developed. Nevertheless, you should be familiar with theterm limbic system because it remains a common usage outside neuroscience andwill be used in studies that will be described in later chapters. We will focus on justthree of the many structures that compose the limbic system — the amygdala, hip-pocampus, and cingulate cortex.

Two functions mediated by the limbic system are memory and emotion. Thehippocampus (named by the Greek word for sea horse, with which it shares astunning resemblance) is a prominent structure in the medial temporal lobe.The hippocampus with nearby regions is responsible for episodic memory. Epi-sodic memory can be thought of as memory of past ‘‘episodes’’ of personal experi-ences, such as what you had for breakfast and where you parked your car. You willlearn in Chapter 13 that human memory is not a perfect digital recorder on whichyou simply press the rewind button to replay previous episodes in your life.

213F. Organization of Subcortical Systems

Humans with damage to the hippocampus and surrounding tissue are unable toform new biographical memories and also may have trouble recalling memories ofpast events before they experienced the unfortunate damage. Impaired formationof memories is called anterograde amnesia, and impaired recall of memories is calledretrograde amnesia.We have all experienced how emotional arousal during an eventseems to enhance recollection of that event. Our memories are formed andretrieved wrapped in an emotional package that helps organize the contentwith the context of the memory. This association between emotion and memoryis mediated by the amygdala (named by the Greek word for almond that it resem-bles). It is a dense collection of neurons nestled in the rostral tip of the temporallobes. The amygdala and hippocampus work together to add emotional tone toepisodic memories such as your first kiss or a traumatic experience. For example, ifthe experience of a stimulus is associated with a fear response, then the amygdalarecreates the fear response at a much later point in time upon the reappearance ofthe original stimulus or even something only resembling the initial stimulus. Forinstance, if a fast moving object is headed your way, you want to immediately moveout of its way. Your fear response (i.e. the fear you attach to memories of previousobjects coming quickly at you) allows you react quickly.

The balance of emotion and memory is complex. On the one hand, the asso-ciation can become so strong that a fear response, for example, results in inappro-priately excessive responses. Often this association will continue despite substantialexperience indicating that the association is invalid, and without the subject’sawareness that the association has been formed. The neurological inability toextinguish the association has been interpreted as insufficient ‘‘top-down’’ controlexerted on the amygdala by regions in the PFC. The phrase ‘‘top-down’’ is usedhere as it is in a business context — the PFC (as ‘‘CEO’’ of the brain) has theresponsibility to determine when certain fear associations are useful (e.g., takecover when you hear a bomb explode on the battlefield) and when they are not(e.g., you don’t need to hide under your desk every time you hear the trash truckrumbling outside). The result of this deficient extinction ability is thought to leadto the anxiety disorder known as post-traumatic stress disorder (PTSD). As you willread in subsequent chapters, much personal injury litigation and some criminaldefense now invokes a client’s PTSD diagnosis.

On the other hand, in other cases too much emotional arousal during a trau-matic event can reduce the ability to recall the memories of the event. Such dis-agreement has found its way into the courtroom. For instance, in the trial of aBosnian-Croatian soldier accused of abuse of a female prisoner, the defense calledexpert witnesses who testified that the woman’s memories were inaccurate due hertraumatic experiences. The experts brought in by the prosecution, however, sug-gested precisely the opposite, that she remembered more vividly and accuratelyher traumatic ordeals. You’ll read more about cases like these in the chapter onmemory.

The final part of the limbic system to introduce is the cingulate cortex. Thecingulate cortex sits directly underneath the frontal and parietal lobes and isinvolved in a wide variety of functions. There are two major subdivisions in thecingulate cortex: the anterior cingulate cortex (ACC) and posterior cingulate cor-tex. However, our interest in the cingulate cortex here is isolated to the ACC.


The ACC has become an area of significant research interest in the last 20years with the introduction of brain imaging. As neuroscientists investigatedother cortical structures using fMRI and PET (these methods will be explainedin Chapter 8), they found that the ACC was involved in a broad variety of cognitiveand emotional functions. The ACC has two more or less separate functionalregions, a dorsal region that is involved in cognitive and executive processesand a ventral region that is involved in emotional processes. Overall, the ACChas been associated with monitoring behavior by judging the consequences ofactions, experience of pain and emotional distress, evaluating choices based onexpected value, evaluating choices in social interactions including moral judg-ments or producing lies, and in planning body movements in complex situations.The role of ACC in regulating emotion has led to experimental therapy of elec-trical stimulation to treat depression. While debate continues about the functionof the ACC, one plausible hypothesis states that the ACC is generally sensitive tothe difficulty of a task when errors are likely and mobilizes through the prefrontalcortex the resources necessary to perform the task correctly.

2. Basal Ganglia

The term basal ganglia refers to a collection of about a half dozen separate collec-tions groups of neurons in the center or base (hence ‘‘basal’’) of the brain thatform a circuit to regulate movement, emotion, and thought. The word ‘‘ganglia’’simply refers to a dense collection of neuron cell bodies. The basal ganglia wereoriginally identified entirely with the motor system of the brain because of itsessential role in producing movements of the body. This function is highlightedby the fact that the symptoms of Parkinson’s disease and Huntington’s disease arisefrom the death of specific neurons in the basal ganglia. However, the basal gangliaare now recognized to play an equally important role in the control of cognition,emotion, and motivation as evidenced, for example, by its contribution to estab-lishment of habits that can become extreme in, for example, addiction orobsessive-compulsive disorder.

3. Thalamus

The thalamus is located in the center of the brain, beneath the cortex, surroundedby (and part of) the limbic system and basal ganglia. Neuroscientists describe thethalamus, perched atop the brainstem, as the gateway to the cerebral cortex.The thalamus consists of neurons that receive inputs from many other parts ofthe brain and send axons to the cerebral cortex. Different parts of the thalamusare connected with different parts of the cerebral cortex in a very organized man-ner. The different parts of the thalamus receive inputs from either different sen-sory pathways or from other parts of the brain, such as the limbic system, the basalganglia, the cerebellum, and the brainstem. Thus, cortical areas in occipital,temporal, and parietal lobes that perform visual processing receive input fromparts of the thalamus that themselves receive input from neurons in the eye.Accordingly, a stroke of this part of the thalamus results in problems with visualperception. Likewise, cortical areas in the frontal lobe that produce body move-ments receive input from parts of the thalamus that themselves receive input from

215F. Organization of Subcortical Systems

neurons in the basal ganglia and cerebellum. A stroke damaging this part of thethalamus results in problems of movement and planning.

4. Hypothalamus & Pituitary Gland

Beneath the thalamus is the hypothalamus (‘‘hypo’’ means below). The hypothal-amus consists of different subdivisions that maintain the internal state of the body.This includes body temperature, hunger, thirst, sexual drive, fatigue, sleep, andcircadian rhythms. Neuroscientists say that the hypothalamus maintains the body’sstate of homeostasis. This is accomplished through neural connections with theautonomic nervous system and hormone signals through the pituitary gland.The latter has been called the ‘‘master gland,’’ because it releases hormonesthat modulate a wide variety of body functions by controlling, in turn, the func-tions of other glands throughout the body.

5. Brainstem — Cerebellum, Pons & Medulla

The brainstem is located between the forebrain and the spinal cord. We can dividethe brainstem into the cerebellum, the pons, and the medulla oblongata (often referredto simply as the medulla).

The cerebellum (‘‘small brain’’) is the conspicuous structure hanging beneaththe occipital lobe (Figure 7.3). The cerebellum is connected to the rest of thebrain through neurons in the pons. The pons is a bulbous part appearing towrap around the brainstem. The cerebellum is necessary to produce preciselytimed and coordinated movements. It is also involved in coordinating thoughts.

The pons and medulla consist of axons traveling down to and up from thespinal cord as well as collections of neurons that are necessary for the most basiccontrol of heart beat, breathing, and consciousness, as well as reflexive actions likevomiting, coughing, sneezing, and swallowing. When we consider the topic ofbrain death in a subsequent chapter, we will learn how important the status ofthe brainstem is when evaluating brain death.

6. Spinal Cord

As we mentioned at the beginning of this chapter, the central nervous system(CNS) is composed of the brain and the spinal cord. Like the brain, the spinalcord is sheathed in meninges, which provide protection along with the spine. Thespinal cord travels from the end of the medulla oblongata through the spinal col-umn, ending near the bottom of the spine.

The spinal cord consists of the axons mediating the two-way transmission ofmotor signals from the brain to the body and sensory signals from the body to thebrain. These two paths are separated in the spinal column, with the motor outputsrunning down the ventral side and the sensory neurons rising up the dorsal side.These axons surround a central core of sensory neurons that receive inputs fromthe body, motor neurons that make muscles contract to produce body movements,and intermediate inhibitory and excitatory neurons that regulate the function ofthese neurons. These spinal cord circuits can operate independently of the cere-bral cortex, and this provides for rapid reflexes that occur substantially faster than


they would if the signal had to be conducted to the brain and back. The skin andmuscles of the body are connected in an orderly map with the spinal cord; thearms are connected to the part of the spinal cord closer to the brain, and the legsare connected to the part of the spinal cord further from the brain. This is whydamage to the spinal cord typically only affects the parts of the body connected tothe spinal cord caudal to the damaged point.

G. BLOOD SUPPLY TO THE BRAIN

It should be evident by now that the brain does a lot of work. From keeping yourheart beating, to helping you answer questions on an exam, your brain is workingaround the clock. You might be wondering at this point what types of fuel the brainneeds to accomplish all of this work. The brain relies on oxygen and glucose sup-plied by blood flow. Comprising just 1/40th of body mass, the brain consumes1/5th of all oxygen. The human brain contains 60,000 miles of blood vessels,large and small. Neuron function and blood flow are very closely related; neuronsinfluence blood vessel size through certain glial cells.

Arteries deliver oxygenated blood, glucose, and other nutrients to brain tissue.Veins take deoxygenated blood and the by-products of metabolism such as carbondioxide and lactic acid back to the heart and respiratory system. The seeminglymiraculous images of brain function that are so important for neurolaw arederived from tiny differences in the magnetic properties of oxygenated and deoxy-genated blood, or by an indirect measure of metabolic activity in which radioactivetracers are injected to visually see relative levels of glucose uptake.

The blood supply to the brain is divided into anterior and posterior arteries.The two main pairs of arteries are the internal carotid arteries and vertebral arter-ies. Pairs of arteries communicating across the midline interconnect the anteriorand posterior blood supplies of each hemisphere. This connection of arteriesforms a circuit that balances blood pressure and provides a path for blood flowto both hemispheres even if one of the major supply arteries becomes occluded.The arteries split into anterior, middle, and posterior branches that further splitinto smaller and smaller branches to supply cortical and subcortical tissue.

The blood that has released oxygen and absorbed waste products collects inthe venous drainage system. This consists of a superficial system and a deep system.The superficial system is composed of gaps called sinuses beneath the dura. Onevery prominent example is the superior sagittal sinus located between the cerebralhemispheres. The deep venous drainage is composed of veins inside the deepstructures of the brain. Smaller veins combine to blood into larger veins thatjoin the confluence of the sinuses to form the two jugular veins.

The brain is exceedingly susceptible to compromises of its blood supply, so thecerebral circulatory system has many safeguards. Failure of these safeguards resultsin cerebrovascular accidents, commonly known as strokes. The particular pattern ofbrain damage following a stroke will depend on which blood vessels are damaged.Blood vessel branching patterns vary across individuals, so even strokes in individ-uals affecting arteries of the same name can produce somewhat different patternsof symptoms.

217G. Blood Supply to the Brain

H. HOW NEURONS COMMUNICATE

This section will describe more of the basics about how circuits of neuronsfunction. No survey of brain function would be complete without this information.As noted above, nervous systems work through the interactions of neurons withother neurons, sensory receptors, muscles, and glands. Neuroscientists differenti-ate between major types of neurons on the basis of what type of signal the neuronsends, and what distance the signal travels. Some neurons have axons that projectto structures other than where their cell body resides. For example, neuronslocated in the frontal lobe of the cerebral cortex send axons to the spinal cordto produce body movements. Also, neurons in one area of the cerebral cortex sendaxons to many other cortical areas. These are just two of the numerous connec-tions that neuroscientists have described. Another pattern of connection is thefeedback loop; most brain structures that send an axon somewhere gets anaxon back from that place.

Neuroscientists have also found neurons that communicate only locally withinstructures. These tend to be smaller neurons with axons that connect only to neu-rons in the local vicinity of the cell body. These local circuit neurons are importantfor tuning the state of activation of the larger neurons that project to otherstructures.

Scientists sort neurons into three major categories, based on the nature of theoutput messages sent to other neurons. Neurons in the first category are calledexcitatory, those in the second category are called inhibitory, and those in the thirdcategory are called modulatory. Every neuron receives a diverse collection of excit-atory, inhibitory, and modulatory inputs. Whether that neuron produces animpulse depends on the pattern of these three inputs over time and over thespace of the dendritic tree in complicated ways that are progressively well under-stood but well beyond the scope of this introduction.

Excitatory neurons commonly use a neurotransmitter called glutamate. Whenglutamate binds to receptors in the dendrites of receiving neurons, processesoccur that increase the probability that the post-synaptic neuron becomes activeimmediately. Most neurons that communicate across structures are excitatory.(This is the same glutamate that is in monosodium glutamate, more commonlyknown as MSG.)

Inhibitory neurons commonly use a neurotransmitter called GABA (gamma-amino butyric acid). When GABA binds to receptors in the dendrites of receivingneurons, processes occur that decrease the probability that the post-synaptic neu-ron becomes active immediately. Most neurons that communicate within a localregion are inhibitory.

Modulatory neurons have longer term and more subtle effects that can eitherincrease or decrease the likelihood of a post-synaptic neuron becoming active inresponse to excitatory and inhibitory inputs. Modulatory neurons can be under-stood to tune the balance of activation in many neurons comprising a circuit orsystem or even the whole brain. These neurons use a variety of neurotransmitters,some of which you may have heard of. The major modulatory neurotransmittersare dopamine, norepinephrine, and serotonin. The cell bodies of these neurons areconcentrated in the brainstem, and their axons travel and branch very broadly


throughout subcortical structures and the cerebral cortex. Thus, they have verybroad influence. These neurotransmitters are the targets of drugs that affectthought, movement, and mood.

We will now describe the mechanics of synaptic transmission. Once again, thiswill be a superficial survey of basic principles. With only a few exceptions that neednot concern us, every neuron signals with only one neurotransmitter. We will treatthe process of synaptic transmission as equivalent for all types of neurons.

When a nerve impulse reaches an axon terminal, it causes the vesicles storingthe neurotransmitter to release their contents into the synaptic cleft. The neuro-transmitters diffuse and eventually may bind to a receptor located in the mem-brane of the post-synaptic neuron. What happens next dictates whether theneurotransmitter effect is excitatory, inhibitory, or modulatory. It is excitatory ifit changes the state of the neuron such that it becomes more likely to produce anerve impulse. It is inhibitory if it changes the state of the neuron such that itbecomes less likely to produce a nerve impulse. Both of these effects occur throughthe receptor changing the state of the membrane in the dendrite to make it eitherdepolarize (if excitatory) or hyperpolarize (if inhibitory).

In contrast to this direct effect, modulatory neurotransmitters exert theirinfluence through second messenger systems within the post-synaptic neuron.These systems are called second messenger because the binding of the neurotrans-mitter to the receptor causes a cascade of chemical reactions within the neuronthat amplifies in magnitude and duration the influence of the neurotransmitter.Researchers have studied a variety of second messenger systems, and they remainthe focus of intense research because they are likely sites of drug influence. Ulti-mately, these second messenger systems influence the inner workings of the neu-ron in ways that can either increase or decrease the responsiveness to otherinfluences. Through second messenger systems, a given neurotransmitter canhave a range of effects. For example, dopamine is heavily concentrated in thebasal ganglia where it has both excitatory and inhibitory effects on different neu-rons depending on the type of receptor in the respective neurons. Because theyhave such spatially and temporally distributed effects, modulatory neurotransmit-ter systems have been identified with general aspects of behavior such as reward,sleep-wake cycle, and mood.

Once the neurotransmitter has been released into the synaptic cleft, it cannotbe left to act forever or the signaling would not be a signal any more. Three majormechanisms exist to remove neurotransmitters from the synapse to limit theirduration of action. First, specific enzymes may convert the neurotransmitter to achemical that will no longer bind to the receptor. Second, the neurotransmittermolecule may be taken up into the presynaptic membrane; this reuptake processallows the neurotransmitter molecule to be recycled and released again. Third,certain neurotransmitters can be removed by glia.

The complex interaction of neurotransmitters and their receptors change thelikelihood that a neural signal will pass between one or another neuron. Throughprocesses like these brains can reconfigure their neural networks with astonishingspeed and in unimaginable combinations to support activities like thinking,feeling, and learning.

219H. How Neurons Communicate

FURTHER READINGNeuroscience is such a popular topic that many introductory books have beenauthored such as:

Sandra Aamodt & Sam Wang, Welcome to Your Brain: Why You Lose Your Car Keys But NeverForget How to Drive and Other Puzzles of Everyday Behavior (2008).

Michael O’Shea, The Brain: A Very Short Introduction (2006).

Neuroscience is taught in undergraduate courses from textbooks like these:Marie T. Banich & Rebecca J. Compton, Cognitive Neuroscience (3d ed., 2010).Mark F. Bear, Barry W. Connors & Michael A. Paradiso, Neuroscience: Exploring the Brain

(3d ed., 2006).Michael Gazzaniga, Richard B. Ivry & George R. Mangum, Cognitive Neuroscience: The

Biology of the Mind (4th ed., 2013).James W. Kalat, Biological Psychology (11th ed., 2013).Dale Purves et al., Neuroscience (5th ed., 2011).

At the graduate level students learn from more detailed and advanced books such as:Eric Kandel, James Schwartz & Thomas Jessell, Principles of Neural Science (5th ed., 2012)Larry R. Squire et al., Fundamental Neuroscience (4th ed., 2012).

History is never a bad perspective on any topic, and the history of neuroscience isfull of fascinating people, events and insights.

Stanley Finger, Origins of Neuroscience: A History of Explorations into Brain Function (1994).

Neuroanatomy is complex, but these books can make it seem simpler:Marion C. Diamond & Arnold B. Scheibel, The Human Brain Coloring Book (1985)Adam Fish, Neuroanatomy: Draw It to Know It (2d ed., 2012).

Finally, this 10-volume encyclopedia provides a comprehensive survey of 32 sepa-rate areas of neuroscience and neurology with language accessible for undergrad-uates and depth appropriate for scientists.

Larry R. Squire, Encyclopedia of Neuroscience (2009).

Online Resources. The Society for Neuroscience provides a very useful and author-itative online resource. The Society for Neuroscience was founded in 1969 and nowhas more than 40,000 members. It not only serves to advance understanding of thebrain and the nervous system, but also promotes educational programs about thebrain and informs legislators and other policymakers about the implications of neu-roscience research for public policy, societal benefit, and continued scientific prog-ress. Its website, www.sfn.org, hosts several excellent resources on basic informationabout the brain including Brain Facts (www.brainfacts.org).

You can also learn more about neuroscience and brain disorders at the web-sites of the various National Institutes of Health such as the:

Nat’l Inst. on Aging, http://www.nia.nih.gov/Nat’l Inst. on Alcohol Abuse & Alcoholism, http://www.niaaa.nih.gov/Eunice Kennedy Shriver Nat’l Inst. of Child Health & Human Dev, http://

www.nichd.nih.govNat’l Inst. on Drug Abuse, http://www.drugabuse.gov/Nat’l Inst. of Mental Health, http://www.nimh.nih.gov/index.shtmlNat’l Inst. of Neurological Disorders & Stroke, http://www.ninds.nih.gov


fundamentals of cognitive neuroscience€¦ · surveys, in chapter 7, basic information about the...

Documents