Neurons and Glia

Home , Axon terminal, Multipolar neuron, Unipolar neuron

INTRODUCTION

THE NEURON DOCTRINE THEGOLGI STAIN CAJAL'SCONTRTBUTTON r Box 2.I O.fSpecial Interest: Advances in Microscopy

THE PROTOTYPICAL NEURON THESOMA Ihe Nucleus r Box 2.2 Brain Food:Expressing One's Mind in the Post-GenomicEra RoughEndoplosmic Reticu/um SmoothEndoplosmic Reticulum ond the GolgiApporotus The Mitochondrion THE NEURONALMEMBRANE THE CYTOSKELETON Microtubules r Box 2.) Af SpecialInterest: Alzheimer's Disease and the Neuronal Cytoskeleton Miuofiloments Neurofloments THEAXON TheAxonTerminal Ihe Synopse AxoplosmicTronsport r Box 2.4 Of SpecialInterest: Hitching a Ride With RetrogradeTtansport DENDRITES r Box 2.5 Of SpecialInterest: Mental Retardationand Dendritic Spines r Box 2.6 Pathof Discovery:Spines and the StructuralBasis of Memory, by William Greenough

CLASSIFYINGNEURONS CLASSIFICATIONBASED ON THE NUMBEROF NEURITES CLASSIFICATIONBASED ON DENDRITES CLASSIFICATIONBASED ON CONNECTIONS CLASSIFICATIONBASED ON AXON LENGTH CLASSIFICATIONBASED ON NEUROTRANSMITTER

GLIA ASTROCYTES MYELINATINGGLIA OTHERNON-NEURONAL CELLS

CONCLUDING REMARKS 24 CHAPTER 2 . NEURONSANDGLIA

V INTRODUCTION All tissuesand organsin the body consistof cells.The specializedfunctions of cellsand how they interact determinethe functions of organs.The brain is an organ-to be sure, the most sophisticatedand complex organ that nature has devised.But the basicstrategy for unraveling its function is no different from that used to investigatethe pancreasor the lung. We must begin by learninghow brain cellswork individually and then seehow they are assembledto work together.In neuroscience,there is no need to sepa- rate mind from brain; once we fully understand the individual and con- certed actionsof brain cells,we will understandthe origins of our mental abilities.The organizationof this book reflectsthis "neurophilosophy."We start with the cells of the nervous system-their structure, function, and meansof communication.In later chapters,we will explorehow thesecells are assembledinto circuits that mediate sensation,perception, movement, speech,and emotion. In this chapter,we focus on the structure of the different types of cells in the nervous system:neurlns andglia. Theseare broad categories,within which are many types of cells that differ basedon their structure, chemistry, and function. Nonetheless,the distinction between neurons and glia is important. Although there are many neurons in the human brain (about 100 billion), glia outnumber neurons by tenfold. Basedon thesenumbers, it might appearthat we should focus our attention on glia for insightsinto the cellular functions of the nervous system. However, neurons are the most important cellsfor the unique functionsof the brain. It is the neurons that sensechanges in the environment, communicatethese changesto other neurons,and command the body's responsesto these sensations. Glia, or glial cells, are thought to contribute to brain function mainly by insulating, supporting, and nourishing neighboring neurons. If the brain were a chocolate-chipcookie and the neurons were chocolatechips, the glia would be the cookie dough that fills all the other spaceand ensures that the chips are suspendedin their appropriate locations. Indeed, the termglia is derived from the Greek word for "glue," giving the impression that the main function of thesecells is to keep the brain from running out of our ears! As we shall see later in the chapter, the simplicity of this view is probablya good indication of the depth of our ignoranceabout glial function. However,we still are confident that neurons perform the bulk of information processingin the brain. Therefore,we will focus 907o of our attention on l0% of brain cells:the neurons. Neuroscience,like other fields,has a languageall its own. To usethis lan- guage,you must learn the vocabulary.After you have read this chapter, take a few minutes to review the key terms list and make sure you understand the meaning of each term. Your neurosciencevocabulary will grow as you work your way through the book.

V THE NEURON DOCTRINE To study the structure of brain cells,scientists have had to overcomesev- eral obstacles.The first was the small size.Most cells are in the range of 0.01-0.05 mm in diameter.The tip of an unsharpenedpencil lead is about 2 mm across;neurons are 40-200 times smaller.(For a review of the met- ric system,see Table 2.1.) This sizeis at or beyond the limit of what can be seenby the naked eye.Therefore, progress in cellular neurosciencewas not possiblebefore the developmentof the compound microscopein the late seventeenthcentury. Even then, obstaclesremained. To observebrain tissue V THE NEURONDOCTRINE 25

METER UNIT ABBREVIATIONEQUIVALENTREAL-WORLDEQUIVALENT

Kilometer km 103m About two-thirds of a mile Meter m lm About 3 feet Centimeter cm l0-2m Thicknessof your little finger Millimeter mm l0-3 m Thicknessof your toenail Micrometerum 10-6m Near the limit of resolution for the light microscope Nanometer nm l0-em Nearthe limit of resolution for the electronmicroscope

using a microscope,it was necessaryto make very thin slices,ideally not much thicker than the diameter of the cells.However, brain tissue has a consistencylike a bowl of Jello: not firm enough to make thin slices.Thus, the study of the anatomy of brain cellshad to await the developmentof a method to harden the tissue without disturbing its structure and an instru- ment that could produce very thin slices.Early in the nineteenth century, scientistsdiscovered how to harden, or "fix," tissuesby immersingthem in formaldehyde,and they developeda specialdevice called a microtome to make very thin slices. These technical advancesspawned the field of histology, the micro- scopicstudy of the structure of tissues.But scientistsstudying brain structure faced yet another obstacle.Freshly prepared brain has a uniform, cream-coloredappearance under the microscope;the tissuehas no differencesin pigmentationto enablehistologists to resolveindividual cells. Thus, the final breakthrough in neurohistology was the introduction of stains that could selectivelycolor some, but not all, parts of the cells in brain tissue. One stain, still used today, was introduced by the German neurologist Franz Nissl in the late nineteenth century. Nissl showed that a classof basic dyes would stain the nuclei of all cells and also stain clumps of material surrounding the nuclei of neurons (Figure 2.1). These clumps are called Nisslbodies, and the stain is known as the Nissl stain. The Nissl stain is ex- tremely useful for two reasons.First, it distinguishesneurons and glia from one another. Second,it enableshistologists to study the arrangement,or cytoarchitecture, of neurons in different parts of the brain. (The prefix qtto-is from the Greek word for "cell.") The study of cytoarchitectureled to the realizationthat the brain consistsof many specializedregions. We now know that each region performs a different function.

FIGURE2.I Nissl-stained neunons. A thin sliceof brain tissuehas been stainedwith cresylviolet. a Nisslstain.The clumps of deeplystained materialaround the cell nucleiare Nissl bodies.(Source: Hammersen, 1980, Fig. 493.) CHAPTER 2 . NEURONSANDGLIA

The Golgi Stain

The Nissl stain, however, does not tell the whole story. A Nissl-stainedneu- ron looks Iike little more than a lump of protoplasm containing a nucleus. Neurons are much more than that, but how much more was not recognized until the publication of the work of Italian histologist Camillo Golgi (Figure 2.2l.In 1873, Golgi discoveredthat by soaking brain tissue in a sil- ver chromate solution, now called the Golgi stain, a small percentage of neurons became darkly colored in their entirety (Figure 2.3). This revealed that the neuronal cell body, the region of the neuron around the nucleus that is shown with the Nissl stain, is actually only a small fraction of the total structure of the neuron. Notice in Figures 2.I and 2.3 how different histological stains can provide strikingly different views of the same tissue. Today, neurohistology remains an active field in neuroscience,along with "The its credo: gain in brain is mainly in the stain." The Golgi stain shows that neurons have at least two distinguishable parts: a central region that contains the cell nucleus, and numerous thin tubes that radiate away from the central region. The swollen region con- FIGURE2.2 taining the cell nucleus has several names that are used interchangeably: CamilloGolgi (1843-l 926). cell body, soma (plural: somata), and perikaryon (plural: perikarya). The (Source:Finger; i 99a, Fig.3.22.) thin tubes that radiate away {rom the soma are called neurites and are of two types: axons and dendrites (Figure 2.4). The cell body usually gives rise to a single axon. The axon is of uniform diameter throughout its length, and if it branches. the branches generally extend at right angles.Because axons can travel over great distancesin the body (a meter or more), it was immediately recognizedby the histologists "wires" of the day that axons must act like that carry the output of the neurons. Dendrites, on the other hand, rarely extend more than 2 mm in

FIGURE2.3 Golgi-stainedneurons. (Source: Hubel, l98B p | 26.)

FIGURE2.4 The basic parts of a neuron. 1, THENEURON DOCTRINE 27 length. Many dendrites extend from the cell body and generally taper to a fine point. Early histologists recognized that because dendrites come in contact with many axons, they must act as the antennae of the neuron to receive incoming signals,or input.

Cajal's Contribution

Golgi invented the stain, but it was a Spanish contemporary of Golgi who used it to greatesteffect. Santiago Ram6n y Cajal was a skilled histologist and artist who learned about Golgi'smethod in 1888 (Figure 2.5). In a remarkable series of publications over the next 25 years, Cajal used the Golgi stain to work out the circuitry of many regions of the brain (Figure 2.6). Ironically, Golgi and Cajal drew completely oppositeconclusions about neurons. Golgi championed the view that the neurites of different cells are fused together to form a continuous reticulum, or network, similar to the FIGURE2.5 arteries and veins of the circulatory system. According to this reticular theory, SantiagoRam6n y Cajal (1852-1934). the brain is an exception to the cell theory, which statesthar the individual (Source:Finger: |994, Fig.3.26.) cell is the elementary functional unit of all animal tissues.Cajal, on the other hand, argued forcefully that the neurites of different neurons are not continuous with one another and must communicateby contact,not continuity. This idea that the neuron adhered to the cell theory came to be known as the neuron doctrine. Although Golgi and Cajal shared the Nobel Prize in 1906,they remained rivals to the end. The scientific evidence over the next 50 years weighed heavily in favor of the neuron doctrine, but final proof had to wait until the development of the electronmicroscope in the 1950s(Box 2.1). With the increasedre- solving power of the electron microscope, it was finally possible to show that the neurites of different neurons are not continuous with one another. Thus, our starting point in the exploration of the brain musr be the individual neuron.

FIGURE2.6 One of Cajal's many drawings of brain circuitry. The letters labelthe differente e- mentsCaiai identifled in an areaof the human cerebralcortex that controlsvoluntary movement.Wewill learnmore aboutthis part of ihe hrrin n Chanrer l4 r Snr rrrc DcFclinc andJones, |998 Fig90 ) 28 CHAPTER2 NEURONSAND GLIA

Advances in Microscopy

The humaneye can distinguish two pointsonly if they are theoreticallimit imposedby the propertiesof microscope separatedby more than about one-tenthof a millimeter lensesand visible light.With the standardlight micro- (100 pm).Thus,we can saythat 100 pm is nearthe limit scope,the limit of resolutionis about 0.I pm. However, of resolutionfor the unaidedeye. Neurons havea diame- the spacebetween neurons measures only 0.02 pm (20 ter of about 20 pm, and neuritescan measureas smallas nm). No wonder two esteemedscientists, Golgi and Ca- a fraction of a micrometer.The light microscope,there- jal, disagreedabout whether neuriteswere continuous fore, was a necessarydevelopment before neuronalstruc- from one cell to the next.Thisquestion could not be an- ture could be studied.But this type of microscopyhas a swered until the electron microscopewas developedand appliedto biologicalspecimens, which occurred only within the past 60 years or so. The electron microscopeuses an electron beam in- stead of light to form images,dramatically increasing the resolvingpower. The limit of resolutionfor an electron microscopeis about 0.I nm-a milliontimes better than the unaidedeye. Our insightsinto the fine structure of the inside of neurons-the ultrostructure-haveall come from electronmicroscopic examination of the brain. Today,microscopes on the leadingedge of technology use laserbeams to illuminatethe tissueand computers to createdigital images (Figure A). Unlikethe traditional methods of light and electron microscopy,which re- quire tissue fixation,these new techniquesgive neuro- FIGUREA scientiststheir first chanceto Deer into brain tissue A lasermicroscope and corrputen (Source: O 7r'npus.) that is still alive.

W THE PROTOTYPICAL NEURON As we have seen, the neuron (also called a neryecel/) consists of several parts: the soma, the dendrites, and the axon. The inside of the neuron is separatedfrom the outside by the limiting skin, the neuronalmembrane, which lies like a circus tent on an intricate internal scaffolding,giving each part of the cell its specialthree-dimensional appearance.Let's explore the inside of the neuron and learn about the functions of the different parts (Figure2.7).

The Soma We begin our tour at the soma, the roughly spherical central part of the neuron. The cell body of the typical neuron is about 20 pm in diameter. The watery fluid inside the cell, called the cytosol, is a salty, potassium- rich solution that is separatedfrom the outside by the neuronal membrane. Within the soma are a number of membrane-enclosed structures called organelles. The cell body of the neuron contains the same organellesthat are found in all animal cells. The most important ones are the nucleus, the rough endoplasmic reticulum, the smooth endoplasmic reticulum, the Golgi apparatus, and the mitochondria. Everything contained within the confines THE PROTOTYPICALNEURON

,.1 'l

\. ll

Mitochondrion

:! \\r' . Nucreus ' '^>*\r,rks"i. ,),/ RoughER a.- $pffisk\'Poryribosomes ^ X't., 1l'\N'' -, l.oo'ot"*"'-,-,,-) ')lt*t-JuyT-- \-\ t----''/-' t)"i-./i,.*'.---N**Y ,j,l' *r-'r< j 't-.-- t o/- \v t r'-Ty -.,' N, i-'ttry {/-a _ ./ z_--1 / ,_ f ,_/- -- 1' ffis;<=-._,]_D;,, -11--".,:.ih' --)-{, {, smoothER // a f; '-,,-\/---7\\- ,/' l/1 \ \-{.- ;^5 'll/A,xon / ,. ( tl ./ hittockl-J\ lJ \ I i ' ----'^-''--\ {'-,\ ,\ ( V/;,)f Microtubub. tlr--,ru . ,-), / f- / il )\\\"\\ 't--/'4i/6'\ 'l li-*"" I / \ \\ ,litli FTGURF2 / The internal structure of a typical neuron. 30 CHAPTER 2 . NEURONSANDGLIA

of the cell membrane,including the organellesbut excluding the nucleus, is referredto collectivelyas the cytoplasm.

The Nucleus. Its name derived from the Latin word for "nut," the nucleus ol the cell is spherical,centrally located, and about 5-10 pm across.It is contained within a double membrane called the nuclearenvelope. The nuclear envelopeis perforatedby pores that measureabout 0.1 pm across. Within the nucleus are chromosomes, which contain the genetic material, DNA (deoryribonucleic acid). Your DNA was passedon to you from your parents,and it containsthe blueprint for your entire body. The DNA in each of your neurons is the same,and it is the sameas the DNA in the cellsof your liver and kidney. What distinguishesa neuron from a liver cell are the specificparts of the DNA that are used to assemblethe cell. These segmentsof DNA are calledgenes. Each chromosomecontains an uninterrupted double-strandedbraid of DNA, 2 nm wide. If the DNA from the 46 human chromosomeswere laid out straight, end to end, it would measure more than 2 m in length. If we were to consider this total length of DNA as analogousto the string of letters that makes up this book, the geneswould be analogousto the individual words. Genescan measureanywhere from 0.1 pm to several micrometersin length. The "reading" of the DNA is known as gene expression. The final product of gene expressionis the synthesisof moleculescalled proteins, which exist in a wide variety of shapesand sizes,perform many different functions, and bestow upon neurons virtually all of their unique characteristics.Protein synthesis, the assemblyof protein molecules,occurs in the cytoplasm.Because the DNA never Ieavesthe nucleus,there must be an intermediary that carries the genetic messageto the sites of protein synthesisin the cytoplasm. This function is performed by another long moleculecalled messenger ribonucleic acid, or mRNA. MessengerRNA consistsof four different nucleic acidsstrung togetherin various sequences to form a chain.The detailedsequence of the nucleicacids in the chain rep- resentsthe information in the gene,just as the sequenceof letters gives meaning to a written word. The processof assemblinga piece of mRNA that containsthe information of a geneis calledtranscription, and the resultingmRNA is calledthe transcript(Figure 2.8a). Protein-codinggenes are flanked by stretchesof DNA that are not usedto encodeproteins but are important for regulating transcription.At one end of the gene is the promoter, the region where the RNA-synthesizingenzyme, RNApolymerase, binds to initiate transcription. The binding of the polymeraseto the promoter is tightly regulatedby other proteins called transcription factors. At the other end is a sequence of DNA calledthe terminatorthat the RNA polymeraserecognizes as the end point for transcription. In addition to the non-codingregions of DNA that flank the genes,there are often additional stretchesof DNA within the gene itself that cannot be used to code for protein. Theseinterspersed regions are called introns,and the coding sequencesare called exons.Initial transcripts contain both introns and exons,but then, by a processcalled RNA splicing, the introns are removed and the remaining exons are fused together (Figure2.8b). In some cases,specific exons are also removed with the introns, leaving an "alternatively spliced" mRNA that actually encodesa different protein. Thus, transcriptionoI a single gene can ultimately give rise to severaldif- ferent mRNAs and protein products. MessengerRNA transcriptsemerge from the nucleus via pores in the nuclear envelopeand travel to the sitesof protein synthesiselsewhere in v rHE PRororYPtcALNEURoN 3 |

FIGURE2.8 Gene transcription. (a) RNA moleculesare synthesized by RNA poylmeraseand then processedinto messenger RNAto carrythe geneticinstructions for proteinassembly from the nucleusto the cytoplasm.(b)Transcription is initi- atedat the promoterregion ofthe geneand stopped at the terminatorregion.The initial RNA mustbe splicedto removethe intronsthat do n.t .odc for nrotcin

Promotor Terminator

I Exon1 O I------T------rranscriPtionI

@ ------T------| RNAprocessing I mRNA - transcript

Cytoplasm

(a)

the neuron. At these sites,a protein molecule is assembledmuch as the mRNA molecule was: by linking together many small molecules into a chain.In the caseof protein, the building blocksare amino acids, of which there are 20 different kinds. This assemblingof proteins from amino acids under the direction of the nRNA is calledtranslation. The scientificstudy of this process,which beginswith the DNA of the nucleusand endswith the synthesisof protein moleculesin the cell, is known as molecularbiology. The "central dogma" of molecular biology is summa- rized as follows:

*"tttttnttot, Ttanslation, DNA 'RNA protein A new field within neuroscienceis calledmolecular neurobiology. Molecu- lar neurobiologistsuse the information containedin the genesto determine the structureand functions of neuronal proteins (Box 2.2).

Rough Endoplasmic Reticulum. Not far from the nucleus are enclosed stacksof membrane dotted with denseglobular structurescalled ribosomes, 32 CHAPTER2 NEURONSAND GLIA

Expressing One's Mind in the Post-GenomicEra

Sequencingthe humangenome-the entire lengthof DNA that comprisesthe geneticinformation in our chromosomes-was a truly monumental achieve- ment,completed in 2003.TheHuman Genome Pro- ject identifedall of the approximately20,000 genes in humanDNA.We now livein what hasbeen called the "post-genomic era," in which informationabout the genesexpressed in our tissuescan be usedto diag- noseand treat diseases.Neuroscientists are now us- ingthis information 0 to tacklelong-standing questions about the biologicalbasis of neurologicaland psychi- atric disorders,as well as to probe deeper into the originsof individuality.Thelogic goes as follows.The Vial of mRNA Vialof mRNA brain is the product of the genesexpressed in it. trombrain 1, frombrain 2, Differencesin gene expressionbetween a normal labeledred labeledgreen brainand a diseasedbrain, or a brainof unusualability, can be used to identifythe molecularbasis of the \,/ observedsymptoms or traits, Mix applied to DNA rnioroarray The levelof gene expressionis usuallydefined as Spot of synthetic the number of mRNA transcriptssynthesized by DNAwith gene- Y/ specificsequence differentcells and tissuesto direct the synthesisof specificproteins.Thus,the analysis of geneexpression v /", requires a method to compare the relative abun- danceof variousmRNAs in the brainsof two groups 7ta - ,;\ of humansor animals.One way to per{orm such a ,//\ .' i,' \ comparisonis to use DNA microorroys,which are ,/ /--" \ Microscopic Gene with Gene with Genewith slide created by robotic machinesthat arrangethousands reduced equivalent reduced of small spots of syntheticDNA on a microscope expression expression expression slide.Each spot containsa uniqueDNA sequence in brain2 in both in brain1 brains that will recognizeand stick to a differentspecific mRNA sequence.Tocompare the geneexpression in FIGUREA two brains,one begins by collectinga sample of Profilingdifferences in gene expression mRNAs from eachbrain.The mRNA of one brain is labeledwith a chemicaltag that fluorescesgreen, and the mRNA of the other brain is labeledwith a tag that fluorescesred.These samples are then applied to the microarray. Highly expressed genes will producebrightly fluorescent spots, and differencesin the relativegene expression between the brainswill be revealed by differencesin the color of the fluorescence (Figure A). T' THE PROTOTYPICALNEURON 33

which measure about 25 nm in diameter. The stacksare called rough endoplasmic reticulum, or rough ER (Figure 2.9). Rough ER abounds in neurons, far more than in glia or most other non-neuronal cells.In fact, we have already been introduced to rough ER by another name: Nissl bodies.This organelleis stained with the dyes that Nissl introduced 100 yearsago. Ror-rghER is a major site of protein synthesisin neurons. RNA transcripts bind to the ribosomes, and the ribosomes translate the instructions contained in the mRNA to assemblea protein molecule.Thus, ribosomestake raw material in the form of amino acidsand manufacture proteins using the blueprint provided by the mRNA (Figure2.10a). Not all ribosomesare attachedto the rough ER. Many are freely floating and are calledfree ribosomes.Several free ribosomes may appear to be at- tachedby a thread; theseare calledpolyribosomes. The thread is a single strand of mRNA, and the associatedribosomes are working on it to make multiplecopies of thc sameprotein. What is the differencebetween proteins synthesizedon the rough ER Nuclear enverope and those synthesizedon the free ribosomes?The answer appearsto lie in the intended fate of the protein molecule.If it is destinedto residewithin Nuclear pore the cytosol of the neuron, then the protein's mRNA transcriptshuns the ribosomesof the rough ER and gravitatestoward the free ribosomes.

RoughER Ribosomes FIGURE2,9 Roughendoplasmic reticulum, or rough ER.

6Plfrltr-

Free / Rough / T

/ mRNAbeing H:4,":'wr translaled

FIGURE2. IO -@ Protein synthesis on a free ribosome and on rough ER. lYessengerRNA (mRNA) bindsto a ribosome,nitiating protein synthesrs.(a) Proteinssynthesized on Newlysynthesized free ribosomesare destinedfor the cytosol, membrane-associated (b) Proteinssynthesized on the rough ER orotein are desti^ecto be e.closedby or inserted rntothe membrane,lYembrane-associated "neffbfane (a) Protein synthesis on a free (b) Protein synthesis on rough ER nrnleinc :rp i^

FIGURE2.II RoughER The Golgi apparatus.This complex organellesorts newly synthesized proteins for deliveryto appropriatelocations in the neuron.

However,if the protein is destinedto be insertedinto the membraneof the cell or an organelle,then it is synthesizedon the rough ER. As the protein is being assembled,it is threadedback and forth through the membraneof the rough ER, where it is trapped(Figure 2.10b). It is not surprisingthat neurons are so well endowed with rough ER, because,as we shall see in later chapters,special membrane proteins are what give thesecells their remarkableinformation-processing abilities.

Smooth Endoplasmic Reticulum and the Golgi Apparatus. The re- mainder of the cytosolof the soma is crowded with stacksof membranous organellesthat look a lot like rough ER without the ribosomes,so much so that one type is calledsmooth endoplasmic reticulum, or smooth ER. Smooth ER is actually quite heterogeneousand performs different functions in different locations.Some smooth ER is continuous with rough ER and is believedto be a site where the proteins that jut out from the membrane are carefully folded, giving them their three-dimensionalstructure. Other types of smooth ER play no direct role in the processingof protein moleculesbut insteadregulate the internal concentrationsof substances ;:;"1 ".! such as calcium. (This organelleis particularly prominent in muscle cells, .: ti where it is calledsarcoplasmic reticulum, as we will seein Chapter 13.) i:,j The stackof membrane-encloseddisks in the somathat lies farthestfrom i:li the nucleusis the Golgi apparatus, first describedin 1898 by Camillo i:i Golgi (Figure2.ll). Thisis a siteof extensive"post-translational" chemical i{ processingof proteins. One important function of the Golgi apparatusis ,i:i.rrl l :,,,1 believedto be the sorting of certain proteins that are destinedfor delivery .\ ,.1 to parts lt',1 different of the neuron, such as the axon and the dendrites. ij;i ---*- The Mitochondrion. Another if.I Fyrut6l very abundant organellein the somais the i1-' lacid I mitochondrion (plural: mitochondria).In neurons,these sausage-shaped i.'ir \\\ \\\ \ structuresmeasure about I pm in length. Within the enclosureof their 'i!-.i \\-:- ProteinI Dietaryand stored outer membraneare multiple folds of inner membranecalled cristae (singu- i$l .r",tr9"t energysources i,:'1 J Iar: crista).Between the cristaeis an inner spacecalled matrix (Figure 2.12a). H] i{1 1,.1 (b) Mitochondria are the site of cellularrespiration (Figure 2.12b). When a il.l "inhales," *r:l mitochondrion it pulls inside pyruvic acid (derived from sugars !:{: if.l:.i FIGURE2.I2 and digestedproteins and fats)and oxygen,both of which are floatingin the The role of mitochondria. (a) Compo- cytosol.Within the inner compartmentof the mitochondrion,pyruvic acid nentsof a mitochondrion,(b) Cellularrespira- F.i tion.ATP is the energycurrency that fuels entersinto a complex seriesof biochemicalreactions called the l(rebsqtcle, \i ,.1 biochemicalreactions in neurons. named after the German-Britishscientist Hans Krebs,who first proposedit i,i1 $* -hi"1 ;''' V THE PROTOTYPICALNEURON 35 in 1937.The biochemicalproducts of the Krebs cycle provide energy that, in another seriesof reactionswithin the cristae(called the electron-transport chain), resultsin the addition of phosphateto adenosinediphosphate (ADP),yielding adenosine triphosphate (ATP), the cell'senergy source. When the mitochondrion "exhales,"17 ATP moleculesare releasedfor every molecule of pyruvic acid that had been taken in. ATP is the energycurrenq) of the cell.The chemical energy stored in ATP is usedto fuel most of the biochemicalreactions of the neuron. For example, as we shall see in Chapter 3, specialproteins in the neuronal membrane use the energy releasedby the breakdown of ATP into ADP to pump certain substancesacross the membraneto establishconcentration differences between the inside and the outside of the neuron.

The Neuronal Membrane The neuronal membrane servesas a barrier to enclosethe cytoplasminside the neuron and to excludecertain substancesthat float in the fluid that bathes the neuron. The membraneis about 5 nm thick and is studdedwith proteins. As mentioned earlier, some of the membrane-associatedproteins pump sub- stancesfrom the inside to the outside. Othersform poresthat regulatewhich substancescan gain accessto the inside of the neuron. An important char- acteristicof neuronsis that the protein compositionof the membranevaries dependingon whether it is in the soma,the dendrites,or the axon. Thefunction of neuronscannot be understoodwithout understandingthe structure and function of the membraneand its associatedproteins. In fact, this topic is so important that we'll spenda good deal of the next four chapterslook- ing at how the membraneendows neurons with the remarkableability to transferelectrical signals throughout the brain and body.

The Cytoskeleton Earlier,we comparedthe neuronal membraneto a circustent that was draped on an internal scaffolding.This scaffoldingis calledthe cytoskeleton, and it givesthe neuron its characteristicshape. The "bones"of the cytoskeleton are the microtubules,microfilaments, and neurofilaments(Figure 2.13). By drawing an analogywith a scaffolding,we do not mean that the cytoskeleton is static.On the contrary,elements of the cytoskeletonare dynamically regulatedand are very likely in continual motion. Your neurons are probably squirming around in your head even as you read this sentence.

Microtubules. Measuring 20 nm in diameter,microtubules are big and run longitudinally down neurites. A microtubule appearsas a straight, thick-walled hollow pipe. The wall of the pipe is composedof smaller strandsthat are braided like rope around the hollow core. Each of the smallerstrands consists of the protein tubulin.A singletubulin molecule is small and globular;the strand consistsof tubulins stuck togetherlike pearls on a string. The processof joining small proteins to form a long strand is calledpolymerization; the resulting strand is called a polymer.Polymerization and depolymerizationof microtubules and, therefore, of neuronal shape can be regulatedby various signalswithin the neuron. One classof proteinsthat participatein the regulationof microtubule as- sembly and function are microtubr,lle:associatedproteins, or MAPs. Among other functions (many of which are unknown), MAPs anchor the micro- FIGURE2.I3 parts Components of the cposkeleton. tubules to one another and to other of the neuron. Pathological The arrangementof microtubules,neuroflla- changesin an axonal MAP, called tau, have been implicated in the de- ments,and microfilaments gives the neuron mentia that accompaniesAlzheimer's disease (Box 2.3). its characterislicshape. 36 CHAPTER2 NEURONSANDGLIA

Alzheimer's Diseaseand the Neuronal Cytoskeleton

Neurites are the most remarkablestructural feature of a Followingher death,Alzheimerexamined the woman's neuron.Their elaboratebranching patterns, critical for brain under the microscope.He made particularnote of "neurofibrilsJ' information processing,reflect the organizationof the changesin the elementsof the cytoskele- underlyingcytoskeleton. lt is therefore no surprisethat ton that are stainedby a silversolution. a devastatingloss of brain function can result when the The Bielschowskysilver preparationshowed very cytoskeleton of neurons is disrupted,An example is characteristicchanges in the neurofibrils.However, Alzheimer'sdiseose, which is characterizedby the disrup- insidean apparentlynormal-looking cell, one or more tion of the cytoskeletonof neurons in the cerebral cor- singlefibers could be observedthat becameprominent tex, a region of the brain crucial for cognitive function. through their striking thicknessand specificimpreg- This disorder and its underlyingbrain pathologywere nability.At a more advancedstage, many fibrils arranged first described in | 907 by the German physicianA. "A parallelshowed the samechanges.Then they accumu- Alzheimerin a papertitled CharacteristicDisease of latedforming dense bundles and gradually advanced to the Cerebral Cortex." Below are excerpts from the Eng- the surfaceof the cell.Eventually, the nucleusand lishtranslation. cy- toplasmdisappeared, and only a tangledbundle of fibrils indicatedthe sitewhere oncethe neuronhad been One of the first diseasesymptoms of a 5l-year-old located. woman was a strong feelingof jealousytoward her As these fibrils can be stainedwith dyes different husband.Very soon she showed rapidly increasing from the normal neurofibrils,a chemicaltransforma- memoryimpairments; she could not find her wayabout tion of the fibril substancemust havetaken olace.This her home, she draggedoblects to and fro, hid herself, might be the reasonwhy the fibrils survivedthe de- or sometimesthought that peoplewere out to kill her, structionof the cell.lt seemsthat the transformation then she would start to scream loudly. of the fibrils goes hand in hand with the storageof an During institutionalizationher gesturesshowed a as yet not closelyexamined pathological product of completehelplessness. She was disorientedas to time the metabolismin the neuron.About one-quarterto and place.From time to time shewould statethat she one-third of all the neurons of the cerebral cortex did not understandanything, that shefelt confusedand showedsuch alterations. Numerous neurons, especially totally lost.Sometimes she consideredthe comingof in the upper cell layers,had totally disappeared.(Bick the doctor as an official visit and apologizedfor not et al.,1987, pp. 2-3.) havingfinished her work, but other times she would start to yell in the fear that the doctor wanted to op- The severityof the dementiain Alzheimer'sdisease is erateon her;or there were timesthat shewould send well correlatedwith the numberand distributionof what him awayin completeindignation, uttering phrases that are now commonly known as neurofibrillorytongles, the "tombstones" indicatedher fear that the doctor wanted to damage of dead and dying neurons(Figure A). In- her woman'shonor. From time to time she was com- deed,as Alzheimer speculated,tangle formation in the pletelydelirious, dragging her blanketsand sheetsto cerebral cortex very likely causesthe symptoms of the andfro,calling for her husbandand daughter,and seem- disease.Electron microscopy reveals that the major ing to haveauditory hallucinations.Often she would componentsof the tanglesare poiredhelicol filoments, long screamfor hours and hours in a horriblevoice. fibrous proteins braided together like strands of a rope Mental regressionadvanced quite steadily.Afterfour (Figure B). lt is now understood that these filaments and a half years of illnessthe patient died.She was consistof the microtubule-associatedprotein tou. completelyapathetic in the end,and was confinedto Tau normally functionsas a bridge between the micro- bed in a fetal position.(Bick et al.,1987, pp. l-2.) tubules in axons,ensuring that they run straightand V THE PROTOTYPICALNEURON 37

FIGUREA Neuronsin a humanbrain with Alzheimers disease. Normal neurons contain neurofllaments but no neurofibrillarytangles. (a) Brain tissuestained by a methodthat makesneuronal neurofilaments fluoresce gr.een, showing viable neurons. (b)The sameregion of the brainstained to showthe pr^esenceof tau withinneurofibrillary tangles, revealed by red fluorescence.(c) Superimpositionof images in partsa andb.The neuron indicated by the arrowheadcontains neurofilaments but no tanglesand, therefore, is healthyTheneuron indicatedbythe largearrow has neurofilaments but alsohas started to showaccumulation of tau and,therefore,is diseased,The neuronindicated by the smallarrow in partsb andc is deadbecause it containsno neurofilaments.Theremaining tangle is the tomb- stoneof a neuronkilled byAlzheimer's disease. (Source: Courtesy of Dr:JohnMorrison and modifled fromVickers et al.,1994.)

parallelto oneanother. In Alzheimer's disease, the tau de- diseaseresearch moves very fast,but the consensustoday tachesfrom the microtubulesand accumulatesin the is that the abnormalsecretion of amyloidby neuronsis soma.This disruptionof the cytoskeletoncauses the the first step in the processthat leadsto neurofibrillary axonsto withenthus impedingthe normalflow of infor- tangleformation and dementia.Current hope for thera- mationin the affectedneurons. peuticintervention is focusedon strategiesto reducethe What causesthe changesin tau?Attention is focused depositionsof amyloidin the brain.Theneed for effective on anotherprotein that accumulatesin the brain of therapyis urgentIn the UnitedStates alone, more than 4 Alzheimer'spatients, calle d omyloid.Thefield of Alzheimer's millionpeople are afflictedwith this tragicdisease.

100nm FIGUREB Pairedhelical filaments of a tangle.(Source. Goedert1996,Fig.2b.) 38 CHAPTER 2 . NEURONSANDGLIA

Microfilaments. Measuring only 5 nm in diameter, microfilaments are about the same thickness as the cell membrane. Found throughout the neuron, they are particularlynumerous in the neurites.Microfilaments are braids of two thin strands,and the strandsare polymers of the protein actin. Actin is one of the most abundant proteins in cells of all types, including neurons, and is believedto play a role in changing cell shape.Indeed, as we shall see in Chapter 13, actin filaments are critically involved in the mechanismof muscle contraction. Like microtubules, actin microfilaments are constantly undergoing as- sembly and disassembly,and this processis regulated by signals in the neuron. Besidesrunning longitudinally down the core of the neurites like microtubules, microfilaments are also closely associatedwith the membrane. They are anchored to the membrane by attachmentswith a meshwork of fibrous proteins that line the inside of the membrane like a spiderweb.

Neurofilaments. With a diameter of l0 nm, neurofilaments are inter- mediate in size between microtubules and microfilaments. They exist in all cells of the body as intermediatefilaments; only in neurons are they called neurofilaments.The differencein name actually does reflect subtle differencesin structure from one tissue to the next. An example of an intermediatefilament from another tissueis keratin, which, when bundled together,makes up hair. Of the types of fibrous structurewe have discussed,neurofilaments most closelyresemble the bonesand ligamentsof the skeleton.A neurofilament consistsof multiple subunits (building blocks) that are organized like a chain of sausages.The internal structure of each subunit consistsof three protein strandswoven together.Unlike microfilamentsand microtubules, thesestrands consist of individual long protein molecules,each of which is coiled in a tight, springlike configuration.This structure makes neurofilaments mechanicallyvery strong.

The Axon So far, we've exploredthe soma,organelles, membrane, and cytoskeleton. However,none of these structuresis unique to neurons;they are found in all the cells in our body. Now we encounter the axon, a structure found only in neurons that is highly specializedfor the transfer of information over distancesin the nervous system. The axon beginswith a region calledthe axon hillock, which tapersto form the initial segmentof the axon proper (Figure2.14). TWonoteworthy featuresdistinguish the axon from the soma: l. No rough ER extendsinto the axon, and there are few, if any, free ribosomes. 2. The protein compositionof the axon membraneis fundamentallydiffer- ent from that of the soma membrane. Thesestructural differences translate into functional distinctions.Because there are no ribosomes,there is no protein synthesisin the axon. This Axon means that all proteins in the axon must originate in the soma.And it is the differentproteins in the axonal membranethat enableit to serveas the "telegraph wire" that sendsinformation over great distances. FIGURE2.I4 Axons may extend from less than a millimeter to over a meter long. The axon and axon collaterals. The axon Axons often branch, and these branchesare called axon collaterals. Oc- functionslike a telegraphwire to send electrical impulsesto distantsites in the nervous casionally,an axon collateralwill return to communicatewith the samecell system.Thearrows indicatethe directionof that gaverise to the axon or with the dendritesof neighboringcells. These informationflow. axon branches are called recurrentcollaterals. V THE PROTOTYPICALNEURON 39

The diameter of an axon is variable, ransng from less than I mm to about 25 mm in humans and to as Iarge as I mm in the squid. This variation in axon size is important. As will be explained in Chapter 4, the speed of the electrical signal that sweepsdown the axon-the nerveimpube-vaies depending on axonal diameter. The thicker the axon, the faster the impulse travels.

The Axon Terminal. All axons have a beginning (the axon hillock), a middle (the axon proper), and an end. The end is called the axon terminal or termlnal bouton (French for "button"), reflecting the fact that it usually appearsas a swollen disk (Figure 2.15). The terminal is a site where the axon comes in contact with other neurons (or other cells) and passes information on to them. This point of contact is called the synapse, a word derived from the Greek, meaning "to fasten together." Sometimes axons have many branches at their ends, and each branch forms a synapse on dendrites or cell bodies in the same region. Thesebranches are collectively called the terminal arbor. Sometimesaxons form synapsesat swollen regions along their length and then continue on to terminate elsewhere. Such swellings are called boutonsen passant("butto'ns in passing"). In either case,when a neuron makes synaptic contact with another cell, it is said to innervate that cell, or to provide lnnervation. The cytoplasm of the axon terminal differs from that of the axon in severalways: l. Microtubules do not extend into the terminal. 2. The terminal contains numerous small bubbles of membrane, called synaptlc vesicles, that measure about 50 nm in diameter. 3. The inside surface of the membrane that facesthe synapsehas a particularly dense covering of proteins. 4. It has numerous mitochondria, indicating a high energy demand.

FIGURE2.15 The aron tennlnal and thc synapse.Axon terminalsform synapseswith the dendrites or somataof other neurons.When a nerve impulsearrives in the presynapticaxon terminal,neurotransmitter molecules are released from synapticvesicles into the synapticcleft, Neurotransmitterthen bindsto specific receptorprcteins, causing the generation of electricalor chemicalsignals in the post- synapticcell, 40 CHAPTER 2 . NEURONSANDGLIA

The Synapse. Although Chapters 5 and 6 are devoted entirely to how information is transferredfrom one neuron to another at the synapse,we provide a preview here. The synapsehas two sid.es:presynaptic and post- synaptic(see Figure 2.15). Thesenames indicate the usual direction of in- "pre" formation flow, which is from to "post." The presynapticside generally consistsof an axon terminal, while the postsynapticside may be the dendrite or soma of another neuron. The spacebetween the presynaptic and postsynapticmembranes is called the synaptic cleft. the transfer of inlormation at the synapsefrom one neuron to another is calledsynaptic transmission. At most synapses,information in the form of electrical impulses traveling down the axon is converted in the terminal into a chemical signal that crossesthe synapticcleft. On the postsynapticmembrane, this chemicalsig- nal is convertedagain into an electricalone. The chemical signal is called a neurotransmitter, and it is stored in and releasedfrom the synaptic vesicleswithin the terminal. As we will see,different neurotransmittersare usedby different types of neurons. This electrical-to-chemical-to-electricaltransformation of information makespossible many of the brain's computationalabilities. Modification of this processis involved in memory and learning,and synaptictransmission dysfunction accountsfor certain mental disorders.The synapseis also the site of action for many toxins and for most psychoactivedrugs.

Axoplasmic Tlansport. As mentioned, one feature of the cytoplasmof axons, including the terminal, is the absenceof ribosomes.Because ribosomesare the protein factoriesof the cell, their absencemeans that the proteins of the axon must be synthesizedin the soma and then shippeddown the axon. Indeed, in the mid-nineteenth century, Englishphysiologist Au- gustusWaller showedthat axonscannot be sustainedwhen separatedfrom their parent cell body. The degenerationof axons that occurs when they are cut is now called Walleriandegeneration. Because it can be detectedwith certain staining methods, Wallerian degenerationis one way to trace axonal connectionsin the brain. Walleriandegeneration occurs because the normal flow of materialsfrom the soma to the axon terminal is interrupted. This movement of material down the axon is called axoplasmic transport, first demonstrateddirectly by the experiments of American neurobiologist Paul Weiss and his col- leaguesin the I940s. They found that if they tied a thread around an axon, material accumulatedon the side of the axon closestto the soma. When the knot was untied, the accumulatedmaterial continued down the axon at a rate of 1-10 mm per day. This was a remarkable discovery,but it is not the whole story. If all material moved down the axon by this transport mechanism alone, it would not reach the ends of the longestaxons for at leasthalf a year-too long a wait to feed hungry synapses.In the late 1960s,methods were developedto track the movements of protein moleculesdown the axon into the terminal. Thesemethods entailedinjecting the somataof neurons with radioactive amino acids. Recall that amino acids are the building "hot" blocksof proteins.The amino acidswere assembledinto proteins,and the arrival of radioactive proteins in the axon terminal was measured to calculatethe rate of transport. Bernice Grafsteinof RockefellerUniversity discoveredthat this /ast axoplasmictransport (so named to distinguish it from slow axoplasmictransport described by Weiss) occurred at a rate as high as 1000 mm per day. Much is now known about how axoplasmictransport works. Material is "walk enclosedwithin vesicles,which then down" the microtubulesof the THE PROTOTYPICALNEURON 4I

FIGURE2.I6 A mechanism for the movement of material on the microtubules of the axon. Trappedtn membrane enclosedvesicles, materal s transportedfrom the somato the axontermina by the acton of the proteinkinesin, whtch "walks" alongm crotubulesat the expenseof ATP

Microtubules

"legs" axon. The are provided by a protein calledkinesin, and the processis fueledby ATP (Figure2.16). I(inesinmoves material only from the soma to the terminal. All movement of material in this direction is calledantero- grade transport. In addition to anterogradetransport, there is a mechanismfor the movement of material up the axon from the terminal to the soma.This process is believedto provide signalsto the soma about changesin the metabolic needsof the axon terminal. Movement in this direction,from terminal to soma, is called retrograde transport. The molecular mechanism is simi- "legs" lar to anterograde transport, except the for retrograde transport are provided by a different protein, dynein. Both anterograde and retrograde transportmechanisms have been exploitedby neuroscientiststo trace connectionsin the brain (Box 2.4).

Dendrites "fiee," The term dendriteis derived from the Greek for reflectingthe fact that FIGURE2.I7 Dendrites receiving synaptic inputs these neurites resemblethe branchesof a tree as they extend from the soma. from axon terminals. A neuron has been The dendrites of a single neuron are collectively called a dendritic tree; rade ro rl-orescegreen, us ng a method each branch of the tree is called a dendriticbranch. The wide variety of shapes that revealsthe distributionof a microtubule- and sizesof dendritic trees are used to classifydifferent groups of neurons. associatedprotein, Axon terminalshave been fluoresce ng 'netl^od Becausedendrites function as the antennae of the neuron, they are rnaceto orange-reo,Ls a to revealthe distributionof synapticvesicles, with thousandsof synapses(Figure 2.17). The dendriticmembrane covered The axonsand cellbodies that contribute pro- under the synapse (the postsynapticmembrane) has many specialized theseaxon term nalsare not visiblein this tein molecules called receptors that detect the neurotransmitters in the photom crograph,(Source: Neuron 0 [Suppl.], svnaDticcleft. 1993,cover image,) 42 CHAPTER 2 . NEURONSANDGLIA

Hitching a Ride With Retrograde Tfansport

Fast anterogradetransport of proteins in axons was retrogradetransport through axons in the skin.However, shown by injectingthe somawith radioactiveamino acids. once insidethe soma,the virus wastesno time in repli- The successof this method immediatelysuggested a way catingmadly, killing its neuronalhost. The virus is then to trace connectionsin the brain.For example,to deter- taken up by other neuronswithin the nervoussystem, minewhere neuronsin the eye sendtheir axons,the eye and the processrepeats itself again and again,usually until was injectedwith radioactiveproline, an amino acid.The the victim dies. proline was incorporatedinto proteins in the somatathat were then transported to the axon terminals.By use of a techniquecalled autoradiography, the location of radioactive axon terminalscould be detected,thereby revealing the fi extent of the connection between the eye and the brain. Researcherssubsequently discovered that retrograde I'm transport could also be exploitedto work out connec- 'n'"""'' Two days later, tions in the brain.Strangely enough, the enzymehorse- after retrograde f transport: radish peroxidase(HRP) is selectivelytaken up by axon terminalsand then transported retrogradelyto the soma. A chemicalreaction can then be initiatedto visualizethe u locationof the HRP in slicesof braintissue.This method is commonlyused to trace connectionsin the brain (Fig- q ure A). Some viruses also exploit retrograde transport to in- fect neurons.For example,the oral type of herpesvirus HRP entersaxon terminalsin the lips and mouth and is then deposit in brain transportedback to the parentcell bodies.Here the virus usuallyremains dormant until physicalor emotionalstress occurs (as on a first date),at which time it replicatesand HRP-labeled neurons returnsto the nerve ending,causing a painfulcold sore. Similarly,the rabies virus enters the nervous system by FIGUREA

The dendrites of some neurons are covered with specializedstructures called dendritic spines that receive some types of synaptic input. spines Iook like little punching bags that hang off the dendrite (Figure 2.18). The unusual morphology of spines has fascinated neuroscientists ever since their discovery by cajal. They are believed to isolate various chemical reactions that are triggered by some types of synaptic activation. spine structure is sensitive to the type and amount of synaptic activity. Unusual changes in spines have been shown to occur in the brains of individuars with cognitive impairments (Box 2.5). william Greenough of the Univer- sity of Illinois at Urbana discovered that spine number is also very sensitive to the quality of the environment experienced during early development and in adulthood (Box 2.6).

FIGURE2. I8 Dendritic spines.This is a computerreconstruction of a segmentof dendrite,showing the variableshapes and sizes of spines.Each spine is postsynapticto one or two axonterminals. (Source:Har-is and Stevens, 1989, cove- image,) THE PROTOTYPICALNEURON 43

Mental Retardationand Dendritic Spines

The elaboratearchitecture of a neuron'sdendritic tree is we accountfor the profound cognitiveimpairment? An a good reflectionof the complexityof its synapticconnec- imoortant clue came in the 1970sfrom the researchof tionswith other neurons.Brain function depends on these MiguelMarin-Padilla, working at Dartmouth College,and highlyprecise synaptic connections, which are formed DominickPurpura, working at the Albert EinsteinCollege duringthe fetal period and are refinedduring infancy and of Medicinein NewYork City.Using the Golgi stain,they early childhood.Not surprisingly,this very complex de- studiedthe brainsof retarded children and discovered velopmentalprocess is vulnerableto disruption.Mentol re- remarkablechanges in dendriticstructure.The dendrites tardotionis said to haveoccurred if a disruption of brain of retardedchildren had manyfewer dendriticspines, and developmentresults in subaveragecognitive functioning the spinesthat they did havewere unusuallylong and thin that impairsadaptive behavior. (FigureA).The extent of the spinechanges was well cor- The use of standardizedtests indicatesthat intelli- related with the degree of mental retardation. gencein the generalpopulation is distributedalong a Dendritic spinesare an important target of synaptic bell-shaped(Gaussian) curve. By convention,the meanin- input. Purpurapointed out that the dendritic spinesof telligencequotient (lQ) is set to be l00.About two- mentallyretarded children resemble those of the normal thirds of the total populationfalls within l5 points (one human fetus.He suggestedthat mental retardationre- standarddeviation) of the mean,and 95%of the popula- flectsthe failureof normal circuitsto form in the brain' tion fallswithin 30 points(two standarddeviations). Peo- ln the 3 decadessince this seminalwork was published' ple with intelligencescores below 70 are consideredto it hasbeen established that normalsynaptic development, be mentallyretarded if the cognitiveimpairment affects includingmaturation of the dendriticspines, depends crit- the person'sability to adapt his or her behaviorto the icallyon the environmentduring infancy and early child- "crit- settingin which he or she lives.Some 2-3% of humans hood.An impoverishedenvironment during an early fit this description. icalperiod" of developmentcan lead to profoundchanges Mental retardation has many causes.The most severe in the circuitsof the brain.However, there is some good forms are associatedwith geneticdisorders. An example news.Many of the deprivation-inducedchanges in the is a conditioncalled phenylketonurio (PKU). The basicab- braincan be reversedif interventionoccurs early enough. normalityis a deficitin the liver enzymethat metabolizes In Chapter 22,we will take a closer look at the role of the dietary amino acid phenylalanine.Infants born with experiencein brain development. PKU havean abnormallyhigh levelof the amino acid in the blood and brain.lf the conditiongoes untreated, brain growth is stunted and severe mental retardation results. Another exampleis Downsyndrome, which occurswhen the Dendritefrom a normalinfant fetushas an extra copy of chromosome21, thus disrupt- ing normalgene expression during brain development. A secondknown causeof mental retardationis acci- dents during pregnancyand childbirth.Examples are maternalinfections with German measles(rubella) and asphyxiaduring childbirth. A third causeof mentalre- tardation is poor nutrition during pregnancy.Anexam- ple is fetol olcoholsyndrome, a constellationof develop- mental abnormalitiesthat occur in children born to alcoholicmothers. A fourth cause,thought to account for the ma.jorityof cases,is environmentalimpoverish- ment-the lack of good nutrition,socialization, and sensory stimulation-during infancy. While some forms of mental retardation have very clear physicalcorrelates (e.9., stunted growth; abnormali- 10pm ties in the structureof the head,hands, and body),most caseshave only behavioralmanifestations. The brainsof FIGUREA these individualsappear grossly normal. How, then, do Normaland abnormal dendrrtes 44 CHAPTER2 NEURONSAND GLIA

Spines and the Structural Basis of Memory

by William Greenough

It is a frequent misconceptionthat scientificresearch re- us to study brain abnormalitiesin fragileX syndrome,the sults in simple,clear answersto questions.The truth is largestcause of inherited mental retardation. "memory that almost every answer results in a whole battery of Do we now,at last,have a protein"l Not yet, new questions.But the researchserves to increaseour becauseFMRP is not a structural or receptor protein,but understandingso that we know how to frame the new a protein that binds mRNA. lt seemsto orchestrate the questions,and try to tackle them. translation of an impressivearray of other mRNAs, few "memory When we first saw polyribosomesbelow synapsesin of which,at first glance,look like proteins."But, "What dendritic spines,we asked: are they doing away if FMRPis a key to how the synapsechanges itself, then from the soma,where mRNA is usuallytranslatedl" An we think we are still on the right track to understand exciting possibilitywas that they might make special synapticremodeling. Indeed, people with fragileX syn- proteinsthat were involvedin remodelingthe synapse. drome haveimmature-appearing synapses. I had seen that in animalsexposed to enriched envi- So we have been wrong a lot of the time, and frus- ronments,new spineswere madeand other spineswere trated on a near-weeklybasis. Nature is full of surprises, remodeledfrom small to larger structures with-pre- but we haveincreased our understandingof synapticpro- sumably-more efficientsignaling. Could these synaptic tein synthesis.Indeed, we are now askingnew questions polyribosomesbe manufacturingcrucial proteins that that may help us understandand possiblytreat the lead- could account for the improved efficiency?Some kind ing inheritedform of mental retardation.lt "memory was worth the of proteins"? effort. I recalleda paper by Hollingsworth,in which he figured out a way to shear off synapseson spines,a preparation calledsynaptoneurosomes (Figure A), and study them bio- chemically.My colleaguelvan JeanneWeiler and I at- LJ tempted to isolatethe mRNA in synaptoneurosomesto qF "memory identifythe crucial proteins."Repeated failures, due apparentlyto mRNA degradation,both frustrated and slowed us. But eventuallywe found that some of the mRNA was beingincorporated into polyribosomes,meaning that protein was being synthesized.We then discovered that this protein synthesiswas strongly increased when we stimulatedthe synaptoneurosomeswith the neurotransmitterglutamate, and we were Synaptic .: able to identify polyribosome,., the glutamatereceptor that was responsible.We now comprex "memory thought we were poised to isolate the proteins" from the newly assembledpolyribosomes. We set up to harvestpolyribosomes from stimulatedand unstimulatedsynaptoneurosomes to see which mRNAs were translatedin responseto glutamate.Aboutthis time, Jim Eberwine(University of Pennsylvania)told us about the 'rt mRNAs he had isolatedfrom singledendrites. We looked s$ for their translationin our new system,and the first one FIGUREA that was increasedby stimulation turned out to be the Electronmicrograph of a synaptoneurosome.Thespine is fragileX mental retardation protein (FMRP).Thishas led about I micronin diameten(Courtesy of WilliamGreenough.) V CLASSIFYINGNEURONS 45

For the most part, the cytoplasm of dendrites resemblesthat of axons. It is filled with cytoskeletal elements and mitochondria. One interesting dif- ferencewas discoveredby neuroscientistOswald Steward.He found that polyribosomescan be observedin dendrites,often right under spines.Stew- ard's research suggeststhat synaptic transmission can actually direct local protein synthesisin some neurons. In Chapter 25,we will seethat synaptic regulation of protein synthesisis crucial for information storageby the brain.

V CLASSIFYINGNEURONS It is unlikely that we can ever hope to understand how each of the hun- dred billion neurons in the nervous system uniquely contributes to the function of the brain. But what if we could show that all the neurons in the brain can be divided into a small number of categories,and that within each category,all neurons function identically? The complexity of the prob- lem might then be reduced to understanding the unique contribution of each category,rather than each cell. It is with this hope that neuroscientists have devised schemesfor classifyingneurons.

Classification Based on the Number of Neurites Neurons can be classifiedaccording to the total number of neurites (axons and dendrites)that extend from the soma (Figure2.19). A neuron that has a single neurite is said to be unipolar. If there are two neurites, the cell is bipolar, and if there are three or more, the cell is multipolar. Most neurons in the brain are multipolar.

Classification Based on Dendrites Dendritic trees can vary widely from one type of neuron to another. Some "double have inspired elegantnames like bouquet cells." Others have less interesting names, such as "alpha cells." Classificationis often unique to a particular part of the brain. For example, in the cerebral cortex (the structure that lies just under the surface of the cerebrum), there are two broad classes:stellate cells (star-shaped)and pyramldal cells (pyramid-shaped) (Figure2.20).

Unipolar

Bipolar FIGURE2.I9 Classification of neurons based on the number of neurites. 46 CHAPTER 2 . NEURONSANDGLIA

Another simple way to classifyneurons is according to whether their dendriteshave spines.Those that do are called spiny, and those that do not are calledaspinous. Thesedendritic classi{icationschemes can overlap. For example,in the cerebralcortex, all pyramidal cells are spiny. stellate cells,on the other hand, can be either spiny or aspinous.

ClassificationBased on Connections Information is deliveredto the nervous systemby neurons that have neurites in the sensorysurfaces of the body, such as the skin and the retina of the eye. cells with these connectionsare called primary sensory neurons. other neurons have axons that form synapseswith the musclesand command movements;these are calledmotor neurons. But most neurons in the nervous systemform connectionsonly with other neurons. According to this classificationscheme, these cellsare all calledinterneurons.

Classification Based on Axon Length Some neurons have long axons that extend from one part of the brain to the otheu theseare calledGolgi type I neurons,or projection neurons. other I'rf'" neuronshave short axonsthat do not extendbeyond the vicinity of the cell body; these are called Golgitype II neurons,or local circuit neurons. In the cerebralcortex, for example,pyramidal cellsusually have long axons that extend to other parts of the brain and are therefore Golgi type I neurons. In contrast,stellate cells have axonsthat never extend beyond the cerebral cortex and are therefore Golgi type II neurons.

Classification Based on Neurotransmitter The classificationschemes presented so far are basedon the morphology of neurons as revealedby a Golgi stain. Newer methods that enable neuroscientiststo identify which neurons contain specificneurotransmitters have resultedin a schemefor classifyingneurons basedon their chemistry.For example,the motor neurons that command voluntary movements all release the neurotransmitter acetylcholineat their synapses.These cells are therefore also classified,as cholinergic,meaning that they use this particular neurotransmitter.collections of cellsthat use a common neurotransmittermake up the brain's neurotransmittersystems (see Chapters 6 and I5).

V GLIA we have devoted most of our attention in this chapter to the neurons. while this decisionis justified by the stateof current knowredge,some neu- "sleeping Y roscientistsconsider glia to be the giants" of neuroscience.one day, they suppose,it will be shown that glia contribute much more impor- tantly to information processingin the brain than is currently appreciated. At present,however, the evidenceindicates that glia contribute to brain function mainly by supporting i neuronal functions. Although their role may be subordinate,without glia, \ the brain could not function properly. Pyramidalcell

FIGURE2.20 Astrocytes Classification of neurons based on The most numerous glia in the brain dendritic tree structure. Stellatecells are called astrocytes (Figure 2.21). andpyramidal cells, distinguished by the Thesecells fill the spacesbetween neurons. The spacethat remains between arrangementof their dendrites,are two types the neurons and the astrocytesin the brain measuresonly about 20 nm of neuronsfound in the cerebralcortex. wide. consequently,astrocytes probably influence whether a neurite can ,,bathing,, grow or retract. And when we speakof fluid neurons in the brain, V GLIA 47 it is more like a soakingfrom a hose than immersion in a swimming pool. An essentialrole of astrocytesis regulatingthe chemicalcontent of this extracellularspace. For example,astrocytes envelop synaptic junctions in the -...:ry brain, thereby restricting the spread of neurotransmitter molecules that have been released.Astrocytes also have specialproteins in their mem- branes that actively remove many neurotransmitters from the synaptic cleft. A recent and unexpecteddiscovery is that astrocyticmembranes also possessneurotransmitter receptors that, like the receptorson neurons, can trigger electricaland biochemicalevents inside the glial cell. Besidesregu- lating neurotransmitters,astrocytes also tightly control the extracellular concentrationof severalsubstances that have the potentialto interferewith proper neuronal function. For example,astrocytes regulate the concentra- tion of potassiumions in the extracellularfluid. FIGURE2.2I An astrocyte. Astrocytesfill most of the spacein the brainthat is not occupiedby MyelinatingGlia neuronsand blood vessels. Unlike astrocytes,the primary function of oligodendroglial and Schwann cells is clear.These glia provide layers of membrane that insulate axons. Boston University anatomistAlan Peters,a pioneer in the electron micro- scopicstudy of the nervous system,showed that this wrapping, called myelin, spiralsaround axonsin the brain (Figure2.22\.Becutse the axon fits inside the spiralwrapping Iike a sword in its scabbard,the name myelin sheathdescribes the entire covering.The sheathis interrupted periodically, Ieavinga short length where the axonal membraneis exposed.This region is calleda node of Ranvier (Figure2.23).

FIGURE2.22 Myelinated optic nerve fibers cut in cross section. (Source:Courtesy of DnAlan Peters,)

Oligodendroglial cells

Myelin sheath

FIGURE2.23 An oligodendroglial cell. Likethe Schwanncells found in the nervesof the body,oligodendroglia provide myelinsheaths around axons in the brain and spinalcord. Cytoplasmof Node of The myelinsheath of an axon is interrupted oligodendroglialcell Ranvier Mitochondrion oeriodicallyat the nodes of Ranvier: CHAPTER2 NEURONSAND GLIA

we will see in Chapter 4 that myelin servesto speed the propagation of nerve impulses down rhe axon. oligodendroglia and Schwann cells differ in their location and some clther characteristics.For example, oligodendroglia are lound only in the central nervous system (brain and spinal cord), while Schwann cells are found only in the peripheral nervous system (parts outside the skuil and vertebral column). Another differenceis that one oligodendroglialcell will contribute myelin to severalaxons, while each Schwann cell myelinates only a single axon.

Other Non-Neuronal Cells

Even if we eliminated every neuron, every astrocyte,and every oligodendroglial cell, other cellswould still remain in the brain. For the sake of com- pleteness,we must mention these other cells. First, special cells called ependymal cells provide the lining of fluid-filled venrricleswithin the brain, and they also play a role in directing cell migration during brain development. secclnd,a classof cells called microglia function as phagocytes to remove debris left by dead or degeneratingneurons and glia. Finally, the vasculatureof the brain-arteries, veins, and capillaries-would still be there.

I CONCLUDING REMARKS

Learningabout the structuralcharacteristics of the neuron providesinsight into how neurons and their different parts work, becausestructllre corre- lates with function. For example, the absenceof ribosomesin the axon correctlypredicts that proteins in the axon terminal mr-rstbe provided by the soma via axoplasmictransport. A large number of mitochondria in the axon terminal correctlypredicts a high energy demand. The elaborate structure of the dendritic tree appearsideally suited for the receipt of incoming information, and indeed, this is where most of the synapsesare formed with the axons of other neunrns. From the time of Nissl,it has been recognizedthat an important feature of neurons is the rough ER. what doesthis tell us about neurons?we have said that rough ER is a site of the synthesisof proteins destinedto be insertedinto the membrane. we will now see how the various proteins in the neuronal membrane give rise to the unique capabilitiesof neurons to transmit, receive,and stitreinformation.

Introduction The Prototypical Neuron transcriptionfactor (p. 30) neuron(p.24) cytosol(p.28) RNA splicing(p.30) glialcell (p.24) organelle(p. 28) aminoacid (p.31) tt1 > cytoplasm(p.30) translation(p. 3 l) UJ: The Neuron Doctrine nucleus(p.30) ribosome(p. Y.t 3 l) histology(p.25) chromosome(p.30) rough endoplasmicreticulum uJ Nisslstain (p.25) DNA (deoxyribonucleicacid) (roughER) (p.33) l-- cytoarchitecture(p. 25) (p.30) polyribosome(p.33) Golgistain (p.26) gene(p.30) smooth endoplasmicreticulum cellbody (p.26) geneexpression (p.30) (smoothER) (p.3a) soma(p.26) protein(p.30) Golgi apparatus(p. 34) perikaryon(p.26) proteinsynthesis (p. 30) mitochondrion (p. 34) neurite(p. 26) mRNA(messenger ribonucleic ATP (adenosinetriphosphate) axon(p.26) acid)(p.30) (p.3s) dendrite(p. 26) transcription(p.30) neuronalmembrane (p. 35) neurondoctrine (p.27) promoter(p.30) cytoskeleton(p.35) CONCLUDINGREMARKS 49

microtubule(p.35) axoplasmictransport (p. 40) aspinousneuron (p.46) microfilament(p.38) anterogradetransport (p.4l) primary sensoryneuron (p.46) neurofilament(p. 38) retrograde transport (p. 4l) motor neuron (p.46) axon hillock(p. 38) dendritictree (p.4l) interneuron(p.46) axon collateral(p. 38) receptor(p.41) axon terminal (p. 39) dendriticspine (p. 42) Glia terminal bouton (p. 39) astrocyte (p.46) synapse(p.39) Classifying Neurons oligodendroglialcell (p.47) terminalarbor (p. 39) unipolarneuron (p.45) Schwanncell (p. 47) innervation(p. 39) bipolarneuron (p.45) myelin(p.47) synapticvesicle (p. 39) multipolarneuron (p.45) node of Ranvier(p.47) synapticcleft (p. 40) stellatecell (p. 45) ependymalcell (p. 48) synaptictransmission (p. 40) pyramidalcell (p.45) microglialcell (p.48) neurotransmitter (p, 40) spinyneuron (p,46)

l. Statethe neurondoctrine in a singlesentence.To whom is this insightcredited? 2. Which parts of a neuronare shown by a Golgi stainthat are not shown by a Nisslstain?

>(, 3. What are three physicalcharacteristics that distinguishaxons from dendritesl ;z-o 4. Of the followingstructures, state which ones are uniqueto neuronsand which are not: nucleus,mitochondria,rough ER,synaptic vesicle, Golgi apparatus. lfJF 5. What are the steps by which the informationin the DNA of the nucleusdirects the synthesisof a &a membrane-associatedprotein moleculel LU l 6. Colchicineis a drug rhat causesmicrotubules to break apart (depolymerize).Whateffect would this drug O haveon anterogradetransport? What would happenin the axon terminal? 7. Classifythe cortical pyramidalcell basedon (a) the number of neurites,(b) the Presenceor absenceof dendriticspines, (c) connections,and (d) axon length. g. What is myelin?What does it do?Which cellsprovide it in the centralnervous systemf

JonesEG. 1999. Golgi, Cajal and the NeuronDoc- PurvesWK. SadavaD, Orians GH, Heller HC. 200I' trine.Journol of the Historyof Neuroscience8: Life:TheScience of Biology,6thed. Sunderland,MA: t7o-t78. Sinauer. dg) PetersA, PalaySL,Webster H deF.l99l .The Fine StewardO, SchumanEM.200 1. Protein synthesis Structureof the NervousSystem, 3rd ed.NewYork: at synapticsites on dendrites.Annuol Review of \= Oxford UniversityPress. Neuroscience24:299 -325. FO d.4 lru u-di