PHYSIOLOGICAL ORGANIZATION OF THE NERVOUS SYSTEM · PHYSIOLOGICAL ORGANIZATION OF THE NERVOUS SYSTEM Surrounding the nucleus is the cell's cyto- plasm, which contains a variety of

PHYSIOLOGICAL ORGANIZATIONOF THE NERVOUS SYSTEM

When male Grayling butterflies are ready tocopulate they fly upward toward females pass-ing overhead. The male s response to femalesis not unerringly accurate , because sometimesthey fly toward other passing objects. This factsuggested to the ethologistTinbergen that thestimulus that is most effective in releasing themale s approach response could be discoveredby controlled experiments. Tinbergen mademodel butterflies, attached them to the line ofa fishing rod , and "flew" them to determine

which were most effective in attracting males.Although females are brightly colored, and

males can see color, color was not an impor-tant feature of the stimulus. Males were at-tracted by dark , large , and irregularly movingstimuli. Furthermore, these characteristicswere mutually reinforcing, which suggested toTinbergen that the nervous system of male

butterflies has a "pooling station" that inte-

grates the different features of the stimulatingobject.

Tinbergen s experiments are an example ofexcellent behavioral research that, although

done with no knowledge or study of the phys-iology of the butterfly s nervous system, still

gives clues about how that system must work.But knowledge of how the process of integra-tion takes place, that is, of how the poolingstation works, involves physiology. This is alsotrue of neuropsychology. Much can belearned about people s behavior through care-ful observations and controlled experimentsbut detailed knowledge of how the nervoussystem controls behavior requires the study ofits physiological organization. This requiresknowing. the structure of cells and how. theywork. Although an extensive knowledge ofelectrophysiblogy (study of neuron activity)and neuropharmacology (study of biochemicalactivity of neurons) is not essential for under-standing neuropsychology, a general under-

standing of them is helpful. The following sec-tions give a brief description of: (1) the

physical features of neurons , (2) the electricalactivity of neurons and the techniques used torecord their activity, and (3) chemical com-munication betWeen cells and the phar-

Kolb, B. , & Whishaw, 1. Q. (1980). Physiological organization of the nervoussyste~. Fundamentals of Human Neuropsychology (pp. 31-42). SanFrancIsco: Freeman.

PHYSIOLOGICAL ORGANIZATION OF THE NERVOUS SYSTEM

macological techniques used to manipulate

their communication.

NEURON STRUCTURE

Neurons are cells that are the integrating unitsof the nervous system, and although they

share many of the characteristics of other cellsin the body, they have special characteristicsthat make them particularly adaptable to theirfunction.

A broad analogy can be drawn between aneuron and a person. Neurons, once formeddo not regenerate, and unless they suffer lethaldamage , they live as long as the person inwhich they are found. Each neuron is sepa-rated from physical contact with every otherneuron , but it bridges this separation by com-municating with a language that is part electri-cal and part chemical. Neurons vary enor-mously in bodily proportions, the differencesmaking each neuron particularly adaptable toits specialized function. Neurons are aggre-gated into communities, or nuclei, each of

which makes a special contribution to behav-ior. Neurons are modifiable: they change theirbehavior with experience; they learn; they re-member; and they forget. At times neuronscan malfunction , causing disruptions in normalbehavior. There are similarities in the behaviorof neurons, but the full significance of theirbehavior can only be understood within thecontext of the community in which they func-tion. In summary, this anthropomorphic

analogy serves to caution us that the functionof a neuron within the context of a workingbrain is not as simple as the neuron is small.

Figure 2-1 shows a neuron schematically.The neuron is enclosed in a specialized mem-brane , and consists of a cell body, processes

called dendrites (from the Greek , meaningtree), a process called an axon (from theGreek , meaning axle), and little end feet onthe terminal branches of the axon. Associated

with each of these parts are other specializedstructures that are described where appropri-ate in the following sections. The dendritescollect information , which is then integrated atthe axon hillock close to the cell body; a sum-mary of the input received by the cell is thenpassed along the axon , through the end feet toother cells. (" Information" is used here looselyto mean any event or events that the cell ac-tively codifies.

Although neurons have these basic struc-tures their configurations differ amongneurons. For example , a sensory cell of the

somatosensory system has one very long den-drite coursing from the skin to a point adja-cent to its cell body, located near the spinalcord. Here the dendrite connects directly to

its axon, which may then travel to the hind-brain. This sensory cell has developed a systemof direct information transmission that re-

quires no modification of the signal betweenreceptor and brain. On the other hand , a motorcell in the inferior spinal cord has a number ofdendrites collecting input and a long axon ex-

tending from the cord to muscles. This cellappears specialized to integrate a variety ofinputs for a specific action.

Between these sensory and motor cells aremany interneurons of various shapes. Somehave a densely arborized dendritic system thatsuggests that their primary function is to col-leCt a great deal of diversified information forintegration (for examples of different neurontypes see Figure 1- 1). Also , although manyneurons communicate chemically-and we

stress this feature of their function-someprobably do communicate electrically.

The Cell Membrane

The cell membrane surrounds the entire celland consists of a double layer of lipid (fat)molecules. These molecules are polar in struc-ture , each having a head and two tails; theheads face outward , the tails face inward. The

NEURON STRUCTURE

AxO"

Axoncollateral

Mitochondria

Node of Ranvier

Glial cell or myelin sheath

Axon

FIGURE 2-1. A typical neuron , showing some of its major physical features.

inner portion made up by the tails is believedto be largely impermeable , thus providing abarrier to free movement of ions through themembrane. Channels exist in the membranehowever, that allow it to be selectively perme-able to ions under appropriate conditions. Pro~teins lie at or near the surface of each layer orpenetrate it completely. The proteins providea structural framework, are involved in thetransport of chemicals across the membrane

and act as receptors for various substances thataffect the transport mechanism.

The Cell Body

The cell body has a nucleus containingchromosomes that code the cell's genetic in-formation in deoxyribonucleic acid (DNA).Within the nucleus there is also a nucleoluswhich is pa,cked with ribonucleic acid (RNA).


Surrounding the nucleus is the cell's cyto-plasm, which contains a variety of structures in-cluding mitochondria, an endoplasmic re-ticulum, ribosomes, Golgi complexes, andlysosomes. Mitochondria are believed to havean energy-producing function. The endoplas-mic reticulum may provide a transport systembetween cytoplasm and nucleus and cytoplasmand the cell wall. Ribosomes are believed tobe the site of protein synthesis in the cell. TheGolgi apparatus may be involved in packagingmaterial to be extruded from the cell, or pack-aging lysosomes that presumably have diges-tive functions within the cell.

The Dendrites

The dendrites are actually extensions of thecell body that allow the neuron to increase thearea of surface upon which it receives informa-tion from other cells. The number of dendritesvaries from neuron to neuron , some having afew, others over 20; and each dendrite maybranch profusely. Dendrites vary from a fewmicrons to millimeters in length and taper asthey branch; some have rough projectionscalled dendritic spines upon which they re-ceive end feet from other cells.

The Axon

The axon originates in the cell body at a tran-sition point called the axon hillock. Its functionis to transmit to other cells information that itreceives from the axon hillock. Each cell hasonly one axon, which varies in length from afew microns to more than a meter in differentcells. Most axons have branches called collat-erals. At the end of the axon and its collateralsare fine terminations called teleodendria. Theteleodendria are covered with little knobscalled end feet, which make junctions withother ce11s.

The End Feet

The end feet terminate in close proximity toother cells. Sherrington coined the termsynapsis (from the Greek , meaning union) forthe "almost" connection between an end footand another neuron; consequently the end feetbecame technically known as synaptic knobsabbreviated to synapses when speaking func-tionally or in more general terms. They con-tain packages of chemical substances thatwhen released will influence the activity ofother cells. End feet may synapse with any partof a neuron; they are called axo-dendriticaxo-somatic, axo-axonic, and axo-synapticdepending upon whether they synapse withrespectively, dendrites, the cell body, axonsor synapses of other cells. Neurons may makeother types of contact with each other (for ex-ample, somas may touch, or axons may touch),but we will limit our description to the mostcommon , axo-dendritic connections.

NEURON ELECTRICAL ACTIVITY

Much of the pioneering research on theneuron s electrical activity, such as that doneby Hodgkin and Huxley, used the giant axonof the squid , on the recommendation of thebiologist Young. This axon measures up to amillimeter in diameter and is a hundred timeslarger than the axons of human nerve cells.The squid' s axon is used to contract musclesthat squirt water out the end of the squid'

body to propel it through the water. Becauseeffective propulsion requires all of the musclesof the body to contract at the same time , thelargest axons, which conduct the fastest , con-nect to the most distant muscles. Because ofits size , the giant axon is easily removed fromthe squid by dissection , and is easy to use forexperiments on how electrical conductiontakes place in axonS.


Probably everyone knows that if a salt is putinto a liquid medium it will dissolve into posi-tive (+) and negative (- ) ions that will even-tually become distributed equally through thesolution. In distributing themselves the ionsrespond to two forces, concentration andcharge, and the equilibrium they obtain repre-sents an equal distribution of both concentra-tion and charge. The membrane of a nerveaxon separates two fluid compartments; the in-tracellular and extracellular fluid, each ofwhich contains many ions. Of these, negativelycharged organic ions (An ) and chlorine ions

(Cn, and positively charged potassium ions(K+) and sodium ions (Na ) are particularly

important in electrical conduction. These ionswould be equally distributed on both sides ofthe membrane if it did not act as a barrier totheir easy passage. It does this in three ways.

First, it provides passive resistance to An - ionsbecause they are simply too large to passthrough it; consequently they are retained inthe intracellular fluid. Second, it is semi-

permeable to the other ions, allowing someof them to pass through more freely thanothers. Normally, K+ passes more freely than

+ (Na , although smaller than K+, is boundmore strongly to water molecules, which addto its bulk). The permeability of the mem-brane also changes in certain situations, allow-ing these ions to pass more freely through themembrane at some times than they can atother times. In particular, the membrane con-tains Na+ channels and K+ channels, whichclose or open to control the flow of these ions.

Third , the membrane contains a pumping sys-tem or Na K+ pump, which exchanges in-tracellular Na+ for extracellular K+. Since themembrane is less permeable to Na+ than to

, Na+ accumulates on the outside of the

membrane. Some K+ flows back out of the cellwhen pumped , to equalize the K+ concentra-tion across the membrane. The unlimitedoutward flow of K+ is checked , however, by

Resting potential

Time (msec)

Na+ - K+ pump

Na+ C

FIGURE 2-2. The nerve membrane , because of itssemipermeability and through the action of the Napump, accumulates anions (An ) and potassium (K+) inits intracellular fluid and sodium (Na ) and chlorine

(CI-) in its extracellular fluid. As a result of the chargedifferences of these ions , the inside of the membranehas a charge of -70 mV compared with the outside.This charge difference is called the resting potential.

the accumulating extracellular charge carriedby the Na+ (like charges repel each other). Asa result of the action of these three processesthere are 350 times as many An - and 20 timesas many K+ on the inside as are on the outsideof the cell membrane , and 9 times as many el-and 20 times as many Na+ on the outside asare on the inside of the cell membrane.

Figure 2-2 shows the distribution of the var-ious ions on the two sides of the axon. Becausethe ions are distributed unequally, and becausethey are charged , there is a voltage across themembrane , produced largely by the high ex-tracellular concentration of Na . If this volt-

age is measured with a voltmeter, with one- ofits poles placed inside and one placed outsidethe cell , the voltage is found to be - 70 mV


(millivolts) in the squid axon (and - 70 to -90m V in different animals) with the inside of theaxon negative with respect to the outside. Ifthis voltage is plotted for a period of time it isfound to be relatively stable signifying, pre-sumably, the constant action of the Napump. In Figure 2-2 the voltage is plotted on agraph. The voltage across the membrane ofthe cell is called the resting potential of themembrane.

Stimulation

There are , of course , normal influences on thecell that change the voltage of the membranein systematic ways. In addition , a wide varietyof external agents such as electrical currentsand chemicals and irritation from manual dis-placement, foreign tissue, etc. , can also pro-duce changes in the membrane voltage. Thenormal processes provide the mechanisms forthe normal functioning of the cell; the otherprocesses more generally lead to various typesof pathology. Despite these differences, boththe normal and abnormal influences act in verymuch the same way; thus, any influence or irri-tation which leads to a change in the voltagecan be called a stimulus, and the processwhether normal or abnormal, can be calledstimulation. In experimental situations stimu-lation is usually provided by giving brief pulsesof electric shock to the axon through smallwires called stimulating electrodes. The re-sponse of the axon is then recorded by measur-ing its voltage change with a voltmeter or os-cilloscope attached to the axon by small wirescalled recording eleqrodes.

Depolarization , Threshold,and Action Potential

When an axon is stimulated with a very smallelectric current, its membrane becomes more

permeable to Na+ and K+, and they movemore freely across the membrane. Conse-quently, N a + enters and K+ leaves the cellcausing the voltage across the membrane todecrease toward 0 mY. (Because K+ alreadymoves more freely across the membrane thanN a + the main change is caused by increasedinward flow of Na ) When this small voltagechange occurs the axon is said to undergo de-polarization. This change in voltage islocal-restricted to the area stimulated-andbrief, so the resting potential of the membraneis rapidly restored.

Although the neuron membrane respondsto weak stimulation by decreasing its permea-bility to ions in a relatively orderly way, itundergoes a peculiar change of behavior ifstimulation is sufficiently intense to cause thetransmembrane voltage to depolarize to about- 50 1Il V. At about this voltage the membranebecomes completely permeable to N a + and

; that is, Na+ rushes into the cell and K+rushes out of it , until the voltage across themembrane falls through 0 m V and reverses toabout + 50 m V. The depolarizaton of themembrane is largely attributable to Na+ in-flux; its repolarization is due to K+ efflux. Thesudden permeability of the membrane occursindependently of any further stimulation oncethe membrane has depolarized to about - 50m V. The loss of permeability is quite briefabout Y2 millisecond , after which normal per-meability is regained , the Na K+ pump re-sumes its action , and the resting potential ofthe membrane is restored. The voltage atwhich the membrane undergoes this au-tonomous change is called its threshold. Thesudden reversal of polarity and the restorationof the resting potential are called an actionpotential. These are displayed graphically inFigure 2-3. One can say, therefore, that the

threshold for eliciting an action potential is-50 mY.


Action potential

:;:. -

Threshold

Stimulation

Time (msec)

FIGURE 2-3. Stimulation of the membrane causes it tobecome more permeable to K+ and Na . As a result thetransmembrane potential declines or depolarizes. Atabout -50 mV, its threshold , the membrane becomescompletely permeable to Na+ and K+ and its chargemomentarily reverses. This reversal is called an actionpotential.

Conduction of the Nerve Impulse

When an action potential occurs in a region ofthe membrane it acts as a stimulus, causingadjacent portions of the membrane to losetheir permeability and undergo a similar volt-age change. Consequently, an action potentialtriggered at one end of an axon will be con-ducted along its length. (Action potentials cantravel in either direction, but they normally

begin at the cell body and travel away from it.This movement of the action potential alongthe length of the axon , shown in Figure 2- , is

called a nerve impulse (or, more colorfullyand descriptively, firing or discharging). Therate at which the impulse travels along the

axon , varying from 1 to 100 meters per sec-ond , is quite slow, but neurons can sustain awide range of firing rates. Usually they firefewer than 100 times a second , but they canfire as frequently as 1000 times per second.

The All-or-None Law

A peculiar property of a neuron s behavior isthat its threshold is stable, and every actionpotential , and hence nerve impulse , once it istriggered, has an identical threshold andheight. These properties of the neuron s be-

havior are formulated in the all-or-none law:action potentials either occur or they do not;there is no in-between condition.

The Origin of the Nerve Impulse

Graded Potentials. So far we have describedthe events that occur on an axon when it isstimulated. What happens on dendrites, whichare normally the origin of the cell' s electricalactivity? Dendrites have a membrane similarto the axons , and a similar resting potentialand they also undergo changes in potentialwhen they are stimulated. But unlike the axonthe dendrites do not produce action poten-tials. If a dendrite is stimulated the voltagechanges from resting potential in proportionto the intensity of the stimulation; the changethen spreads along the dendrite away from thepoint of stimulation , getting smaller with dis-tance (as the size of a wave in water decreaseswith distance from its source). These voltagechanges undergone by dendrites are calledgraded potentials , which can occur as a de-crease in transmembrane voltage (depolariza-tion) or an increase (hyperpolarization), de-pending upon the nature of the stimulation.How each of these occurs will be discussedafter we have described how graded potentialstrigger the nerve impulse on the axon.


;::. -

Propagation of action potential

- - ~ ++++++++++++++++ +++++++++;::. - + + + + + + + + + + + + + + - -

N~ + + + + + + + + + + + + + +

K+

++++++++++++++

- - t-

++++++++++++++

FIGURE 2-4. Because an action potential on one part of the axon stimulates adjacent areas of theaxon to produce one , the action potential is propagated along the axon. (After Katz, 1967.

Spatial and Temporal Summation. Becausedendrites respond to stimulation with gradedpotentials they have some interesting proper-ties. If a dendrite is stimulated ' at two pointsin close proximity the graded potentials pro-duced at each point will add. If the twostimuli are identical the graded potential willbe twice as large as would occur with only onestimulus. If the tWo stimuli are given at widely

different points on the dendrite the graded po-tentials will dissipate before they reach eachother and will not add. Stimuli given at inter-mediate distances will produce additivegraded potentials, but only at the points thatreceive the potentials from both sources.Also , the potentials will be smaller becausethey decay with distance. Similar rules applywhen one stimulus hyperpolarizes the mem-


brane and one depolarizes the membranewith the difference that the graded potentialssubtract. This property of adjacent graded po-tentials to add and subtract is called spatialsummation.

Another type of change that can occur ondendrites is called temporal summation. Thegraded potential of a stimulated dendrite willdecay with time after the stimulus has termi-nated. A second stimulus given some timelater at the same site will produce a similarresponse. If the second stimulus is given soonafter the first, the potentials will add , becom-ing larger than either is alone. The strength ofthe graded potential will be determined by thestrength of the two stimuli and the intervalbetween them. If one stimulus hyperpolarizesthe membrane and the other depolarizes themembrane , then the two will subtract, and thegraded potential will accorpingly be decreasedin size.

If the features of spatial and temporal sum-mation of graded potentials are considered , itis possible to see how the nerve impulse isgenerated. It will be . recalled that the thresh-old for an action potential is -50 mY. If theentire dendritic system is influenced so that itis depolarized to -50 m V, and if this gradedpotential spreads over the cell body to a pointadjacent to the axon , then the necessary condi-tions for eliciting an action potential will bemet. In fact, the point of transition betweenthe cell body and axon , called the axon hill-ock (Figure 2- 1), is the site where the nerveimpulse originates. As long as this area is de-polarized below - 50 m V by spread of gradedpotentials, the cell will fire. However, ifgraded potentials are not sufficiently strong todepolarize the axon hillock to threshold , thecell will not fire. In summary, therefore, the

origin of axonal firing can be traced to the in-fluence of graded potentials from the den-drites of the cell.

The Origin of Graded Potentials:The Synapse, EPSPs, and IPSPs

The idea \that chemicals playa role in thetransmission of information from one neuronto another, from a neuron to a muscle, or froma neuron to a body organ originated with theexperiments of Otto Loewi in 1921. He stimu-lated the nerves going to a frog heart

, col-lected a fluid perfused through the ventriclesof the heart, and transferred it to the heart ofanother frog. The activity of the second heartwas changed by introduction of the fluid in thesame way that the activity of the first heart waschanged by electrical stimulation. The stimu-lated nerve had been releasing a chemical , andit was the chemical , not some direct action ofthe nerve , that was causing the heart s activityto change. It is now widely accepted thatneurons communicate chiefly through theagency of the chemicals they release when theyfire. These chemicals, each known as aneurotransmitter, are released by the endfeet of the neuron.

Figure 2-5 shows a diagram of an end foot.The end foot is s~parated from other neuronsby a very small space called the synaptic space.

The membrane of the end foot is called thepresynaptic membrane, and the membraneit synapses with is called the postsynapticmembrane. Penetrating the end foot from theaxon are neurofibrils, which may transportprecursor chemicals for the manufacture ofneurotransmitters into the end foot. There aremitochondria that provide energy for metabolicprocesses. There are also two types of vesiclesin the end foot: storage granules, which arepresumed to be long-term storage sites forneurotransmitters; and synaptic vesicleswhich hold neurotransmitters for immediate

use. On the postsynaptic membrane there arespecialized proteins that act as receptors for theneurotransmitter. The synapse functions in the


Neurofibrils

Storagegranules

PO"'Y"""" receptors

Synaptic cleft

FIGURE 2-5. Diagram of the major features of a syn-apse.

following way. When a neuron fires, somesynaptic vesicles release their neurotransmit-ter content into the synaptic space. Theneurotransmitter binds weakly to the recep-

tors on the postsynaptic membrane, afterwhich it is quickly washed away by extracellu-lar fluid and is destroyed or taken back intothe presynaptic membrane for reuse.

How neurotransmitters, released by thefiring of a presynaptic neuron, produce gradedpotentials in a postsynaptic neuron has beenclarified in part by Eccles and his coworkerswho stimulated the axons of presynapticneurons while recording from a postsynapticcell body. Postsynaptic graded potentials fol-lowed each volley of presynaptic stimulation.These postsynaptic potentials, or PSPs, had avery small amplitude, 1 to 3 m V; but depend-ing upon which presynaptic axOns were stimu-

lated, they consisted either of depolarization

or hyperpolarization of the poStsynaptic mem-brane. Because depolarizing PSPs of courseincrease the probability of the neuron firing,they are called excitatory postsynaptic potentials

or EPSPsj and because hyperpolarizing poten-tials decrease the probability of the neuron fir-ing, they are called inhibitory postsynaptic po-

tentials or IPSPs. It is now accepted that theEPSPs and IPSPs are produced by the actionof neurotransmitters on the postsynaptic re-ceptors of the cell. Neurotransmitters fromcertain synapses called excitatory neuro-

transmitters are responsible for EPSPs, whileother neurotransmitters called inhibitoryneurotransmitters, are responsible for IPSPs.

Eccles suggests that excitatory neurotransmit-ters produce EPSPs by making the membraneslightly more permeable to Na , which entersthe cell, lowering the transmembrane voltage.Inhibitory neurotransmitters, on the otherhand , make the membrane more permeable toK+ and Cl+ ions; K+ flows out and Cl- flowsinto the cell, raising the transmembrane volt-age.

It can now be seen that the origin of thegraded potentials of dendrites can be traced tothe release and action of neurotransmittersfrom the end feet of other neurons. It will beremembered that there are thousands of endfeet synapsing with the dendrites and cellbody of anyone neuron; thus, the summedgraded potential of the cell is produced by theaction of all of these inputs. The integration ofthese inputs by spatial and temporal summa-tion determines whether the neuron will fire ornot. If EPSPs predominate, and if there areenough of them to produce depolarization tothreshold at the axon hillock, the neuron willfire. If IPSPs predominate the neuron will notfire.

Factors Determining Nerve Impulse Speed

The nerve impulse does not travel at exactly thesame speed in all neurons. At least two factorsaffect speed. One factor is resistance to currentalong the axon. Impulse speed is increased asresistance is decreased; and resistance is most


g -

1"'+

"'+

Node of Ranvier

Axon

Schwann cell

FIGURE 2-6. The nerve impulse jumps from one inter-Schwann cell space, called a node ofRanvier to the next. This process, saltatory conduction greatly speeds impulse transmission.

effectively decreased by increase of the axonsize. Thus, large axons conduct at a faster ratethan small axons. Were the nervous system torely only on this procedure , axons would haveto be cumbersomely large. An alternative pro-cedure has evolved that uses the glial cells toaid in speeding propagation. Schwann cells inthe peripheral nervous system and oligoden-droglia in the central nervous system wraparound some axons , forming a compact sheathof myelin (from the Greek , meaning marrow)against the cell membrane, as shown in Fig-ure 2-6. Between each glial cell the membraneof the axon is exposed by a gap called a nodeof Ranvier. In these myelinated axons thenerve impulse jumps along the axon fromnode to node , a type of conduction called sal-tatory conduction (from the Latin , meaningskip). Saltatory conduction is an extremely ef-fective way of speeding the impulse because asmall myelinated axon can conduct as rapidly

. as an unmyelinated axon 30 times as large.

The Integration of Neural Activityand Information Processing

It can be seen from the preceding sections thatthe dendritic system of the neuron sums theactivity from many other neurons by produc-ing graded potentials that will determinewhether or not the neuron fires. The firing ofthe neuron is an all-or-none response, whichwill continue for as long as the firing thresholdof the axon hillock is maintained. But how dothese series of activities code information?

It is now thought that the nervous systemworks by a combination of analogue (howmuch) and digital , or binary (yes-no), princi-ples. Analogue functions are the property ofthe dendritic system , and digital functions arethe property of the axons. We can see howthese principles determine behavior if we re"turn to the opening description of the maleGrayling butterfly behavior. Recall Tin-bergen s suggestion that in the male butterfly


nervous system there is a pooling station thatintegrates the different features (dark, large

and irregular) of a stimulus object to deter-mine whether or not the male will approachthe stimulus. Theoretically, all of the male but-terfly s behavior could be accounted for by theactivity of one central neuron. The dendritesof that neuron would serve as the pooling sta-tion and the axon would be the system thatinitiates an approach response. If three chan-nels of input converged upon the dendrites(one signalling darkness, one size, and onemovement), simultaneous activity in the sepa-rate channels signalling dark , large , and irregu-lar would produce EPSPs that when summedwould trigger axonal firing and thus approachby the butterfly. Activity in only one channelmight not be sufficient to fire the neuron; ac-tivity in two channels might be sufficient toproduce a response , if input in each were par-ticularly intense. At any rate, it can be seenthat the analogue function of the dendrites willintegrate the various sources of input, whilethe digital activity of the axon will determinewhether or not approach is to occur. Ofcourse , the analogue feature of dendritic in-tegration can be put to many types of use , andthe digital properties of the axon can be ex-pressed in many codes (frequency, pattern offiring, etc.).

Many other factors contribute to informa-tion processing. Synapses proximal to the axonhillock may have special access to influencecell firing. Inhibition or excitation by more dis-tal axosynaptic connections may allow formore subtle control of intercell communica-tion. Some synapses may also change structur-ally with use or disuse , thus becoming increas-ingly or decreasingly effective in communicat-ing. These factors, and many others, are be-yond the scope of the present discussion, butthey do contribute to the brain s incredible

synthesizing and storage abilities.

ANALYZING THE BRAIN THROUGHITS ELECTRICAL ACTIVITY

Because the activity of nerve cells has an elec-trochemical basis the activity can be recordedwith instruments sensitive to small changes inelectrical activity. The several techniques forrecording the brain s electrical activity include:(1) intracellular unit and extracellular unit re-cording, (2) electroencephalographic (EEG)recording, and (3) evoked potential (EP) re-cording. Relating each of these types of activ-ity to behavior can be used as a way of deter-mining the function of particular br3.in areasand as a way of determining the normality offunction in a given brain area. Because of itselectrochemical mode of activity, the brain canalso be artificially stimulated with electricalcurrent. This technique has been used as amethod of analyzing the function of differentareas, as a possible source of therapy, and as amethod of producing experimental models ofdiseases such as epilepsy.

Unit Recording

If small wires or pipettes containing an ionizedconducting solUtion are inserted into the brainso that their tips are placed in or near a nervecell , the changes in a single cell's electrical po-tentials, i. , unit activity, can be recorded inrelation to some indifferent electrode orground. Intracellular recordings are madefrom electrodes with very tiny tips, less than111000 of a millimeter in diameter, which areplaced in the cell, whereas extracellular record-ings are made when an electrode tip is placedadjacent to one or a number of cells. Bothtechniques require amplification of the signaland some type of display. The cell's activity iseither displayed on an oscilloscope for photo-graphing or recorded on a tape recorder forcomputer analysis. In many experiments the

ANALYZING THE BRAIN THROUGH ITS ELECTRICAL ACTIVITY

signal is played through a loudspeaker so cellfiring is heard as a beep or pop. Both record-ing techniques require considerable skill toperform because it is difficult to place the elec-trode in or sufficiently close to the cell withoutkilling it , and when a cell is "captured " it is

often difficult to hold it for more than a fewminutes or hours before the signal is lost.

U nit recording techniques provide a par-ticularly interesting insight into the brainfunction. For example , cell records obtainedfrom the visual cortex of cats and monkeysreveal that each cell has a preferred stimulusand a preferred response pattern. Some cellsfire to horizontal lines, others to diagonal linesand still others fire only to lines that areoriented in a special way and that also move ina particular direction. Unit recording tech-niques have also been used to analyze such

abnormal cell activity as occurs in epilepsy. Inepilepsy, the activity of cells becomes syn-chronized in an abnormal pattern, and an un-derstanding of epilepsy depends in part uponanalyzing and controlling this feature of thecell' s behavior. Much of the information ob-rained with unit recordings has of necessity

come from experiments performed on anes-thetized animals. Future research is likely torepeat these tests in freely moving animals toconfirm and elaborate upon the findings.

EEG Recording

A simple technique for recording electrical ac-tivity of the brain was developed in the early1930s by Hans Berger. He found that it waspossible to record "brain waves" from thescalp. These waves, called electroencepha-lograms, or EEGs, have proved - to be avaluable tool for studying problems such assleep-waking, for monitoring depth of anes-thesia, and for diagnosing epilepsy and braindamage.

To record a person s EEG a small metal discis attached to the scalp, and the change in elec-trical activity in the area of this electrode iscompared to some electrically neutral zonesuch as the ear lobe. The electrical changesrecorded on the scalp are rather small, usuallymuch less than a millivolt, so they must beamplified for display on an oscilloscope or on apaper chart recorder called an electroen-cephalograph, or EEG machine.

The electrical activity recorded from thescalp is the sum of all neural activity, actionpotentials, graded potentials etc. , but it mostly the measure of the graded potentials ofdendrites. As a result, it represents thesummed dendritic activity of thousands ofnerve cells and can only be considered a rathergeneral measure of the brain s activity. Al-though it can be used as a crude index of thebrain s level of excitation it tells very littleabout the activity of single cells as such. Dur-ing a given EEG pattern any particular singlecell may be . active or inactive.

It was originally thought that each cytoar-chitectonic area of the brain had its own pat-tern of EEG activity, but it is now recognizedthat variations in patterns do not correlateclosely with cytoarchitectonic areas. Figure 2-shows the characteristic resting rhythms ob-

tained from different parts of the cortex. Thepatterns are obtained only under optimal con-ditions, when the person is awake, restingquietly, with eyes closed. The dominantrhythm of the posterior cortex is an 8-to-cycles/second waveform called the alpharhythm. The dominant rhythm of the precen-tral and postcentral sensorimotor area is a 20-to-25 cycles/second beta rhythm. The sec-ondary frontal areas have a 17-tO-20 cycles/second beta rhythm. And the tertiary frontalarea has 8-to~12 cycles/second waves.

The resting EEG patterns de synchronize orflatten into a low-voltage asynchronous activ-

PHYSIOLOGICAL ORGANIZATION OF THE NERVOUS SYSTEM · PHYSIOLOGICAL ORGANIZATION OF THE NERVOUS SYSTEM Surrounding the nucleus is the cell's cyto- plasm, which contains a variety of

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