On the physiology of dendrites

CilRKBNTS

IN MODli,KN II!O,,GGY

3 (,969)

‘P--M.

NORTH-HOLLAND

PUBLfSHlNG COMPANY, AMST,%DAR(

fii,ds czeeated.

1. iNTRODUCTlON Scver~l mechanisms have been suggested as capable of modifying the excitability of spinal motoneurons. :‘orne evidence of the possible role of dendrites in modulating tbls excitability has been presented jBrookhart and Kubota, 1963; Garcia Raw, 1968; lGareia Ramos sod liufzar SBnchez, 19G99a,b).If dendritic activity is largely responsible for the slow components of the spioal cord potenlials, then it would be ioterestmg to study the excitability changes in a matoneuron pool during the developmcot of those siow compooenls as revealed by cxtsacellular records taken wi?h Iine electrodes. The spiwl motoow~ons were activated via different pnlhways, including s&nuli applied directly dtrough the exploring electrode. It was expected that the analysis of the excitability changes in ;ll tboso different conditions miglit throw some light on this problem. Incidentally, farther evidence is presented to show the deodritic contribution to the spinal cord potentids.

2:.METHODS The experiments wcrc carded out on fourteen car% anestbP;tizr
more under peoiobarbital(30 mg/kg i.p.). The lower lumbar spinal segments we,e exposed. The dorsal sod ventral roots of b were dissected free for stimuls tions or recording. The mo‘or area of the brain cortex and tho posterior dorsal arpect of tbe cerebellum were exposed on one si& by corresponding small openings in the skull. A fine-tip metal-filled pipette fGest&nd et al..

1959) was inwted at the matoneuron pool of the L.7 spinal segment contralateral to the side of the brain and webellar corticcssxpwd. The electrical records were taken between the exploring electrode and a large silver plate uodzr the skin of the lumbar wound. A Grcss PM preamplifier wasuwd, with a time conlaot of 0.3 to 1 96~.For the inlramlbdar records, 3 M KCMlled microelcctrodea, with about 3OM52 resistance, were used. They were inxrted through the lateral column, the cord being rotated by pulling the dun and tying it to a frame. A ftcld effect t,ramistar (Gs PLOmicroejectrode zz~plifl~r) war employed in the tint stage. The records were displayed on the screen of a SMA Tektronix oscilloscope and photo graphed with n Grass kymograph camera. The stimulating or recording electrodes for the brain n?d cercbellar cortices were baUt!otred silversilver chloride wires. For the apinsl roots similar hook&aped wires enclosed in lhort piew of polyethylene tubing were employed. The exposed spinal cord was

ON THL PHYSIOLOGY

protected by a layer of small c”,,“,, belts. 1 he animals were kept werm by meam of an infrared lamp placed above-the preparation. The obvxvations were made under g&mix and artificial raptratio,,. The stimuli we,e rectangular pulses from e Grass S8 stimulator, passed through Grass SIU 4678 isolatio,, units. Their durations were of 0.01 m~ec for the spinal roots, and of 0.2 msec for the brain and cerebella cortex and for those indirectly applied 1” the motoneu*on pool.

3. RESULTS

In the ex~racellular records taken with the metaifilled pipettes, there often appeared some potential waves which were interpreted as unitary dendritic potentials (brief excursions in tig, 1). The larger of these potential waves were about 200 pV; values of 500 pV, however, were observed in some instances. The largest vah,es we,e recorded with the saline-filled pipettes and near the motoneuron pool (fig. 2). These unitary dendritic potentials, d wwes, had the following chawteristics. They appeared in definite loci and could be lost by displacing for a short distance the tip of the electrode, although in some cases they could be followed, gradually incieasing “I de-reasing in amplitude, for mope than 200 microns. The patterns were variable, either iiiaioiy negative “I, more usualI:,, mabdy pa&it&ewaves. With the metal-tilled pipettes they usually showed a tripharic pattern (fig. 1). They were commonly followed by a protongcd positive awe, dhkh. in tbr: records taken with the salinef&d electrodes, showed a relatively fast rising phase end a slow decay (fig. 2). They may appear “spcntencously” or, mo~f often, as response:; to stimuli applied. They could sppear singly o, ar trains of variable duetions (fig. 1). The frequency of [be OScillationr in the trains varied from 45 f> 70 per sec. When forming part of a renponre to suli raspmal or dorsal mot stimulation, they appeared during the &vel”pme”t of the slow component of the spinal poteutials. In some cases they appeared to contribute to tbe formation of the Bow component. With repetitive stimulalio~~,the lit,& ofd WIVDS could increase and the latency become shorter for frcquenciet up !a 10 per BCCapplied for a brief period. If the

OP DENDKITLS

period.of stimula!inn was prolonged, however. or the frequency was increased, the opposite Grcurred. in n’ost cases, the d potentials could not follow a froquency of 100 per set for more than P Jw sec. The d wwes which occurred after antidromic stimulation could drop ir. all-or-nothb?;manner after repetitive stimulation (fig. 2Gj, Without first decreasing in amplitude. They showed a threshold when the strength of the stiuti was varied; incr:asingthc intensity abow this thretiold did not increase tile amplitude of ‘he d wave, although the latency could shorten (fig. 2A and R).

These observations were made with saline-f&d pipettes in order to record from the motoneuron dc:.drites activated by antidromic volleys. The o&r+ electrode was inserted through the lateral column while applying 8 stimulus to the ventral root every 1.6 sec. The majority of the successfulwnetrations took place rather far from the motoneuron pools; in wire cases at a depth of only 0.8 mm from the cord surface. Responses related to the stimyti were obtained also from thr som8ta, but they wem discarded when identified by their short latency, their amplitude of

80 to LOOmV and their brief temporal course. Only the responses w!th latencies longer than 2 mzec were analyzed. As illustrated in fig. 3, in which examples of two different imp&d aUs are shown, the d potentiils appeared as positive waves followed by a prolonged negative afterpotential. During the period in which the electrode was inside the cell, in most casc~ for axera mln, the amplitude of the positive wave could z.howsmnc variations io the wccepsivo responea The larger the amplitudes, the geater theii duration (fig. 3H). In every case, they wefe tested by increasing ths rate of stimulation, which led to changes in amplc tude and latency similar to thor dcraibed above for the extracellular records. At rates of kza than 5 pet set thz successive potentills recaded intracellukly may show an increase In amplitude, with shortening of their latency md duration. When 4 frequency of stimulation was increased, the opposite effects were observed. They could follow frequencies up to 100 par sac for P few PC. Before they f&d, they could appear RJconstituted by tbe sum of .!wo or more waves (fig. 39. In mllcases they showed P complete rccovety @hen tested later with single Lmclts.

3.3.l?~epotentinlfields

of the componenrs

spinal respoaseer to onridromic

ojlhe

stimdario~~

The fields of distribution of the componentsof these resp&es were followed m records taken along the electrode tracks. A typical example is illustrated in lhe graphs of fig. 4, which allow the comparison of the distribution of d waveletr with those of the initial positive component, attributed to the axon potenl~als, and of the first neg;ltive wave, attributed to the activalion of the somete and the first d potential (Lorente de No, 1947). The fart decey of the d waves indicates that their foci are limited in extension. The inversion of their electrica sign suggests thsr they are blocked at some point along the dendrittc branches and recorded as a field potential beyond that point. 3.4. nte excitability

charqcs

ufmomneuronsin

r&rim to the slow emnponen~s reryonses a, 0 vtwtralfcms

of the spinal

The changes of the excitabilily of the motoneurons during the development of the spinal potential besponer to ringleahock stimtdallon of the motcr area of the brain, the spinal anterior column, the cerebellar cortex and the corresponding dorsal root were studied 8s follows. Test slimuli were applied, either to ihc

wne dorsal root (figs. 5 sod 6, A and B) or directly to the exp1orir.gelecttode (fig. 6, C aod D). The conditioning and the tat !.hockr were applied at different intervel!; and the paits repeated evecy 2.5 sec. The strength of the test stimuli applied to the dorsal loot was adjusted either to give only 3 monosynaptic response in the ventral root, or to evoke also a multisynaptx r&x. The increases ip excitability recorded were usually greater when the intensity of the conditioning shock was stronger, or, rather, when the amplitude of the spinal potential evoked hy that stimulus we5 larger. When tested with the monosynaptis refk .es, the excitabdity of the motoneurons showed sim : i chnng?s m all instances. There was up initial period of increase, short-lasting (less than I5 msec) when the cooditioninp .‘mcks were applied to the fame dorsal root (fig. 6, A and B).and more prolonged (30-40 msec) after previous tingle-shock stimulation of the motor and the cerebellar cortices(fia. S. A end B). This incredrc ME followed by a d&se, interr&cl by or mixed with a second period of increase which occurred during the development of t!ie wand negative component (Nz). This late increase was very smell or was not presenl in the animals anssthetized with pentobubital. When the test stimuli were

78

J.GARCIA RAMCG

applied to the brain or to the cerebeUar corticer, and the conditioning shock HBI given to the dorsal root, the changes in excitability were quantitatively similar, although the perhd of depression appeared more marked (Ii& 5, C and D), The excitability of the spinal neurons itivolved in lhe mullisynaptic reflex responsesusually showed a more molonaed initial increase (fin. 6Ak This increase was more marked and showed less variability in the individual responsesto the test shocks after a corn

_

_

,

I

plete transection of the spinal cord at the level of L4 (fig. 6a). P’hen the test shocks were applied direztly lo the

recording the rerponser in the correapondtng dorsal rool. or in a split branch of the dorsal root and ap plying the conditioning shwk to the other bundle. The clearest changes were seen during lhe develop ?montof the spinal responrc lo a dorsal root volley (fig. 6D), in which P prolonged(l(n mwc) increw was observed. Similar &angel WCICseen when the direc( stb&ilation to the ventral focus was osed for both the conditioning sod the tex pulaer I( L interesting to mention that large baeasea in excite bility were obrewed when a train of test shocks was

explxing electrode in the ventral focus. the same sequence of motoneuron excitability changs appeared (fig. K), although the changes were alwys of small magnitude. In some of these cares, the axcit?bility of

applted with a high fwquency (Wall, 1958). In these cases,the effects could be &o mote prolonged in time (ftt 7). Finally, changes in motoneuroo excitability were atso observed by applying the wnditiooing shock lo some oftbe fibers Ll the ventral root. For this pur-

the afiemt

pose, the ventral root was split into two halves. One

termi&

was simdltaneouely teaed by

ON THE PHYSIOLOGY OF DENDRITES

of them was wed to apply a maximal conditioning stimulus and the other to record the responsesto the test shocks applii dircctiy to the ventral LKW. An increase ofexcitability of about I%% was swn during the initial 10 to 15 an*.

dritic activation. They differ from the action potenti;lls of axons and somat mainly in their prolonged temporal course and their failure to follow h$h frequencies of stimulation. The fact that they may change

4.1. Ilw rlrehical si&nsof dmdritic activation The potential waves described io sections 3.1 and

in temporal course and amplitude in successivere~ponser may be attributed to differences in the extent of the surface of the dendritic tree activated. The triphasic patterns observed sometimes in the extraceUt_darrecords(fig. 1) strongly suggestthat the d waves are of the nature of propagated action potentials. The observation thrt with a relatively high fre-

3.2 GUI bs into~pmted as the electrical riSns of den-

quency of stimulation they may appear as constituted

4. DISCUSSION

suggests that the d wavesmay

be actlva ted directly tbrowh fine twminalrendine. on axodendritic wtw ECP. 0;; the other hand, the brief lalcncier with which d WBW may appearin the ventral focus after Mtidromic stimulation (t:ga. 2,3 and 4) strongly suggest that they can also be wtivated tbrougb propagzxtion from the somars. This wggcrtion is supported by the

ON THE PHYSIDLDG-

prese”ce of an inflection in their rising phase, as show” il the intmceU”lar records (fig. 3). Evidence has bee” given in support of the inference that the slow components of the spinal potential responses are due to a temporal and spatial summation of fusion of d mte,itiats &a& Ramos. 1965: Garcia Ramor and H&r Stich& 1969a,2). Record; such as those illustrated in fig. I strongly support this idea. An interesring obsenation is that d waves may appear as trains of discharges. This repeated activity may well be Lue to their activation by the repetitive hing of pome of the “ewms which have endings on dendrites. The possibility exists, however, that dendrites could also jhow repetitive fiiing in response t” a single stimulus. I” any czw, it teems that their dig charge is facilitated by previous activity. Some of the records, such as tl.xe in tig. 1, may be interpreted in this way. It ban been reporwd previously (Garcia Ra”lO8, 19681, that this fac!litatio” occ”,s with r&dlively low rates of stbnulathm. The two modes of activation of dendrites, together with the possibility of repetitive firing of interneurons, or of the dendrites themselves, cauld explain the prolonged potential changes observed in the spinal wrd in mponie to a single volley of incoming bnpulses. Fcrsard nnd Matthews (1939) have shown that o w&kvelopcd dorral root potential may foUow the activation Iofa single affwent iiber. The first nqativc slow component (N,) of the spinal purentials could be due to the s”m of the propagated dcndritic action potentials of tbe proximal dendritic branches activated either in a somatofugal direction or through direct axodendritic synapses. The late slow positivity e mybe men as the sum of the positive afterpotenti& of the= stmctwes. TL raid ul and pry longed poritivity wai present in most of tile intmcellular records and in some of the extracellular ones. The second slow negative component (Nz) may be due mainly to the activation of tke dendritic brvrhes either througit axodmdritic syrapres “I by propagation in a somatohrgd direction. The second slow poaitivity of the spinal potential responses may also be the result of Bm~zha”ii similar to that mentioned above, i.e., the sum of the positive dfterpoten. tials w&h follow the dendritic rctivity.

OF DENDRITES

81

4.2. The effects oj’dendritic acriviry on the exitahilily of the m~toneumn~ The whole rerics of events following ;he stimulation of a pathway ending at the spinal cord creates a widespread field in which electtricaland chemical change could affect many stmctwes, including even some elements not directly involved. For those directly involved, long!asfing electronic effects could certainly modify their subszquent behavior. The histological eviknce of the high density of synaptic connections on the dendritlc branches, together with the ‘lifferent marginsof safety for prapagation in tbe bran&x of different diamters, strongly mpports the idea that Ihe modulatiag role of dendrites upon neuron tiring capacity may be quite bnportont, as suggested by Brookhart and Kubota( 1963). The electrical changes which follow the activation of the proximal branches of dendrites could act on the mmata by electratonic effects. During the dendritic depolarization. the excirability of the soma would increase and repetitive firkg may occur On the other hand, during the late hyperpdarization the opposite effects may be expected. The late and m”re prolonged electrical changes in the tine arborizatiors may affect the soma mainly by the effects of the electricill fields they create. Inhibitory effects may therefae take place ot: xwisc Ihar, by direct synaptx hypcrpolarizing potentials. Charlges of the types mentioned may be proPent at the interneuron level when two successive volleys reach those newons. This was the explawtio” proposed by Garcia Ranms and Huizar Sgnchez (19698. b) for the deproo sion of the motoneuron activity observeii when tw” volleys of supraspinal origin were made to interact ilt the spinal cord. Changes in excitability of the motonewm somata, aside from the b&f excitability cycle that immediately follows their activation, car. be produced by several mechanisms: (a) the spatial and temporal algebraic mm of t!:e synaptic impulses impinging upon the soma; (b) the electrotonic effects of the potential chmges occurring in their own dendritic branches “ear the soma; the electrical field created during activation of their distant dendritic arborizutions;(d) the electricai fields due 13 activity in noighbcring elements; and (e) changeeain the chemical composition inside “I

outside the cell. When testing that excitability we shoobl consider the possibility of changes occurag in the affemnt terminals throueh which this exciiabiliiy ISmeasured. These &&es may be :affected by similar fectors. In Cxli?I to analyze the expsrimental results, the fol!owing assumptions are made: (1) For a given expainrental condition the number and excitatcry value of tbs impulses reaching Bgiven neuron is constant. T&is far from being tme, but is fairly acceptable rwtktically for an anesthetized and curarized animal in a constant environment. This assumption ISbased on the relative cactitancy of the spiral potential rerponss to a given stimulus of suprathreshold strength and on the reprodwi%lity af any given change; (2) The slow potential changes are priourily due to dendritic eetivity;(3) The only significant variations in chemical com_poritioninside and outside the rAls arc the ionic changes related tothr electrical signs of the newour activity. Other chemicd changes exist but, as far as is known, they become important only during high degrees of activil) (Rosenbluoth and Garcia Ramos, 3%6);(4j Excitability changes in &rent terminals do not play an important .ole, since the results were qualitatively aimiku whetbrr the motonboroo excitability was tested e:ther via the rame or a different synaptic pathway, or by direct stimulation through the exploring electrode. Thus, in the following an;llysis only the possible role of the factors(b), (c) and(d) mentioned aboto will be considered. Under the experinental candirions of the present study, three types of activation of a motoneuron pool may be distingokhed: (i) Direct and short l&stingact& vatioo: the antidromic volleys and perhaps the stimuletion through the exploring electrode inserted in the motone~~ronpool. In thb type, the dendritic tree would be mainly activated through ~~npulsespropagated from the some. The electrotunic effects during this propagation would be exgpcted to be of relatively short duration:(ii) .~ . _Indirect activation thiounh one or more interneurons: stimulation of the motor area of the brain cortex and of the spinal anterior cohtmo above the La segment. The characteristics of this type arc that the impulses reaching the motoneuronr will be significantly disputed in tie, end that a gnat proportion of the syoapses involved arc of the den. dritic kind. The latter inference is based on the disproportion betwee. the large amplitude of the spinal

potential

and the meager response in the ventral

(fig.I):(iiii Inthethird type. the motoneurons

root

were activated both via mono- and pcfysynaptic pathways; monosynaptic activation was more important when the stimuli were applied to the dorsal root while polysynaptic pathways were more important when the stimuli were applied to the cerebellum. This group &red both tbe characteristics of the other two. Monosynaptic pathways probably end mrinly on ax* somat& synapses. The comesponding spinal potential b small in proportion with the high number of mot* neurons activated. Following the direct activation of the motoneurons either by ‘vay of an antidromie shock or by rlimulation ttuough the exploring electrode, there were changes in excitability IS indicated by the responses to a second stimulus to the spinat ekctrode. There was a defmite increase of excitability for about I5 msec and a kss clear late reduction prolonged for 100 mPec or more. These changes could be interpreted as due fist to P catelectr>tonic effect produced by the activation of the dendrites proximal to the soma, and then to an vleleotrotonk aIT& due to the hyperpolarizing p tent8 and to the feld created by the depoluization of the fI den&ilk arboriuttons. This interpretation assumes that the activation of dendrites pr~esser in a somstofugal direction (Garcfa Ramos, ISaS). That it cao be also an effect of the ckcuical fiild was shown by the experiment of tk split ventral mot. There wan a small iocreoncin the excitability which affected the motoneurone not activated by the conditioning shock (section 3.4). This effect was confumed by the observation of a paraUe1incrwre in the amplitude of the nspon~ recorded in tbc dorsal root. In this tespect it is interestiog to mention that the stbnulur directly applied ?Jtrou$t lhe exploring ekv trade has its own effects on the responses to a following similar alimldus(fl& 7). In this cue, some facilitation after previous activity of the fme affcrent terminals is suggested by the larger ckan~.x obtained with 200 per IC (fig. 7A). Rob&dy. the spinal electrimd t-iildwas “0 largm than with the StimStioo at 100 per set, sinceit is produced by activity in elemutts not able to follow high fx!quelEiea (Car& Ramor. 1968). For the indirect activation ofmotoncilrons, the sitiatioo has to be mope complex. Stiiulttion of the motor cortex in the cat activatea the motoneurons

ON THE PHYStOLOGY

interneurons

located at the level of the latninaeV and VI of Rexed (Kuypers, 1964: Garcia Ramos and HuizarStichez, 1969a, b). Severalfactors intervenein these conditions: the impulseswhich reach the ictemeurunsare dispersedin time; these neuronsmay be able to dischargerepetitively(fig. 1); the distributionof the svna”tic _ . contacts to different sites of th-zmotoneurons is irregular. Another factor which may play a role is that theremay be more important changesof the excitabilityof the afferent temdnalrbecauseof the ratherlargeelectricalor chemicalevents that take place in tbe spinalcord duringtbe activationof the severalstructuressti mulated by the corticc-spinal volley of impulses. It is not surprising. therefore. that the changes in tbe excitabiity of motoneurons, tested through the mane synaptii response to a week dorsal mot volley, should be complex. ADillustratedin f& SA, there is a” increaseduringthe early part of tbe spinzl potential recorded a1 tbc mOto”eurO”pool. Later, the changes wem variable.but a signifiiantreductionwas the most importat limdir,g,followed by a secad incma&.These changes are compatiblewith -he mechanisnupostulated above. Activationof the proximal dendriticbrvlches after motoneumn discharge will add to the ekctrotonic effects of their prolonged synaptic bombardment. The late changescan be the result of the following hypcrpolarizingpotentiis and the dcpokiation of the fm uborizations. effects which will tend to raim the thresholdfor firing,md the prolo”ged activity of htterncuronrwhich will :end to maintainthe excitability levelhigh.Similarvariationr cccwred after the applicationof a conditioning rti”~td”rto the cerebellarcortex (fig. 58). The effects of the directarebcUomotoneuron fibers did not s&“ifkantlyalter the picture.This can be explained by the fact that them ia only a mlau “umberof these directconnections(Car& Ran106and Huiw S&nchez, 196911.b). The ch~ngn in motoneuronexcitability following a dorrplroot volley have ken amply WuJied(Creed et al., 1932, for early studiea).Recently the prolonged inhibitionhas been mpinlyattributedto presynaptic depolarizltio” of the afferentterminals(Eccles, 1964) md based WIthe import& increamof excitabilityof thcs~ tenninnlr.Thcfact that similartypes of inhibi. ti0n OEEulwhen the condilioning01 testingstimuli were lppkd thmugb differentpathways, ineludingthe tbm&

OF DENDRlTES

83

activation of motoneumns through interneurons (fig. 5. C and D), is hardlycompa!iblewith the idea of a presynaptictype of inhibition.In all cases there is an increase in excitability of the afferent terminals, such as is illustrated in fig. 6D, but the temporalcourse and duration?f tbis changet usuallydifferentfrom that of the decrease&& ?.autoneuronexcitability. Depolarization of the fine terminal ce1tainl.vOCCUIE, but it can be reasonably explainedby the effects of the electrical field. This idea is wpported by the fact tha’t the increase in excitability may affect the acttie as well as the non-active fibers and also the motoneuron somata not previously activated (section 3.4). The more marked late reduction in motoneuron excitability after a conditioning stimulus to the dorsal root (fig. 5, C. D) could be due to the shorter duration of the excitatory or facilitatory effects of the interneuron discharges following the conditioning stimulur. This inferenceis based on the smallmagnitudeof the late negativityof the corresponding spinal potential. It has been shown that the number of such discharges evoked by the test volley is reduced (Garcia Ramos and H&r Sinchn, 1%9b). Another factor may be the smaller exciting value of the interneuron die charges consequent to the test stimuli. Compatible with this idea is the observation that wizenthe motcneuron excitability was tested by direct stimulation through the exploring electrode, the changes were also of small magnitude (fig. SC). The importance of the prolonged interneuron discharges for the changes described is suggested by, the differences observed after spinal transest:on (fig. 6, A and B:)since, in a spinal animal, the number of impulses impinging on the i~terneurons decreases. In this respect, it is importan! to point out that the marked late increase in motoneuron excitability which appears in fig. 5 A and B was not observed in the animals anesthetized with barbiturates. It has been repeatedly mentioned here that the different methods of testing the excitability gave someivhat different results. The differences, however were only quantitative. The smallest changes were obwwed when the test stimuli were applied directly through the exploring electrode (fg 6C). This may bo due to the fact that the neumn~ teated were not exactly the same 85 those involved in the conditioa in&response, and that the neurcet~which change in tiring level could bc too few at the low of htflucnce

04

JCARCIA

of the stimulated site. On the other hand, the decrease in excitebiliiy appeared less impMant when the tests were made through polysynapdc pathways (fig. 6, A er,dB). This could be explained, first, by the retatively large number of multisynaptic endings activated by the test shock. and second, by the different bcati ,n of these endings on the. neurons, since probably most of them form axoden&itic synapses which would be more influenced by the factitalmg electrotonic effect of the depolarization of the fu: branches. In addition, and this is probably the most important factor, since the impulses from the multisynapdc pathways reach the mOto”E”Iom later than thosz trawuing over the monosynaptic tracts, they will ariive within the initial pried of increase in excitability of those elements. The fundamental similarity ir;.the temporal course of the excitability changes of motoneumns, following volleys of impulses in diffe1ent pathways, suggests that the processes involwd are similar in the various instances. It is sug@ed that the electrical events whiih DCEUIat the dendrites with a specific time squaw 8% the most important factors that determine the changer observed.

ACKNOWLEDGEMENT I wish to express my gratitude to Dr. Artum Rosenblueth for bis cwUtuous helR and advice.end to Mrs. Virginia T.Roaer.blueth for her assistance in preparing the manuscript.

RAMGS RBFERENCES J.M. md K.Kulwta,1963,Sadies af the infqn tive functienof the metor neweue,hog. BrainRIL 1. 38.

Bmdlm.t,

eat, ActaPbysiol.Lalbleam.19, in Pras. Garct Ramos I. and P.Hti Sinchez,1%9b, ExtnccsUlv tham., ampted for P&Iication. Gertelmd,R.C.,B.Hov&ed,J.Y.l.cttvinand W.Pitt8,1959, A commenton micmeleclrodes,Rec. Inst. RadioEegn 41. 1856. Lore& de No,R., 1947,Actionpotentialof the motonuwon~ of the hype@rulrn”cleur,1. CelLCcmq.PhyU.29. 207. Mattbwx,B.H.C. 1966,S@e fiber rtivatien ofeeelral newoyssystem,ia: Cm&s Muscuhrdfcnnu aad meta contrel (NOal Sym@m I) Mtqvist md WIW, Stc‘cLl,0lm, D.221.