Philadelphia, Pa. 19104, U.S.A.

J. Physiol. (1971), 218, pp. 515-532 515With 11 text-ftgure"Printed in Great Britain

DEPOLARIZATION OFTHE TOOTH PULP AFFERENT TERMINALS IN THE BRAIN

STEM OF THE CAT

BY W. I. R. DAVIES,* D. SCOTT, JR., K. VESTERSTR0MAND L. VYKLICKYt

From the Department of Physiology, University of Pennsylvania,Philadelphia, Pa. 19104, U.S.A.

(Received 15 September 1970)

SUMMARY

1. Tooth pulp afferent fibres belonging exclusively to the AA groupwere stimulated bi-polarly with electrical pulses applied to the dentineand the central effects of the stimulation were examined in the nucleusof the spinal trigeminal complex of anaesthetized cats.

2. Field potentials evoked by single pulses to the upper or lower caninetooth pulp were explored over the nucleus of the spinal trigeminal tract inthe region 5 mm caudally to the obex up to 8-5 mm rostrally to the obex.They were found to be restricted to a region of 5-5-8-5 mm rostrally to theobex, 4-5mm laterally from the mid line, with a maximum amplitude at adepth of 5+ 0-5 mm.

3. Antidromic action potentials were recorded from the tooth pulpafferents. The stimulating micro-electrode was inserted in the region ofthe medulla from which distinct field potentials could be recorded byorthodromic stimulation.

4. Excitability of the central terminals of the tooth pulp primaryafferents was increased when the test stimulus was preceded either by aconditioning volley in low threshold afferent fibres of the infraorbitalnerve or in Ad fibres of another tooth.

5. Single pulses applied to the afferent fibres of the tooth pulp producedchanges in the excitability of central terminals of the fast conductingafferent fibres of the infraorbital nerve.

6. The greatly increased excitability of central terminals of tooth pulpprimary afferent fibres was accompanied by a small decrease in their

* Present address: Department of Periodontology, University College HospitalDental School, Mortimer Market, London W.C. 1.

t Present address: Laboratory of Cellular and Comparative Neurophysiology,Czechoslovak Academy of Sciences, Institute of Physiology, Praha 4-Kr6, Bud&jovicka 1083, Czechoslovakia.

21-2

W. I. R. DAVIES AND OTHERS

synaptic efficiency as estimated from the changes of the post-synapticcomponent of the field potential evoked by their stimulation.

7. It is concluded that an analogous presynaptic control mechanism ispresent at the central terminals of the tooth pulp primary afferent fibresas is known for the fast conducting cutaneous system. But a difference inthe specifity of the action from this afferent system is suggested. Whileactivity from Aa fibres is very effective in evoking primary afferentdepolarization at both fast conducting trigeminal fibres and slow con-ducting Ad system, the Ad fibre activity is effective only at the latter,and not at the former.

INTRODUCTION

Depolarization of the central terminals of primary afferent fibres (PAD)has been extensively studied in the spinal cord and evidence has accumu-lated to show that it is associated with presynaptic inhibition (cf. Eccles,1964). Stimulation of fast conducting cutaneous fibres accounts pre-dominantly for PAD of the same afferent group in the spinal cord (Eccles,Schmidt & Willis, 1963), and similar findings have been obtained fromtrigeminal afferent fibres in the medulla oblongata (Darian-Smith, 1965;Stewart, Scibetta & King, 1967; Vyklicky, Maksimova & Jirousek, 1967).

There is still controversy on the effects produced by C fibre stimulation.Selective excitation of unmyelinated C fibres was reported to producehyperpolarization of the fast conducting cutaneous fibres in the spinalcord (Mendell & Wall, 1964; Dawson, Merrill & Wall, 1970), but otherfindings indicate that depolarization may also be produced using a dif-ferent experimental arrangement (Zimmermann, 1968; Franz & Iggo,1968; Vyklicky et al. 1967).There is, however, very little information on primary afferent de-

polarization which may be produced at the central terminals of the slowconducting afferent system belonging to the A5 group. It is not known towhat extent selective stimulation of this afferent group may contribute tothe depolarization of the central terminals of the Acc fast conductinggroup. This lack of information is mainly due to the difficulty of evaluatingthe effects produced at central terminals of the A& fibres, since Aa andA6 fibres are intermingled in peripheral nerves and dorsal roots. Theexistence of a presynaptic control mechanism at the terminals of A4afferent fibres would be of importance, since these fibres are undoubtedlyconcerned with mediation of pain sensation (Heinbecker, Bishop &O'Leary, 1933) and moreover it has been suggested that modulation ofafferent activity by presynaptic inhibition may play an important role inthe mechanism of pain (Melzack & Wall, 1965).

However, the tooth pulp afferent fibres 1-7 , in diameter and amaximum

516

TOOTH PULP PRIMARY AFFERENT DEPOLARIZATION 517

conducting velocity 30-45 msec represent an isolated and rather homo-geneous group of Ad fibres (Brookhart, Livingston & Haugen, 1953), andpain is the only modality of sensation which, to our knowledge, has beenreported by their stimulation (Livingston, 1943).The aim of this study was to explore the projection of the tooth pulp

primary afferent fibres in the medulla oblongata and to learn if there isa similar presynaptic control mechanism at the central endings of theAd primary afferent fibres to that already described for Aa fibres in thespinal cord and medulla oblongata. A part of the results has already beenreported in preliminary form elsewhere (Vyklicky, Davies, Vesterstr0m &Scott, 1970).

METHODS

Twenty cats (24-4.2 kg) anaesthetized with pentobarbitone sodium 45 mg/kg,intraperitoneally, were used in the experiments. Subsequent intravenous doses of10-15 mg were administered, when required, to maintain the anaesthesia. The cat'shead was fixed in a Horsley-Clarke stereotaxic apparatus. The posterior fossa wasexposed and posterior parts of the cerebellum were sucked away to gain access tothe medulla oblongata.Two small cavities were drilled into the dentine of the opposite sides of the upper

or lower canine of both under a dissecting microscope to a depth approximately03-0 1 mm from the tooth pulp and filled with Ringer agar gel. Tungsten electrodesof 20-100 #t uninsulated tip were inserted into each of the two holes and used eitherfor stimulation or recording from nerve fibres in the tooth pulp. The interelectroderesistance varied in individual experiments between 30 kQl up to 200 kM. Great carewas taken to prevent drying of the floor of the cavity or short circuiting of theelectrodes by saliva (for details see Scott & Stewart, 1965). This technique proveduseful for stimulating exclusively afferent fibres in the tooth pulp without anyadmixture of the fast conducting fibres in the periodontal membrane or gingiva.For current escape to the surrounding tissue the strength of the stimulus had to beincreased at least ten times above the intensity necessary for maximum amplitudeof the field potential. In that case, there was a sudden decrease of the latency andfrom the surface of the medulla a complex trigeminal potential was recorded,characteristic of the response when low threshold fibres in the cutaneous trigeminalbranches were being stimulated (see Fig. 1).A bi-polar electrode was placed on a dissected cutaneous branch of the infra-

orbital nerve for stimulation. In the experiments in which changes of the excita-bility in A a afferent terminals of the infraorbital nerve were studied, the eye on theleft side was removed, one branch of the infraorbital nerve was dissected in theorbit and placed on a pair ofAg/AgCl electrodes for recording, while the other branchwas used for stimulation. An Ag/AgCl monopolar electrode with a ball of less than1 mm diameter at the tip was placed on the surface of the medulla 3-4 mm belowthe obex and superficial to the nucleus of the spinal trigeminal tract to monitor allresponses from the surface of the medulla. The reference electrode was an Ag/AgClwire inserted into the neck musculature.The technique of Wall (1958) for studying changes in the excitability of the central

terminals of the primary afferent fibres in the spinal cord was adopted for studyingchanges in the excitability of primary afferents supplying the tooth pulp. Glasscapillary micro-electrodes of about 5-7 ,u tip were filled with 4 m-NaCl and used


either for recording the field potentials within the medulla or for antidromic stimula-tion of central endings of the trigeminal afferent fibres. An electrometer preamplifierwas used in connexion with a preamplifier for recordings with glass capillary micro-electrodes. Animals were immobilized with gallamine triethiodide and artificiallyventilated with a mixture of 02 (98%) and C02 (2 %). Blood pressure was monitoredin all experiments and ranged between 100-130 mm of mercury (systolic) in thedifferent experiments. Body temperature of the animal was maintained by heatingpads and kept between 37-38° C.

RE1SULTS

Localization of the tooth pulp primary afferent terminalsin the medulla oblongata

No direct evidence on the projection of the tooth pulp primary afferentsinto the medulla oblongata is, as yet, available. Therefore a systematicsearch for field potentials evoked by selective stimulation of the toothpulp primary afferent fibres in the trigeminal nuclear complex in themedulla was necessary.

Employing monopolar recording no detectable potential deflexioncould be recorded from the surface of the medulla at the level of the obexor caudally when a supramaximal stimulus was applied to the tooth pulpof the upper or lower canine teeth (Fig. lA, lower trace). This was instriking contrast to the potentials recorded from the surface of the medullawhen a stimulus was applied to other trigeminal branches containing, inaddition, fast conducting Aoa fibres. As demonstrated in Fig. 1B supra-threshold stimuli to the infra-orbital nerve produced typical complexsurface trigeminal potentials (Hammer, Ternecki, Vyklick' & Wiesen-danger, 1966) even when the corresponding field potentials recorded witha micro-electrode from a depth of 5 mm, 7 mm rostrally to obex (Fig. lBupper trace) were of lower amplitude than those evoked by stimulatingthe tooth pulp (Fig. IA upper trace). By systematic tracking with micro-electrodes in the rostrocaudal and mediolateral planes of the medullaoblongata, it has been found that field potentials evoked by stimulationof the tooth pulp in the upper or lower canine teeth could be recorded onlyin a region between 5 5 and 8 5 mm rostrally to the obex in the rostro-caudalplane and between 4 and 5mm laterally from the mid line. The records froma tract in this area are shown in Fig. 2A. The amplitude of the maximumnegative deflexion, measured from the base line, was plotted against thedepth at four different planes rostral to the obex in Fig. 2B. It can beseen that the maximum of the negativity of the field potentials was reachedat a depth of 5 + 0 5 mm. Stimulation of the tooth pulp nerve never pro-duced any sign of electrical activity between the planes reaching 5 mmrostral and 5 mm caudal to obex.Employing micro-electrodes of smaller tip diameter (2-3,), it was

518

TOOTH PULP PRIMARY AFFERENT DEPOLARIZATION

8

J100,V

2 msecFig. 1. Field potentials from the nucleus of the spinal trigeminal tract andsurface potentials from the medulla oblongata. Upper traces: field potentialsrecorded from the trigeminal nuclear complex with a capillary micro-electrode placed 7 mm rostrally to the obex and 5 mm from the mid line at adepth of 5 mm. Lower traces: monopolar recordings from the surface of theoblongata 3 mm below obex. A, stimulation of the upper canine tooth; B,stimulation of the infraorbital nerve. Calibration 200 #sV belongs to theupper traces and 100 RV to the lower traces.

& A 45

8 6 4 2mm S-16 Elull 111111-l

V11 i n S 86420mm

1. t9]8 10 S 75

E-0-5

Depth 5 5.5 6 4-5 5 5.5 6 45 5 5mmFig. 2. Field potentials in the rostral subdivision of trigeminal nuclear com-plex evoked by stimulation of tooth pulp afferents. A, recordings fromvarious depths in the medulla oblongata 8 mm above obex and 5 nunlaterally from the mid line evoked by stimulation of the upper canine tooth.B, diagrams the maximum negativity of the field potentials plotted againstthe depth at four subsequent tracks as indicated in scheme (left diagramobtained from recordings in A).

519

W. I. R. DAVIES AND OTHERSpossible to record extracellularly discharges of individual neurones inaddition to the field potential produced by stimulation of either the toothpulp primary afferents or the infraorbital nerve. Although no systematicstudy was made, a certain degree of specificity with respect to the afferentsource, stimulated, was found for the neurones in the nucleus activated byelectrical stimulation of tooth pulp afferents. As shown in Fig. 3A, Band C, the units discharged by stimulating the tooth pulp afferents atshort latency (2.5-3 msec) were activated rather specifically by fibres ofonly one tooth and not by impulses conveyed in the infraorbital nerve or

]50Au 2 mseeX~~~~~~~~

IF

Fig. 3. Extracellular recordings of units from various parts of the rostraltrigeminal nuclear complex (upper traces). Recordingsfromthe surface ofthemedulla 3 mm below obex (lower traces). Traces in each horizontal line arefrom the same point during tracking with the micro-electrode. A, D, F, I:stimulation of the upper canine. B, E, G, H, J: stimulation of the infra-orbital nerve. Note: the strength of stimulus was increased from G to H,and in J just suprathreshold stimulus was applied.

Calibration 100 uV applies to all upper traces, 50 ,sV applies to lowertraces A, B, and C, and 200 #uV to lower traces D to J.

520


afferent fibres of another tooth. Such specificity was less apparent forunits exhibiting longer latency (4-6 msec) as these could be dischargedfrom the tooth pulp afferents (Fig. 3D, F) as well as from higher thresholdfibres in the infraorbital nerve (Fig. 3E, H), but not from the low thresholdafferents (Fig. 3G). However, medially, outside of the nucleus, neuroneswere frequently encountered which were activated from the tooth pulpafferents (I) as well as from low threshold fibres ofthe infraorbitalnerve (J).

Thus, the afferent fibres from the tooth pulp, consisting of a homo-geneous population of slow delta fibres, appear to terminate in a smalldiscrete region of the rostral part of the spinal trigeminal nuclear complex.From the measurements of the co-ordinates at which distinct field po-tentials were consistently encountered, this area would correspond to therostral subdivision of the nucleus of the spinal trigeminal tract as definedby Eisenman, Landgren & Novin (1963).

Antidromic action potentials from the tooth pulp primary afferentsAntidromic action potentials in afferent fibres supplying the tooth pulp

were recorded by using tungsten electrodes inserted in small cavitiesdrilled in the dentine as described by Scott & Stewart (1965). When carewas taken to prevent drying of the dentine, stable potentials were obtainedfor many hours either by stimulating the alveolar nerve or that locus ofthe medulla in which distinct field potentials could be recorded by ortho-dromic stimulation.The form of antidromic potentials evoked by stimuli to the central

terminals of tooth afferent fibres is shown in Fig. 4. When the strength ofstimulus applied to the micro-electrode in the medulla was gradually in-creased, the action potential ofthe shortest latency appeared first (Fig. 4A),while action potentials of longer latency contributing to the formation ofthe latter parts of the complex antidromic potentials did not appear untilhigher intensity of the stimulus was used (Fig. 4B, C).

Conduction velocity of 35-40 m/sec for the fastest fibres was obtainedin eight experiments. This value is in agreement with that alreadypublished by Brookhart et al. (1953). We are uncertain about the con-duction velocity of the slowest afferent fibres, but from latencies of indivi-dual components of the antidromic potential, it seems likely to be lessthan half that obtained for the fastest fibres.

Changes in the excitability at the central terminals ofthe tooth pulp afferent fibres

Changes in the excitability at the central terminals of primary afferentfibres have been introduced as a measure of their depolarization in thespinal cord by Wall (1958). This method has also been successfully applied

W. I. R. DA VIES AND OTHERS

for studying depolarization of the central terminals of the fast conductingtrigeminal afferent fibres (Darian-Smith, 1965; Hammer et al. 1966;Stewart et al. 1967) and has, therefore, been employed for the primaryafferents supplying tooth pulp.

A

|B

I msecU~~~~~~~0U

Fig. 4. Antidromic potentials from the tooth pulp primary afferents.Antidromic potentials recorded from the tooth pulp of the upper canine.Cathodal pulses of increasing strength from A to C were applied to themicro-electrode in the rostral part of the trigeminal nuclear complex.Position of micro-electrode was determined for maximum orthodromicresponse as shown in Fig. 2.

The technique is diagrammatically shown in Fig. 5E. The micro-electrodewas introduced into the region of maximal amplitude field potentialsevoked by orthodromic stimulation of the tooth pulp afferents (Fig. 5Fb).Then the stimulating and recording sites were reversed and cathodalpulses of 0-1 msec duration were applied to the micro-electrode whileantidromic action potentials were recorded by tungsten electrodes insertedin the two holes of a canine tooth. The antidromic potentials recordedfrom the tooth are shown in the upper traces in all records. Lower tracesare monopolar recordings from the surface of the medulla above thetrigeminal nuclear complex 3 rum caudal to the obex. Fig. 5A is the testvolley alone. In the subsequent records it was preceded by a conditioningvolley applied to the infraorbital nerve at varying intervals (Fig. 5B,

522

TOOTH PULP PRIMARY AFFERENT DEPOLARIZATION 52315 msec; 5C, 70 msec; 5D, 220 msec). From these records it can be seenthat a conditioning volley in the infraorbital nerve produced markedenhancement of the antidromic potential recorded from the tooth pulp.As antidromic test potentials were of complex form, changes in their

magnitude were determined by comparison with potentials evoked by thetest stimulus alone and expressed as percentages of the area (measured byplanimetry), which was covered by the most prominent part of the anti-dromic potential (see horizontal bar in Fig. 5B) rather than as percentagesof the peak amplitude. In the graph attached to this Figure, increaseof the area covered by the antidromic potential was plotted against thetime interval between the conditioning and test stimulus. It can be seenthat the time course of increased excitability is very similar to that whichhas already been described for other primary afferent fibres in the spinalcord (see Eccles, 1964).Cutaneous branches of the trigeminal nerve consist of fibres of various

conduction velocity (Darian-Smith et at. 1965) and, thus, unequal thresholds

ATest B Area 1c *meD -

00,10 Me I20ms100msec

200 F, I ISOOpV4J~~~~~~~~~~~~~~~ a

a~~~~~~see

660- .150~50 00ms

100~50 100 IS o

Fig. 5. Changes of the excitability at the central terminals of the toothpulp primary afferent fibres ofthe upper canine following avolley ofimpulsesin the infraorbital nerve. Upper recordings: A, antidromic action potentialfrom the tooth pulp of the upper canine evoked by a submaximal cathodalpulse delivered to the glass capillary micro-electrode. The micro-electrodewas introduced in the rostral part of the trigeminal complex at a pointwhere field potentials could be recorded by stimulation of the upper canine(Fb) or the infraorbital nerve (Fa). B, C and D: increase of the antidromicpotential at three selected intervals following a conditioning volley in theinfraorbital nerve. Lower traces: monopolar recordings from the surface ofthe medulla 3 mm below obex. Note differences of the time scale for theupper and lower traces. In the diagram the relative increase of areas coveredby the antidromic potential between the time limits indicated in B wereexpressed as percentages of test A and plotted against time intervalbetween the conditioning and test pulse.


for electrical stimuli can be expected. It was therefore attempted toestimate the extent to which the low and high threshold trigeminalafferent fibres contribute to the depolarization of the tooth pulp afferentterminals.The threshold for the infraorbital nerve was estimated from the tri-

geminal potential recorded from the surface of the medulla as the firstdeflexion visible at high amplification and the interval 25 msec betweenthe conditioning and testing stimulus was chosen as it was optimum for

I Test Area

i msec AMTIS42msec 3.7xTh 'r

-150 ;i ;OO'4Fig. 6. Changes of excitability at the central terminals of tooth pulp primaryefferent fibres produced by increasing strength of the conditioning stimulus.Upper traces: A, antidromic test potential recorded from the tooth pulp ofthe upper canine. B and C demonstrate increase of the antidromic potentialwhen conditioned by a volley in the infraorbital nerve at an interval of25 msec. Intensity of the conditioning volley was 3-7 x threshold in B and10 x threshold in C. Lower traces: monopolar recordings from the surface ofthe medulla 3 mm below the obex.

the increase of excitability. Strength of the conditioning stimulus to theinfraorbital nerve was then gradually increased from the threshold up toa maximum of about ten times that value. Specimen records in Fig. 6demonstrate examples of increased excitability for conditioning stimuli3-7 x and 10 x threshold. However, significant increases in excitabilitywere obtained when the strength of the conditioning shock was only 1-2times threshold, indicating that impulses in the low threshold Aa afferentfibres in the trigeminal nerve account for the depolarization of the centralterminals of the tooth pulp afferent terminals.

Changes in excitability at the central terminals of the tooth pulp primaryafferent fibres produced by a volley of impulses in the nerve fibres supplying

the pulp of another toothThe high threshold Ad fibres in the infraorbital nerve can be stimulated

if the strength of a stimulus is increased above the maximum for Aac fibres.However, it is difficult to estimate the contribution of the activity of Adgroup to the depolarization of the tooth pulp primary afferents, as most ofit is already produced by stimulation of the low threshold afferent fibres(see Fig. 6). Nevertheless, the effects of selective stimulation of AMfibres can

524


be estimated when a volley of impulses in afferent fibres of another toothpulp is employed for conditioning. Recordings in Fig. 7A, B, C demonstratethe increased antidromic action potentials recorded from the tooth pulpof the lower canine when the test (D, E, F) was preceded at three selected

t10msects{ 2Omsoc%=

500 10 5

0~~~~~~~10#0

10mec s20meq~

150

000

100~~~~~~no b sop00 15

Fig. 7. Changes of excitability at the central terminals of the tooth pulpprimary afferent fibres induced by a volley in afferent fibres supplying thepulp of another tooth. Upper traces: A, B and C recordings of the anti-dromic potential recorded from upper canine when conditioned by a volleyin nerve fibres supplying the tooth pulp of lower canine at three selectedintervals. Traces D, E and F are the corresponding test antidromicpotentials. Lower traces are monopolar recordings from the surface of themedulla 3 mm below obex. The time scale 10 msee applies to lower tracesA, B, D and E, 20 msee to lower traces C and F, 1 msec to all upper trace.In the diagram the relative increase in area covered by antidromic potentialplotted against the time interval between the conditioning and teststimulus.

intervals by a volley in afferent fibres from the upper canine tooth. In thegraph attached to this Figure the ratios of the area covered by the anti-dromic potential as measured by planimetry were plotted against the timeinterval between the conditioning and test stimulus. It can be seen that the

W. I. R. DA VIES AND OTHERS

intensity and the time course of the increased excitability of the toothpulp afferent terminals produced by a volley in Ad fibres of another toothis comparable to that elicited by a volley in the infraorbital nerve (Fig. 5).

Effects of impulses in tooth pulp primary afferents on the excitability ofcentral terminals of the fast conducting fibres in the infraorbital nerve

It has been demonstrated that under certain experimental conditionsactivation of the high threshold unmyelinated C fibres may exert oppositeeffects at the central terminals of the Aa cutaneous fibres in the spinal cordto those produced by activation of the low threshold cutaneous fibres(Mendell & Wall, 1964; Dawson et al. 1970). It was therefore of interest tolearn if there are any significant differences between the high thresholdmyelinated AA fibres and the low threshold Ax fibres in respect to thePAD elicited at the Ax cutaneous fibres of the infraorbital nerve. Themicro-electrode in the medulla was positioned to record the field potentialsevoked by stimulation of nerve fibres in the upper canine tooth pulp orin the infraorbital nerve, as shown in Fig. 5F. Subsequently submaximalcathodal pulses delivered through this electrode produced antidromicaction potentials in one branch of the infraorbital nerve, which then servedas a test for studying the changes of the excitability produced by a volleyin the nerve fibres of the upper canine.From the graph in Fig. 8, in which the relative peak amplitude of the

antidromic spike was plotted against the time interval between the con-ditioning and test stimulus, it can be seen that the changes of excitabilityin both directions (hyper- and hypoexcitability) did not exceed 10 %.

This finding was in striking contrast to the effects exerted at the centralterminals of the AA trigeminal afferents when the conditioning stimuluswas applied to another branch of the trigeminal nerve. Fig. 9 demonstratesthe changes of the excitability at the central terminals of the afferentfibres ofthe same branch ofthe infraorbital nerve (micro-electrode positionthe same as in Fig. 8) when a conditioning stimulus was applied to anotherbranch of the infraorbital nerve instead of to the tooth pulp. The changesof excitability were plotted as percentages of the control amplitudes of theantidromic spike and related to the interval between the conditioning andtest stimulus. The maximum relative increase in amplitude of the anti-dromic potential, which exceeded 200% of the control, was reached around25 msec after a conditioning stimulus, but the total duration of increasedexcitability was detectable for a period exceeding 200 msec. Thus, thetime course of the increased excitability at the Ax trigeminal terminalsproduced by a volley in the infraorbital nerve is in good agreement withthe previous studies dealing with the same afferent system (Darian-Smith, 1965; Vyklick' et al. 1967; Stewart et al. 1967).

526


Changes of the post-synaptic field potential which might beattributed to presynaptic inhibition

In the theory of presynaptic inhibition, it was proposed that depolariza-tion of the primary afferent terminals is accompanied by lowering of theirsynaptic efficiency (Eccles, 1964). An attempt was therefore made to find

150,

0*

100 0

50 100msec

Fig. 8. Changes of excitability of the central terminals in the fast conductingfibres in the infraorbital nerve to a conditioning volley in the tooth pulpafferent fibres. Diagram demonstrates excitability changes at the centralterminals expressed as percentages of peak amplitude of the antidromicpotential against the interval between the conditioning and test stimulus.

out if there are any changes in the post-synaptic component of the fieldpotentials evoked by stimulation of tooth pulp afferents, which could berelated to the time course of increased excitability at their terminals.The effect of a volley in the infraorbital nerve on field potentials evoked

by stimulation of the nerve fibres in the tooth pulp of the upper canine isdemonstrated in Fig. 10. The amplitude of the conditioned responses atthree selected intervals in A, B, C can be compared with the correspondingtest responses D, E, F. In the graph the percentages of the controlamplitude of the field potential were plotted against the time intervalbetween the conditioning and test stimulus. A decrease of the amplitudeof the field potential was found at very short intervals up to 5 msec, andat the intervals between 15 and 70 msec. However, the depression of thefield potentials was small and never exceeded 20 %.When the conditioning volley originated in the afferent fibres supplying

528 W. 1. R. DAVIES AND OTHERS

the pulp of another tooth, an early profound depression of the field po-tential lasting up to 8 msec was observed, but during the period of in-creased excitability of afferent terminals only a slight depression was seen

250

200a,

AWo4SooIf

150

inn

* 0

* 0

0*@0

*0

0 0

0

0

0

*

50 100 150 200 250

msec

Fig. 9. Excitability changes at the central terminals of the infraorbitalnerve induced by a volley of impulses in another branch of the same nerve.

Micro-electrode was inserted at the same position as in Fig. 8. The increaseof peak amplitude of the antidromic potential was plotted against the timeinterval between the conditioning and test pulse.

I rIC@

000 ---

404.C

0",0

0o

10 20 30 40 50 60 70 80msec

Fig. 10. For legend see opposite page.

Ia a a

A"..$A'r"

160~

TOOTH PULP PRIMARY AFFERENT DEPOLARIZATION 529(Fig. 11). Thus a conditioning volley in the afferent systems which accountsfor a high increase of excitability of tooth pulp afferents (see Figs. 5 and 6)results in relatively small depression of the post-synaptic component of thefield potentials evoked by their stimulation. The initial short latencydepression was not analysed further, but it can be expected that it is dueto occlusion or short latency post-synaptic inhibition of the secondaryneurones.

A 4 msecB C ICF-msec iD ~20m-sec

E F G H

&100\t-Ti10 20 30 40 50 60 70 80

Fig. 11. Depression of post-synaptic field potentials from the tooth pulpinduced by a volley of impulses in afferent fibres of another tooth. Uppertraces: field potentials recorded from the rostral division of the spinaltrigeminal complex. Lower traces: same as in Fig. 10. A, B.aandD; thetestvolley in the afferent fibres of the upper canine tooth was preceded by avolley in the afferent fibres from the tooth pulp of the lower caninetooth. E,F, G and H are corresponding controls. Time scale 4 msec applies to A,B, E and F, 10 msec to C and G and 20 msec to D and H. In the diagramchanges ofamplitude ofthe field potentials were plotted against time intervalbetween the conditioning and test stimulus.

Legend to Fig. 10.

Fig. 10. Post-synaptic field potentials evoked by a volley in the upper canineconditioned by a stimulus to the infraorbital nerve. Upper traces: fieldpotentials recorded from the rostral subdivision of the spinal trigeminalcomplex. Lower traces: monopolar recordings from the surface of themedulla 3 mm below obex. In A, B and C the test volley in the afferentfibres ofthe upper canine was preceded by a volley in the infra-orbital nerve.D, E and F are the corresponding controls. In the diagram the changes ofamplitude of field potentials were plotted against the time interval betweenthe conditioning and test pulse. The time scale 4 msec applies to A, B, Dand E, 10 msec to C and F.


DISCUSSION

Our experiments indicate that primary afferent fibres from the toothpulp of the upper and lower canines of the cat terminate in a small area inthe rostral part of trigeminal nuclear complex in the medulla. This con-clusion is based on three facts revealed in our study.

First, field potentials evoked by stimulation of tooth pulp primaryafferents could be recorded only in the area which, according to stereotaxicco-ordinates, corresponds to the rostral subdivision of the nucleus of thespinal trigeminal tract (Eisenman et al. 1963).

Secondly, antidromic action potentials in the tooth pulp primaryafferents could be elicited only if the micro-electrode was inserted in theregion from which distinct field potentials were recorded by their ortho-dromic stimulation. No antidromic action potentials in the tooth pulpprimary afferents could ever be elicited by electrical pulses applied to themicroelectrode inserted in the caudal part of the medulla oblongata,although exploration was made in the rostrocaudal plane reaching as faras 5 mm caudal to the obex.

Thirdly, the fact that we have seen maximal changes in the excitabilityof primary afferent fibres in the area corresponding to the maximumamplitude of the field potentials evoked by their orthodromic stimulationmay also be considered as further evidence about the locus of theirterminations, as it has been suggested that maximal depolarization islocated near the endings of primary afferent fibres (Eccles, 1964).However, there are inaccuracies involved in the readings of co-ordinates

and the variability of individual preparations as pointed out by Eisenmanet al. (1963). Therefore, histological controls are desirable for furthermore precise anatomical localization of individual neurones activatedfrom the tooth pulp primary afferent fibres.

If it is assumed that pain is the only modality of sensation subserved bythe tooth pulp primary afferent fibres (Livingston, 1943), this localizationseems to be rather surprising in view of the studies in man demonstratingthat transaction of the spinal trigeminal tract in patients sufferingunbearable facial pain may result in cessation of pain, and analgesia offace and mouth, even when the transaction is performed caudal to theobex (Kunz, 1964). It would be of importance to know if patients withspinal trigeminal tractotomy caudal to the obex report tooth pain incircumstances when stimulation of dentine receptors can be expected. Ifthis is the case, it would be possible to suggest that human toothpulp primary afferents project rather rostrally in the spinal trigeminalcomplex. Alternatively, if there is analgesia of the tooth after tractotomyperformed caudal to the obex, appreciable differences in the projection

530


of the tooth pulp afferents should be expected in different mammalianspecies.However, in the cat such a projection oftooth primary afferents would be

expected, as Eisenman et al. (1963) described units in the rostral sub-division of the nucleus of the spinal trigeminal tract in the cat which couldbe activated exclusively by painful stimuli, and it has been reported in anearlier study (Gerard, 1923) that transaction of the tractus spinalis n. tri-gemini at various levels in the cat did not abolish the nociceptive reactionto stimulation of the cornea until the level reached the mid-pontine area.

Increased excitability of the central terminals of the Ad afferent fibressupplying the tooth pulp was found following a volley of impulses eitherin low threshold Aa afferent fibres of the infraorbital nerve or in Ad fibresof another homolateral tooth. The time course of the increased excitabilityis comparable to that previously described at the central terminals of thefast conducting cutaneous nerve fibres in the spinal cord (Eccles et al.1963) and at the central terminals of the fast conducting afferent fibres inthe branches of the trigeminal nerve (Darian-Smith, 1965; Stewart et al.1967; Vyklick' et al. 1967). It is therefore likely that a similar presynapticcontrol mechanism exists at both AA and Aa afferent terminals.However, there is an indication of the differential role of the tooth pulp

Ad fibres in evoking PAD at the central terminals of the fast conductingand slow conducting A afferent system. The evidence is presented inFigs. 8 and 9 demonstrating that increased excitability at the centralterminals of the fast conducting trigeminal fibres may be produced by avolley in the same afferent group of another trigeminal branch, but not byimpulses arriving in the medulla in tooth pulp afferent fibres. Small changesin excitability demonstrated in Fig. 8 could only be due to spontaneousvariations of excitability.The explanation of the low effect in evoking PAD in the fast conducting

trigeminal afferents by impulses in tooth pulp nerves is uncertain. Thesmall number of afferent fibres in the tooth pulp does not seem an adequateexplanation, because a pronounced increase in excitability at the centralterminals of afferent fibres supplying tooth pulp can be induced bystimulation of a nerve from another tooth (Fig. 7). A more likely explana-tion seems to be that the decreased efficiency of the transmission fromA& fibres to the Aa terminals may result from the relative specificity in theinteraction of the two classes of afferent fibres.A pronounced increase in excitability of the tooth pulp afferents has

been found associated with only small changes in the efficiency of theirsynaptic transmission (Fig. 5, 7, 10 and 11 respectively). However, werealize that the method of comparing amplitudes of focal potentials, whichwas employed in these experiments for estimation ofthe synaptic efficiency,


is limited in sensitivity and accuracy of evaluation and that these resultsdo not therefore invalidate the original suggestion by Eccles (1964) thatthe PAD may play an important role in processing sensory information.

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DARIAN-SMITH, I., MUTTON, P. & PROCTOR, R. (1965). Functional organisation oftactile cutaneous afferents within the semilunar ganglion and trigeminal spinaltract of the cat. J. Neurophyeiol. 28, 682-694.

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