Exp. Eye Res. (1961) 1, 160-167

Sensory Mechanisms and Intraocular Pressure

E. S. PERKINS

Institule oj Ophthalmology, London, England

(Received 21 July 1961)

Recordings from the long ciliary nerves of cats and monkeys demonstrated fibreswhioh responded to a. rise in intraocular pressure by an increase in frequency of poten­tials which deoreased when the pressure was reduced. Such fibres were found in 6 out of22 cats and lout of 4 monkeys. In no animal was there an exact parallel between thechange in pressure and the frequency of the spikes. Using mioro-eleebrodes the regionof the spinal nuoleus of the fifth cranial nerve subserving corneal sensation was ex­plored in the oat. The only responses to changes in intraocular pressure recorded fromthis region were bursts of spikes after raising the intraocular pressure rapidly. Nosustained disoharges were obtained. It is concluded that these experiments fail todemonstrate any direct afferent pathway for information concerning the level of intra­ocular pressure in the animals studied.

Introduction

There is now a considerable amount of evidence that under experimental conditionsin animals, stimulation of some areas of the diencephalon can produce acute changesin intraocular pressure (von Sallmann and Lowenstein, 1955; Gloster and Greaves,1956, 1957). The efferent pathways by which the effects are transmitted have notbeen fully elucidated, but decreases in intraocular pressure are probably mediated bythe sympathetic system (Greaves and Perkins, 1952). Increases in intraocular pressurehave been found to occur in rabbits following stimulation of-the fifth nerve (Perkins,1957) and recently Gloster (1961) has suggested a pathway via the seventh cranialnerve and the greater superficial petrosal nerve which may be responsible for some ofthe e:ffecta of diencephalic stimulation in the cat and rabbit.

These results lend some support to the view that the normal intraocular pressure isunder the control of the central nervous system. Any direct form ofnervous control­such as that for systemic blood pressure-would require a sensory or afferent pathway,a regulatory centre in the central nervous system and an e:fferent pathway. Lessattention has been devoted to the afferent aspects of such a system than to thecentral and efferent mechanisms. The hypothetical requirements of a sensory pathwayare, first, baroceptors in the eye and, second, a sensory pathway to the centralnervous system.

There is some histological evidence for specialized nerve endings in the eye. Vrabec(1954), for example, described the innervation ofthe human trabeculum as arranged ina lattice pattern with nerve endings as loops or free terminals. Kurus (1955) describedganglion oells in the choroid of human eyes which were of three types: large onesconnected directly or indirectly with each other and with structures like end plates;two smaller types, one less than half as large; and unipolar and minute bipolar cells.This ganglion network is interspersed with leaf-like and other structures, reminiscentof Krause's capsules and Meisner's taste buds. The system is connected with theciliary nerves and the ciliary ganglion and was thought by Kurus to have a baro­regulatory function.

160

SENSORY MECHANISMS AND INTRAOCULAR PRESSURE 161

Holland, von Ballmann and Collins (1956) studied the innervation of the angle ofthe anterior chamber in rabbits and found a plexiform arrangement of delicatepreterminal and terminal axons which were interwoven in different places and endedas free axonal filaments. They arose from two systems of nerve fibres, the irido-ciliaryplexus and the nerves which supply the cornea. Free nerve endings were shownbeneath the endothelium of Schlemm's canal and within the trabecular meshwork thatforms the inner wall of this structure. Dieter in 1940 published a brief report on actionpotentials in short ciliary nerves which were obtained from increasing the intraocularpressure either by pressing on the cornea or by injecting fluid into the anteriorchamber. He considered that pain receptors were not responsible for the potentialsbut gave too few details for an accurate assessment of his results. In the same yearTower (1940), while investigating the pattern of innervation of the cornea in cats,reported background spike potentials from the long ciliary nerves which disappearedwhen the intraocular pressure was reduced by paracentesis. Raising the pressureartificially produced much electrical activity in the nerve which was thought not toarise from the cornea as the pattern of discharge was different from that of cornealfibres.

Further studies on the ciliary nerves were reported by myself and by von SaUmannand Macri at the Josiah Maay Conference on Glaucoma in January 1958. The latterauthors' results were published more fully later (von SaUmann, Fuortes, Macriand Grimes, 1958). They placed electrodes on the ciliary nerves of cats and monkeysand recorded the electrical responses while changes were induced in the intraocularpressure. In 8 out of 37 experiments with cats and 2 out of 6 with monkeys changesof intraocular pressure were associated with electrical responses in the ciliary nerves.The frequency of impulses in the nerves was found to increase in a manner roughlyproportional to the pressure increments, though in no case could strict linearity beestablished between the height of the pressure and the rate of the spike-potentialsrecorded.

PART I

RECORDINGS FROM THE CILIARY NERVES

1. Methods

In my own experiments the animals were anaesthetized with nembutal and the headfirmly held in the stereotactic instrument designed by Gloster and Greaves (1957). Theskull was exposed and the bony vault removed on one side. The cerebral hemisphere onthis side was then removed and the roof of the orbit opened.

On entering the orbit the ophthalmic division of the fifth nerve in the cat divides intotwo branches. One, corresponding to the frontal branch in man, crosses the third andfourth nerves and runs forward laterally and then above the levator palpebrae muscle.The other branch, corresponding to the naso-ciliary in man, runs forward over the thirdnerve and the optic nerve between the levator muscle and the medial border of the superiorrectus. Just before crossing the fourth nerve it gives off two fine filaments, the medial andlateral long ciliary nerves. The lateral nerve joins with a short ciliary nerve from the ciliaryganglion. The medial nerve runs with short ciliary nerves to the globe. The long ciliariescontain sensory fibres to the cornea and iris and also sympa.thetic fibres to the pupil.

One or both long ciliary nerves were separated from the nasal branch of the :fifth nerveand cut centrally to give as great a length as possible from which to record. One of thenerves or part of one nerve was then. laid on platinum electrodes and the whole bathedin warmed, oxygenated paraffin. The electrodes were connected to a cathode-follower

162 E. S. PERKINS

input and a sensitive a.c, amplifier which fed a power amplifier and loud-speaker and alsoa cathode-ray oscilloscope. Under favourable conditions spontaneous activity could bepicked up from the nerve and touching the cornea produced bunches of spikes as shownin Plate l.

A needle, connected through a tap to a reservoir of saline and a recording manometer,was then inserted into the anterior chamber. The manometer used consisted of a perspexchamber with a glass diaphragm attached to the anode of an RCA transducer valve(No. 5734). Slight movements of the anode of this valve alter the characteristics of thevalve, increasing or decreasing its total resistance. By means of a suitable amplifiercircuit these changes can be converted to changes of voltage and can be displayed on thesecond beam of the oscillograph so that the action potentials and the pressure changescan be observed and photographed simultaneously. In addition it has been found veryconvenient to record the action potentials on a tape recorder so that they can be replayedand studied after each experiment.

The experiment is not easy and many technical failures were initially encountered.Recording of action potentials from touch receptors in the cornea was used as a criterionof technically adequate experimental conclitions and was achieved in 22 cats and 4monkeys.

2. Results

When touch fibres were demonstrated successfully, the anterior chamber was oon­nected to the saline reservoir at a pressure of25 em saline and the tap closed. Record­ing commenced and the reservoir of saline was raised to the required pressure and thetap opened. The change in intraocular pressure was shown by the recorcling man­ometer and correlated with the pattern of discharges obtained from the nerve.

When the pressure was raised, for example from 25 em to 65 em saline, itwas commonto find a burst of activity following immediately, but dying down over the next 5 sec.This probably came from fibres which adapt quickly and, from the fact that theactivity was mostly abolished when a local anaesthetic was applied to the cornea, itis likely that it was produced by slight movements of the eye associated with theinjection of fluid.

The most convincing results have been those in which a single spontaneously dis­charging fibre has been isolated. In these cases it has been possible to measure therate of discharge and correlate the rate with variations in intraocular pressure. Thesespontaneous potentials do not appear to arise from touch receptors in the cornea asthey are usually unaffected by corneal stimuli. Their site of origin is not yet deter­mined. Certainly many of them are not associated with pressure receptors as they maycontinue to discharge at a steady rate in spite of changes in intraocular pressure.

In 6 out of 22 cats and lout of 4 monkeys, fibres were found which responded to arise in intraocular pressure by an increase in rate of discharge which decreased whenthe pressure was reduced.

The rate of discharge was usually less than lO/sec at 25 em of saline, rising toabout 40/sec at 60 em of saline. In one cat a rate of 74 spikes/sec was obtainedat a pressure of 40 em saline.

Graphic representations of the results from 4 cats are shown in Fig. 1. Photographsof the actual oscilloscope traces of the results in the monkey are shown in Plate 2 (a)and (b).

It will be seen from these figures that although the rate of discharge is not quitedirectly proportional to the pressure, in every case there is a correlation between thesefactors. In spite of this correlation it cannot be assumed. without further evidence

1secPLATE 1. Recording from long ciliarv nerve: response to touching cornea.

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Lo.P.Raised fro m25 to 35cm

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PLAT E 2(a). Rooording from long ciliary nerve of a monkey. Haising the intraocular pr essure from35 em saline to 65 em sali ne in 10 em steps . Upp er line represent s int raocular pr essure 118 recordedby transducer ,

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PI.AT~ 2(b). Recording from long ciliary nerve of a monkey . Lowering th e intraocular preasuro fromli5 em saline to 25 ern saline in 10 ('1Il steps. Upper line represents lntrnocular pressure as record ed bytransducer.

1sec

PLAT E :1. Responses to to uch iug the corne a recorded fro m s pinn l root of fif t h ne-rve nucleus in t ho eat.

10 sec

P L A '1'I' 4. Recording hom spina l root of IHth nerve nucleus in a cat : response obtained from raisingt he intraocular pwssure from 25 em sali ne to 55 em saline. Lower line indic ates duration of raisedpressure.

1sec

PLATE 5. R ecording from spinal root of fifth nerve nucleu s in a <'ILt : res ponse to rnpid r ise in int ra.ocu lar pressure,

SENSORY MECHANISMS AND INTRAOCULAR PRESSURE 163

that the potentials arise from baroceptors in the eye although this is a reasonableexplanation of the results. It is possible that when fluid flows into the anteriorchamber the iris and lens are pushed backwards, and this movement might stimulateend organs in the iris or ciliary body; and if these were slowly adapting receptors theymight produce a rather similar pattern of discharge.

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FIG. 1. Recordings from long ciliary nerves in tho oat. The graphs show the relationship betweenthe frequency of spike potentials and the intraocular pressure for four animals.

The great difficulty under these experimental conditions was that these fibres wereonly found in approximately one in four animals, and even then a slight movement ofthe recording electrodes was enough to lose the fibre. Direct approach to the ciliarynerves therefore did not result in easily repeatable results and the possibility ofrecording from other areas of the sensory pathway remote from the eye wasconsidered.

PART II

RECORDINGS FROM THE NUOLEUS OF THE FIFTH ORANIAL NERVE

Such responses to changes of intraocular pressure as had been recorded from theciliary nerves appeared to be closely related to fibres subserving corneal sensitivity,and it seemed possible that a similar close relationship might extend to the sensorynucleus of the nerve. Olinical studies and a few experimental reports suggested thatE

164 E. S. PERKINS

the ophthalmic branch of the fifth nerve ended in the spinal root of the fifth nervenucleus and, in the experiments to be described, this part of the nucleus was exploredto locate the nuclear representation of the cornea. Recordings were made from thisand adjacent areas and the responses to changes in intraocular pressure investigated.

1. Methods

Cats were anaesthetized with nembutal intraperitoneally, a tracheal cannula insertedand the animal placed in the prone position with the head held in a stereotactic holder.The skin and muscle were dissected from the occipital region and the skull and uppercervical vertebrae exposed. Bone was removed to expose the cerebellum and the upperpart of the spinal cord. The vertebral arch of the atlas was removed ill some experiments.The dura was kept intact as long as possible.

Steel micro-electrodes were mounted in a holder with a micrometer adjustment foradvancing the electrode. The micrometer was attached to a lathe-bed allowing controlledmovements in the antero-posterior and lateral directions. The electrode holder could alsobe tilted in an antero-posterior plane.

The dura mater was then incised to expose the posterior pole of the cerebellum and theupper part of the spinal cord as far ventrally as the posterior roots of the first cervicalnerve.

As the normal stereotactic co-ordinates could not be used as far ventrally as this, theelectrode was first positioned in the midline at the point where this was crossed by thetip of the cerebellum. This corresponds to a plane about 1 mm posterior to the obex(Gordon and Paine, 1960) and was considered the zero point for movements of the elec­trode. The electrode was tilted to an angle of 30° from the vertical to compensate for theslope of the eat's neck so that the electrode insertions into the spinal cord were approxi­mately vertical.

A cathode-follower was mounted on the electrode holder and recorded activity wasconducted through a pre-amplifier to a Tectronics oscillograph. Part of the output of thepre-amplifier was led to one channel of a dual-channel tape recorder, the other channelbeing used for a microphone so that comments on the experiment could be recordedsimultaneously. The tape recorder also served as an audio-amplifier for monitoring pur­poses. As the main interest in this work was the frequency of the discharges, photographsof the direct oscilloscope tracings were rarely done at the time, but selected parts of thetape recording could be played back into an oscilloscopeand photographed when required.

The exposed area of the cord was searched with the micro-electrode and when responsesfrom tactile stimulation of the cornea were recorded, a cannula connected to a reservoirof saline was introduced into the anterior chamber of the appropriate eye. The intraocularpressure was then varied by raising and lowering the reservoir.

At the end of the experiment steel marker needles were inserted at known distancesfrom the midline and a current from a 1.5 V cell applied for one minute. The cat was thenkilled and perfused through a cannula in the heart with saline followed by 4% formalsaline and 1% potassium ferrocyanide. Thc head was removed and left in 4% formalsaline and 1% potassium ferrocyanide for 4 days. The brain stem was then dissected outand tho tissue dehydrated and embedded in paraffin wax. Sections were cut and stainedwith haematoxylin and eosin.

2. Results

Localization within the spinal nucleus of the fifth nerve

The first problem was to localize the position of the ophthalmic division of thetrigeminal nerve and to find within this area that part of the nucleus associated withthe cornea.

SENSORY MECHANISMS AND INTRAOCULAR PRESSURE 165

The general arrangement of the ophthalmic division was found to agree with thatreported by Darien-Smith and Mayday (1960). In the six cats studied, responses totactile stimulation of skin supplied by the ophthalmic division were obtained in anarea extending from approximately 2 moo to 5 rom posterior to the obex, 3 to 4.5 romlateral to the midline, and 1 to 4 mm in depth. Responses from the mandibulardivision were usually obtained more superficially in this area and, as penetration ofthe electrodeincreased, responses were obtained from the infraorbital region and lowerlid and palpebral conjunctiva. Slightly deeper penetration usually enabled responsesfrom the cornea to be recorded. The region subserving the cornea was limited inindividual animals to an area approximately 1.5 to 3 rom beneath the surface of thecord and extending antero-posteriorly and laterally approximately 1 rom. Thisposition was nsually in close relation to the entry of the most anterior twig of theposterior root of the £rst cervical nerve. If all the corneal responses obtained in 5animals are summated, the total volume ofthe region is larger owing to the individualvariation in size of the spinal cord of the cats and the fact that the zero point chosen(the midline at the posterior tip of the cerebellum) varied in position in differentanimals. Using this zero and considering all 5 animals together, corneal responseswere obtained in an area I to 3.5 rom posteriorly and 3 to 4.5 mm laterally. Themajority of the corneal responses were obtained at a depth of approxirnately2 mmbut occasional responses resulted at 1.5 moo and as deep as 4 mm, The approximateposition of the points from which recordings of corneal sensations were obtained areshown for one animal in Fig. 2.

Approximately2·5mm caudal

toobex

(0)

Approximately3·0 mm cauda I

to obex

(b)

o I 234! I I I I

mm

Approximotely3·5mmcaudal

to obex

(c)

FIG. 2. Diagrammatic representation of points at which potentials were elicited by touching the cornea.

On two occasions, touching the iris with the tip of the cannula after anaesthetizingthe cornea produced responses in the regionsubserving cornealsensation. Penetrationof the electrode deeper than the region of corneal responsesresulted in responsesbeingobtained from the upper lid and supra-orbital region.

It seems therefore that the ophthalmic division of the fifth nerve in the cat isrepresented inversely in the dorsa-ventral layers of the spinal nucleus. The infra­orbital region and lower lid are most superficial and the upper lid and supra-orbitalregion deepest, while the cornea and iris lie between these areas.

166 E. S. PERKINS

Responses to changes in intraocular pressureCorneal. responses were obtained on 44 occasions in the 5 animals studied. The

typical response to touching the cornea is shown in Plate 3. In most instances spon­taneous activity could be recorded from the area associated with the corneal responses,but in no case was any significant alteration in the frequency of the spikes obtained byraising or lowering the intraocular pressure over a range of 10-50 em saline.

The only pressure-sensitive responses obtained in this series of experiments consistedin the appearance of a burst of spikes on rapidly raising the intraocular pressure. Thedischarge continued with lessening frequency for some seconds but usually ceasedabout ten seconds after raising the pressure. In one Ortwo experiments an irregular dis­charge at a lower frequency continued while the pressure remained raised. Loweringthe pressure occasionally evoked another brief burst of activity but more often noresponse was obtained. Examples of these e:ffects are shown in Plates 4: and 5.

These results were not considered sufficiently encouraging to continue the experi­ments.

PART III

Discussion

The difficulty of the direct approach to the ciliary nerves in the living animal whichprompted investigation of the spinal nucleus in these experiments has been circum­vented by Lele and Grimes (1960) by using an isolated perfused preparation of eyeballsresected together with nerves and blood vessels. More reproducible results were ob­tained in this way but, as in the experiments described above, spontaneous .aotivitythat could be related to resting pressure levels was not observed in any preparation.Lele and Grimes also observed an immediate response on raising the intraocularpressure followed by a tonic discharge at a lower frequency than that of the initialdischarge. In approximately 20% of their preparations the pressure-evoked responsegradually declined to zero over two or three minutes. This pattern of events is clearlysimilar to that obtained from the spinal root of the :fifthnerve nucleus and is probablydue, as Lele and Grimes suggest, to stretching of the nerve endings located in theouter coats of the eye when the intraocular pressure is raised.

There is therefore at the moment no direct evidence of any activity in the afferentpathways from the eye that conveys information concerning the resting level ofintraocular pressure. Even such responses as are evoked by rapid changes in pressureappear to adapt quickly and would therefore be of little value for regulatory purposes.

The importance of negative findings in an investigation of this type is alwaysdifficult to assess. It is possible that the recording techniques employed failed todisplay activity in fine fibres which might be more closely related to levels of intra­ocular pressure. It is also possible that the central representation of baroceptors isnot to be found in the ordinary sensory root of the fifth nerve nucleus, but perhapsbelongs to the areas in which proprioceptive sensations, for example, are represented.

The results to date do not encourage the prospect of a direct nervous control ofintraocular pressure as such, but this does not mean that nervous influences are notinvolved in the formation of aqueous humour and its elimination from the eye.

The evidence for an active secretory process in the formation of aqueous in theciliary epithelium is now strong and it may be that it is the rate of formation ofaqueous that is controlled rather than the pressure induced in the eye by the influx

SENSORY MECHANISMS AND INTRAOCULAR PRESSURE 167

of aqueous. In this case the chemical composition of the aqueous humour mayinitiate reflex changes in production. As short-term experiments have failed to showgood evidence of a reflex arc controlling intraocular pressure, the effects of neuralactivity on the long-term secretion of aqueous may provide a more worthwhilesubject for further study.

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Darian-Smith, 1. and Mayday, G. (1960). Exp. N eurol. 2, 290.Dieter, W. (1940). Ber. dtsch. ophih, Ges. 53, 53.Gloster, J. (1961). Brit. J. Ophthal. 45, 259.Gloster, J. and Greaves, D. P. (1956). Proc. R. Soc. ffIed. 49, 675.Gloster, J. and Greaves, D. P. (1957). Brit. J. Ophthal. 41, 513.Gordon, G. and Paine, C. H. (1960). J. Physiol. 153, 331.Greaves, D. P. and Perkins, E. S. (1952). Brit. J. Ophthal. 36, 258.Holland, M. G., von Sallmann, L. and Collins, Eo M. (1956). Amer. J. Ophthat. 42, 148.Kurus, E. (1955). Klin. uu. Augenheilk. 127, 198.Lele, P. P. and Grimes, P. (1960). Exp. Neural: 2, 199.Perkins, E. S. (1957). Brit. J. Ophthal. 41, 257.Perkins, E. S. (1958). In Glaucoma. Trans. 3rd. Conf. Josiah Maoy Jr. Foundation, ed. by

F. W. Newell, p. 143. New York, 1959.Tower, S. S. (1940). J. Neurophy8iol. 3,486.von Sallmann, L. (1958). In Glaucoma. Trans. 3rd. Conf. Josiah Maoy Jr. Foundation, ed. by

F. W. Newell, p. 181. New York, 1959.von Sallmann, L., Fuortes, M. G. F., Maori, F. J. and Grimes, P. (1958). Amer. J. Ophthal.

45(2),211.von Sallmann, L. and Lowenstein, O. (1955). Amer. J. Ophthal. 39(2), 11.Vrabeo F. (1954). Ophthalmologico" Bo,sell28, 359.