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J. Physiol. (I943) I02, 3I3-3I8 6I2.858.2:6I2.0I4.42

THE MICR,OPHONIC EFFECT OF TELEOST LABYRINTHSAND ITS BIOLOGICAL SIGNIFICANCE

BY Y. ZOTTERMAN, From Karolinska Institutet,Fysiologiska Institution, Stockholm

(Received 29 April 1943)

In the present paper a series of experiments is described dealing with elec-trical effects led off from the labyrinth of the pike, Esox lucius, and the burbot,Lota vulgaris. The sacculus of these fishes produces a microphonic effect, asoriginally found by Adrian, Craik & Sturdy [1938] for the pike and the eel aswell as for reptiles and amph7ibians. The microphonic effect of the inner earfirst observed by Wever & Bray [1930] in mammals seems thus not to belimited to species which possess a fully developed cochlea.The Wever & Bray effect has been subjected to many studies concerning

its origin. Several ideas have been put forward. Thus it has been attributedin turn to the nerve fibres, to the hair cells of the organs of Corti, and toReissner's membrane or any other polarized membrane of the cochlea. Thefact that a very similar effect can be led off from the sacculus of these fishesoffers special means for solving, in principle, the problem concerning thestructures which are responsible for the phenomenon. In surviving prepara-tions of the pike and burbot the boundaries of the macular tissue can be seenby the naked eye so that the sacculus of these fishes, particularly of the latter,offers especially good facilities for - exploring the site of generation of themicrophonic effect.

Besides this phenomenon the impulses elicited from the posterior semi-circular canal have been studied by leading off the action potentials from thenerve strand running from the posterior cupola.

TECHNIQUE AND PROCEDURE

After decapitation, the head was divided by a sagittal section in the midline.The labyrinths of both sides were now open to free inspection. That part ofthe eighth nerve which spreads radially to the macula sacculi can easily bedissected free from the surrounding tissue (Fig. 1, rs). The electrodes were

placed on isolated strands of this nerve. The nerve running to the posteriorampulla (Fig. 1, rap) was similarly isolated and in order to'avoid interferencefrom the lagena the branch running to its macula (Fig. 1, rl) was severeil. Thenerves had to be handled with the greatest care in order to avoid mechanicaldamage. Repeated failures to obtain working preparations in the beginningseemed to be due to mechanical damage. The electrodes used, were of thecommon Ag-AgCl type, and the electrical response was recorded by- means ofa resistance-capacity coupled amplifier and a cathode-ray oscillograph. Theorigin of the sacculus effect was explored with the aid of a micro-electrode anda micro-manipulator kindly put at my disposal by Prof. R. Granit.

~~~~~~~~4 /

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Fig. 1. The labyrinth of Esox lucius, after G. Retzius [1884]. ac, the acoustic nerve; r8, nervebranch running to the macula sacculi, m8; rap, nerve branch to the posterior ampulla;rl, branch to lagena.

In order to study the impulses produced in the posterior cupola the pre-paration was fixed on a cork disk in a gyroscopic device of the kind describedby Ross [1936] in his study of the frog's labyrinth. This apparatus- made itpossible to study the effects of angular' displacements in all planes.

RESULTSThe sacculu, effect

In'the preparations used, the electrical response from the sacculus and that.from the nerve strands running to the macula sacculi can be studied separately.With'one electrode on the nerve and the other on the sacculus the two effectsobtained in response to a tuning-fork of 60 cyc./sec. in contact with theoperation, table can be seen combined as in Fig. 3 A. There is a sinusoidaloscillation which follows the frequency of the fork. On these waves a numberof spikes can be seen occurring fairly regularly at the end of the rising phase

314 Y. ZOTTEP4MAN

MICROPHONIC EFFECT OF TELEOST LABYRINTHS 315,

*. a -'-i-D ''"

X_'Fig. 2. Diagram showing arrangement of electrodes in experiments upon the saccuhws. I, II

and III, Ag-AgOl electrodes; I and II, placed upon a nerve strand running to the maoulasacculi; III, electrode placed upon the. sacculus. IV and V, micro-electrodes; IV, placedupon the macuia; V, placed outside the macular region.

a

Fig. 3. Electrical effects from- the sacculus nerves; Esox lucius. The two upper records show theresponse to a tuning-fork of 60 cyc./sec. I-A one electrode was placed on the nerve andthe other on the sacculus. In B both electrodes were plaeed on the nerve. The lower recordsshow the response to a light tap on the preparation table with coTresponding electrodearrangements. Time marker, 50 cyc./sec.

316 Y. ZOTTERMAN

of each wave. When both electrodes are placed on the nerve as in Fig. 3 B,the sinusoidal current nearly vanishes, while the spikes remain, appearingregularly as single spikes or brief volleys of spikes at the frequency of thestimulus. In Fig. 3 C, D tl e effect of a light tap on the preparation tablewith corresponding electrode arrangements can be seen.

The origin of the sacculus effect

As already stated the large sacculus ot the burbot offers special facilitiesfor exploring thq site of the microphonic phenomenon. For this purpose theeffect was picked up by a micro-electrode, which was shifted albng the surface

T~~~~~~~~~~~~ T

Fig. 4. Records of the microphonic effect of the sacculus of Lota vulgaris8. The effect was pickedup by a micro-electrode placed A outside, B inside the macular region.

of the 'sacculus. As the electrode approached the macular region the effectincreased, and when it passed the border of the macula there was regularlya very strong increase in*the amplitude, which often exceeded by ten timesor more the value obtained by leading off from a point outside the macularregion. The difference can be seen in Fig. 4.

This observation strongly suggests that the microphonic effect is generatedin the macula itself. It thus raises the much debated question whether themicrophonic phenomenon is purely an incidental feature or whether it servesa definite physiological purpose. 4When using strong stimuli the effect led off from the sacculus may attain

values of several millivolts, which indicates that the intrinsic potential mostlikely reaches values of hundreds of millivolts or more. Thus even fairly weak

MICROPHONIC EFFECT OF TELEOST LABYRINTHS 317stimuli must set up potentials within the macula, which are of such a mag-nitude that they can hardly leave the nerve fibres within the macula unaffected.The records A and C of Fig. 3 show how the impulses are set up at the end ofeach rising phase in accordance with this view. It suggests that the micro-phonic effect, obviously originating within the macula, could be regarded asa true generator potential, which is set up within the receptor cells and whichin its turn excites the nerve fibres concerned. In their recent experimentalanalyses of the effect of electric potentials applied to afferent nerve fibres,Granit & Skoglund [1943] have shown how it is possible to reproduce thephenomena observed in afferent fibres in response to natural stimuli byapplying potentials of various gradients and strengths to the nerve fibre itself.The microphonic effect from the sacculus which might be called the maculareffect could then be regarded as a generator potential similar to the retinalpotential. Thus it seems very likely that the cochlear phenomenon serves asimilar purpose. There are so far no facts which seem to be contradictory tothis view.

The response from the posterior ampullaThe effect of accelerated rotation about all axes of the labyrinth was

observed in eight surviving preparations of the pike and in five prepara-tions of the burbot. In both species an outburst of impulses was elicited byangular displacements when the ampulla was leading. Fig. 5 shows a record

Fig. 5. Action potentials led off from the nerve strand running to the right posterior ampullaof Esox luciu in response to anticlockwise rotation about a transverse axis. The recordsshow the beginning and the end of the response.

of the action potentials from the nerve of the right posterior ampulla of theburbot in response to an anticlockwise rotation about a transverse axis. Whenit is rotated in the opposite direction no immediate effect can be seen, butthere is a quite definite after-discharge when the movement is stopped. Thisafter-discharge is generally well pronounced in fresh preparations biit dimin-ishes gradually as the preparation ages, and it vanishes earlier than the directresponse to ampulla-leading displacements.

In most preparations there has been very little spontaneous activity. Asseen in Fig. 5 there is no activity when the preparation is at rest. In otherpreparations, as shown in Fig. 6, where a spontaneous activity was observed,this activity was not inhibited when the ampulla was exposed to relative

movements of the endolymph in the direction of the ampulla. In this respectthe results obtained from the ampullar nerves of the pike and the burbotdiffer from the findings by Lowenstein & Sand [1936, 1941] in the elasmobranchlabyrinth.

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Fig. 6. Record from the nerve to the posterior ampulla of Lota vulgarisin rdsponse to rotary movements.

SUMMARY1. The microphonic effect of the sacculus has been studied in surviving

preparations of the labyrinth of the pike and the burbot. Evidence has beenproduced which suggests that the saccular effect is generated within themacula, and the biological significance of the phenomenon considered as agenerator potential discharging the nerve fibres is briefly discussed.

2. The electric response from the nerve of the posterior ampulla is of phasicnature occurring in response to angular displacements in one direction only.Rotation in the opposite direction does not produce any immediate effect in'these fishes but-only an after-discharge when the movement is stopped.

In condusion I wish to thank Dr Hj. Koch for his valuable assistance in the earlier part ofthis research carried out in 1938.

REFERENCES

Adrian, E. D., Craik, K. J. W. & Sturdy, R. S. [1938]. Proc. Roy. Soc. B, 126, 435.Granit, R. & Skoglund, R. [1943]. J. Neurophysi8i. (in the Press).L6wenstein, 0. & Sand, A. [1936]. J. exp. Biol. 13, 416.LMwenstein, 0. & Sand, A. [1941]. J. Phy&iol. 99, 89.Retzius, G. [1884]. Da eh6rorgan der Wirbeliere. Stockholm.Ross, D. A. [1936]. J. Physiol. 88, 117:Wever, E. G. & Bray, C. W. [1930]. Proc. nat. Acad. Sci., Wash., 16, 344.

318 Y. ZOTTERMAN