EXCITATION OF AUDITORY RECEPTORS 



577 



the acoustic stimulus (fig. 15). The summatins; poten- 

 tials are proportional not to any instantaneous value 

 of the acoustic signal but to a root-mean-square value, 

 integrated over a very short time. Thus the summating 

 potentials reproduce appro.ximately the form of the 

 envelope of the original acoustic signal. The positive 

 and the negative summating potentials are opposite 

 in sign. They can be separated by the greater vulner- 

 ability of the positive summating potential to oxygen 

 lack and other injury and by the more apical site of 

 generation of the negative response. The range of 

 linear response of the summating potentials has not 

 yet been determined. 



The cochlear microphonic is generated at the 

 cuticular surface of the hair cells. This is clearly 

 proved by exploration with microelectrodes. The 

 microphonic, and in all probability the summating 

 potentials also, seem to reflect the bending of hairs in 

 the appropriate direction. It is believed that, at 

 intensities high enough to evoke the summating poten- 

 tials, some kind o*^ mechanical rectifying or detector 

 action takes place in the inner ear to cause an asym- 



BASAL 



250 TURN n 



APICAL 



1000 



2000 



8000 



PAIRED ELECTRODES, SCALAE VESTIBULI AND TyMPANI, 

 IN EACH TURN 



FIG. 15. Cochlear microphonic responses to 'tone pips' of 

 various frequencies recorded simultaneously from the basal, the 

 second and the apical turn. The wave form of the acoustic sig- 

 nals is accurately reproduced. The time delay (phase difference) 

 between the second and apical turns and the basal turn demon- 

 strates the traveling wave pattern. The failure of 1000, 2000 

 and 8000 cps waves to reach the apical turn and of 8000 cps to 

 reach the second turn demonstrates acoustical analysis. The 

 displacements of the base line in the 8000 cps responses are 

 summating potentials. [From Tasaki (17).] 



metrical, persistent one-way bend in the hairs of 

 certain cells. In some cases the bending is probably 

 across, in others lengthwise of, the organ of Corti. 

 [A theory that includes this and several other aspects 

 of the electrophysiology of the cochlea has recently 

 been published elsewhere (4).] 



Both the cochlear microphonic and the summating 

 potentials are continuously graded responses, linearly 

 related, up to a limit, to the intensity of the acoustic 

 stimulus and with no true ' threshold' like that of all- 

 or-none action potentials. No evidence of anv all-or- 

 none response in the sen.sory cells or of a refractory 

 period has been found, even when the cochlear micro- 

 phonic was recorded from an electrode inside a hair 

 cell. Both the microphonic and the summating poten- 

 tials show very little or no fatigue or adaptation. 



The cochlear microphonic 'appears', in the sense 

 that it reaches a root-mean-square value of a micro- 

 volt or thereabouts, at a much lower sound pressure 

 level than the summating potential (except at the 

 extreme high-frequency limit of response). The in- 

 crease is linear with the sound pressure level up to 

 about 90 db relative to 0.0002 microbar in the guinea 

 pig but varies somewhat with frequency. The response 

 then increases more slowly and usually goes through a 

 maximum. At low frequencies harmonic distortion 

 (peak limiting) occurs within the cochlea at even 

 lower levels than in the iTiiddle ear. For high fre- 

 quencies, however, the sinusoidal wave form of the 

 microphonic is maintained even when the increase of 

 amplitude with intensity has become nonlinear. This 

 curious behavior is in sharp contrast to the peak 

 limiting seen at low frequencies. 



The summating potentials are not directly related 

 to the nonlinearity of the mechanism of the cochlear 

 microphonics although they may happen to first ap- 

 pear at sound pressure levels which lie near the 

 beginning of nonlinearity. With increasing intensity 

 the summating potentials do not reach a maximum 

 but continue to increase up to limits that are set only 

 by acoustic injury to the organ of Corti. 



The cochlear microphonic, like the endocochlear 

 potential, is closely dependent on an adequate oxygen 

 supply. In anoxia both fall almost in parallel, and the 

 minor differences are well explained by the changes in 

 electrical resistance of Reissner's membrane, etc., 

 which also occur in anoxia. This parallelism is a 

 strong argument for a cau.sal dependence of the micro- 

 phonic on the endocochlear potential, but neverthe- 

 less the microphonic may also be abolished by certain 

 injuries that leave the endocochlear potential un- 

 affected. Two such injurious procedures are a) injec- 



