576 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Reissner's 

 membrane 



CELLS OF THE 

 ENDOLYMPHATIC 

 WALL 



ENDOLYMPHATIC 

 SPACE 

 + 80mV 



STRIA 

 VASCULARIS 



BLOOD 

 ■' VESSELS 



SPIRAL 

 LIGAMENT 



BONE 



FIG. 14. Distribution of the positive endocochlear potential'. The 'endolymphatic space' of the 

 scala media is shown in heavy outline. The negative intracellular potentials are also indicated. The 

 tectorial membrane is omitted and only one external hair cell is shown. [From Tasaki (16).] 



(in anesthetized, moribund guinea pigs). Full recovery 

 requires only a few seconds after a large single gasping 

 inspiration. It is also abolished rapidly by injection of 

 cyanide or azide into the scala tympani or scala 

 media. It is not immediately affected by injection of 

 isotonic potassium chloride, choline chloride, or po- 

 ta.ssium glutamate into the scala tympani or scala 

 vestibuli. It does fail, although less rapidly than with 

 cyanide, following surgical injury to the scala media 

 or the injection into the scala media of a solution that 

 differs substantially in ionic content from the analytic 

 figures for endolymph given in table i. 



The endocochlear potential is modified by displace- 

 ment of structures within the cochlear partition. Dis- 

 placement of the basilar membrane toward the scala 

 tympani, as by injection of fluid into the scala media 

 or an inward movement of the stapes, causes an in- 

 crease in the positive potential by as much as 5 or 10 

 mv. Movement in the opposite direction, as by out- 

 ward movement of the stapes, causes even greater 

 reductions in the potential. Movements of Reissner's 

 membrane alone are not effective, but movements of 

 the tectorial membrane relative to the reticular 

 lamina, when it is manipulated by a microneedle, 

 produce just such changes in potential. The changes 



are related to displacement, not to velocity, and are 

 sustained as long as the displacement is maintained. 



The source of the endocochlear potential has been 

 identified positively. It is the stria vascularis (6). The 

 changes in endocochlear potential described above 

 are clearly associated with the organ of Corti, almost 

 surely with the hair cells, but the resting positive 

 endocochlear potential is not generated there. Per- 

 haps a separate electric response to mechanical move- 

 ment occurs in the hair cells and simply adds to the 

 potential that is produced by the generator in the 

 stria vascularis, or perhaps the potential of scala 

 media is modified by a change of the electrical re- 

 sistance to the continual leakage current that must 

 flow from stria \'ascularis through the hair cells. 



Cochlear Microphonic and Sumrnatino Potentials 



The cochlear microphonic and two summating 

 potentials (positive and negative) are all electric 

 responses to acoustic stimulation. The cochlear micro- 

 phonic is linearly proportional, up to a limit, to the 

 displacement of the cochlear partition and thus, 

 indirectly, to the instantaneous acoustic pressure. 

 The microphonic thus reproduces the wave form of 



