EXCITATION OF AUDITcmV RECEPTORS 



0/0 



TABLE I . Composition of Spinal Fluid, 

 Perilymph and Endolymph 



Results (means and standard errors of means) of analyses 

 of spinal fluid, perilymph and endolymph, made by Smith 

 et al. (10). The endolymph was collected from the utricle, 

 but cochlear endolymph gave similar although less reliable 

 results. 



ELECTRIC RESPONSES OF INNER EAR' 



Action Potentials 



The final output of the inner ear is ner\e impulses 

 in the auditory nerve. If these impulses are well 

 enough synchronized into definite groups or volleys, 

 as in responses to clicks or to successive sound waves 

 of a low-frequency tone, the corresponding action 

 potential waves can easily be recorded. They appear 

 clearly when one electrode is on the round window 

 and the other on the neck, but special placements 

 are needed to record the action potentials without 

 contamination by the other electric responses of the 

 cochlea. With the u.sual electrode placements, the 

 potentials are recorded as the impulses pass through 

 the modiolus and just before they enter the internal 

 auditory meatus. 



The action potentials represent the familiar all-or- 

 none 'spike' responses of nerve fibers. They show defi- 

 nite thresholds and are followed by refractory 

 periods. One consequence of the refractory period is 

 the phenomenon of 'masking.' The synchronized 

 action potentials in response to a click or tone of 

 moderate intensity are much reduced if a moderate 

 random noise is presented at the same time. The 

 noise stimulates the nerve fibers at random and the 

 refractory periods prevent the usual synchronized 

 responses of many fibers. 



Other details of the nerve response in relation to 

 parameters of the stimulus will be given below. The 

 present point is that nerve action potentials are one of 



' See especially the papers of Davis (3, 4) and of Tasaki el 



al. (15, 16, 17) . 



the electric responses of the ear and that they are in 

 all ways similar to 'axon spikes' elsewhere. 



Intracellular Potentials 



Nearly all cells show a negative intracellular poten- 

 tial. Explorations of the cochlea and the auditory 

 nerve with very fine microelectrodes reveal these intra- 

 cellular potentials, ranging from —60 or even —80 

 mv relative to the potential of the perilymph in large 

 cells such as Hensen's or Claudius' down to — 20 or 

 so in the cells of Reissner's membrane. The exact 

 value seems to be a function of the amount of injury 

 caused by the microelectrode, the greater the injury 

 the lower the value. The hair cells, like the nerve 

 fibers and supporting cells, are electrically negative 

 internally. 



Endocochlear Potential 



The interior of the scala media, the endolymph, is 

 electrically positive relative to the perilymph in the 

 scala vestibuli and the scala tympani, and to the spiral 

 ligament and extracochlear tissues in general. This 

 potential is -|-8o mv (fig. 14). It is encountered 

 abruptly at the point where the exploring electrode 

 enters the endolymphatic space, although a relatively 

 large (15 ju) electrode pushed through the stria vascu- 

 laris usually reaches this potential level in a series of 

 two or more steps. The change of potential in going 

 from the interior of a hair cell through its cuticular 

 layer into the scala media is from — 70 to -f-Bo, or 

 about 150 mv. 



The endocochlear potential, formerly known as the 

 endolymphatic potential, is so designated because it 

 seems to be practically confined to the endolymphatic 

 space of the cochlea. The corresponding potential 

 within the utricle is not more than -I-5 mv. The endo- 

 utricular potential is hardly more than the difference 

 in potential found in the perilymph between the 

 helicotrema and the basal end of the scala vestibuli 

 or scala tympani. The latter potential gradient may 

 well be due to unequal leakage through Reissner's 

 membrane or other parts of the endolymphatic wall, 

 but in any case it implies a considerable continuing 

 current flow, dependent presumably on continuing 

 metabolic activity. 



The endocochlear potential is in fact closely de- 

 pendent on an adequate oxygen supply It falls, re- 

 versibly, to a very low level at the stage of asphyxia 

 that is reached in extreme Cheyne-Stokes respiration 



