68 PHYSIOLOGICAL TRIGGERS 



of readily available energy (i) that can somehow be valved or modulated to do 

 the work of exciting the nerve tibers or endings. Such a mechanism was sug- 

 gested several years ago by the writer as a working hypothesis (6, 7) and, al- 

 though some of the details of that hypothesis have been shown by experiment 

 to be untenable or at least very improbable, the general outline is still plausible 

 enough to warrant repeating it once more. 



The amplifier hypothesis states that the immediate stimulus that excites the 

 nerve fibers (or endings) is electrical, and specifically that it is the so-called 

 'cochlear microphonic' The electrical energy is supposed to be derived from a 

 pre-existing electrical potential known as the 'endolymphatic potential' by 

 virtue of a change in the electrical resis ance across the upper surface of the 

 hair cells in the reticular lamina. The change of electrical resistance is supposed 

 to be caused by the movement or bending of the hair cells where they arise from 

 this upper surface. The change in resistance varies the current flow through the 

 hair cells, and the part of this current that flows from the hair cell outward 

 through the nerve ending and fiber is assumed to be the stimulus to the nerve. 



Now let us consider the electrical phenomena in more detail. The cochlear 

 microphonic has been recognized for about 25 years. It appears as an alter- 

 nating electric potential that reflects faithfully, within limits, the pattern and 

 amplitude of the mechanical movements of the cochlear partition. It is sym- 

 metrical and it shows no true threshold, no all-or-none character and no re- 

 factory period. It appears across the basilar membrane (cf. fig. 3), and a variety 

 of experiments associate it c'early with the hair cells (5). Even more specifically, 

 it is associated with the 'upper' end of the hair cell, because when a hyperfine 

 micropipette is pushed through the organ of Corti in this region from the scala 

 tympani the phase of the cochlear microphonic is reversed just at the point 

 where the pipette enters the endolymphatic space (11). We have, then, as a 

 matter of experimental observation, an electric counterpart of the mechanical 

 movement located anatomically at just the point to which we were led by our 

 analysis of the mechanical events in the cochlear partition. It is not the locus 

 but the mechanism and the assumed function of the cochlear microphonic that 

 are hypothetical. 



We may note in passing that it had been widely assumed, ever since the 

 microphonic was first identified, that the microphonic stimulates the nerve. The 

 present writer was for many years inclined to be skeptical and to regard the 

 microphonic as probably an epiphenomenon. The chief reason for this skepti- 

 cism was the rather long latency, about i millisecond, between the microphonic 

 and the first clearly recorded action potential of nerve. It now appears, however, 

 that the main action potential is recorded as the impulses pass through the 

 internal auditory meatus (cf. fig. 3), and its latency can easily be ex{)lained as 

 conduction time. Furthermore, an electrode placed close beneath the spiral 

 lamina, where the nerve fibers enter the bony structure of the central core of the 

 cochlea, reveals an early electric i)otential that seems to signal the start of nerve 



