582 



HANDBOOK OF PHVSIOLOOY 



NEUROPHYSIOLOGY I 



tudinal bending of iheir hairs (19a). The differential 

 stimulation of the two sets by the different directions 

 of bending allows possibilities, through inhibitory 

 neural interactions within the central nervous system, 

 of sharpening the 'place' aspect of frequency discrimi- 

 nation. 



The large traveling waves are known to produce 

 eddies in the cochlear fluids on the apical side of the 

 position of maximal amplitude. The forces that pro- 

 duce eddies we beliexe also tend to cause an unsym- 

 metrical longitudinal shift or 'creep' of the tectorial 

 membrane relative to the organ of Corti. .Such a 

 shift would cau.se a one-way longitudinal bending of 

 the hairs. This is a mechanical rectifying action, and 

 it allows the cochlea to 'detect' efficiently and respond 

 with nerve impulses to high-frequency acoustic signals 

 above 2000 per sec. Just as the cochlear microphonic 

 is the electrical sign of a symmetrical vibratory bending 

 of the hairs, we believe the negative summating poten- 

 tial is the electrical sign of an asymmetrical, rectified 

 longitudinal shift. This shift is strongest on the apical 

 side of the position of ma.ximal excursion. The sus- 

 tained bend of the hairs presumably acts as a steady 

 stimulus to the hair cells that are affected, but com- 

 pared to the alternating shearing movements revealed 

 by the cochlear microphonic this mechanism is rela- 

 tively insensitive. The rectifying action, as revealed 

 by the negative summating potential, continues to 

 increase, however, after the vibratory movements, 

 and with them the cochlear microphonic, have 

 reached their maximum. The rectifying action, no 

 matter how it is produced, seems to be a mechanism 

 that significantly extends the dynamic range of the 

 ear. 



The complete theory, as presented elsewhere, con- 

 siders the mechanism of limitation of crosswise bend- 

 ing (and with it the cochlear microphonic) in more 

 detail and it also includes a second rectifying action, 

 associated with the crosswise bending, that depends 

 on the viscous properties of the tectorial membrane. 

 This second mechanical rectifying action and conse- 

 quent one-way bias of the hairs is invoked as the basis 

 of the positive summating potential, but this extension 

 of the theory as well as a possible inhibitory action of 

 the positive summating potential is admittedly more 

 speculative than the postulate of the longitudinal 

 "shift' and its production of the negative summating 

 potential. 



The association of the cochlear microphonic with 

 the bending of the hairs seems very well established. 

 The mechanism that connects the two is completely 

 obscure, however. A vague suggestion that the 



mechanical distortion changes the electrical resistance 

 of the upper ends of the hair cells has been offered but 

 without supporting evidence (fig. 20). Whatever the 

 mechanism, the bending of the hairs is supposed to 

 account for not only the cochlear microphonic but 

 also for both of the summating potentials. But these 

 three electrical responses, it should be noted, are 

 observed phenomena, not theories. 



Consideration of the extreme .sensitivity of the ear, 

 and afso the fact that the summating potential per- 

 sists indefinitely if a static displacement of the tectorial 

 membrane relative to the reticular lamina is main- 

 tained mechanically, leads to the conclusion that the 

 energy of the electrical responses is derived from the 

 metabolism of the tissues, not from the acoustic 

 stimulus. The latter .serves merely to 'valve' the flow 

 of energy from the biological source. The result is an 

 amplifier action in the sense organ prior to stimulation 

 of the nerve fibers. 



The endocochlear potential has been hailed as the 

 obvious 'pool of biological energy' that is tapped by 

 a valving action of the hair cells (3). Its mechanism is 

 completely unknown but it seems to be a imicjue prop- 

 erty of the cochlea. Its analogue in the utricle is not 



MODEL OF 

 COCHLEAR EXCITATION 



"POLARIZED RtLA^f' 

 OR OTHER DETECTOR 

 THAT TRIGGERS THE 

 NERVE IMPULSE 



FIG. 20. An electrical model of excitation of nerve impulses 

 in the cochlea. Additional batteries', not shown in the diagram, 

 are located at the cell membranes of the hair cells and of the 

 nerve endings. The return circuit from nerve endings to the 

 stria vascularis is not restricted to the narrow anatomical path 

 indicated in the diagiam but is diflfuse through all intervening 

 tissues except the scala media. [From Davis (3).] 



