EXCITATION OF AUDITORY RECEPTORS 



571 



Flg.7 



A<* = ^:200cps 



20 



22 



24 26 28 



Distance from stapes in millimeters 



30 



32 



50 100 200 400 800 1600 2400 5000 



Frequency in cps 



FIG. 7. A traveling wave on the cochlear partition for a 200 cps tone. The solid line shows the 

 pattern at one instant, the line with short dashes a quarter of a period later. The envelope shows 

 the maximal displacement at each point. [From von Bekesy (19).] 



FIG. 8. Resonance curves for si.x points on the basilar membrane. The solid curves represent 

 measurements by von Bekesy; the dashed curves, theoretical calculations by Zwislocki. [From von 

 Bekesy (22).] 



apex (fig. 9). At about lOO cps in man, it is very close 

 to the helicotrema. At 2000 cps there is very Httle 

 movement beyond the mid-point of the cochlear par- 

 tition. The extreme basal end of the partition, how- 

 ever, moves in response to all frequencies within the 

 audible range. 



The unsymmetrica! traveling wave pattern of 

 movement, with its rather flat maximum of amplitude 

 and its abrupt apical reduction in activity, has been 

 shown to be a necessary and predictable consequence 

 of the principles of acoustic resonance in a system 

 such as the cochlea with gradation of stiffness, mass, 

 damping and coupling (21, 26). The traveling wave 

 pattern has been reproduced in appropriate physical 

 models and it has been observed directly in the 

 cochlea under the microscope with stroboscopic illu- 

 mination (20) and inferred from electrical recordings 

 (17) (see fig. 15). It allows the cochlea to act as a 

 mechanical frequency analyzer because the extent of 

 activity and position of maxima vary as functions of 

 frequency. It introduces additional features, such as 

 asymmetry, progressive time and phase lag, and sig- 

 nificant longitudinal as well as transverse bending of 

 the cochlear partition, that contribute to the pattern 

 of neural excitation that results from the movements 

 of the partition. 



Fine Structure of Organ of Corti^ 



The organ of Corti consists of sensory cells that are 

 known as ' hair cells' because of their tufts of hair-like 



SOOcps 200 cps 100 cps 



50 Cps 



25 Distance from 3Q 

 stapes in millimeters 



n 

 35 



* = 0i d:^:::- 



' See especially the review by Davis (4). 



FIG. 9. Amplitude and phase angle of movements of the 

 cochlear partition for four different frequencies as a function of 

 distance from the stapes. At 50 cps the partition moves sub- 

 stantially in phase throughout. [From von Bekesy (19).] 



processes that extend into the scala media and sup- 

 porting cells. The tectorial membrane, in which the 

 outer ends of the hairs are imbedded, is an important 

 accessory structure (fig. 2). It is obviously the ana- 

 logue of the otolithic membrane of the utricle and of 

 the cristae of the semicircular canals, sense organs 

 that are sensitive to mechanical acceleration. 



The ends of Deiters" cells that face the scala media 

 form a stiff but openwork plate, the reticular lamina. 

 The hair-bearing ends of the hair cells are firmly held 

 in the openings of this lamina; their opposite ends, 

 surrounded by the nerve endings of the auditory nerve, 

 rest in cup-like supports that are also part of Deiters' 

 cells. Between their upper and lower ends the external 

 hair cells hang free in a fluid-filled space. The so- 

 called ' rods of Corti' form, with the basilar membrane 



