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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



on which they rest, a stiff triangular supporting struc- 

 ture for the inner end of the reticular lamina. The 

 outer edge of the lamina rests on a softer cushion of 

 Hensen's cells. The outer portion of the basilar mem- 

 brane, between Hensen's cells and the spiral ligament, 

 carries the lower cuboida! cells of Claudius. 



The flask-shaped hair cells forming a single row 

 along the inner edge of the reticular lamina are known 

 as internal or inner hair cells. There are about 3500 of 

 them, each about 12 |i in diameter. The smaller (8 fi) 

 cylindrical external hair cells are arranged in three 

 or four rows external to the tunnel of Corti. There are 

 aiiout 20,000 of them in each ear. 



Innervation of Hair Cells'^ 



The afferent neurons of the auditory nerve art- 

 bipolar cells. The cell bodies, about 28,000 in each 

 ear, are arranged in a long spiral ganglion parallel to 

 the organ of Corti but within the bony modiolus 

 Their axons pass inward to the hollow core and 

 thence, as the cochlear portion of the eighth cranial 

 nerve, through the internal auditory meatus, to the 

 cochlear nucleus of the medulla. The axon-like den- 

 dritic processes pass outward through the sieve-like 

 bony and fibrous habenula perforata into the organ 

 of Corti (fig. 2). They are myelinated up to the 

 habenula perforata. Some of them, the internal radial 

 fibers, pass directly to the internal cells and innervate 

 one to three cells. Others cress the tunnel of Corti to 

 the external hair cells. Some of these are radial fibers 

 with a restricted area of distribution but most of them 

 run apically or basally, or in both directions, for as 

 much as several millimeters as the external spiral 

 fibers. Each fiber innervates many external hair cells 

 but not cver\- cell along its course, and each cell 

 typically receives more than one nerve fiber. The 

 plan of innervation is illustrated in figure 10. The 

 ner\c endings around the lower ends of the hair cells 

 appear under the electron-microscope as well-devel- 

 oped structures rich in mitochondria. 



In addition to the afferent fillers, an efferent olivo- 

 cochlear bundle from the contralateral olivary nu- 

 cleus runs lengthwise of the organ of Corti as the intra- 

 ganglionic bundle within the modiolus and just 

 peripheral to the spiral ganglion (fig. 2). These 

 efferent fibers distriiiute to the organ of Corti and 

 apparently innervate the hair cells, particularh' the 

 inner hair cells. 



' See especially the papers of Dasis (4) and Wever (23). 



Fine Movcmenis of Organ of Corti 



The fine movements of the organ of Corti and the 

 tectorial membrane have been observed under the 

 microscope by stroboscopic illumination and described 

 in some detail by von Bekesy (20). In any one seg- 

 ment the basilar membrane, organ of Corti, tectorial 

 membrane, and usually Reissner's membrane also, 

 move in phase with one another. The basilar mem- 

 brane is fibrous and elastic, and basically it deter- 

 mines the traveling wave pattern of vibration de- 

 scribed above. The cells of Hensen form a soft cushion 

 supporting the stiffer plate of the reticular lamina. 

 The tectorial membrane is hinged like the cover of a 

 book along the edge of the limbus. It is composed of a 

 system of diagonal fibers and also a jelly-like sub- 

 stance It is a viscous elastic system that yields to slow 

 movements but is quite resistant to quick movements. 

 It returns rather slowly after being displaced. 



Apparently, as the basilar membrane bulges 'up- 

 ward' or 'downward' (fig. 11), the stiff reticular 

 lamina tends to rock on the support of the rods of 

 Corti around an axis at the attachment of the basilar 

 membrane to the bony modiolus. The tectorial mem- 

 brane swings on its attachment to the limbus The 

 result is a shearing action between the tectorial mem- 

 brane and the reticular lamina (fig 12). The 'hairs' 

 arise from the cuticular plates of the hair cells which 

 are set firmly in the reticular lamina, and their outer 

 ends are firmly imbedded in the tectorial membrane. 

 Therefore, as the basilar membrane bulges, the hairs 

 are bent. The force of the movements of the cochlear 

 partition is rather efficiently concentrated on this 

 shearing action. 



The movement described above is associated with 

 an approximately radial displacement of Hensen's 

 cells, as seen under the microscope, and a correspond- 

 ing radial or slightly diagonal bending of the hairs. 

 This movement is characteristic on the basal side of 

 the position of maximal amplitude. On the apical 

 side, however, due to the shorter wavelength of the 

 traveling wave and sharper longitudinal bending of 

 the basilar membrane, a longitudinal mo\ement pre- 

 dominates and the hairs are presumably bent longi- 

 tudinally instead of radially (fig. 13) 



The exact significance of these different directions 

 of mo\cment in relation to the excitation of nerve 

 impulses by the hair cells is still a matter of specula- 

 tion, but the bending of the hairs is the final and 



