66 



PHYSIOLOGICAL TRIGGERS 



The tectorial membrane contains a system of fibers as well as the more 

 familiar jelly-like substance. It is described by Bekesy (3) as stiff and resistant 

 to quick vibrations, although yielding easily to slow movements. We now know 

 that it is attached to the organ of Corti near its inner border (fig. 4) and, by a 

 web-like extension, along the outer border also. The cilia or 'hairs' of the hair 

 cells are firmly imbedded in it. Its inner edge is hinged like the leaf of a book 

 along the projecting edge of the rigid limbus. The tectorial membrane is a very 

 significant mechanical structure. Its axis of rotation lies considerably above the 

 axis of rotation of the organ of Corti beneath it. We have already noted that the 

 latter apparently swings up and down as a relatively rigid structure. 



The net result of this mechanical arrangement seems to be a sliding or shear- 

 ing movement between the tectorial membrane and the reticular lamina, and 

 the hairs are bent from side to side as a result (fig. 6). Bekesy (3) points out that 

 this arrangement serves to deliver the force of the movement of the cochlear 



RETICULAR 



LIMBUS 



BONE BASILAR 



Fig. 6. Movement of the organ of Corti and the tectorial membrane, based on description 

 by von Bekesy (3). The shearing action between these two stiff structures bends the hairs of 

 the hair cells. 



partition very efficiently to the hair cells. It also seems obvious that the attach- 

 ments of the tectorial membrane to the organ of Corti must serve to limit the 

 amplitude of this movement and thus protect the hairs against further stress 

 when a certain amplitude has been reached. 



We may sum up this interpretation of the basic mechanical movements of the 

 cochlear partition with the inference that the final critical event is a bending of 

 the hairs of the hair cells. It is here, in the hairs or in the cells from which they 

 arise, that we should look for a trigger action if such an action is interposed 

 between the last mechanical event and the initiation of impulses in the nerve 

 fibers. 



The amplitudes of the mechanical movements in the cochlea are extremely 

 small, particularly for faint sounds near the threshold of hearing. The greatest 

 conceivable amplitudes are only of the order of a fraction of a millimeter even at 

 the ear drum, and it seems that the movements we have described in the coch- 

 lear partition are less than those of the drum membrane, rather than greater. 

 The best estimates of the amplitude of movement of the drum membrane at 



