58 



MATTER WAVES: SOUND AND ULTRASOUND 



times simply by amplification of the signal reaching the tympanic mem- 

 brane, sometimes, although less commonly, directly to the cochlea by stimu- 

 lation of the bone structure which surrounds it. Surgery is often necessary 

 to free the "frozen" lever system. 



Reissner s membrane 



bone 



auditory 

 ner ve 



tec torial 

 membrane 



transducer 

 cells 



basilar 

 membrane 



COCHLEA 



non-elastic 

 oining f i ber s\ 



auditoi y 

 nerve end ings 



Figure 3-5. Schematic Drawing of Cross-section of the Cochlea, the Inner Ear. The 

 three scalae are separated by deformable membranes. The transducers are fastened to 

 the tectorial membrane by fibers. Relative motion between the tectorial membrane and 

 the basilar membrane causes stretching of the transducer cells, resulting in change in 

 membrane permeability, and therefore ionic composition and membrane potential. This 

 change activates the nerve endings attached to the cells, and the impulse is carried down 

 the auditory nerve to the brain. 



The Sense of Touch And Other Mechanoreceptors 



A magnificent array of mechanoreceptors (as well as photo-, chemo-, and 

 thermal receptors) is displayed by the human body. These bring in informa- 

 tion from the environment, and then provide a feedback of information con- 

 cerning an action taken. The most sensitive transducers, other than those 

 in the ear, are found on the tip of the tongue and on the tips of the fingers, 

 although mechanoreceptors are located all over the body, so closely spaced 

 that no pressure change on the surface, above some threshold value, goes 

 undetected. 



They all have three parts in common: (1) a mechanism for transmitting a 

 pressure change to the receptor cell; (2) the deformable receptor cell, the 

 deformation of which (apparently) changes its cell membrane potential at a 

 point intimately associated with (3) a specialized ending of a nerve cell's 



