CHAPTER XXIII 



Excitation of auditory receptors' 



H A L L O W E L L DAVIS Central Institute for the Deaf, St. Louis, Missouri 



C: H A P T E R CONTENTS 



Introduction 



Auditory Information 



Range and Differential Sensitivity 



Significance of Bitemporal Location of Ears 

 General Plan of Ear 

 Functional Anatomy and Acoustic Properties of Ear 



Middle Eai-: Acoustic Impedance Matching 



Tympanic Reflex 



Frequency Characteristics of Ear 



Mechanical Properties of Inner Ear Structures 



Traveling Wave Pattern of Cochlear Partition 



Fine Structure of Organ of Corti 



Innervation of Hair Cells 



Fine Movements of Organ of Corti 

 Blood Supply and Fluids of Inner Ear 



Blood Supply 



Fluids 

 Electric Responses of Inner Ear 



Action Potentials 



Intracellular Potentials 



Endocochlear Potential 



Cochlear Microphonic and Summating Potentials 

 Auditory Nerve Impulses 



Volleys and Latencies 



Single Fiber Activity 



Efferent Inhibitory Action 

 Theory of Aural Action 



Transmission of Auditory Information 



INTRODUCTION 



Auditory Information 



THE EARS ARE SENSE ORGANS specialized for excitation 

 by airborne vibratory energy. They belong in the 



' This work was supported by a contract between the Central 

 Institute for the Deaf and the Office of Naval Research. Re- 

 production in whole or in part is permitted for any purpose of 

 the United States Government. 



general class of mechanoreceptors, together with the 

 organs of touch, pressure, stretch and equilibrium. 

 They are exteroceptors; the source of the acoustic 

 energy is in general external to the body. They serve 

 to transmit information concerning the character of 

 the physical source as revealed by the rates of vibra- 

 tion, the intensity, the epoch and the overall temporal 

 pattern of .such vibrations. The ears also give informa- 

 tion indirectly as to the direction from which the 

 sound waves arrive. 



Range and Differential Sensitivity' 



The lower frequenc\- limit of ' hearing' is usually 

 set arbitrarily anywhere from 20 to 50 cps. Hearing 

 merges gradually into sensations of touch, vibration, 

 'flutter', etc. The upper limit is about 20,000 cps in 

 young ears but falls off with age. Differences in fre- 

 quency of less than one per cent may be recognized. 

 The dynamic range is very great, covering more than 

 12 logarithmic units (120 db) on the scale of acoustic 

 energy (see fig. 6) from a lower limit close to the 

 physical background noise of thermal energy (Brown- 

 ian movement) up to limits set by acoustic injurv to 

 the sense organ. Differential .sen.sitivity for intensity is 

 in the order of magnitude of a tenth of a logarithmic 

 unit, i.e. one db. Absolute differences in time of ar- 

 rival of .sound wavesat the twoearsas small as 10 msec, 

 sensed in terms of the direction of the source, can be 

 detected by practiced observers. 



One physiological problem of hearing is to under- 

 stand how the sense organ achieves such sensitivity, 

 dynamic range and discrimination. Another is the 

 means by which it encodes in nerve iinpulses the in- 



^ See especially the papers of Stevens & Davis (11) and of 

 von Bekesy & Rosenblith (22). 



565 



