-------------------------------------------------------------------------------- TITLE: Normal Auditory Physiology SOURCE: Dept. of Otolaryngology, UTMB, Grand Rounds DATE: November 8, 1989 RESIDENT PHYSICIAN: Lane F. Smith, M.D. FACULTY: Chester L. Strunk, M.D. DATABASE ADMINISTRATOR: Melinda McCracken, M.S. -------------------------------------------------------------------------------- "This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." Normal Auditory Physiology I. Historical Perspective A. Egypt 1. Edwin Smith Surgical Papyrus (3000 - 2500 B.C) mentions temporal bone fractures. B. Greece 1. Aristotle (384 -322 B.C.) and Plato (427 - 347 B.C.) a. Implanted Air Theory 1. Purer air which never mixes with outside air 2. Sound is the shaking of air in our ear 3. Dominates auditory phys. theory for 2000 years. 4. Idea supported by anatomists (Galan) who use dried temporal bones. 5. Finally proven false by Cotungo of Naples in 1760 who discovers the cochlea is filled with fluid 2. Hippocrates (460 - 377 A.D.) a. Temporal bone of extraordinary hardness b. Tympanic Membrane as thin as a cobweb C. Galen 1. Introduced term labryinth 2. Followed Facial N. through Temp. Bone 3. Recognized Pinna as collector of sound D. Vesalius 1. Reports Malleus and Incus, but fails to notice Stapes E. Other Italian Anatomists 1. Eustachi 2. Ingrassi discovers Stapes 3. Fallopia a. Discovers movement of TM and Ossicles b. Gives Tympanum as name to ear drum c. Discovers Fallopian canal F. Alfonso Corti (1822 -1876) 1. Discovered the sensory epithelium resting on basilar membrane, the Tectorial Membrane, the Stria Vascularis and the Spiral Ganglion G. Heimholtz (1821 - 1894) 1. Student of physics, medicine, physiology, psychology, and music 2. Numerous medical contributions a. conservation of energy in muscular work b. Invented ophthalmoscope 3. Reasonance theory of how the ear hears frequency H. Georg Von Bekesy (1899 - 1972) 1. Nobel prize winner 1961 for medicine 2. Demonstrated the "Traveling Wave" along the basilar membrane 3. Modern day Mossbauer technique confirms his work I. Place and Frequency theories 1. Place theory located site of analysis of frequency to a site on the basilar membrane 2. Frequency theories stated that all hair cells along the basilar membrane are capable of responding to any given frequency and that pitch was perceived by the number of times the nerve fired a. Rutherford's Telephone Theory b. Wever's Volley Theory II. Sound A. Caused by a vibrating system producing oscillation 1. Must have mass 2. Must have elasticity 3. Alternation between kinetic and potential energy 4. Any vibration capable of producing sound (air, vocal cords water and stringed instruments) B. Movement of air causing sound 1. Think of thousands of particles which bunch together and and then separate and then bunch together and separate again etc. (ie. oscillate) 2. Hence sound wave considered as the movement through the air of a change in ambient air pressure 3. Mathematically the is described as a sine wave 4. Periodic motion a. occurs if events of the wave formation are regularly repeated b. simplest form of periodic motion is called simple harmonic motion c. non-periodic sound waves consist of random fluctuations of pressure we call noise d. fundamental frequency is the lowest frequency present, each harmonic is a division of this frequency C. Frequency 1. The number of cycles of mechanical events each second. a. corresponds to the wavelength of the sine wave b. measured in Hertz 2. Corresponds with subjective aspect of sound we call pitch D. Amplitude or power 1. refers to the amount of vibratory displacement 2. Described by Decibels (or sound intensity which is Watts/meter squared) 3. Corresponds to the subjective aspect we call loudness 4. Can also be described in terms of pressure E. Decibels 1. An expression of sound intensity or pressure level to a reference level which is a ratio of an exponent of the number 10 (ie. as its log to the base 10) 2. Uses logarithmic scale a. because ears detect such an enormously wide range of pressure levels (the change from from threshold pressure to max pressure varies 2. High frequency sounds most affected 3. The closer the sound the greater the effect. C. Accuracy of of sound localization to within 2 degrees 1. Accuracy is best for sounds in the horizontal plane 2. We localize continuous sounds by phase differences 3. The pinna funnels sounds and assists in localization IV. Some facts about normal hearing A. Range of frequencies from about 20 Hz to 20,000 Hz with greatest sensitivity in the 1000 Hz range B. Loudness from -5 dB to 140 dB (a 100 trillion fold change) C. The minimum increase in intensity detectable by the ear is 0.4 dB D. The average male voice in conversation is about 120 Hz average female voice about 250 Hz E. Threshold varies with frequency, being lowest (or most sensitive) at about 2000 Hz V. The Auricle and External Auditory Canal A. Pinna; increases the intensity of sound by about 3-4 dB at 4 KHz B. Concha: produces a gain of 5 - 10 dB at 4-5 KHz, but a a loss of 5 dB at 10 KHz C. External auditory canal 1. Reasonates and amplifies sound at 3 - 5 KHz with peak at 3.5 KHz 2. Can amplify 5 to 10 dB D. Total amplification 0 to 20 dB depending on direction and frequency of sound, best at 2000 - 6000 Hz VI. The Middle Ear A. Works mainly as an impedance transformer. Sound must be transmitted from the medium of air to liquid perilymph B. Tympanic Membrane 1. Anatomically 21 times the size of the oval window 2. Functionally (since it is an inverted cone) 14:1 mechanical advantage over the oval window. (some sources say up to 17:1) 3. Acts to separates sound transmission of oval window from round window C. Ossicles 1. Malleus and Incus act as a functional unit. Lever action gives mechanical advantage of 1.3 to 1 2. Stapes a. Amplitude of vibrational displacement is extremely minute. Millionths of a millimeter (10 to the -9th meter) at mod. sound intensities, (10 to the -12 meter at threshold) b. Motion of stapes: acts as a piston at low amplitudes (soft sounds) and high amplitudes it acts like a trap door with a rocking motion about a vertical axis through posterior edge D. Total mechanical advantage provided by the middle ear is about 17 - 18 times (ie. 1.3 x 14 = 18) K.J. Lee states it is 22.2 VII. Hearing by the Inner Ear A. Anatomy 1. Inner Ear a. Osseous labyrinth --- perilymph b. Membranous labryinth --- endolymph c. Reissner's membrane, Scala Vestibuli, Scala Media, Scala Tympani, Helicotrema, Bony Modiolus, Osseous Spiral Lamina, Organ of Corti etc. 2. Organ of Corti a. Sits on the basilar membrane, about 3,500 in number b. Tectorial membrane c. Outer hair cells (12,000) i. three to five in each organ of corti ii. steriocila projects into tectorial membrane iii. only 5% of afferent neurons of the cochlear nerve synapse here, (mostly unmyelinated divergent fibers.) iv. efferent fibers from olivocochlear bundle synapse directly on hair cells. v. appear to modify transmission by the inner hair cells. d. Inner hair cells (3,500) i. steriocilia do not attach to tectorial membrane ii. 95% of afferent neurons of cochlear nerve synapse here, (mostly large type I myelinated convergent fibers.) iii. Efferent fibers from the olivocochlear bundle synapse on the afferent nerve that innervate these cells. iv. cells most responsible for the transmission of nerve impulse to sound stimulus. e. Supporting epithelium (dieter's cells) and other supporting cells (Hensen's, and Claudius) 3. Basilar Membrane a. 35mm long b. increases in width from base (0.08mm) to apex (0.5mm) c. decreases in stiffness from the base to the more flexible apex resulting in a hundred fold change in compliance. B. The Travelling Wave 1. The Travelling Wave Envelope a. Wave increases in amplitude until it reaches a maximum at a specific place b. Each area along the basilar membrane codes for a specific frequency (tonotopic organization) high frequency at base, low frequencies at apex c. The area of maximal deflection is the region most activated. d. Displacement of the basilar membrane at threshold is picometers (ten to the minus 12th) C. The Hair Cells 1. Activated by the movement of Basilar and Tectorial membrane's sheering force. a. The outer hair cells steriocilia are attached to the tectorial membrane which moves b. The inner hair cells steriocilia are moved by the inertia of the surrounding endolymph 2. Steriocilia transfers the mechanical energy of movement into electrical energy a. Endolymph +80mv relative to perilymph and is called the endocochlear potential b. Interior of hair cells is -70 relative to perilymph for for a total potential difference of 150mv between endolymph and interior of the hair cell c. There is a standing current of 5 microamps flowing through the cell d. Transduction channels i. Probably located at the apical end of the steriocilia ii. a small number are always open iii. Movement of the steriocilia opens the transduction channels allowing mostly K+ (and some other small positively charged molecules) to enter the cell which causes depolarization of the cell iv. Deplorization opens the calcium channels in the Basilar aspect of the cell allowing calcium to flow in v. Calcium influx causes vesicles in the base of the cell to fuse with cell membrane and release their neurotransmitter vi. Neurotransmitter diffuses across the synaptic space exciting the neuron e. Only small amount of movement of the hair cell needed for transmission to occur. At threshold the response begins when the steriocilia are moved 100 picometers (trillionths of a meter) or about the size of some large atoms. D. Cochlear Potentials 1. Resting potential: direct current potentials which are generated without acoustic stimulus 2. Summating potentials: also a direct current potential but is only generated during acoustic stimulation 3. Cochlear Microphonic potentials: are alternating current potentials that appear only during acoustic stimulation. (these are felt to be generated by the hair cells) 4. The Action Potential: also an alternating current potential but is generated by the nerves rather then the inner ear (unlike the aforementioned 3) E. The Acoustic Reflex 1. Mediated by connections of the Cochlear Nuclei, the Superior Olivary Neucleus, and the Motor Nucleus of the Seventh Nerve 2. Probably only involves the Stapedius muscle in humans (not the tensor tympani) 3. Bilateral reflex 4. occurs 80 - 85 db above threshold of hearing 5. Latency to onset of reflex (150 msec) so won't protect against loud bursts of sound 6. Serves to increase the dynamic range of the ear by reducing the transmission of continuous loud sounds 7. The reflex also occurs during vocalization and may serve to attenuate the sounds in our ears during vocalization VIII. THE PLACE VOLLEY THEORY (or the modern theory of how we understand frequency) A. At low frequencies, less then 250 Hz frequency is mostly coded by rapidity of neurons firing B. At higher frequencies (up to 1000 hz) groups of adjacent hair cells are firing in groups, but slightly out of phase with each other so that the total number of impulses per second is extremely high and frequency is determined by the rapidity of firing C. Frequency is also coded (and mainly coded) by the position of the hair cells which are most stimulated by the traveling wave of the basilar membrane D. Therefore the brain interpets frequency by the position of hair cell that is stimulated and it's frequency of firing ( THE PLACE-VOLLEY THEORY ) IX. Efferent Input A. Mediated by the Olivocochlear bundle providing efferent innervation to the Cochlea 1. This bundle contains fibers from the Lateral Superior Olivary Nucleus and fibers surrounding the Medial Superior Olivary Nucleus 2. It runs along the base of the vestibular nerve and enters the cochlea in the vestibulocochlear anastomosis (Bundle of Ort) B. Medial Olivocochlear Neurons 1. Made up of neurons medial, ventral and anterior to the Medial Superior Olivary Nucleus 2. Project predominantly to the contralateral cochlea and terminate on the outer hair cells 3. Its neurotransmitter is Ach C. Lateral Olivocochlear Neurons 1. Originate in the lateral Superior Olivary Nucleus 2. Project mainly to the ipsilateral cochlea and terminate on the afferent neurons of the inner hair cells (not on the hair cells themselves) 3. It's neurotransmitter is also Ach D. Function is to inhibit afferent fibers. Possibly to inhibit fibers one half octave below the stimulating frequency and thereby help sharpen our ability to hear different frequencies. E. May assist in sound localization F. May protect the ear in acoustic trauma X. Neural Pathways A. Tonotopic pattern of basilar membrane maintained throughout auditory pathway B. Extensive bilaterality and interconnection throughout the pathway beginning at the level of the Superior Olives C. Cochlear Nerve 1. Bipolar ganglion cells whose nuclei are located in the spiral ganglion of the Modiolus 2. Enters brainstem at the Pontomedulary junction where fibers bifurcate to enter the Ventral and Dorsal Cochlear Nuclei D. Ventral Cochlear Nucleus: It's axons form the Trapezoid Body (and some synapse there) and cross over to the other side of the brainstem through tegmentum to the Superior Olivary Complexes E. Dorsal Cochlear Nucleus: It's axons reach the Superior Olivary Complexes on the opposite side through the Dorsal Acoustic Stria F. Superior Olivary Complexes: receive fibers from both ipsilateral and contralateral Cochlear Nuclei G. Lateral Leminiscus 1. Fibers from the Superior Olives (and some from the Cochlear Nuclei) ascend through the Lateral Leminiscus to the Inferior Colliculus 2. Some fibers synapse in the Neucleus of the Lateral Leminiscus H. Inferior Colliculus 1. Fibers from the Inferior Colliculus form the Brachium of the Inferior Colliculus as they project to the Medial Geniculate Body 2. The Inferior Colliculus also connects with the contralateral Medial Geniculate body via the Commissure of the Inferior Colliculus I. Medial Geniculate Body: fibers from the Medial Geniculate Body form the Auditory Radiations and pass through the Sublenticular part of the internal capsule on their way to the Superior Temporal Gyrus J. Superior Temporal Gyrus (Auditory Cortex) 1. Tonotopic organization a. High frequency sounds represented in the posterior medial aspect of the Auditory Cortex b. Low frequency sounds represented in the anterior lateral part of the Auditory Cortex 2. By the time information gets here it has been already extensively processed K. REVIEW OF THE MAJOR NUCLEI IN THE AUDITORY PATHWAY COCHLEAR NUCLUS (Ventral and Dorsal) to SUPERIOR OLIVE to INFERIOR COLLICULUS to MEDIAL GENICULATE BODY to AUDITORY CORTEX (SUPERIOR TEMPORAL GYRUS) --------------------------------------------------------------------- BIBLIOGRAPHY 1. Warr, B. W., et al; Organization of the efferent fibers: The Lateral and Mesial Olivocochlear Systems; Neurobiology of hearing: The Cochlea, Raven Press, chapter 18, 1986: pp 333 - 348. 2. Ludman, H., Physiology of hearing and balance; Mawson's Diseases of the Ear: fifth edition, Year Book Medical Publishers, chapter 3, 1988: pp 74 - 92. 3. Abbas, P. 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