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In The Name Of God Hearing System physiology Characteristics of Sound Waves 1) Frequency: number of cycles of sound waves passing a stationary point per second (Hertz = Hz; sec-1) - range of human hearing is ~ 20 - 20,000 Hz - range of human voice is ~ 350 - 3500 Hz 2) Intensity: amplitude of a sound wave (decibels = dB) - expressed in terms of sound pressure - sound intensity is proportional to square of pressure dB = 10 log intensity of a particular sound wave intensity threshold for human hearing Normal conversation 60 dB Airplane 100 dB Damaging sound 120 - 140 dB Anatomy of the Ear Outer ear consists of: 1) Pinna - collects sound waves 2) External Auditory Canal - conducts sound waves to middle ear Middle Ear consists of: 1) Tympanic membrane (eardrum) - vibrates in response to sound waves 2) Auditory ossicles 3 bones which mechanically transduce tympanic membrane vibrations to the inner ear a. Malleus (hammer) - tympanic membrane to incus b. Incus (anvil) - malleus to stapes c. Stapes (stirrup) - incus to oval window of cochlea 3) Eustacheon (auditory) tube - equalizes pressure between middle ear and the environment 4. Two muscle • Tensor tympani muscle: anchored to bone at one end is attached to the malleus • stapedius muscle: extend from a fixed anchor of bone and attached to stapes contraction reduces ossicular mobility • contraction therefore: reduce the intensity of lower frequency sound transmission by 30 to40 decibels that its called Attenuation reflex • function attenuation reflex 1. Protect the cochlea from damaging vibrations caused by extensively loud sound 2.To mask low frequency sound in loud environments 3. Decrease a person hearing sensitivity to his or her own speech Function of the Ossicular Chain I. Impedance Matching - effective transfer of sound energy from air to fluid Sound pressure in the middle ear is amplified by 2 mechanisms: 1. Ossicular act like a lever system : displace oval window against cochlear fluid Force increase = 1.3 fold 2. Surface area of oval window = 1/17 surface area of tympanic membrane Force increase = 17 fold NET RESULT: Ossicles increase sound pressure 22 fold Inner ear consists of: 1) Semicircular canals important for sense of balance and equilibrium 2) Cochlea - responsible for sound detection, discrimination and transduction into neural signal From: W.F. Ganong, Review of Medical Physiology, 19th ed. Appleton & Lange, 1999 Cochlear structures: BASE 1) Cochlear duct - fluid-filled tube within cochlea 2) Scala media - endolymph filled space containing the sensory apparatus; delimited by Reissner’s and basilar membranes (high [K+] !) 3) Scala vestubuli - perilymph filled space above scala media 4) Scala tympani - perilymph filled space below scala media From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993. apex Length: 34 mm Dia. 2mm 5. Helicotrema - distal opening between scala vestibuli and scala tympani 6. Oval window - membranous opening of scala vestibuli, joins with stapes 7. Round window - membranous opening of scala tympani 8. Basilar membrane: Organ of Corti - sensory detection apparatus From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993. Properties of basilar membrane 0.5 mm 0.04 The basilar membrane of the cochlear duct is stiff and narrow close to the oval window. It becomes wider and more flexible near its distal end. Some important features of the Organ of Corti: 1) Auditory hair cells (AHC) sensory receptor cells, have stereocilia projecting from their apical surface 2) Tectorial membrane glycoprotein-rich flap, stereocilia tips imbedded in its surface 3) Afferent nerve fibers - synapse with AHCs, carried in the vestibulocochlear nerve From: Berne & Levy, Physiology, 3rd ed., Mosby Year Book, 1993. Auditory Hair Cells (AHC) The Specialized Auditory Receptor Cells 2 Populations: 1) Inner AHCs - ~ 3500 in number - arranged in a single row - provide basic auditory info to CNS 2) Outer AHCs - ~ 12000 in number - arranged in 3 parallel rows - fine tuning of auditory signal - have a limited motility, and shorten slightly in response to certain tones - amplifies the sound wave Sound Wave Transduction in the Cochlea 1. Sound waves enter via oval window; round window bulges in response (fluid is not very compressible) 2. Basilar membrane and Organ of Corti vibrate in response AHC Stereocilia Structure / Function 1) Rows of stereocilia are of constant diameter and taper at their base 2) Stereocilia act as rigid rods 3) Have several stretch-activated nonselective cation channels 4) Stereocilia tips are joined by a protein tip link Mechanism of transduction Upward Bending: - stereocilia bend away from limbus, toward tallest stereocilia - cation channels open, AHC depolarizes - VG Ca++ channels open; AHC releases NT (glutamate) Downward Bending: - stereocilia bend toward limbus, away from tallest stereocilia - cation channels close, AHC hyperpolarizes - VG Ca++ channels closed; no NT release Discrimination of Auditory Signals The two physical characteristics of sound that we can discriminate are frequency and intensity. 1) Place Principle of Frequency High frequency - cause vibration of the basilar membrane at the base of the cochlea where the membrane is narrow and stiff Low frequency - cause vibration of the basilar membrane at the apex of the cochlea where the membrane is wide and more compliant 2. Volley or frequency principle: It was used for determination low freq. sound from 20 to 200 HZ It cause volley of impulse synchronized at same at same freq. Discrimination of Auditory Signals (continued) 3) Mechanisms for discrimination of LOUDNESS Louder sound firing rate of AHCs stimulation of special “high threshold” AHCs amplitude of basilar membrane vibration number of stimulated AHCs Determination of the direction sound 1.By the time lag between the entry of sound into one ear and into the opposite area. • it is benefit at Frq.<3000 Hz •The medial superior olivary nucleus has important role in determination time lag between acoustic signal entering the tow ear 2.Differnce between the intensity of the sound in the two ear • the intensity mechanism operated best at Frq.>3000 Hz • it is concern with Lateral superior olivary nucleus Figure 9.19 Pathways in the auditory system Subcortical Mechanisms of Sound Localization The lateral and medial superior olives react to differences in what is heard by the two ears Medial – arrival time differences Lateral – amplitude differences Both project to the superior colliculus The deep layers of the superior colliculus are laid out according to auditory space, allowing location of sound sources in the world; the shallow layers are laid out retinotopically Tonotopic mapping/organization • The organization of frequency in terms of place. http://tonks.disted.camosun.bc.ca/courses/psyc290/brain/tonotopic.GIF • Info is carried through auditory system in frequency channels. Two Streams of Auditory Cortex Auditory signals are conducted to two areas of association cortex Prefrontal cortex Posterior parietal cortex Anterior auditory pathway may be more involved in identifying sounds (what) Posterior auditory pathway may be more involved in locating sounds (where) Perception of different characteristics of sound Frequency Starts at the basilar membrane and frequency sharpening occurs throughout the auditory pathway Intensity Starts at the hair cells (OHC are stimulated by weaker stimulus) Frequency of impulses Direction Inter-aural time difference Pattern recognition Cortical function Interpretation of speech Complex cortical phenomenon Electrical stimulation in Wernicke’s area of a conscious person occasionally causes a highly complex thought. . The types of thoughts that might be experienced include complicated visual scenes that one might remember from childhood, auditory hallucinations such as a specific musical piece, or even a statement made by a specific person. For this reason, it is believed that activation of Wernicke’s area can call forth complicated memory patterns that involve more than one sensory modality even though most of the individual memories .may be stored elsewhere Hearing abnormality A. Conductive Defects 1. Otitis - inflammation of the external or middle ear 2. Otosclerosis - calcification of the stapes 3. Tympanic perforation (broken eardrum) 4. Foreign body insertion B. Sensorineuronal Defects 1. 2. 3. 4. Drug-induced - aminoglycoside antibiotics Congenital Infections - syphilis, measles, mumps, meningitis, flu Acoustic neuroma - benign tumor of vestibulocochlear nerve