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HEARING ANATOMY The auditory system • Peripheral auditory system – Outer ear → – Middle ear → – Inner ear • Central auditory system – Brainstem nuclei → – Auditory centers in the cortex Peripheral auditory system Inner ear Outer ear Middle ear Peripheral auditory system (PAS) Central Auditory System (CAS) Inner ear (PAS) PAS: Outer Ear • Auricle/Pinna – Localisation PAS: Outer Ear • External auditory meatus • Ear canal • Auditory canal – Funnels and resonates sound • Cerumen • Cilia Middle ear (PAS) Ossicular chain PAS: Middle Ear • Tympanic Membrane – Ear drum – Pressure conversion • Ossicular Chain – Malleus, Incus, stapes – Amplifies • Eustachian Tube – Equalizes Protection mechanism of middle ear • Acoustic reflex – Stapedius muscle • Attached to stapes – Tensor tympani muscle • Attached to malleus – Stiffens the ossicular chain to protect from loud sounds. • http://www.youtube.com/watch?v=qW1z-DjOB2Q The Eustachian Tube • From middle ear to nasopharynx • Equalizes pressure • Sits closed at nasal entrance PAS: Inner Ear • Cochlea – Hearing • Vestibular Apparatus (SemiCircular Canals) – Balance Amplification via 3 mechanisms • Ear canal – resonates speech frequencies (2,500Hz – 4,000Hz) and these amplify the sound 2-4 times • Ossicular chain – Leverage power of ossicular chain multiplies tympanic vibration slightly • Ear drum → stapes – Difference in size between tympanic membrane and footplate of stapes – Amplifies sound up to 14 times Helicotrema Reissner’s membrane Round window Scala media/ cochlear duct Scala vestibuli Scala tympani Basilar membrane The cochlea (inner ear – PAS) Tonotopical organization of the organ of corti Central Auditory System (CAS) • Auditory Nerve •30,000 fibres from Cochlea • Brain Stem Nuclei •“cross-over point” • Auditory Cortex •Heschl’s gyrus Inner ear (PAS) Process of Hearing History • Bartolomeo Eustachi (1510-15-74) – – – – ‘tuba auditiva’ Tensor tympani Stapedius Inspiration for Hamlet • Alfonso Corti (1822-1876) – Studied over 200 cochleas – Developed techniques to visualize different structures in the cochlea – First to describe the organ of corti Physics of Sound • Displacement of air particles • Intensity = loudness (amplitude) – Unit of measure is the decibel (dB) • Frequency = pitch (number of cycles per second) – Unit of measure is Hertz (Hz) Pure tone vs complex sound Types of Hearing Loss http://www.muschealth.com/video/Default.aspx?videoId=10266&cId=11&type=rel Portion of Auditory System Affected: Conductive Loss Sensorineural Loss Mixed Loss Ear Affected: Unilateral Bilateral Conductive Hearing Loss Results from interference of sound waves that would normally pass through the outer and middle ear. Can be completely corrected (“reversed”) in most cases. Sensorineural Hearing Loss Results from damage to the cochlea or the auditory nerve and upwards (CAS). The electrical transmission of the sound is affected. Can generally not be fully corrected (irreversible). Mixed Hearing Loss Results from interference of sound waves through outer and/or middle ear AND the electrical transmission of the sound. Degree of hearing loss • Severity • • • • • • 0-25dB – 26-40dB 41-55dB 56-70dB 71-90dB 90+dB NONE – MILD – MODERATE – MODERATE-TO-SEVERE – SEVERE – PROFOUND Hearing Loss: Demographics • 10.5% of NZ population experience hearing loss (approx 450,000 people) • NZ European 10% • Maori 12% • • • • • 2 per 1000 newborns (well-baby) 4 per 100 newborns (NICU) 4% of population below 45-years of age 30% of population over 65-years of age 36% of population over 70-years of age Deafness Defined: The permanent and profound loss of hearing in both ears with an auditory threshold of more than 90 decibels. Hard of Hearing: Mild to Severe loss of hearing. Types of Deafness Congenital & Adventitious Language & Deafness Prelingual & Postlingual Risk Factors for Hearing Loss • Early Age Factors: – Genetically inherited (50% of all hearing impairments “run” in families) – Prenatal Disease (rubella, meningitis) – Prematurity (less than 37 weeks gestation, LBW<1500 grams) – Chronic Otitis Media – Medications (Gentamicin) Risk Factors for Hearing Loss • Later Age Factors: – Noise (cochlear damage) – Ageing (Presbycusis) – Drugs (asprin, Lasix)