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CHAPTER 14.3 PAGES 456-461 Hearing and Equilibrium HEARING AND EQUILIBRIUM • the ear • is associated with two separate functions: • hearing • equilibrium • can be divided into three sections • the outer ear • the middle ear • the inner ear HEARING AND EQUILIBRIUM HEARING AND EQUILIBRIUM The Outer Ear • comprised of the • pinna • the external ear flap • collects the sound • auditory canal • carries sound to the eardrum. • lined with specialized sweat glands that produce earwax. • earwax traps foreign particles and prevents them entering the ear. HEARING AND EQUILIBRIUM The Middle Ear • begins at the tympanic membrane and extends toward the oval and round windows. HEARING AND EQUILIBRIUM • the tympanic membrane is a thin layer of tissue that receives sound vibrations, also known as the eardrum. HEARING AND EQUILIBRIUM • the air filled chamber of the middle ear contains three small bones called ossicles, which include the: • mallus (the hammer) • incus (anvil) • stapes (stirrup) • the ossicles amplify and carry sound in the middle ear. HEARING AND EQUILIBRIUM • sound vibrations that strike the eardrum and are first concentrated within the solid malleus. • vibrations are then transmitted to the incus and finally to the stapes HEARING AND EQUILIBRIUM • the stapes strikes the membrane covering the oval window in the inner wall of the middle ear • the oval window is an oval shaped hole in the vestibule of the inner ear, covered by a thin layer of tissue • sound is amplified by concentrating the sound energy from the large tympanic membrane to the smaller oval window. HEARING AND EQUILIBRIUM • the eustachian tube • an air-filled tube of the middle ear that equalizes pressure between the external and internal ear. • approximately 40 mm in length and 3 mm in diameter. • extends from the middle ear to the mouth and chambers of the nose. • equalizing your ears on a plane by yawning or swallowing allows air to leave your middle ear through the eustachian tube. HEARING AND EQUILIBRIUM The Inner Ear • has three distinct structures, the: • vestibule • semicircular canals • cochlea HEARING AND EQUILIBRIUM • the vestibule • a chamber found at the base of the semicircular canals that provides information about static equilibrium • involved in balance • connected to the middle ear by the oval window. • houses two sacs • the utricle • the saccule HEARING AND EQUILIBRIUM • the utricle and saccule contain granules called otoliths that allow us to detect gravity (linear movement) and head movement. • three semicircular canals arranged at different angles helps identify body movement (3 canals for 3 axis of movement) (Three semicircular canals contain fluid and allow us to detect angular acceleration such as the turning of the head) Dynamic eq. vid: balance HEARING AND EQUILIBRIUM • The cochlea • a coiled structure of the inner ear that responds to various sound waves and converts them to nerve impulses. • shaped like a spiralling snail’s shell. • contains rows of specialized hair cells that run the length of the inner cannal. • the hair cells respond to sound waves and convert them into nerve impulses. HEARING AND EQUILIBRIUM Hearing • sound like light must be converted into an electrical impulse before you can interpret it. • you ear is so sensitive that you can hear a mosquito even though the sound energy reaching you ear is less than one quadrillionth of watt. The average light in the house uses a 60 watt bulb. HEARING AND EQUILIBRIUM • hearing begins when sound waves push against the eardrum, or tympanic membrane. • the vibrations of the eardrum are passed on to the three bones of the middle ear: the malleus, the incus, and the stapes • arranged in a lever system the three bones are held together by muscles and ligaments. HEARING AND EQUILIBRIUM • the bones concentrate and amplify the vibrations received from the tympanic membrane (they can triple the force) • during excessive noise a protection reflex mechanism goes into effect. • the muscles that join the bones together contract and restrict the movement of the malleus reducing the intensity of movement. • at the same time a second muscle contracts pulling the stapes away from the oval window. HEARING AND EQUILIBRIUM • the oval window receives vibrations from the ossicles. • as the oval window pushes inwards, the round window, located immediately below the oval window moves outward. • this triggers waves of fluid within the inner ear. • the cochlea receives the fluid waves and converts them into electrical impulses, which you interpret as sound. HEARING AND EQUILIBRIUM • the hearing apparatus within the cochlea is known as the organ of Corti. • it comprises a single inner row and three outer rows of specialized hair cells anchored to a basilar membrane. • the hair cells respond to vibrations of the basilar membrane. • vibrations in the fluid on either side of the basilar membrane cause the membrane to move. • the hairs on the cells bend as they brush against the tectorial membrane. HEARING AND EQUILIBRIUM HEARING AND EQUILIBRIUM • the movement of the hair cells stimulates sensory nerves in the basilar membrane • Auditory information is the sent to the temporal lobe of the cerebrum via the auditory nerves. HEARING AND EQUILIBRIUM • The ear conveys information about: • Volume, the amplitude of the sound wave • Pitch, the frequency of the sound wave • The cochlea can distinguish pitch because the basilar membrane is not uniform along its length • Each region vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex http://health.howstuffworks.com/adam200010.htm Hearing test 27 3) TREATMENTS FOR HEARING LOSS • Conductive hearing loss: caused by wax build-up, middle ear infection, punctured eardrum • Sensorineural hearing loss: auditory nerve severed, or cochlear hair cells damaged. Treatments: • Hearing aid: amplifies sound and transmit to eardrum. • Cochlear implants: converts sound (speech processor) to electrical impulses that are sent to auditory nerve. 30 Tinnitus – ringing in ear; “phantom sound” • Malfunction of cochlea, transmitted to brain • Temporary, after a rock concert • Hair cells are damaged, bent, or destroyed • Loss of hair cells as we age • Other causes: • TMJ, damage to inner ear by trauma, ear infection, tumors, use of ototoxic drugs (antibiotics, anticancer drugs, or antimalarial drugs) – damage to hair cells or auditory nerve or both 31 31