CHAPTER 14.3 PAGES 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

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: balancebalance

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

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 htm

27 Hearing test

30 3) TREATMENTS FOR HEARING LOSS HEARING 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.

31 Tinnitus 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