Download - Chapter 9: Hearing and Equilibrium
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Chapter 9: Hearing and Equilibrium
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Equilibrium and Hearing• Both of these senses are provided
by the internal ear which is located in the temporal bone.
• Equilibrium informs us of our body’s position in space
• Hearing enables us to detect and interpret sound waves
• Both use hair cells which are mechano receptors
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Anatomy of the Ear
• Divided into three anatomical regions– External ear• Collects and direct sound waves toward middle ear
– Middle ear• Amplify sound waves and transmit them to inner ear
– Internal ear• Contains the sensory organs for hearing and
equilibrium
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External Ear
• Includes the auricle or pinna which surround the entrance to the external acoustic meatus
• Ends at the tympanic membrane.
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Middle Ear
• Connected to the nasopharynx by the auditory tube (eustachian tube).
• Encloses and protects the auditory ossicles which connect the tympanic membrane to the internal ear.– Malleus: attached to tympanic membrane– Incus: middle bone– Stapes: attached to the oval window of the inner
ear.
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Internal Ear
• Senses of equilibrium and hearing are provided by the receptors within the internal ear.
• These receptors are protected by the bony labyrinth which is fused with the temporal bone
• The bony labyrinth surrounds the membranous labyrinth which is a collection of tubes and chambers.
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• The membranous labyrinth is filled with endolymph and between the bony and membranous labyrinths is another fluid called perilymph.
• Bony labyrinth has three parts– Vestibule: receptors for gravity and acceleration– Semicircular canals: rotation of the head.– Cochlea: hearing.
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Equilibriium• Dynamic equilibrium : aids us maintaining our
balance when the head and body move suddenly
• Static equilibrium: maintains our posture and stability when the body is motionless.
• Semicircular canals monitor rotational movement of the head which is part of dynamic equilibrium
• Structures in the maculae respond to gravity and linear acceleration.
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Hearing
• The receptors for hearing are hair cells similar to those of equilibrium.
• Their placement in the cochlea shields them from stimuli other than sound
• The auditory ossicles convert the pressure waves of air to pressure pulses in the perilymph at the oval window.
• The pressure pulses stimulate hair cells along the cochlear spiral.
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• The frequency (pitch) of the perceived sound is determined by which part of the cochlear duct is stimulated. (units hertz)
• The intensity (volume) of the perceived sound is determined by how many hair cells at that location are stimulated. (units decibels)
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6 steps
• 1. Sound waves arrive at the tympanic membrane.
• 2. Movement of the tympanic membrane causes displacement of the auditory ossicles.
• 3. The movement of the stapes at the oval window establishes pressure waves in the perilymphs of the inner ear.
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• 4. The pressure waves distort the basilar membrane on their way to the round window of the tympanic duct.
• 5. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane.
• 6. Information about the region and intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII.
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Aging and the Senses
• Smell: olfactory receptor cells are regularly replaced by cell division but this decreases with age. The receptors also become less sensitive.
• Taste: reduction in number and sensitivity of taste buds. Begin life with around 10,000 taste buds but number declines quickly after age 50.
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Vision and age
• With age the lens loses its elasticity and stiffens. Seeing objects close up becomes a problem—called presbyopia.
• Cataracts: loss of transparency in the lens.• Gradual loss of rods with age: need more light
to read.• Macular degeneration: growth and
proliferation of blood vessels in the retina.
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Hearing
• The tympanic membrane loses some elasticity.• It becomes difficult to hear high pitched
sounds.• Progressive hearing loss that occurs with aging
is presbycusis.
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