Exercise 27
BI 232

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Functions: Hearing and
Equilibrium
Mechanoreception: because the ear receives mechanical vibrations and translates them into nerve impulses
Static equilibrium: able to determine nonmoving position
Dynamic equilibrium: motion is detected

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Vestibular Portion
Cochlear Portion

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Malleus
Incus
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Stapes

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Composed of three areas:
Cochlea
Vestibule
Semicircular Ducts
(canals)
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Cochlea- snail shaped
Contains sensory receptors for hearing, known as the organ of Corti (spiral organ)
Sensory hair cells are found in all receptor organs of the inner ear which contain long microvilli, called stereocilia
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These can be stimulated by gravitational forces in the vestibule, turning movements in the semicircular canals or sound waves in the cochlea

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oval window
vestibular duct helicotrema round window tympanic duct
Cochlear duct containing the Organ of
Corti
• Stapes pushes on fluid of vestibular duct at oval window
• At helicotrema, vibration moves into tympanic duct
• Fluid vibration dissipated at round window which bulges
• The central structure is vibrated (cochlear duct)

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Vestibulocochlear nerve sends impulses to the auditory cortex of the temporal lobe of brain and interpreted as sound

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Consists of the utricle and saccule
Involved in the interpretation of static equilibrium and linear acceleration
Regions known as maculae, which consist of hair cells with stereocilia and a kinocilium grouped together in a gelatinous mass called otolithic membrane and weighted with calcium caronate stones called otoliths

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As the head is accelerated or tipped by gravity, the otoliths cause the cilia to bend, indicating that the position of the head has changed.
Visual cues play a part in this also
When visual and vestibular cues are not synchronized, a sense of imbalance or nausea can occur

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Semicircular canals contain sensory receptors called crista and detect change in acceleration or deceleration.
Dynamic equilibrium
3 semicircular ducts, each at 90 degrees to one another
Filled with endolymph and has an expanded base called an ampulla
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Inside are clusters of hair cells and supports cells
(crista ampullaris)
These cells have stereocilia and a kinocilium enclosed in a gelatinous material called the cupula.
As the head is rotated, the endolymph pushes pushes against the stereocilia.

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Conductive hearing loss occurs when sound is not conducted efficiently through the outer ear canal to the eardrum and the bones of the middle ear.
Sensorineural hearing loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear to the brain.
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Ring tuning fork and place on center of head. Ask the subject where they hear the sound.
Interpreting the test:
Normally, the sound is heard in the center of the head or equally in both ears.
Sound localizes toward the poor ear with a conductive loss
Sound localizes toward the good ear with a sensorineural hearing loss
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Place the vibrating tuning fork on the base of the mastoid bone.
Ask patient to tell you when the sound is no longer heard.
Immediately move the tuning fork to the front of the ear
Ask the patient to tell you when the sound is no longer heard.
Repeat the process putting the tuning fork in front of the ear first

 Normally, someone will hear the vibration in the air
(in front of the ear) after they stop hearing it on the bone
 Conductive hearing loss : If the person hears the vibration on the bone after they no longer hear it in the air.
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Similar to the Rinne Test
Strike the tuning fork and place it on the mastoid process.
With your other hand close off the auditory canal with pad of finger.
A person with normal hearing or one with sensorineurial hearing loss will hear the sound better when ear canal is closed.
A person with conductive hearing loss will not notice a change in sound

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Have lab partner sit with eyes closed.
Strike the tuning fork with a rubber reflex hammer above head.
Have partner describe to you where the sound is located.
Try the following locations: behind head, right side, left side, in front of head, below chin

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Unexpected changes that move the body away from a state of equilibrium cause postural reflexes to compensate for that change.
Important for maintaining the upright position of the body.
Negative feedback mechanisms
Find an area w/o obstacles
Stand on tiptoes and read lab manual
Lab partner should give a little nudge to left or right (not too hard)

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Tests visual responses to changes in dynamic equilibrium.
Place subject in a swivel chair with four or five students close by.
Subject sits in chair and tilts head forward about 30 degrees
Spin the chair about 10 times
Notice twitching of the eyes
(nystagmus) after stopping.

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Tests static equilibrium
Subject stands with back to the wall.
Don’t lean on wall
Stand for 1 minute and have partner watch for swaying
Do the same exercise again but have subject close eyes

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Identify structures on models
View and identify structures on cochlea slides
Make sure that you understand the tests
What cranial nerve is being innervated with the tests performed in lab?