Ear, Hearing and Equilibrium

Exercise 27BI 232

Introduction 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

Hearing

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Vestibular Portion

Cochlear Portion

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Middle Ear Ossicles (Bones)

Stapes

Malleus Incus

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Vestibular Complex

Inner Ear Composed of three

areas: Cochlea Vestibule Semicircular Ducts

(canals)

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Labyrinth 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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• The stapes strikes the oval window of the cochlea

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• 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)

helicotrema

vestibular duct

tympanic duct

round window

Cochlear duct containing the Organ of Corti

Cochlea Uncoiled

oval window

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Cochlea

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

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Organ of Corti

Vestibule 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

Vestibule 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

Inner Ear 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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Ampulla of the semicircular canals 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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Types of Hearing Loss

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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Weber Test

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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Rinne Test

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

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Rinne Test 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.

Bing Test 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

Sound Location 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

Postural Reflex Test 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)

Barany’s Test 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.

Romberg Test 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

The End 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?