Anatomy and Physiology of Hearing

sound exposure is the ____ leading cause of hearing loss

2nd

we have 2 ears for sound ____ and ____

localization and lateralization

our hearing range is from ____ to ____ Hz

16 to 20,000 Hz

sounds > 120 dB are damaging to the ____ in the cochlea

cilia (hair cells)

sounds that are >140 dB:

structure of the ear cannot tolerate it and will come apart

what is the pinna

• cartilage covered with skin

  • attached to skull via 3 extrinsic muscles/ligaments, cartilage, and skin

  • innervation from CN V (trigeminal)

the ear starts to develop at ____

6 weeks gestation

by the 12th week the ____ begins to fuse and assume the classic shape

auricle/pinna

by week 20, the ____ has moved to its appropriate location on the side of the head

outer ear

ear malformation can be related to ____

genetic syndromes, exposure to teratogens, or intrauterine positioning

external auditory meatus

• opens at pinna and ends at the tympanic membrane

  • innervated by CN V, CN VII and CN X

• has a “bend” to protect the tympanic membrane

outer 1/3 of the ear is ____ and medial 2/3 is ___

cartilaginous 1/3, osseous 2/3

cereumen helps to _____

lubricate and clean the ear canal and rotect the middle ear from fungi/bacteria

what is the function of the outer ear?

• collect sound and channel it to the middle ear via the EAM

  • pinna collects and reflects sound based on location of stimulus

  • protect ears

    • provide lubrication

EAM filters and boosts higher frequency sounds by approximately ____

10-12 dB

microtia

abnormally small pinna (usually also have small or absent EAM)

EAM stenosis

narrowed external auditory meatus

atresia

absent EAM (usually unilateral but can be bilateral)

how does sound transfer in the middle ear?

transfers sound vibration from the air filled ear canal to the cochlea (fluid filled) by changing the acoustic energy into mechanical vibration

tympanic membrane

• thin, concave, cone-shaped membrane

• seperates the outer ear and the middle ear

• pearl-gray in color when healthy

• has a cone of light in the lower quadrant

  • susceptible to rupture from middle ear infection or trauma

middle ear cavity

6- walled air filled cavity located in the petrous portion of the temporal bone

• approximately size of sugar cube

middle ear cavity is divided into categories, what are they?

• atic

• tympanic cavity proper

• roof

  • floor

what nerve innervates the middle ear cavity?

• auriculotemporal nerve (branch of mandibular nerve CN V3)

osscicular chain

malleus, incus, stapes

what is the point of the ossicular chain?

• creates change in the fluid-filled cavity (footplates of the stapes)

middle ear development

1. 6-7 weeks structures of the ossicles begin to form

2. 8 weeks tympanic ring appears and is fully ossified by 19 weeks

3. 16-22 weeks rapid ossification started with the incus and malleus, followed by the stapes

4. 23-24 weeks ossicles are essentially fully ossified and attain adult size

5. by 35 weeks, the TM reaches full-term size and shape

6. by 37 weeks the middle ear cavity matures and ossicles reach adult configuration

oval window

where sound enters into the inner ear

• occupied by the footplate of the stapes

round window (fenestra rotunda)

circular opening in the basal end of the cochlea

• important role in fluid displacement from movement of the stapes footplate in the oval window

tensor tympani

• Draws the malleus medially and anteriorly to increase tension on the TM

• Increased tension reduces the pliability and movement of the TM, limiting

mechanical sound propagation

• Innervated by the mandibular branch of the Trigeminal nerve (CN V3)

stapedius

• pulls stapes posteriorly and tilts the footplate to limit movement in the oval window

  • innervated by the facial nerve (CN VII)

sound propagation through the middle ear

• OE funnels acoustic energy into the ear

• acoustic energy causes the tympanic membrane to move/vibrate

• vibration of the tympanic membrane pushes the malleus, the incus, the stapes

  • the footplate of the stapes pushes in and out of the oval window of the cochlea

impedance matching

Sound is amplified by approximately 30

dB

• The TM is larger than the ossicles, so

the force concentrated on each ossicle

creates much higher pressures

• The concave shape of the TM funnels

vibrations to the center (the umbo)

where the malleus attaches, adding

more pressure

• The ossicles also act as a lever system,

amplifying the force at the stapes

problems with middle ear

-perforations of tm

• otitis media

  • tympanosclerosis

inner ear

Fluid-filled cochlea that converts

mechanical vibrations into fluid

pressure waves, then electrical

signals to CN VIII

Contains vestibular system (balance

organ)

funcitonal divisons of inner ear

• Semicircular canals and otolithic organs

• Organs for balance

• Cochlea

• Organ for hearing

parts of the inner ear

Vestibule (continuous with semicircular

canals and cochlea)

• Oval window perforates the lateral wall

and is the opening to the middle ear

• Semicircular canals (superior, posterior,

and lateral)

• Movement of fluid within the canals

registers forward-backward, side-to-

side, and rotation of the head

• Proprioceptive sense of where we are

in space

• Organ of balance

• Cochlea

• Spiral shaped canal that coils 2.5 turns

(~1.5” long when straightened)

ducts of the cochlea

Scala vestibuli: starts at the oval window

and runs to the helicotrema

• Helicotrema is at the apex of the

cochlea and allows the scala vestibuli

and the scala tympani to share fluid

• Scala tympani: lower duct that runs from

the helicotrema down to the round window

• Scala media: middle canal that houses the

organ of Corti (organ for hearing)

• Basilar membrane forms the

foundation for the organ of Corti and

stretches across the cochlea

development of inner ear

Week 6 Cochlear duct begins to differentiate

• Weeks 6-10 cochlear duct grows and spirals

• Week 10 epithelium inside cochlear duct differentiates into organ

of Corti, comprising hair cells and tectorial membrane

• Weeks 12-16 scala vestibuli and scala tympani develop

• 2nd trimester cartilaginous capsule surrounding membranous

cochlea begins to ossify into bony labyrinth

sound propagation in the cochlea

The stapes footplate in the oval window

pushes against and creates a pressure wave

• The round window is displaced (bulges

outward into ME) to allow for fluid

displacement

• The basilar membrane ripples in response to

the pressure change, the ripple travels along

the basilar membrane until it reaches the

appropriate frequency band

• Hair cells (stereocilia) are bent/sheared by the

pressure wave, causing them to release

chemical signals that trigger an electrical

response in the auditory nerve

• The electrical signals travel via CN VIII to the

brain

vestibulocochlear nerve

• Roots arise from vestibular and cochlear

nuclei in the brainstem

• Vestibular root  vestibular nerve

(balance)

• Cochlear root  cochlear nerve

(audition)

• Posterior (dorsal) branch of the cochlear

nerve carries information about high-

frequency tones

• Anterior (ventral) branch of the cochlear

nerve carries information about low-

frequency tones

retrocochlear problems

Sensorineural hearing loss is caused

by damage beyond the cochlea,

typically involving the auditory nerve

or brainstem

• Acoustic neuroma/ Vestibular

schwannoma (benign tumor on CN

VIII)

• Multiple sclerosis

• Auditory neuropathy

• Vascular abnormalities

auditory neural pathways

• Primary pathway

• Spiral Ganglia

• Cochlear nuclei (ventral and

dorsal)

• Superior Olivary Complex

• Lateral Lemniscus

• Inferior Colliculus

• Medial Geniculate Body

• Auditory Cortex

• Non-lemniscal pathway

(unconscious perception

including attention, emotional

response, and auditory

reflexes)