Hearing Anatomy

General

• Distinguish all sounds of speech as well as 1/2 million other sounds

• Locate direction in which sounds come from

• We can detect a sound so weak that it moves the ear drum only 1/10 the diameter of a molecule

Outer Ear: Acoustic System

• Two Parts:

– Auricle or Pinna- Visible, sound gathering part

– Ear Canal or External Auditory Canal-• Tube conveying sound waves to the ear drum; • Secrete cerumen (prevents ear canal from drying

out); • Lined with cilia (small hairs) that filter dust• Tube is about an inch long• Tympanic membrane stretched across one end

(separated external canal from middle ear)• Air-filled (functions as a closed tube resonator)

Pinna

Outer Ear Middle Ear

Eustachian Tube

Ear Canal

Inner Ear

Auditory Nerve

Ear

Middle Ear: Mechanical System

• Air-filled cavity within the temporal bone (hardest bone in the body)

• Where acoustic energy of air pressure waves are converted to mechanical vibrations– Conversion occurs at the tympanic membrane which

vibrates in response to the changes in air pressure that travel down the ear canal.

– For the TM to vibrate properly:• Air pressure inside the middle ear must be the same as the

Patm outside

Middle Ear: Mechanical System

• But…If the acoustic pressure wave that travels down the ear canal also traveled into middle ear, the 2 waves would cancel out and the TM would not vibrate.– A mechanism that closes the middle ear

cavity to vibratory pressure changes, but permits it to open and adjust to changes in Patm.

Eustachian Tube

• Middle ear pressure equalizing- Plane; “Pop”

• Runs from middle ear to the nasopharynx

• Normally closed at nasal entrance

– Closure prevents direct transmission of breathing and speech sounds to the middle ear

– Opened for pressure adjustments by velopharyngeal muscle

Eustachian Tube: Infections

• Children- Repeated middle ear infections

• Frequent & severe- Damage the auditory apparatus and interferes with the child’s language development

• Reason: ET short and horizontal compared to adult

– Orientation makes it easy for infections to spread from sinuses, upper resp tract or throat by way of ET

– Spread assisted by enlarged adenoids & lymphoid tissue (blocks opening of ET)

Eustachian Tube in Infants & Adults

Tympanic Membrane

• Ear Drum- marks boundary between outer and middle ears

• Extremely thin, three layered sheet of tissue

• Epithelial lining of the external canal continues as external layer of TM; inner membrane provided by middle ear

• Middle layer-fibrous tissue providing the structure

• TM is 55 mm2

• Umbo- distant point of attachment of the inner ear TM to one of the bones of the middle ear

Tympanic Membrane

Umbo

Cone ofLight

“As viewed from theEAM”

Auditory Ossicles

• Bridges gap across the middle ear– Connecting the outer ear to the inner ear by 3 tiny

bones also called ossicular chain

• Malleus: Largest; Provide point of attachment with TM; shaped like a primitive club or mallet; length only 9 mm

• Incus: Middle bone; resembles an anvil; Intermediate communicating link; 7 mm

• Stapes: Resembles a stirrup; 3rd bone of ossicular chain; smallest ossicle

Tympanic Membrane & Auditory Ossicles

TympanicMembrane

Malleus Incus

Stapes

Middle EarEar Canal or

External AuditoryMeatus

Pinna

Outer Ear Middle Ear

Eustachian Tube

Ear Canal

Inner Ear

Auditory Nerve

Ear

Middle Ear Muscles

StapesVibration

TympanicVibration

Middle Ear

Mal

leus Incus

Inner Ear

AnnularLigament

Stapedius Tendon

TensorTympaniTendon

TensorTympaniMuscle

Amplifying Sound

• Biggest amplification source of acoustic pressure – Difference in size of ear drum and stapes

• Footplate (bottom part of stirrup) of stapes fits into oval window (Hole in the bone of inner ear)– Attached by annular ligament

– Separates inner ear (fluid) from middle ear (air)

– Converts mechanical energy into hydraulic waves

Amplifying Sound

• Footplate is 14x smaller than the area of the ear drum– All air pressure waves against the large TM is

concentrated on small footplate

• Impedance mismatch occurs:– Must be overcome to transfer acoustic vibrations into

hydraulic waves. HOW?– 1) Amplification of speech frequencies (2,500-4,000)

in ear canal– 2) Leverage of ossicular chain– 3) Mechanical force by stirrup is 14 x that of TM

Amplification of Sound

OssiclesInnerEar

Leverage Power of Ossicles

Amplifying Sound

• So…– 1) Pressure at footplate of stapes is 14 x greater

than TM

– 2) Resonation of speech frequencies in ear canal multiplies pressure at entrance of ear canal 2-4x before it reaches ear drum

– 3) Therefore, Pressure multiplied 28-56x before reaching stapes

– 4) Result= Amplification of sound

Reading/Assignments

• Seikel: Pgs. 533-548

• Maue-Dickson: Pgs. 253-264