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)

Ear Inner Ear Ear Canal Middle Ear Outer Ear Eustachian Tube Pinna Auditory Nerve Middle Ear Outer Ear Eustachian Tube

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 “As viewed from the EAM” Cone of Light

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 Malleus Incus Stapes Ear Canal or External Auditory Meatus Middle Ear Tympanic Membrane

Ear Inner Ear Ear Canal Middle Ear Outer Ear Eustachian Tube Pinna Auditory Nerve Middle Ear Outer Ear Eustachian Tube

Middle Ear Muscles Inner Ear Malleus Middle Ear Stapes Vibration Incus Malleus Stapes Vibration Annular Ligament Tensor Tympani Tendon Tympanic Vibration Stapedius Tendon Middle Ear Tensor Tympani Muscle

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 Ossicles Inner Ear 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