Lecture 16 – Anatomy of Hearing.docx

Auricle (pinna)

The visible part of the external ear that helps localize sound sources and has evolved for this purpose.

Tympanic membrane (eardrum)

Separates the outer ear from the middle ear, transmits sound vibrations. Consists of three layers of tissue, oval-shaped and 10 mm in diamter

External auditory meatus

The ear canal that amplifies frequencies between 2 kHz and 5 kHz and terminates at the tympanic membrane. 7mm in diameter and 2.5 cm long. lined with hair cells and cerum (wax) to protect by trapping dirt and insects

Ossicles

transmit sound vibrations to the inner ear, held in place by ligaments

Stapedius muscle

The smallest muscle in the human body, pulls the stapes posteriorly.

Cochlea

A snail-shaped structure in the inner ear responsible for transforming sound waves into neural impulses.

Oval window

Connects the middle ear to the cochlea, allowing for the transmission of sound vibrations through the stapes.

Umbo

point of attachment for malleus in center of tympanic membrane

Layers of the tympanic membrane

Responsible for initiating mechanical impedance-matching process of middle ear

-First layer: outer (cuticular) layer

-Second layer: intermittent (fibrous) layer

-Third layer: inner (mucous) layer

Malleus

largest ossicle (9mm long and 25mg), attachment with tympanic membrane

Incus

shaped like and anvil (7mm and 30 mg), intermediate link of ossicular chain, incus-malleus joint is a saddle joint

Stapes

final and smallest bone in ossicular chain (4mg), transmits from eardrum to oval window. Incus-stapes joint is ball and socket joint

Tensor typani

pulls malleus anterior and medial

Sound attenuation in the middle ear

the muscles of the malleus can be tensed to dampen low frequencies. Activated before speaking or once inner ear processes noise

Inner ear pressure

ossicles convert pressure from air (elastic) to water of internal ear (incompressible liquid, leading to pressure increase about 20 times (necessary to generate activation of liquid and prevent reflection)

Eustachian tube

connects from middle ear to nasopharynx to allow pressure equilization

Scala vestibuli

upper passage of cochelea, connect to middle ear through oval window

Scala tympani

lower passage of cochlea

Oval window

o   The pressure waves from the middle ear (stapes) reach the cochlea through the oval window à longitudinal pressure waves are generated in internal ear fluid through scala vestibuli to the apex

Round window

pressure waves return via scala tympani to round window, which serves as pressure release (incompressible fluid)