Hearing Anatomy

Ear Protection

• Middle Ear:– Contains 2 smallest striated muscles in the

body-• Tensor Tympani (stiffens ear drum)• Stapedius (draws stapes away from oval window)

– Contraction is the basis of Acoustic Reflex-• Protects against damaging loud sounds• Tune ear to respond to selectively high speech

frequencies• Protects us from our own voices when too loud

Ear Protection

• How does the acoustic reflex work?– Pits two auditory muscles against each other

(they pull on opposite ends of ossicular chain)

– Tensor tympani stretches ear drum tight; more resistant to large vibrations of sound

– Stapedius attaches to stapes (antagonist to tensor tympani)and stiffens ossicular chain and acts against transmission of loud sounds.

– At extremely loud sounds the stapedius changes axis around which the stapes vibrates

Stapes

Stapes Movement

Vestibular Canal

Tympanic Canal

Uncoiled CochleaCochlear Partition

(Basilar Membrane)

Incus*Stapes shifts to midline relieving pressure on inner ear

Inner Ear: Hydraulic System

• Cavern with 2 exits close together:– Oval and sealed with a movable door (footplate

of stapes)

– Round, sealed and flexible membrane (round window)

– Off to one side- Passageway that spirals upward for 2 1/2 turns before it ends (cochlea)

– Off in the other direction is the vestibular mechanism- organ which maintains balance & detects bodily movement.

Utr

icle

Sac

cule

Ves

tibu

le

Oval Window

Round WindowCochlea

Semicircular Canals

Vestibular Mechanism

Inner Ear:Bony Labyrinth

*Filled with perilymph

Inner Ear

• Oval Window: The opening in the inner ear to which the stapes fits

• Round Window: A membrane sealed opening in the inner ear that relieves pressure at the oval window by the vibratory movement of the stapes

• Semi-Circular Canals: 3 fluid filled canals by which turning movements of the head are detected

• Vestibular Mechanism: The acceleration and equilibrium mechanism

Inner Ear

• Vestibule: The central room into which the oval window opens that connects to both vestibular mechanism and auditory receptors

• Saccule: The membranous cavity in the vestibule that detects forward and sideways movement

• Utricle: The membranous cavity that opens into the semicircular canals and detects forward and sideways movement

Cochlea

Helicotrema

Cochlea Canals UncoiledRoundWindow

Reissner’sMembrane

Scala VestibuliCochlear Duct

Scala Tympani

Basilar Membrane

Cochlea

• Cochlea: The spiral-shaped organ of hearing in the inner ear

• Scala Vestibuli: The perilymph filled canal extending from vestibule to the apex of the cochlear spiral

• Scala Tympani: The perilymph filled canal extending from the apex of the cochlea to the round window

• Helicotrema: The isthmus of the apex of the cochlea through which perilymph can flow from scala vestibuli to scala tympani

Cochlea• Cochlear Duct: The portion of the membranous labyrinth,

containing the auditory sensory receptors, forms partition

between scala vestibuli and scala tympani

• Basilar Membrane: The partition tuned to different

frequencies along its length, on which the organ of Corti

rests, separates cochlear duct from scala tympani

• Reissner’s Membrane: Thin partition separating the

cochlear duct from the scala vestibuli

Membranous Labyrinth

Utr

icle

Membranous Labyrinth

Bony Labyrinth

Saccule

Cochlear Duct* Filled with endolymph

Inner Ear: Neural System

• Organ of Corti- Mounted on the basilar membrane along its entire length– Converts hydraulic energy into bioelectric

energy

– Immersed in the endolymph that fills cochlear duct

– Above is Reissner’s membrane, separating the sealed duct from the vestibular canals

– Below the basilar membrane, terminating at round window

Inner Ear: Neural System

• Basilar membrane:– Membrane stretched between outer wall of bony

labyrinth and the bony core around which the cochlear channels spiral

• Organ of Corti:– Between 15,000 & 20,000 auditory nerve receptors are

contained in the organ of Corti– Each receptor has its own hair cells arranged in four

rows:• one row of..Inner (3,000)

• three rows of...Outer (12,000)

Nucleus

Cilia

Basilar Membrane

Nerve Endings

PhalangealProcess

Hair Cells

Hair Cells

• Rests on phalangeal cells

• Each hair cell has a phalangeal cell

• Inner row and three outer rows of phalangeal cells– outer-Deiter’s cells

• Cilia is not nerve cell but movement generates neural response

• Each inner hair cell has 30-60 cilia

• Each outer hair cell has 75-100 cillia

• Organ of Corti may contain a million or more cilia

Hair Cell Support

• Any movement of the cilia generates a neural auditory signal– Hair cells are firmly buttressed

– No accidental movement of cilia (signals without sound)

– Inner row of cells- Border cells of Held

– Outer rows- Cells of Claudius & Cells of Hensen

– Between inner & outer rows are pillars of Corti

Reading/Assignments

• Seikel: Pgs.548-558

• Dickson: Pgs. 265-281