Download Hearing Anatomy

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 Movement
Incus
*Stapes shifts to
midline relieving
pressure on inner ear
Vestibular Canal
Tympanic Canal
Uncoiled Cochlea
Cochlear Partition
(Basilar Membrane)
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.
Semicircular
Canals
Vestibular Mechanism
Inner Ear:
Bony Labyrinth
Round Window
Oval Window
Cochlea
*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
Reissner’s
Membrane
Round
Window
Cochlea Canals Uncoiled
Scala Vestibuli
Cochlear 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
Bony Labyrinth
Membranous Labyrinth
* 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)
Hair Cells
Cilia
Nucleus
Phalangeal
Process
Basilar Membrane
Nerve Endings
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