1. Hearing Anatomy

2. 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

3. 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

4. Stapes Movement
Stapes
Incus
*Stapes shifts to midline relieving pressure on inner ear
Vestibular Canal
Tympanic Canal
Cochlear Partition
(Basilar Membrane)
Uncoiled Cochlea

5. Ear Protection Stapes Movement-
Aquarium with 3 doors; two are hinged on the side like swinging doors, other is hinged in the middle like a revolving door
Doors have flexible seal that permits them to swing in or out without leaking
Push door in, the incompressible liquid pushes out other door equal amount
Revolving door; push one side and the other sides moves the same distance (swing doors stay in same place because of equalization by revolving door)
Shifting vibratory axis of the stapes toward midline, reduces the pressure applied to the fluid in the inner ear, reducing the damaging effects of very loud sounds.

6. 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.

7. Inner Ear: Bony Labyrinth
Vestibular Mechanism
Semicircular
Canals
Inner Ear:
Bony Labyrinth
Utricle
Vestibule
Saccule
Cochlea
Round Window
Oval Window
*Filled with perilymph

8. 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

9. 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

10. Cochlea Helicotrema Cochlea Canals Uncoiled Round Window Cochlear Duct
Reissner’s Membrane
Round
Window
Cochlea Canals Uncoiled
Scala Vestibuli
Cochlear Duct
Scala Tympani
Basilar Membrane

11. 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

12. 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

13. Membranous Labyrinth Bony Labyrinth Membranous Labyrinth Utricle
Saccule
Cochlear Duct
* Filled with endolymph

14. 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

15. 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)

16. Hair Cells Cilia Nucleus Phalangeal Process Basilar Membrane
Nerve Endings

17. 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 cilia
Each outer hair cell has cillia
Organ of Corti may contain a million or more cilia

18. 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

19. Reading/Assignments
Seikel: Pgs
Dickson: Pgs