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

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.

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.

Inner Ear: Bony Labyrinth Vestibular Mechanism Semicircular Canals Inner Ear: Bony Labyrinth Utricle Vestibule Saccule Cochlea Round Window Oval Window *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 Uncoiled Round Window Cochlear Duct 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 Utricle 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)

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