Auditory & vestibule periphery



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Auditory & vestibule periphery:



  • Composed of Osseous Labyrinth space within petrous bone lined by mesh-like tissue matrix filled with Perilymph (Extracellular fluid= high Na+ & low K+)

Osseous labyrinth

  • Surroundes membranous labyrinth filled with Endolymph (Intracellular fluid= high K+ & low Na+)

Labyrinthine artery [basilar artery branch]- blood supply to inner ear
Osseous Labyrinth: semicircular canals, vestibule, & cochlea

1) Semicircular canals-



  • 3 bony tubes [right angles to other 2] & emerging/ending in vestibule

  • Ampulla- swelling at each end of the semicircular canals

2) Vestibule



  • Space connected to semicircular canals posterioly and the cochlear anteriorly

  • Vestibule aqueduct- smaller bony channel

  • Stapes- insert in oval window = part of wall of vestibule

3) Cochlear

  • Spiral, 2.5/2.75 turns

  • Modiolus- bony central axis [inner means toward it, outer means away from it]

    • Gives off osseous spiral lamina= indents innner edge of cochlea

    • Vestibularcochlear Nerve VIII & Spiral ganglion [peripheral ganglion for auditory system- bipolar cells]

    • Outside wall: Spiral ligament = dense connective tissue



  • Each Cochlea: Divided into 3 compartments

    • In Osseous Labyrinth: Scala Vestibuli [SV] (oval window connection) & Scala typmpani [ST] (round window connection)

    • Membranous Labyrinth: Scala Media – middle compartment

    • Helicotrema= small connection of scala vestibule with scala typmpani at Apex of cochlea

  • Perilymp – Secreted by blood vessels in connective tissue of osseous labyrinth


Membranous Labyrinth

  • Continuous series of epithelially lned membrane channels from otocyst contaning endolymph

  • Occluding zonules- keep endolymph & Perilymph from mixing

  • Most epithelium lining= squamous BUT in sensory regions – complex

  • Semicircular ducts- Ampulla swelling at 1 end [houses Crista ampullaris = dectect rotational movements of head]

  • Utrical ---connecting duct---Saccule

    • Connecting duct gives off endolymphatic duct and sac [pressing on meninges] within vestibular aqueduct

    • Utrical & Saccule= Detect linear acceleration

    • Saccule connected by small duct to Cochlear duct

  • Cochlear duct (Scala Media) – In cochlea

    • Spiraling, trianglular shaped

    • Roof- formed by Vestibular (Reissner’s) membrane [2 simple squamous epithelial sheets separated by basal lamina]

      • Top layer = osseous labyrinth

      • Bottom layer= membranous labyrinth [with occluding zonules]

    • Floor –from by Basilar membrane (from ossous labyrinth)

      • Organ of Corti- Transduce auditory signals

    • Spiral limus- ridge in angle btw organ of corti & vestibular membrane

      • Make tectorial membrane: function w/ organ of corti: transduction

      • Supported by Osseous spiral lamina [part of osseous labyrinth]

    • Outer wall- Contains Stria Vascularis [pseudostratified vascularized epithelium: secretes Endolymph]

    • Spiral ligament- connective tissue layer [part of osseous labyrinth]



Sensory Areas of Ear= Crista ampullaris [semicircular ducts], sensory maculae [utricle & saccula], & organ of corti [cochlear duct]

General Sensory cells (hair cells): stereocilia [long microvilli] & single kinocilium (basal body if missing)

  • Kinocilium: cilium-like structure: 9 doublet microtubules (non motile)

    • Lost during development

    • Stereocilia: have Organ pipe arrangement- longest one near kinocilium or basal body

  • Stereocilia/kinocilia embedded in some type of membrane

    • K+ channels: Open mechanically via connection of adjacent sterocillia

  • Kinocilium location: Determines functional polarity = steriocillia moves: Toward kinocilium= depolarization / away = hyperpolarization

  • Depolarization= release more neurotransmitter upon CN VIII nerve Afferent sensory ganglion cells) Increase signal to: Vestibular/auditory brain centers

  • Hair cells: Influenced by axons coming from brainstem [Efferent endings]


1) Crista ampullaris- ridge running perpendicular to semicircular duct axis

  • Sensory surface [columnar epithelium]: 2 types of cells

    • Tall columnar supporting cells- secretory granules

    • Sensory Hair cells- Stereocilia and kinocilia in cupula [large gel membrane- acts like sail]

      • Head rotates endolymph move within semicircular duct push against & deviate cupula = deflection of stereocilia [polarity dependent on direction of deviation]

2) Sensory maculae- [Utricle & saccule]

  • 2 types of cells (like crista ampularis)

    • Stereocilia embedded in Otolithic Membrane =small gell with Otoliths/otoconia [crystalline inclusions in its surface]

    • Crystalline inclusions- increase mass of membrane to augment effect of acceleration on membrane & thus sterocilia

3) Organ of Corti – HEARING, runs alon entire length of cochlear duct floor

  • Specialized hair cells & supporting cells

    • Hair cells- stereocilia and basal body, NO kinocilium

      • Transducer mechanical energy into sound Freq

      • Freq response= continuous gradient along organ of corti [at Base of cochlea= transduce high freq & at Apex of cochlea= tranduce low freq]

    • Cross-section= single row of inner hair cells (most important) & 3-5 rows of outer hair cells separated by Inner tunnel/tunnel of Chordi [space]

    • Hair cells have sterocilia in Tectorial Membrene [bases of hair cells DO NOT REST on basilar membrane]

    • Each hair cell base: Supported by Phalangeal cell & a process of that cell surrounds the apex of the hair cell

    • Phalangeal cell & Pillar cells: lining inner tunnel = contain microtubules and intermediate filaments to support Inner tunnel/tunnel of Chordi

      • Allows phalangeal cells to form rigid Reticular lamina

        • Apical portion of hair cells are locked by zonula adherens & zonula occludens junctions

    • Tectoral membrane- most hair cell stereocilia embedded: contain keratin-like protein & rigid [relationship uncertain]

    • Basilar membrane- organ of corti rest on=dense mat of collagenous fibers- not rigid

    • Sound = compression of waves in perilymph shifting of two rigid plates to deviate from stereocilia produced receptor potential in sensory cells

    • Neurotransmitters: released upon Afferents of Spiral ganglion cells [some transverse inner tunnel]



Clinical

  • Vertigo= dizziness- disturbance in vestibular function [infection of vestibule or lesions of CNS/PNS]

  • Menieres disease- swelling of membranous labyrinthdizziness, tinnitus, low freq hearing loss, motion sickness (more common) caused by over stimulation of Utricle/saccule

  • Hearing loss

    • Conductive- break in conduction pathway from atmosphere to inner ear

      • Ankylosis= Disruption of tympanic membrane

      • Otosclerosis: increase bone growth & fusion of ossicles [Beethoven]

    • Sensorineural- loss of hair cells or VIII nerve fibers [cochlear implant/sign language]

      • Presbycusis (most common)- poor high freq perception due to age related loss of hair cells at basal turn of cochlea








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