Karnataka PGET ENT MCQ Answer 5

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The Correct Answer is A

Ear development

  • three distinct parts in embryo
    • external ear: sound-collecting organ
    • middle ear: sound conductor from external to internal ear
    • internal ear sound waves --> nerve impulses; registers changes in equilibrium
  • external ear: 3 parts
    • EAM: first pharyngeal cleft
    • Eardrum or tympanic membrane
      • 3 layers of cells
        • ectodermal epithelial lining of bottom of EAM
        • endodermal epithelial lining of tympanic cavity
        • intermediate layer of connective tissue
      • major part attached to handle of malleus
    • Auricle/external ear
      • Develops from 6 mesenchymal proliferations at dorsal ends of 1st and 2nd pharyngeal arches
        • 1st arch=top of ear
        • 2nd arch=bottom of ear
      • Swellings=auricular hillocks
        • Fuse to form definitive auricle
        • As mandible grows, ear is pushed upward and backward from horizontal position on neck
  • Middle ear:
    • Tympanic cavity and auditory tube forms from 1st pharyngeal pouch
    • Ossicles: from cartilages of 1st and second arches
      • 1st: malleus and incus
        • tensor tympani (muscle of malleus)—CN V
      • 2nd arch: stapes
        • stapedius muscle—CN VII
      • embedded in mesenchyme for most of gestation
      • 8th month: mesenchyme degenerates; endodermal epithelial lining of tympanic cavity envelops them and connects them to wall of cavity in mesentery-like fashion
  • Inner Ear
    • 4th week: thickening of surface ectoderm on both sides of hindbrain
    • thickenings=otic placodes
      • invaginate to form otic vesicles (otocysts)
      • divide into ventral component and dorsal component
        • ventral componentàsaccule and cochlear duct (organ associated with hearing)
        • dorsal componentàutricle/semicircular canals (organ associated with equilibrium) and endolymphatic duct
  • cochlear duct
    • 6th week: initiates as outgrowth of saccule
    • penetrate surrounding mesenchyme in spiral fashion (2.5 turns)
    • surrounding mesenchymeàcartilage
      • undergoes vacuolization
    • scala vestibule and scala tympani form, surround cochlear duct
      • filled with perilymph to receive mechanical vibrations of ossicles
      • mechanical stimuli activates sensory cells in cochlear duct
      • impulsesàauditory fibers of CN VIII
  • semicircular canals
    • utricle: three flattened outpocketings, which lose central core
    • three semicircular canals formed
      • slightly different angles to each other
      • sensory cells arise in ampulla at one end of each canal, in utricle and saccule
      • change in position of body generates impulse in cell
      • impulses carried to vestibular fibers of CN VIII

Category: Karnataka PGET 2007 MCQs



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