Karnataka PGET ENT MCQ Answer 5

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 1^st and 2^nd pharyngeal arches
• 1^st arch=top of ear
• 2^nd 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 1^st pharyngeal pouch
• Ossicles: from cartilages of 1^st and second arches
• 1^st: malleus and incus
• tensor tympani (muscle of malleus)—CN V
• 2^nd arch: stapes
• stapedius muscle—CN VII
• embedded in mesenchyme for most of gestation
• 8^th month: mesenchyme degenerates; endodermal epithelial lining of tympanic cavity envelops them and connects them to wall of cavity in mesentery-like fashion
• Inner Ear
• 4^th 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
• 6^th 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