Updated: 5/11/2021

Auditory System

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  • The ear can be divided into the
    • outer ear
      • involved in directing sound into the auditory canal
    • middle ear
      • an air-filled space that contains
        • tympanic membrane
        • auditory ossicles (which amplifies sound) which is comprised of the
          • malleus
          • incus
          • stapes
            • this inserts into the oval window and therefore when the ossicles vibrate it displaces the fluid within the inner ear
          • note that the
            • tensor tympani muscle decreases movement of the malleus
              • this muscle is innervated by the
                • tensor tympani nerve off of CN V3 (mandibular division of the trigeminal nerve)
            • stapedius decreases movement of the stapes
              • this muscle is innervated by the
                • stapedius nerve off of CN VII (facial nerve)
            • this is in response to loud noise
        • Eustachian tube 
    • inner ear
      • a fluid-filled space that consists of a
        • bony labyrinth that is comprised of the
          • cochlea
            • structures
              • scala vestiubli, tympani, and media
            • perilymph is located outside the ducts
              • the scala vestibuli and tympani contain this fluid which has an
                •  sodium (Na+) concentration
          • vestibule
          • semicircular canals
        • membranous labyrinth that is comprised of the
          • cochlear duct
          • utricle
          • saccule
          • semicircular canals
          • note that endolymph is located inside the ducts
            • the scala media contains this fluid which has an
              • potassium (K+) concentration
      • organ of Corti
        • located on the basilar membrane in the cochlea and contains
          • hair cells with protruding cilia that are embedded in the tectorial membrane
            • these hair cells synapse to primary sensory neurons in which their cell bodies are found in the spiral ganglion
        • tonotopy
          • high frequency vibrations are best received by the cochlear base (by the oval window)
            • due to its thin and rigid nature
          • low frequency vibrations are best received by the cochlear apex (by the helicotrema)
            • due to its wide and flexible nature
  • Auditory pathway
    • external ear → tympanic membrane → middle ear ossicles → oval window → labyrinth
      • scala media (cochlear duct) also receives vibrations, which result in
        • movement of the basilar and tectorial membrane leading to
          • activation of mechanoreceptor cilia on hair cells which in turn
            • excite primary sensory neurons (which have their cell bodies in the spiral ganglion) and ultimately will have axons sent into the cochlear nerve (of CN VIII)
    • from the cochlear nerve, electrical impulses subsequently 
      • arrive at the dorsal and ventral cochlear nuclei and then
        • bilaterally ascends superior olivary nuclear complex inferior colliculus medial geniculate nucleus primary auditory cortex

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