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Updated: May 11 2021

Auditory System

  • Overview
  • Introduction
    • 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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