2015 Hearing & Balance

Cochlear Anatomy

Critical fact icon The sound pressure wave delivered to the oval window by the stapes will ascend from the base of scala vestibuli to the apex where it is transferred to scala tympani via the helicotrema; it will then descend to the round window (also in the base of the cochlea) where the pressure will be relieved. Scala vestibuli and scala tympani contain perilymph; scala media contains endolymph.
  • the cochlea has a bony core called the MODIOLUS

  • the modiolus defines the medial direction in the cochlea (the outside wall is lateral)

  • the bony SPIRAL LAMINA is represented by the threads of the screw

  • the cochlea has a base (= screw head) and an apex (=point)
  • two membranes extend from the spiral lamina to the lateral wall of the cochlea, dividing the bony cochlea into 3 compartments: scala vestibuli, scala tympani and scala media (= cochlear duct)

  • the scalae wind around the modiolus ~3 times, defining apical, middle and basal turns

  • scala vestibuli and scala tympani are joined at the apex of the cochlea at the HELICOTREMA

  • the oval window (stapes footplate) terminates scala vestibuli; the round window terminates scala tympani
    • although the oval and round windows are structurally part of the vestibule (i.e., they are openings in that part of the osseous labyrinth), functionally they are part of the cochlea

Midmodiolar section of cochlea
  • one turn of the cochlea is outlined in light blue

  • the BASILAR MEMBRANE divides scala tympani from scala media
    • it supports the organ of Corti where the hair cells are found
    • its medial attachment is the SPIRAL LAMINA
    • its lateral attachment is the SPIRAL LIGAMENT

  • the VESTIBULAR = REISSNER's MEMBRANE divides scala vestibuli from scala media

Spiral ganglion location
  • the peripheral processes of the bipolar primary afferent neurons leave the hair cells in the organ of Corti, and enter the modiolus through the habenula perforata in the spiral limbus

  • the cell bodies of the neurons are in the SPIRAL GANGLION which is found Roesenthal's canal in the modiolus, on the inner surface of the spiral lamina

  • the central processes of the auditory nerve fibres run through the core of the modiolus as they make their way to the brainstem

  • apical fibres are found in the core of the nerve bundle, and basal fibres toward the outside, preserving a tonotopic relationship (more on that in the section on place coding)
Email: Dr. Janet Fitzakerley | ©2015 University of Minnesota Medical School Duluth | Last modified: 8-feb-15 11:17 PM