Vestibulocochlear Nerve (CNVIII)

  • There are 2 parts to CNVIII
    • Vestibular nerve – Vestibular (balance) sensation
    • Cochlear nerve – Auditory (sound) sensation

Structure

  • CNVIII originates from two separate nuclei (that are located very close together)
    1. Vestibular nuclei between pons and medulla
    2. Cochlear nuclei in the medulla
  • Passes through the cerebellopontine angle (CPA) into the internal acoustic meatus (IAM) along with CNVII
  • Splits at the IAM into 4 different nerves:
    • Vestibular nerve3 divisions going to vestibular apparatus coming off at the vestibular ganglion (within the IAM in the temporal bone)
      • Superior division (to utricle and anterior/lateral semicircular canals)
      • Inferior division (to saccule) – commonly affected by Acoustic Neuroma
      • Nerve from posterior semicircular canal
      • The cell body of bipolar neurones lies in the vestibular ganglion (within the IAM)
    • Cochlear nerve – sensory to the spiral organ and so is responsible for hearing
      • Cell body of bipolar neurones in the spinal ganglion
Anatomy of CNVIII, note the logical branching pattern and destinations in the lateral view. Image from Moore & Dalley.

Pathways of transmission

Sound Transmission
  • Sound waves reach the ear
  • They are then transmitted along the external acoustic meatus
  • Causes vibration of the tympanic membrane (between external and middle ear)
  • Auditory ossicles in middle ear move due to this vibration
  • Stapes (part of the auditory ossicles) vibrates another membrane (oval window, causing the round window membrane to move out)
  • Causes waves in the perilymph of the cochlea in the inner ear
  • Leads to movement of cochlear inner hair cells
  • These hair cells stimulate action potential transmission through cochlear nerve

Balance Transmission
  • Changes in head orientation causes movement of endolymph fluid in the three semicircular canals/ otolithic organs
  • This excites receptors in the vestibular apparatus
  • Causes action potential in the vestibular nerve

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