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 nerve – 3 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

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