CorpusHuman Anatomy
NervousNerve

Vestibulocochlear nerve (CN VIII)

Nervus vestibulocochlearis

Also known as: CN VIII, eighth cranial nerve, auditory nerve, acoustic nerve, statoacoustic nerve

A purely sensory nerve (SSA) with two parts. The cochlear nerve carries hearing and the vestibular nerve carries balance and head movement, both from the inner ear through the internal acoustic meatus to the brainstem at the cerebellopontine angle.

Show on body map Ear — Coronal Section

Overview

ComponentsSSA only (plus efferent olivocochlear fibres)
GangliaSpiral (cochlear); vestibular (Scarpa)
Cochlear nerve fibres≈30 000
Entry/exitInternal acoustic meatus

The cochlear nerve is formed by the central processes of bipolar neurons in the spiral ganglion, which lies in the modiolus of the cochlea. These neurons are mainly type I neurons innervating the inner hair cells. The vestibular nerve comes from bipolar neurons of the vestibular (Scarpa) ganglion in the internal acoustic meatus. They supply the maculae of the utricle and saccule (linear acceleration and gravity) and the cristae of the semicircular canals (angular acceleration). The two parts join in the meatus and run with CN VII and the labyrinthine artery. They enter the brainstem at the cerebellopontine angle, lateral to the facial nerve.

Cochlear fibres end in the dorsal and ventral cochlear nuclei. From there the auditory pathway runs through the superior olivary complex, lateral lemniscus, inferior colliculus and medial geniculate nucleus to the primary auditory cortex (Heschl gyri). Because input crosses at several levels, each ear is represented bilaterally above the cochlear nuclei. As a result, unilateral deafness indicates a lesion of the ear, the nerve or the cochlear nuclei, not of the cortex. Vestibular fibres end in the four vestibular nuclei (superior, inferior, medial, lateral), and some pass directly to the flocculonodular lobe of the cerebellum. The vestibular nuclei drive:
• the vestibulo-ocular reflex via the MLF;
• posture via the vestibulospinal tracts.

Efferent olivocochlear fibres also travel in the nerve and modulate the gain of the outer hair cells.

Location

Inner ear (cochlea, vestibule, semicircular canals) → internal acoustic meatus → cerebellopontine angle (pontomedullary junction).

Parts & subdivisions

  1. Cochlear nerve
    Hearing. Bipolar neurons of the spiral ganglion relay to the cochlear nuclei. Tonotopic organisation is preserved throughout the pathway.
  2. Vestibular nerve
    Balance, linear and angular acceleration, and head position. It has superior and inferior divisions. Its projections go to the vestibular nuclei and cerebellum, and onward to the vestibulo-ocular and vestibulospinal pathways.
  3. Clinical testing
    Hearing: • Whispered voice or finger rub in each ear. • Rinne test (512 Hz fork, mastoid vs ear canal). Normally air conduction is better than bone conduction ("Rinne positive"). In conductive loss bone conduction is better than air conduction. • Weber test (fork on the vertex). In conductive loss the sound lateralises to the affected ear; in sensorineural loss it lateralises to the normal ear. Vestibular: • Look for nystagmus. • Head impulse test (abnormal with a peripheral lesion). • Romberg test and gait. • Dix–Hallpike manoeuvre for benign paroxysmal positional vertigo (BPPV).
  4. Lesion signs
    Ipsilateral sensorineural hearing loss, tinnitus, vertigo, nystagmus (fast phase away from a destructive peripheral lesion), imbalance and nausea.

Anatomy

Course

Spiral and vestibular ganglia → internal acoustic meatus (posterior to the facial nerve; cochlear part anteroinferior) → crosses the cerebellopontine angle cistern → enters the brainstem at the pontomedullary junction → cochlear and vestibular nuclei.

Function

  • Hearing (sound transduced by cochlear hair cells)
  • Detection of head rotation (semicircular canals) and of linear acceleration and gravity (utricle, saccule)
  • Afferent limb of the vestibulo-ocular and vestibulospinal reflexes that stabilise gaze and posture

Clinical significance

Vestibular schwannoma (acoustic neuroma)

A benign Schwann cell tumour of the vestibular division in the internal acoustic meatus and cerebellopontine angle. It presents with unilateral progressive sensorineural hearing loss and tinnitus, with less vertigo than expected because of central compensation. As it enlarges it may cause:
• loss of the corneal reflex (CN V), an early sign;
• facial weakness (VII);
• ipsilateral ataxia;
• obstructive hydrocephalus.

Bilateral vestibular schwannomas are diagnostic of neurofibromatosis type 2. Unilateral sensorineural hearing loss should prompt an MRI of the internal acoustic meatus.

Acute vestibular neuritis

Probably viral inflammation of the vestibular nerve. It causes sudden, severe, continuous vertigo with nausea, horizontal nystagmus and an abnormal head impulse test, but no hearing loss. When hearing is also affected the condition is called labyrinthitis. A normal head impulse test in acute vertigo is a red flag for posterior circulation stroke (HINTS examination).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.