Vestibulocochlear Nerve (CN VIII): Anatomy And Pathway | Kenhub

Origin

Vestibulocochlear nerve Nervus vestibulocochlearis 1/3 Synonyms: Cranial nerve VIII, CN VIII , show more...

The vestibulocochlear nerve, or the eighth cranial nerve (CN VIII), is the sensory nerve which consists of two divisions. Each emerge from their respective roots:

• The vestibular root (gives rise to the vestibular nerve)
• The cochlear root (gives rise to the cochlear nerve)

The roots arise from the vestibular and cochlear nuclei located in the brainstem. The fibers from both roots merge to form the vestibulocochlear trunk of the nerve that is found in the posterior cranial fossa in the petrous part of the temporal bone. When the vestibulocochlear nerve reaches the inner ear, it again splits into the vestibular and cochlear part which supply target tissues of the inner ear. Since they provide special sensory functions (balance and hearing) this nerve would be functionally categorized as special somatic afferent (SSA) instead of the general somatic afferent.

Since there are two functional components of the vestibulocochlear nerve, it has two groups of nuclei: the vestibular and the cochlear nuclei. They are found within the vestibular area, a space in the lateral corner of the rhomboid fossa of the brainstem. They contain the bodies of the SSA neurons.

Vestibulocochlear nerve (CN VIII) Explore study unit

Vestibular nuclei

Superior vestibular nucleus Nucleus vestibularis superior 1/4 Synonyms: Bechterew’s nucleus, Nucleus Bechtrew , show more...

The vestibular nerve has four nuclei placed in the lateral corner of the rhomboid fossa and they are all special somatic afferent (SSA):

• The superior vestibular nucleus - Bechterew
• The lateral vestibular nucleus - Deiters
• The inferior vestibular nucleus - Roller
• The medial vestibular nucleus - Schwalbe

These nuclei belong to the vestibular component of the vestibulocochlear nerve and play a role in the function of balance, spatial orientation and modification of muscle tone.   The vestibular nuclei are aligned within the rhomboid fossa in such a way that they form two vertical columns, one medial and one lateral. In the medial column is the medial nucleus, whereas the inferior, lateral and the superior nuclei form the lateral column.   These nuclei contain the bodies of second-order neurons of the vestibular pathway. First-order neurons are located within the vestibular ganglion at the fundus of the internal acoustic meatus. The peripheral fibers of these neurons conduct signals from the receptors of the utricle, saccule and semicircular ducts, whereas the central fibers form the vestibular component of the vestibulocochlear nerve, which extends between the inferior peduncle of the cerebellum and the spinal tract of the trigeminal nerve. Each fiber splits into the superior and the inferior branch just before its ending. The superior branches end within the superior and the lateral vestibular nuclei, whereas the inferior branches terminate in the medial and the inferior vestibular nuclei. Medial and inferior nuclei send afferent fibers to the cerebellum that travel through the inferior cerebellar peduncle. They innervate the flocculus and nodulus and supply the cerebellum with informations necessary for coordinating of postural adjustments. 

The vestibular nuclei share multiple connections with the gray matter of the spinal cord, oculogyric nuclei (the nuclei of the CN III, IV and VI), the reticular formation and the cerebellum. Signals from the vestibular ganglion and the cerebellum travel to the vestibular nuclei, and from there go to the motor nuclei of the spinal cord.

Vestibulospinal tract Tractus vestibulospinalis 1/2 Synonyms: none

The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract which goes to the ipsilateral lumbosacral segments of the spinal cord and is important for the maintenance of posture and walking.

From the medial vestibular nucleus, the medial vestibulospinal tract emerges. This tract synapses with the neurons of laminas VII and VIII of the cervical and thoracic segments of the spinal cord. This pathway decussates partially and joins the major associative pathway of the brainstem – the medial longitudinal fasciculus – through which it influences the movements of the neck and the upper limbs as a response to the stimuli that reach the vestibular nuclei.

Cochlear nuclei

The cochlear nerve has two nuclei, and both are special somatic afferent (SSA):

Anterior cochlear nucleus Nucleus cochlearis anterior 1/4 Synonyms: Ventral cochlear nucleus, Nucleus cochlearis ventralis

• The posterior (dorsal) cochlear nucleus
• The anterior (ventral) cochlear nucleus

These nuclei are the most lateral of all the cranial nuclei. They are located medially to the inferior peduncle of the cerebellum in the lateral angle of the rhomboid fossa.

In these nuclei are the bodies of second-order neurons of the acoustic pathway. The first-order neurons of the acoustic pathway are located within the cochlear spiral ganglion in the inner ear. The dendrites (peripheral extensions) of these neurons receive the stimuli from the receptors in the organ of Corti, whereas their axons (central extensions) together form the cochlear part of the vestibulocochlear nerve.

The cochlear nerve enters the brainstem and terminates within its cochlear nuclei. Each cochlear fiber has two branches:

• The anterior (ventral) branch that conducts the impulses caused by the tones of the low frequency and terminates in the anterior cochlear nucleus
• The posterior (dorsal) branch which carries the information about the high-frequency tones, and ends within the posterior cochlear nucleus

The neurons of the cochlear nuclei give rise to striatal fibers which create stripes of white matter. These stripes communicate with the tertiary centers of the nervous system in which gathered information is integrated, interpreted, and consciously perceived. The posterior nucleus sends two stripes: the posterior and the intermediate cochlear stripes, whereas the anterior nucleus sends the third, the anterior cochlear stripe. All three of these stripes pass through the reticular formation of the pons, cross to the contralateral side of the brain at the level of the inferior part of the pontine tegmentum, and finally, at the level of the superior olivary nucleus, all three stripes merge to form the lateral lemniscus. Nearly half of the striatal fibers from the posterior and the intermediate cochlear stripes do not participate in the forming of the lateral lemniscus, nor do they cross to the contralateral side. Instead, they synapse with the ipsilateral superior olivary nucleus and join the ipsilateral lateral lemniscus.