It is often hard to memorize everything about each cranial nerve - the fact that they are twelve in total and various types isn't helpful either. There are many known facts about them, so students easily get lost in all of that information and forget why they even have to know them.
For this reason, every important fact about the vestibulocochlear nerve in this article will be backed with its clinical importance. This way you can easily remember everything you have to know about the anatomy, course and the damage of the vestibulocochlear nerve.
|Type||Special somatic afferent (SSA)|
|Origin||Rhomboid fossa of the brainstem|
Vestibular for balance
Cochlear for hearing
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.
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. 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.
The cochlear nerve has two nuclei, and both are special somatic afferent (SSA):
- 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.
Course and relations
After the merging of the vestibular and cochlear root, the trunk of the vestibulocochlear nerve leaves the brain through the posterior cranial fossa travelling lateral to the abducens nerve and the facial nerve. The vestibulocochlear nerve then extends anteriorly and laterally. Together with the facial and the intermediate nerve, it enters the internal acoustic meatus. At the fundus of the internal acoustic meatus, the nerve splits into its two roots, vestibular and cochlear. Each roots takes a different pathway and has different connections within the inner ear.
The cochlear root penetrates the anterior lower quadrant of the fundus of the internal acoustic meatus and through the longitudinal canals of the modiolus, it reaches the spiral canal of the modiolus in which the spiral ganglion is placed. The vestibular root connects to the vestibular ganglion, and it lies at the fundus of the internal meatus.
The vestibular root passes through the vestibular ganglion, and after that, it splits at the fundus of the internal acoustic meatus to the following branches:
- The utriculo-ampullary nerve
- The saccular nerve
- The posterior ampullary nerve
The utriculo-ampullary nerve passes through the apertures on the posterior upper quadrant of the fundus of the internal acoustic meatus, and, after it enters the vestibule, it splits into its three branches:
- The utricular nerve which innervates the macula of the utricle
- The anterior ampullary nerve which innervates the crista ampullaris of the anterior membranous ampulla
- The lateral ampullary nerve which innervates the crista ampullaris of the lateral membranous ampulla
The saccular nerve penetrates through the apertures in the posterior lower quadrant of the fundus of the internal acoustic meatus and enters the vestibule, where it innervates the macula of the saccule which detects somatosensory stimuli.
The posterior ampullary nerve passes through its own aperture at the fundus of the internal acoustic meatus and then innervates the crista ampullaris of the posterior membranous ampulla.
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All of the structures of the inner ear are connected to each other, therefore, any lesions of the vestibular system may cause cochlear disorders and vice versa. The most common disorders of the vestibulocochlear nerve are:
- Conductive deafness which occurs when there is a defected conduction of sound waves to the endolymph, meaning that it is external ear deafness. Several conditions may cause conductive deafness, but the most common is the blockage of the external acoustic meatus, injuries of the tympanic membrane, fluid in the tympanic cavity or the fixation of the footplate of the stapes. Some types of conductive deafness can be corrected with hearing aids as they help transmit sound from the outer or middle ear to the vestibulocochlear nerve . Conductive deafness caused by fluid in tympanic cavity or the fixation of the footplate of the stapes may require a surgical procedure.
- Sensorineural deafness is a deafness which results from the disease of the cochlea or the vestibulocochlear nerve, meaning that it is the middle ear deafness. It can be distinguished from conductive deafness after performing both Rinne and Weber hearing tests. Sensorineural deafness is usually treated with cochlear implants which work with patient’s remaining sense of hearing by amplifying sounds.
- Nystagmus presents as slow eye movements in one direction followed by quick eye movements in the other. Nystagmus can be a physiological condition that occurs as a person moves progressively in one direction. Because of this, the eyes need to catch up, so slow eye movements in the opposite direction are followed by rapid movements in the same direction. Just like when you're looking through the window while travelling on a bus. But if the nystagmus is happening without the proper stimuli (progressive one directional movement), it is always pathological and indicates a lesion of the vestibular system, the cerebellum or the medial longitudinal fasciculus.
- Tinnitus presents as ringing in the ear. A ringing sound that is in the low-frequency range is associated with conductive deafness. On the other hand, if it is associated with problems of the middle or the inner ear (sensorineural deafness), the ringing sound is in high-frequency range. Tinnitus usually occurs after exposure to loud sounds (e.g concerts), but also can be associated with vascular occlusions, in which case it has a pulsatory character and requires further examination. Conductive deafness tinnitus usually disappears after treatment, but sensorineural tinnitus can even amplify with a progressive loss of hearing due to the primary disease.
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