VESTIBULOCOCHLEAR NERVE: PATH, FUNCTION, PATHOLOGIES - ANATOMY AND PHYSIOLOGY - 2025

The vestibulocochlear nerve is the eighth cranial nerve. This nerve consists of a vestibular portion and a cochlear portion. The VIII cranial nerve is basically sensory and, therefore, carries afferent information.

The vestibular portion conducts afferent impulses related to balance and posture from the vestibular labyrinth in the inner ear; It includes the semicircular canals, the utricle, and the saccule. The cochlear portion transmits the auditory impulses from the cochlea of ​​the inner ear.

Anterior view of the human brain showing the cerebral nerves. VIII. N. vestibulocochlearis (Source: John A Beal, PhD Dep't. Of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport

The history of the discovery of the vestibulocochlear nerve and its functions dates back to ancient Greece in the 6th century BC. C., period in which the Greek philosopher and mathematician Pythagoras discovered that sound was a vibration of the air. Then it was discovered that sound waves move the eardrum and these vibrations are transmitted to the inside of the ear.

Seven centuries later, in 175 AD. C., the Greek doctor Galen discovered that the nerves transmitted sound to the brain. This eminence also recognized the acoustic nerve as an independent nerve and discovered that it was composed of two different parts, one acoustic and the other that he defined as static.

Later, Rafael Lorente de Nó (1902-1990) described the VIII cranial nerve pathways and the structure of the vestibular nuclei. The name vestibulocochlear nerve was reflected in the third edition of the 1966 International Anatomical Payroll; this name arose by consensus, since the name had to reflect the double function of the nerve.

Journey

Vestibulocochlear nerve fibers originate from bipolar cells located in the cochlear and vestibular spiral ganglia. The nerve runs between the pons and the bulb in a groove and postero-laterally with respect to the facial nerve.

The peripheral processes go to the vestibular and cochlear receptors, and the central ones to the brain stem. The facial nerve (VII) and its intermediate nerve, the vestibulocochlear nerve (VIII) and the internal auditory artery (labyrinthine) circulate through the internal auditory canal.

Course of the cochlear portion of the VIII cranial nerve

The cochlea is the human auditory receptor. It is a spiral duct located in the petrous portion of the temporal bone, at the base of the skull. The cochlea receives double innervation by afferent and efferent fibers connected to the hair cells.

The auditory afferent fibers, after entering the brainstem at the vestibulocochlear nerve, branch into the medulla oblongata in the cochlear-ventral and cochlear-dorsal nuclei. This zone forms the primary receptor area.

In the cochlear nuclei of the bulb, neurons sensitive to different frequencies are arranged such that there is a tonotopic distribution in the nucleus. This pathway, in addition to the cochlear nuclei of the bulb, connects with other nuclei.

Cochlea cross section (Source: Fred the Oyster via Wikimedia Commons)

These nuclei are: the superior olive complex, the nucleus of the lateral lemniscus, the inferior quadrigeminal tubercle and the medial geniculate body. There, signals from the ear interact on their way to the cerebral cortex.

Finally, the pathway reaches the medial geniculate body and from there projects to the cerebral cortex in the primary auditory area, which corresponds to Brodmann's area 41 in the temporal lobe. Despite the multiple interconnections of the pathway, most of the information that reaches one of the auditory cortices comes from the contralateral ear.

Path of the vestibular portion of the VIII cranial nerve

There are several vestibular structures that possess specific mechanoreceptors. The saccular structures called the utricle and saccule contain areas called macules and respond to linear acceleration.

The utricle connects with the superior, horizontal, and posterior semicircular canals. In the widening of these ducts are the bullae where the specialized receptors are, the ridges, which respond to angular acceleration.

The vestibule receives a double innervation. Vestibular afferent fibers from bipolar neurons whose bodies are in the vestibular ganglia and efferent fibers from the brain stem.

The afferent axons connect to the vestibular hair cells that are mechanoreceptors of the labyrinth. The macula contains hair cells arranged in different directions, this causes that under certain movements of the head some fibers increase the firing frequency and others reduce it.

Diagram of the macular region of the saccule in the inner ear. Hair cells, otolithic membrane and otoliths are visible (Source: via Wikimedia Commons)

As this information reaches the central nervous system, the system can learn the location of the head.

The vestibular nuclei receive information from the afferent fibers from the vestibular receptors. These nuclei are found in the pons and in the brain stem. There are four: one superior, one medial, one lateral and one inferior.

The vestibular nuclei receive information from the spinal cord, the cerebellum, the reticular formation, and higher centers. These nuclei also have projections toward the medulla, toward the common ocular motor, the cerebellum, and the reticular formation.

Each vestibular nerve terminates in the ipsilateral (same side) portion of the vestibular nucleus and in the flocculonodular nucleus of the cerebellum. The fibers that come from the semicircular canals terminate in the superior and medial vestibular nuclei and project towards the nuclei that control eye movements.

The fibers of the utricle and saccule terminate in the lateral nuclei and project to the spinal cord. The vestibular nuclei also project to the cerebellum, the reticular formation, and the thalamus, and from there to the primary somatosensory cortex.

Features

The receptors for hearing and balance are found in the ear. The outer ear, the middle ear, and the cochlear portion of the inner ear are responsible for hearing. The semicircular canals, the utricle, and the saccule of the inner ear are responsible for balance.

The receptors of the semicircular canals detect angular acceleration, those of the utricle detect horizontal linear acceleration, and those of the saccule detect vertical linear acceleration.

Pathologies

Deafness of nervous origin is one of the pathologies that affect the cochlear root of the VIII cranial nerve. This may be due to the use of gentamicin, an ototoxic antibiotic that can damage the stereocilia of the receptor cells or destroy them. Prolonged exposure to noise can also damage stereocilia and cause deafness.

Vascular lesions of the medulla oblongata that affect the connections of the auditory pathway or tumors of the VIII cranial nerve can cause deafness of nervous origin.

The "motion sickness" is produced by excessive stimulation of the vestibular system, characterized by nausea, changes in blood pressure, sweating, paleness and vomiting. These symptoms are due to reflexes mediated by connections in the brain stem and the flocculonodular nucleus of the cerebellum.

Reference

1. Ganong, WF, & Barrett, KE (2012). Ganong's review of medical physiology. McGraw-Hill Medical.
2. Netter, FH (1983). The ClBA Collection of Medical Illustrations, Vol. 1: Nervous System, Part II. Neurologic and Neuromuscular Disorders.
3. Putz, R., & Pabst, R. (2006). Sobotta-Atlas of Human Anatomy: Head, Neck, Upper Limb, Thorax, Abdomen, Pelvis, Lower Limb; Two-volume set.
4. Spalteholz, W. (2013). Atlas of human anatomy. Butterworth-Heinemann.
5. Standring, S. (Ed.). (2015). Gray's Anatomy ebook: the anatomical basis of clinical practice. Elsevier Health Sciences.
6. Wiener, CM, Brown, CD, Hemnes, AR, & Longo, DL (Eds.). (2012). Harrison's principles of internal medicine. McGraw-Hill Medical.