Table of Contents
Most people think that the ears are meant for hearing only. No doubt, audition is a very special function performed by the cochlear apparatus of the inner ear. But there is another very important function that ears perform also. The ears help in maintaining the balance of the body. Children do a common practice while playing, spinning their bodies along their axis; after sometime, when they stop and try to stand erect, for a while, they can’t maintain the balance of their bodies and tend to fall. This is due to the disturbance of the fluid present in the vestibular apparatus of the inner ear. This vestibular apparatus tells the brain about the position and orientation of the head in space. The vestibular and cochlear apparatus information is transmitted to the central nervous system via the vestibulocochlear nerve. In the following article, we will try to learn about the vestibulocochlear nerve, its origin, course, classification, and functional components. We shall also discuss the nuclei of the vestibulocochlear nerve and the auditory and vestibular pathways. We shall have a look at auditory and vestibular reflexes. Last but not least, we shall discuss some of the diseases related to the vestibulocochlear nerve.
We know that there are 12 pairs of cranial nerves that provide nerve supply to the structures present in the head and neck region of the body. Only the tenth cranial nerve is an exception that provides innervation to the viscera in the chest and abdominal cavity as well. Vestibulocochlear nerve is the eighth cranial nerve (CN VIII). It carries two special sensations, i.e., the sense of hearing from the auditory receptors of the inner ear to the auditory cortex of the forebrain. Auditory signals are received, integrated, and analyzed in the auditory cortex. If the auditory cortex gets damaged, it can result in auditory hallucinations, just like visual or olfactory hallucinations. The second important sensation carried by the vestibulocochlear nerve is concerned with the head’s balance, position, and orientation in space. The vestibulocochlear nerve is exclusively sensory in nature and does not contain any motor fibers.
Origin and course of the vestibulocochlear nerve
The vestibulocochlear nerve emerges from the brain stem at its anterior aspect in the groove between the pons and the medulla oblongata lateral to the facial nerve. In the posterior cranial fossa, it runs laterally, and enters into the internal acoustic meatus along with the facial nerve. The facial nerve enters the facial canal of the temporal bone, while the vestibulocochlear nerve enters the inner ear to innervate the cochlea. The complete sensory pathway for auditory and vestibular sensations is explained later.
Classification and functional components
The vestibulocochlear nerve is purely a sensory nerve. It contains special somatic afferents for two different sensations, both are special sensations. These fibers carry these sensations to the central nervous system.
Nuclei related to the vestibulocochlear nerve
The nuclei of the cranial nerves are the collection of cell bodies of neurons forming that cranial nerve. There are six nuclei of the vestibulocochlear nerve. These nuclei are briefly discussed below:
Cochlear nuclei are two in number, an anterior cochlear nucleus, and a posterior cochlear nucleus. These nuclei are present in the lower part of the pons where the inferior cerebellar peduncle originates. These nuclei receive afferent fibers from the organ of Corti of the cochlea. Efferent fibers ascend upward and reach the forebrain through the lateral lemniscus.
There are four vestibular nuclei; 1) superior vestibular nucleus, 2) inferior vestibular nucleus, 3) lateral vestibular nucleus 4) medial vestibular nucleus. These nuclei collectively form the vestibular system. They receive afferent fibers from the utricle, saccule, and semicircular canals and from the cerebellum via the inferior cerebellar peduncle. Efferent fibers from the vestibular nuclei go to the cerebellum, spinal cord, and the nuclei of the 3rd, 4th and 6th cranial nerves via the medial longitudinal fasciculus. Efferents also go to the cerebral cortex.
When sound waves strike the tympanic membrane, it vibrates. These vibrations are transmitted to the cochlea by the three small bones called ear ossicles; malleus, incus, and stapes. In the cochlea, there is fluid. Ear ossicles are connected to the cochlea by the footplate of stapes in the oval window. Vibrations of the stapes are transmitted to the oval window and then to the cochlear fluid. Movement of the fluid produces vibrations in the hair cells of the organ of the Corti (a special membranous organ containing hair cells that serves as the auditory receptors and are connected to the cochlear neurons). Cell bodies of these bipolar neurons of the cochlear nerve are present in a spiral-shaped ganglion, named as spiral ganglion present in the cochlea. From the spiral ganglion, the central processes of these neurons ascend upward and synapse in the cochlear nuclei present in the lower pons at the level of the inferior cerebellar peduncle. From the cochlear nuclei, second-order neurons ascend upward and synapse in the trapezoid body and inferior olivary nucleus. Then third, order neurons ascend upward in the form of a bundle known as lateral lemniscus and relay in the medial geniculate body. Some fibers of the lateral lemniscus also relay in the inferior colliculus. These play an important role in some reflexes. The medial geniculate body passes information to the auditory area via acoustic radiations. The primary auditory area is located in the superior temporal gyrus of the forebrain. All the auditory information is ultimately received in the auditory cortex. There is a secondary auditory area which is concerned with the recognition and interpretations of the sounds based on past experience (auditory memory).
Descending fibers for auditory control
Some fibers from the auditory cortex go down bilaterally and end at various levels of the auditory pathway (i.e., cochlear nuclei, trapezoid body, etc.) as well as at the organ of Corti. These fibers provide auditory feedback and may inhibit sound reception. They can also modulate the sound reception in different ways, i.e., sharpening the auditory signals, enhancing the desired auditory signals as well, and suppressing the undesired signals.
In the utricle, saccule, and semicircular canals, there is fluid. Movements of this fluid with the positional changes of the head cause impulse generation in the hair cells of the equilibrium apparatus in the inner ear. The hair cells’ movements cause the cells to depolarize, and neuronal impulses are fired. These impulses are carried by the peripheral processes of the first-order neurons to their cell bodies which are located in the vestibular ganglion. A vestibular ganglion is present in the internal acoustic meatus. From the vestibular ganglion, the central processes of these cells carry these impulses to the vestibular nuclei present in the midbrain. Efferent fibers from the vestibular nuclei go in different directions. Some of the fibers descend down to the spinal cord and form the vestibulospinal tract that takes part in balance reflexes. Some fibers enter the cerebellum via the inferior cerebellum peduncle. Some of the ascending fibers go to the thalamus and relay to the ventral posterior nucleus of the thalamus. We know that the thalamus is the main sensory collection center of the brain. From the thalamus, third-order neurons go to the vestibular area of the frontal cortex. This area is located in the postcentral gyrus just above the lateral fissure. The vestibular area in the frontal cortex involves conscious perception and control of the balance of the head.
Another important route of the efferent fibers from the vestibular nuclei is to the medial longitudinal fasciculus. Through this fasciculus, the vestibulocochlear nerve is connected with the nuclei of cranial nerve III (oculomotor nerve), IV (trochlear nerve), and VI (abducens nerve). The medial longitudinal fasciculus participates in the conjugate eye movements. It provides the pathway for the coordinated movements of the head and the eyes so that the eyes can be kept fixed on some object while the head is moving. This also involves maintaining the body’s balance using visual information.
The cochlear nerve transmits auditory stimulus to the central nervous system from the organ of Corti, the hearing organ in the inner ear. Different characteristics of the auditory stimuli are transferred, like the pitch of the sound, the amplitude of the sound, the distance of the sound source, etc., and are all transmitted as an intrinsic ability of the nerve impulse. The central nervous system receives all the signals, analyze them, and interprets the meaning of the sound heard along with its characteristics.
The utricle and saccule in the inner ear detect the position of the head and the changes in the static equilibrium state. They also play an important role in detecting the linear acceleration of the head when it moves in a linear way. Semicircular canals, on the other hand, detect the rotational movement of the head. The semicircular canal system also helps the central nervous system maintain the body’s equilibrium state when walking or running due to its predictive function. By sensing the head’s motion, this system tells the brain that disequilibrium is about to occur, so some necessary adjustments should be made to prevent the person from falling off the ground.
When a stimulus with high amplitude (a loud sound) is transmitted to the inner ear and then to the central nervous system, a reflex occurs known as the attenuation reflex, which attenuates that high amplitude stimulus by locking the ear’s ossicular system. In this reflex, the afferent pathway is formed by the cochlear part of the vestibulocochlear nerve, which takes the input to the brain. From the brain, descending signals (efferent pathway) travel from the CNS via the motor component of the facial nerve to the stapedius muscle and via the motor component of the trigeminal nerve to the tensor tympani muscles. When these muscle contract, they pull the ear ossicles towards each other, resulting in locking the system due to increased rigidity. This helps to prevent damage to the cochlea from the high-amplitude vibrations of loud sounds. Another benefit of this reflex is that it masks the low-frequency sounds present in the environment and helps in concentrating on the desired sound source, reducing the background interferences.
It is a motor tract formed in the anterior white column of the spinal cord by the descending fibers from the cerebellum and the vestibular nuclei. These fibers synapse with the anterior motor neurons. This pathway activates the extensor muscles and inhibits the activity of flexor muscles. To maintain equilibrium, there is a balance between the vestibulospinal and rubrospinal tract activity. The rubrospinal tract activates the flexors and inhibits the extensors. In normal day-to-day activities, these systems work subconsciously along with the proprioceptive pathways and cerebellar programs.
Diseases of the vestibulocochlear nerve
There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the vestibulocochlear nerve and vestibulocochlear pathway.
The inability to hear is known as deafness. Deafness can be of two types; 1) conduction deafness due to conduction disorders in which sound waves cannot be transmitted to the organ of Corti. 2) nerve deafness
Nerve deafness is due to damage to the cochlear nerve or cochlear apparatus. This is usually permanent, and hearing devices can not work in case of nerve deafness. The nerve damage causes trauma to the skull, acoustic neuroma (lesion in the internal acoustic meatus), acute labyrinthitis (inflammation of the membranous labyrinth or cochlea), ischemic injury, diabetic neuropathies, and other systemic nervous system disorders, i.e., multiple sclerosis, etc.
There is currently no cure for nerve deafness.
Lesions in the midbrain (i.e., tumor of the midbrain) that involve the vestibular nuclei, trapezoid body or lateral lemniscus result in conditions like nerve deafness. Similarly, cerebral ischemia, infarction or other injuries involving the auditory cortex (bilateral temporal lobe lesions) result in deafness.
Vertigo and Nystagmus
We know that nuclei of the vestibulocochlear nerve are connected with the nuclei of cranial nerve III (oculomotor nerve), IV (trochlear nerve), and VI (abducens nerve). These three nerves supply the extraocular muscles. A disease of Vertigo (giddiness) or nystagmus is caused by damage to the vestibular component of the vestibulocochlear nerve. Nystagmus due to vestibular lesions is the fast rhythmical oscillations of the eyeball. This is due to the loss of reflex control of the semicircular canals on the extraocular muscles. Causes include Meniere’s disease, ischemic injury, lesions of the midbrain, brainstem, tumors, etc. The caloric test can check the function of the vestibulocochlear nerve.
The vestibulocochlear nerve (CN VIII) is the eighth cranial nerve. It is a purely sensory nerve and contains special somatic afferents as its functional components for two different special senses. The vestibulocochlear nerve emerges from the brain stem in the groove between the pons and the medulla oblongata. It runs laterally and enters into the internal acoustic meatus along with the facial nerve. The vestibulocochlear nerve enters the inner ear to innervate the cochlea. There are six nuclei of the vestibulocochlear nerve; two cochlear nuclei (superior and inferior) and four vestibular nuclei (superior, inferior, medial, and lateral). Sound stimulus is perceived by the hair cells of the organ of the corti and sent to the CNS via the auditory pathway; spiral ganglion, cochlear nuclei, trapezoid body, lateral lemniscus, medial geniculate body, acoustic radiation, and primary auditory area. Movement and position of the head are detected by the utricle, saccule, and semicircular canals and send impulses to the vestibular nuclei. Efferents are passed to the cerebral cortex (conscious perception of head movement and position), cerebellum (subconscious control of body position, orientation, and equilibrium), spinal cord via vestibulospinal tract (activates extensors and inhibits flexors of the body), and the cranial nerves 3, 4 and 6 via medial longitudinal fasciculus (coordinated head and eye movements).
The cochlear part transmits sound stimuli to the CNS. The vestibular part carries information regarding the position of the head and its movements to maintain the body’s equilibrium both in static as well as dynamic states. The vestibulocochlear nerve also forms the afferent pathway of the attenuation reflex.
Diseases related to the vestibulocochlear nerve include nerve deafness and vertigo. Damage to the cochlear part can be caused by trauma to the skull, acoustic neuroma, acute labyrinthitis, ischemic injury, diabetic neuropathies, multiple sclerosis, etc. Midbrain tumors or bilateral temporal lobe lesions can also result in deafness. Currently, no treatment is available for nerve deafness. Damage to the vestibular component can cause Vertigo (giddiness) or nystagmus. This is due to the loss of reflex control of the semicircular canals on the extraocular muscles. Causes include Meniere’s disease, ischemic injury, lesions of the midbrain, brainstem, tumors, etc.
Ropper, AH, Brown RH. Victor’s Principles of Neurology, 8th ed. McGraw-Hill, 2005
Standring S (ed.) Gray’s Anatomy, 39th edition. Elsevier Churchill Livingstone, 2005Image source: Origin of the vestibulocochlear nerve