The vestibular system is a somatosensory portion of the nervous system that provides us with the awareness of the spatial position of our head and body (proprioception) and self-motion (kinesthesia). It is composed of central and peripheral portions.
The peripheral portion of the vestibular system consists of the vestibular labyrinth, vestibular ganglion, and vestibulocochlear nerve (CN VIII). The vestibular labyrinth is comprised of proprioceptive components located in the inner ear;
- The semicircular canals, which contain the cells that detect angular acceleration of the head;
- The utricle and saccule, which contain the cells that detect the linear acceleration of the head and position of the head in space (spatial orientation).
The stimuli from these receptors are conveyed to the vestibular ganglion. From here, they travel through the vestibular portion of the vestibulocochlear nerve (CN VIII) into the central portion of the vestibular system; the vestibular nuclei in the brainstem. The vestibular nuclei send projections into the cerebellum, spinal cord, thalamus, and nuclei of the oculomotor (III), trochlear (IV) and abducens (VI) nerves. Via these connections, the vestibular system contributes to the adjustments of the head and neck movements, as well as the posture and balance of the whole body, vestibulo-ocular reflex and eye movements.
This article will discuss the anatomy and function of the vestibular system.
|Definition||A proprioceptive somatosensory system which mediates the sense of balance and position of the head relative to other body parts and the surroundings.|
Central: vestibular nuclei
Peripheral: Semicircular canals, otolithic organs, vestibular ganglion, vestibulocochlear nerve (CN VIII)
|Functions||Reflex eye movements (vestibulo-ocular reflex), posture and balance of head and neck|
- Labyrinth and semicircular canals
- Otolithic organs
- Hair cells
- Vestibular ganglion
- Vestibular nerve
- Vestibular nuclei
- Clinical anatomy
Labyrinth and semicircular canals
The vestibular labyrinth is a bony cavity located within the petrous portion of the temporal bone. It consists of the bony framework for the cochlea as well as the three semicircular canals. The bony labyrinth houses the three semicircular canals and the two otolithic organs (the utricle and saccule). Moreover, it contains the cochlea which is a part of the hearing apparatus.
The semicircular canals are the three membranous channels located within the bony semicircular ducts of the labyrinth. They are located in three planes, with each canal making an angle of approximately 90 degrees with the other. Thus, the semicircular canals are the:
- Anterior (superior), located in the sagittal plane
- Lateral (horizontal), located in the transverse plane
- Posterior (inferior), located in the frontal plane
The terminal part of each canal ends with a dilation called the ampulla, which opens into the vestibule. The ampulla of each semicircular canal contains a cluster of mechanoreceptor cells called the crista ampullaris. Each crista is composed of the special sensory receptor cells, called the hair cells. Given that the semicircular canals are filled with endolymph, the movements of this fluid stimulate the hair cells. In this way, each semicircular canal detects when the head moves during the rotational acceleration along its corresponding plane. In other words, the semicircular canals detect head movements such as nodding up and down, shaking side to side, or tilting left and right.
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The otolithic organs are the two membranous cavities that lie in the bony vestibule of the inner ear. Namely, they are the utricle and saccule.
- The utricle lies in the posterior part of the vestibule. On one end, it communicates with the semicircular canals, whilst on the opposite end forms a utriculosaccular duct with the saccule. This duct passes through the temporal bone and reaches the posterior surface of the petrous part of the temporal bone.
- The saccule lies anterior to the utricle and is significantly smaller than it. Besides joining the utriculosaccular duct, the saccule communicates with the cochlea by ductus reuniens.
The utricle and saccule contain the clusters of hair cells as well, except that in them, the clusters are called the macula of the utricle and the macula of the saccule. These neurosensory areas respond to the stimulation from the endolymph to detect the linear movements of the head, as well as its position in space while the head is not moving but the rest of the body is. The cells of the utricle are specialized to detect movement in the horizontal plane, while the saccule detects vertical movement.
The hair cells are the motion receptors of the inner ear, embedded within the walls of the semicircular canals and otolithic organs. They are cylindrical in shape and feature many stereocilia on their apical ends. Each cell also has a single kinocilium, which sits on the lateral most end of the apical surface.
- In the semicircular canals, the hair cells project from the cristae ampullares towards the cavity of the canals. The apical lining of the hair cells in the semicircular canals is covered by a gelatinous mass called the cupula.
- The cupula of the utricle and saccule has an additional superficial fibrous layer called the otolithic membrane which contains numerous crystals of calcium carbonate called otoliths or otoconia.
The hair cells within the vestibular organs are oriented in a way that the whole vestibular apparatus is sensitive to movements in all directions.
The rotational movements of the head in one of the planes causes the endolymph within the semicircular canals to move in the opposite direction by inertia. The endolymph then flows against the cupula, which distends across the apical surface of the hair cells, moving the stereocilia toward the kinocilium. These ciliary movements open the transduction cell channels, triggering the excitation of the cells. The movement of stereocilia in the opposite direction reduces the cell activity. The reason why the linear movements don’t stimulate the cristae ampullares is that the linear movements happen in the planes in which the endolymph always hits the cupula from both sides, eventually not causing any displacement nor hair cell stimulation.
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The hair cells of the otolithic organs function slightly differently. The otolithic membrane makes the layer over the hair cells a lot heavier than the cupula in the semicircular canal. Once the head is linearly tilted, the gravity causes the otolithic membrane to shear from the apical surface of the relevant macula. The shearing movement stimulates the cilia and triggers an action potential. The reason why the rotational movements don’t stimulate the maculae of the otolithic organs is that the flow of the endolymph alone is not enough to cause the shearing of the otolithic membrane, thus it never moves the cilia.
The action potentials from the cristae and maculae are carried by the corresponding branch of the vestibular nerve;
- Crista ampullaris of the anterior duct transmits signals via the anterior ampullary nerve.
- Crista ampullaris of the posterior duct transmits signals via the posterior ampullary nerve.
- Crista ampullaris of the lateral duct transmits signals via the lateral ampullary nerve.
- Macula of the utricle transmits signals via the utricular nerve.
- Macula the saccule transmits signals via the saccular nerve.
The utricular, anterior ampullary, and lateral ampullary nerves join to form the utriculo-ampullary nerve. Ultimately, the utriculo-ampullary, saccular and posterior ampullary nerves synapse within the vestibular ganglion.
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The vestibular ganglion (ganglion of Scarpa) lies within the fundus of the internal auditory meatus. It is a cluster of bipolar sensory neurons, that are the first-order neurons of the vestibular pathway.
The peripheral processes of vestibular ganglion cells comprise the nerve fibers that receive the stimuli from the hair cells of the otolithic organs and semicircular canals, respectively. The central processes of the vestibular ganglion comprise the fibers of the vestibular portion of the vestibulocochlear nerve (CN VIII).
The vestibular nerve transmits the equilibrium impulses from the vestibular apparatus. It leaves the inner ear through the internal auditory meatus and enters the posterior cranial fossa. Then, it synapses with the vestibular nuclei in the brainstem.
Moreover, some of the fibers synapse directly with the cerebellum as well. This makes the vestibular nerve unique, as it is the only cranial nerve whose first-order neurons synapse with the cerebellum directly.
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The vestibular nuclei are the four nuclei that lie within the rhomboid fossa of the brainstem. They contain the second-order neurons of the vestibular pathway that synapse with the vestibular portion of the vestibulocochlear nerve. Namely, the vestibular nuclei are:
- The superior vestibular nucleus (of Bechterew)
- The lateral vestibular nucleus (of Deiters)
- The inferior vestibular nucleus (of Roller)
- The medial vestibular nucleus (of Schwalbe)
The superior and medial vestibular nuclei receive most of the inputs from the cristae ampullares of the semicircular canals. The inferior and lateral nuclei receive the remaining fibers from inferior semicircular canals, as well as from the utricle and saccule. The vestibular nuclei integrate inputs from the peripheral vestibular structures, contralateral vestibular nuclei, cerebellum and the other sensory systems (the visual and somatosensory systems).
Connections with the nuclei of the cranial nerves
The superior and medial vestibular nuclei send fibers that join the medial longitudinal fasciculus (MLF). Via this pathway, they synapse with the motor nuclei of the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves, including the interstitial nucleus of Cajal and the nucleus of Darkschewitsch.
This way, the vestibular system mediates the reflexive activity of the extraocular muscles. More precisely, the vestibular system mediates the vestibulo-ocular reflex, in which the movements of the eyes are adjusted to the movements of the head.
The vestibulo-ocular reflex is an involuntary motor activity mediated by the vestibular system which serves for adjusting the eye movements while the head moves in the horizontal plane. It serves for fixing the gaze during head repositioning. The reflex arc is mediated by the medial longitudinal fasciculus (MLF) in the following way;
- Once the head is turned sideways (e.g. to the left), the endolymph in the vestibular organs falls behind to the right due to inertia and stimulates the hair cells of the left horizontal semicircular canal.
- The information is conveyed to the vestibular nuclei via the ipsilateral vestibulocochlear nerve.
- The neurons from the superior and medial vestibular nuclei join the MLF and reach the nucleus of the contralateral abducens nerve. This way, the contralateral lateral rectus muscle contracts and pulls the eyes in the opposite direction to that of the head.
- Simultaneously to the abducens stimulation, some MLF fibers cross the midline and synapse with the oculomotor nucleus. This stimulates the medial rectus muscle on the opposite eye, resulting with the conjugated movement of both eyes in the horizontal plane.
- At the same time, the ipsilateral medial rectus muscle is inhibited so that it doesn’t compete with its contralateral counterpart.
To reiterate, once a person turns their head while wanting to fix the gaze on a static object, the vestibulo-ocular reflex enables the eyes to turn in the opposite direction so that the person can maintain focus on the perceived object.
Connections with the spinal cord
The lateral vestibular nucleus sends axons via the lateral vestibulospinal tract. This tract synapses with interneurons along the entire length of the spinal cord, adjusting the posture of the body and tone of extensors according to the vestibular stimuli (vestibulo-spinal reflex).
The medial and inferior vestibular nuclei project their fibers via the medial vestibulospinal tract. This tract terminates within the cervical spinal cord, adjusting the posture of the head and neck (vestibulo-cervical reflex).
Connections with the cerebellum
The vestibular inputs connect with the cerebellum in two ways;
- The second-order neurons from the vestibular nuclei project to the inferior olivary nucleus via the vestibulo-olivary tract. From here, the vestibular inputs are relayed via the lateral part of the inferior cerebellar peduncle (restiform body) into the ipsilateral cerebellar vermis, flocculus and nodulus. This connection enables the joint modulation of balance by the cerebellum and vestibular system.
- A portion of the first-order neurons from the vestibular ganglion of Scarpa pass through the medial part of the inferior cerebellar peduncle (juxtarestiform body). They enter the cerebellum as the mossy fibers and synapse directly with the ipsilateral vestibulocerebellum, vermis and fastigial nucleus. These connections enable cerebellar awareness of the vestibular sensations and promote necessary movement modifications by the cerebellum.
Connections with the cortex
The superior and lateral vestibular nuclei project to the ventral posterior nuclei of the thalamus to synapse with the third-order neurons of the vestibular pathway. The thalamus then relays the signals to the primary vestibular cortex (Brodmann area 3a), located in the cortex of the parietal lobe adjacent to the primary motor cortex. This area integrates the information from the vestibular system with other proprioceptive systems and passes that information directly to the primary motor cortex (Brodmann area 4). From here, the motor response to the proprioceptive stimuli is generated.
Other connections between the vestibular system and the cortex are still being investigated. So far, the two particularly interesting cortical areas have been identified;
- One of them lies posterior to the primary somatosensory cortex;
- The second one lies between the somatosensory cortex and motor cortex
These cortical areas process the vestibular and visual stimuli, and they are particularly active when a person undergoes rotatory movements with eyes closed.
Motion sickness is a condition characterized by nausea and vomiting due to the travelling, it is most often seen in some people travelling by vehicle. It is due to fluctuations in the maculae.
Vertigo is a condition in which a person has a false sensation that either him or surroundings are in motion . It can cause nausea, dizziness, sweating and vomiting. This condition is associated with vestibular malfunction. Vertigo is the common symptom of Vestibular neuritis (labyrinthitis), Meniere’s disease and VIII nerve damage.
This is caused by blockage in the cochlear aqueduct and symptoms are tinnitus, hearing loss and vertigo, sense of pressure in the ear, sound distortions and noise sensitivity. This disease results in endolymphatic volume with ballooning of the cochlea duct, utricle, and saccule.
Caloric reflex test
This is a test of the vestibulo-ocular reflex. It is done by pouring cold or warm water into the external auditory canal using a syringe. The temperature difference between the body and the water creates a convective current in the endolymph of the horizontal semicircular canal. Hot and cold water produce currents in opposite directions and therefore a quick horizontal eyes movement in the opposite directions. If the water is warm, the endolymph in the ipsilateral horizontal canal rises, thereby increasing the firing rate of the afferent vestibular nerve. This will result to the eyes turning towards the contralateral ear, with a quick horizontal eyes movement to the ipsilateral ear. If the water is cold, the endolymph in the semicircular canal falls, thereby decreasing the firing rate of the afferent vestibular nerve. This will result to the eyes turning towards the ipsilateral ear and a quick horizontal movement of the eyes to the contralateral ear. If the caloric test is done on patients with cerebral damage, the fast phase of quick horizontal eyes movement will be absent as this is controlled by the cerebrum. Also pouring cold water into the external auditory canal of brain-damaged patients will result in quick horizontal eyes movement toward the contralateral ear.
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