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Definition
detects position and motion of the head in space by integrating information from peripheral receptors located in the inner ear. The vestibular system provides the nervous system with information that is important in two areas:
1. reflex control of eye movements (the vestibulo-ocular reflex, VOR)
2. reflex control of upright posture (the vestibulospinal reflex (VSR) |
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Term
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Definition
oriented so they can detect angular acceleration in three planes (horizontal, anterior, and posterior) and direction of head movement.
2. Both ends of each semicircular canal terminate in the utricle, and each canal has an enlargement called the ampulla, where it joins the utricle.
3. In one region of the ampulla, the epithelium of the canal is thickened and contains specialized receptor cells, the vestibular hair cells.
4. A gelatinous mass called the cupula surrounds the hair cells. |
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| Both ends of each semicircular canal terminate in the _______, and each canal has an enlargement called the ______, where it joins the utricle. |
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| hair cells are stimulated only during |
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| changes in the rate of motion ( e.g., during acceleration of the head). |
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| Vestibular hair cells consist of a single tall ________ and 40-70 shorter ________. |
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| Bending of the hairs towards the kinocilium causes |
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| depolarization (leading to an increase in the firing rate of CN VIII). |
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| deflection in the opposite direction (away from the kinocilium) causes |
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| hyperpolarization (and a decreasing firing rate in CN VIII). |
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| The otolith organs provide the nervous system with two types of information: |
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| linear acceleration and head position with respect to gravitational forces. |
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| Hair cells of the utricle respond most to |
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| linear acceleration and to tilting the head forward and backward. |
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| hair cells of the saccule respond most to |
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| covered by a gelatinous substance in which tiny stones called... |
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| otoliths (also called otoconia: composed of calcium carbonate), are embedded. This makes the gelatinous substance heavier than the surrounding fluid. |
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| CN VIII (vestibular component) is composed of bipolar neurons with cell bodies located in |
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Definition
vestibular ganglia. The central portion of these axons terminate and synapse in one or more of the vestibular nuclei in the medulla:
A. Lateral vestibular nucleus (Deiter's nucleus)
B. Medial vestibular nucleus
C. Superior vestibular nucleus
D. Inferior vestibular nucleus |
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Term
| Superior vestibular nucleus (SVN) |
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Definition
A. Receives input via CN VIII from ampullae of semicircular canals, maculae in utricle and saccule, cerebellum
B. Ascending axons of the super vestibular nuclei join the ascending component of the MLF. These neurons terminate and synapse in motor nuclei of CN III, IV and VI, and participate in the another peripheral response to vestibular stimulation, the vestibulo-ocular reflex (described below) for the coordination of head and eye movements to maintain stable gaze while the head is moving. (travel with axons of the MVN)
C. Axons also project to the cerebellum and are important for postural control |
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| Lateral vestibular nucleus (Deiter's nucleus, LVN) |
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Definition
A. Receives input via CN VIII, primarily from the utricle
B. Neurons respond selectively to tilting of the head and linear acceleration
C. Also receives input from the vermis of the cerebellum
D. Gives rise to the lateral vestibulospinal tract (LVST) which has a pronounced facilitatory effect on antigravity muscles (UE flexors and LE extensors) and participates in one of the peripheral responses to vestibular stimulation known as the vestibulospinal reflex which helps maintain balance and upright posture. |
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| Medial vestibular nucleus (MVN) |
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Definition
A. Input mainly from ampullae of semicircular ducts
B. Descending neurons give rise to the medial vestibulospinal tract (MVST), which join the descending component of the medial longitudinal fasciculus (MLF). The MVST neurons terminate primarily in cervical and upper thoracic segments of the SC. The descending pathway participates in reflex control of neck movements.
C. Ascending neurons of the medial vestibular nuclei join the ascending component of the MLF. These neurons terminate and synapse in motor nuclei of CN III, IV and VI, and participate in the another peripheral response to vestibular stimulation, the vestibulo-ocular reflex (described below) for the coordination of head and eye movements to maintain stable gaze while the head is moving. (travel with axons of the SVN) |
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| Inferior Vestibular Nucleus (IVN) |
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Definition
A. Receives input from semicircular ducts, saccule, and utricle and from the vermis of the cerebellum
B. Contributes some neurons to both vestibulospinal pathways (LVST and MVST) and to the cerebellum.
C. Function: to integrate peripheral input from the vestibular apparatus with input from the cerebellum
D. Important for postural control. |
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| Vestibulo-Ocular Reflex (VOR) |
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Definition
help maintain a stable visual image on the retina while the head is moving
2. Some of the cells in both the medial and superior vestibular nuclei located in the medulla participate in the VOR.
3. The function of these nuclei can be tested by turning the head to the left or the right. The eyes rotate in the opposite direction. This helps to maintain the visual field in the horizontal plane. |
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A. Rotation around vertical axis induces normal nystagmus (to and from oscillations of eye movements, named for fast component).
B. Subject sits on rotating chair with neck flexed approximately 30 degrees to place horizontal semicircular canals in plane of rotation. Subject is rotated (i.e., to right) about 10 turns within 20 seconds, and suddenly stopped.
C. Normal subject will have horizontal nystagmus to the opposite side (i.e. to left), show dysmetria (past-pointing) and tend to fall to the same side (right), and experience a sensation of turning (vertigo) to the opposite side (left). Nystagmus lasts 15-40 seconds.
D. When nystagmus occurs spontaneously, it is a sign of CNS lesion affecting the vestibular system including the semicircular canals, otolith organs, vestibular pathways, and the vestibulocerebellum |
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Term
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Definition
A. External auditory meatus is irrigated with either cold or warm water. Convection currents are set up in endolymph in vestibular apparatus due to temperature differential. Movement of endolymph stimulates the vestibular nerve and induces nystagmus.
B. In normal subjects, cold-water irrigation results in nystagmus to the opposite side; hot water irrigation results in the reverse reactions (mnemonic COWS: cold, opposite; warm, same)
C. In comatose patients, no nystagmus is seen. With brainstem intact (but still comatose), eyes deviate to the side of cold irrigation. |
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Definition
A. Sensation of irregular or whirling motion; an illusion of movement when there is no movement.
B. May result from vestibular disease or cerebellar disease. |
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| Meniere's Syndrome (Meniere's Disease) |
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Definition
A. Inner ear disease associated with increased production of endolymph (or decreased absorption of endolymph) and increased endolymphatic fluid pressure.
B. Episodic attacks of vertigo, tinnitus (ringing in ears), hearing loss, nausea, vomiting, and sensation of fullness and pressure in the ear.
C. Characterized by horizontal nystagmus (fast phase to the opposite ear), past-pointing, and falling toward the affected side. |
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Definition
A. Inflammation of labyrinth possibly due to bacterial or viral infections or to toxicity (e.g., due to alcohol, quinine, salicylates)
B. Symptoms are same as Meniere's disease. |
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| Benign Paroxysmal Positional Vertigo (BPPV or BPV) |
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Definition
A. Most common cause of recurrent vertigo; elicited by certain head positions.
B. Not associated with tinnitus or hearing loss.
C. Presumably due to dislodging and dislocation of the utricular macular otoliths to the cupula of the posterior semicircular duct-- also called cupulolithiasis of the posterior semicircular duct. Canaliths (i.e., the otoliths that have fallen into the semicircular canals) may free-float in the semicircular canal, causing abnormal movements of the endolymph. This condition is called canalithiasis.
D. Canaliths could also stick to the gelatinous cupula in the ampulla and falsely stimulate the hair cells to signal movement when there is no actuall movement. Adherence of the otoliths or otoconia to the cupula is called cupulolithiasis.
E. Canaliths can often be successfully repositioned by a series of maneuvers of the head, and the symptoms of vertigo are alleviated. |
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Term
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Definition
A. Common cause is a conflict in sensory input from visual and vestibular receptors (e.g., sense of movement when there is none)
B. Provocative stimuli may be visual and/or vestibular.
C. Signs of motion sickness include:
1. visual and postural instability
2. pallor
3. diaphoresis
4. excessive salivation
5. headaches
6. anxiety
7. nausea
8. vomiting
D. Patients respond well to habituation training in which repetitive vestibular and visual stimuli are presented. The abnormal responses to the provocative stimuli are gradually diminished, and the patient experiences less motion sickness. |
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| Vestibular Schwannoma (Acoustic Neuroma) |
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Definition
A. Hallmark of neurofibromatosis 2 (NF2), an autosomal dominant disease characterized by formation of multiple tumors in the central and peripheral nervous system.
B. Bilateral (often) acoustic neuromas often found in cerebellopontine angle (at exit of CN VII and VIII from the brainstem)
C. Symptoms include vestibular and/or cochlear (CN VIII) dysfunction (vertigo, dizziness, tinnitus, hearing loss, facial weakness, loss of corneal reflex due to CN VII involvement). |
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| Vestibular dysfunction (vertigo and imbalance) may also occur after implantation of cochlear prosthesis to improve hearing. |
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