Vestibular system

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Vestibular system

The system that subserves the bodily functions of balance and equilibrium. It accomplishes this by assessing head and body movement and position in space, generating a neural code representing this information, and distributing this code to appropriate sites located throughout the central nervous system. Vestibular function is largely reflex and unconscious in nature.

The vestibular labyrinth is located within the inner earenlarge picture
The vestibular labyrinth is located within the inner ear

The centrifugal flow of information begins at sensory hair cells located within the peripheral vestibular labyrinth. These hair cells synapse chemically with primary vestibular afferent nerve fibers, causing them to fire with a frequency code of action potentials that include the parameters of head motion and position. These vestibular afferents, in turn, enter the brain and terminate within the vestibular nuclei and cerebellum. Information carried by the firing patterns of these afferents is combined within these central structures with incoming sensory information from the visual, somatosensory, cognitive, and visceral systems to compute a central representation of head and body position in space. This representation is called the gravito-inortial vector and is an important quantity that the central nervous system employs to achieve balance and equilibrium. See Brain, Nervous system (vertebrate), Postural equilibrium, Reflex

The vestibular labyrinth is housed within the petrous portion of the temporal bone of the skull along with the cochlea, the organ of hearing (Fig. 1). The receptor element or primary motion sensor within the labyrinth is the hair cell (Fig. 2). Hair cells respond to bending of their apical sensory hairs by changing the electrical potential across their cell membranes. These changes are called receptor potentials, and the apical surface of the hair cell thus functions as a mechanical-to-electrical transducer. The frequency of the resulting action potentials in the VIIIth cranial (vestibulocochlear) nerve encodes the parameters of angular and linear motion. See Biopotentials and ionic currents, Ear (vertebrate), Synaptic transmission

Otolithic macula at restenlarge picture
Otolithic macula at rest

Hair cells are the common sensory element in both the angular and linear labyrinthine sensors as well as within the cochlea. The particular frequency of energy that hair cells sense within these diverse end organs arises because of the accessory structures surrounding the hair cells. Thus, angular motion is sensed by the semicircular canals, linear motion by the otolith organs, and sound energy by the cochlea.

The primary afferents innervated by hair cells are the peripheral processes of bipolar neurons having cell bodies located in Scarpa's ganglion within the internal auditory meatus. The central processes of these cells contact neurons in the brainstem of the central nervous system. The vestibular nuclei complex is defined as the brainstem region where primary afferents from the labyrinth terminate. It is composed of four main nuclei: the superior, medial, lateral, and descending nuclei. The axonal projections of vestibular nuclear neurons travel to all parts of the neuraxis, including the brainstem, cerebellum, spinal cord, and cerebrum. See Motor systems

In all vertebrates, there is an efferent system that originates from cell bodies within the central nervous system and terminates upon labyrinthine hair cells and primary afferents. The efferent vestibular system is presently a subject of intense study but undoubtedly is in place to enhance vestibular function. It is interesting that evolution felt it necessary to modify incoming vestibular information before it could enter the central nervous system.

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Monothermal caloric testing in the screening of vestibular function. Aviat Space Environ Med 2010;81(4):369-74.
Test of AN complicated with vestibular function disorders: Nieolet NystarTM Plus electronystagmograph (USA) was employed to record electronystagmograms and to perform vestibular function examinations.
There is no evidence to insist that vestibular functions will directly correlate with audiometric changes,[sup][26] and the use of vestibular testing in evaluation of the disease remains controversial.
Video nystagmography-caloric testing can confirm unilateral loss of vestibular function (usually incomplete), but this is nonspecific.
The management of vestibular neuritis involves (1) symptomatic treatment with antivertiginous drugs to reduce vertigo, andnausea/vomiting, (2) causal treatment with corticosteroids to improve the recovery of peripheral vestibular function, and (3) physical therapy (vestibular exercises and balance training) to improve central vestibular compensation [67].
Studies using objective measures of vestibular function after blast TBI provide more specific information about the true prevalence and nature of vestibular dysfunction.
Material is organized in sections on electroencephalography; magnetoencephalography; electromyography, nerve conduction studies, and related techniques; evoked potentials and related techniques; bladder, bowel, and sexual dysfunction; tests of vestibular function; and electrophysiologic evaluation in special situations.
Furthermore, it is unclear whether exposures to lead and cadmium affect balance or vestibular function in the general population.
Although the association between inner ear abnormalities and progressive sensorineural hearing loss is well known, (1-10) vestibular signs or loss of vestibular function in these patients are often unrecognised by medical practitioners.
Our findings add to growing evidence that fall risk is increased in breast cancer survivors and that vestibular function may underpin associations between breast cancer treatment and falls."
Researchers in mental health and neurological sciences then discuss such aspects as dizziness at the interface of psychiatry and neuro-otology, neurological causes of dizziness and vertigo, a modified vestibular rehabilitation program, benign paroxysmal positional vertigo in old age, arrhythmogenic causes of dizziness, cognitive deficits following complete bilateral loss of vestibular function and their relationship to the hippocampus, vibratory induced nystagmus in superior semicircular canal dehiscence and in otosclerosis, and pediatric vertigo.
People who have compromised vestibular function lose their sense of balance and increasingly need to watch where they place their feet to prevent stumbling of falling.