The vestibular system: structure and function.

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•The inner ear consists of a bony capsule known as the ‘otic capsule’ within the petrous portion of temporal bone. Anteriorly there is the snail like cochlear, in the middle there is the vestibule and posteriorly the three semicircular canals.

 

•The otic capsule is filled with fluid known as perilymph and suspended in the perilymph is a membranous labyrinth. These are delicate arrangements of sacs and tubes filled with a different fluid known as endolymph.

 

•The three membranous semicircular canals which occupy the corresponding bony structures are known as the anterior, posterior and horizontal semicircular canals. They are set at right angles and each represent a plane in space.

 

•The anterior end of each canal is dilated to form its ampulla. This contains a patch of neuroepithelium called the crista. The hairs of the crista are displaced following angular acceleration in that plane which triggers signal transduction along the vestibular nerve. 

 

•The canals come together at the vestibule which contains two further  significant regions; the utricle and the saccule. These contain a patch of neuroepithelium known as the macula. The macula contain calcium carbonate particles called  otoliths  and these are used to detect gravitational pull  (utricle) and linear acceleration (saccule).

 

•Nerves from the ampulla, utricle and saccule unite to form the vestibular nerve, the ganglion of which lies in the internal auditory meatus.

 

•Vestibular fibres from cranial nerve VIII terminate  in the vestibular nucleus in the medulla oblongata.  Axons then run to numerous areas of the CNS such as the spinal cord, the cerebellum, the cerebral cortex and the nuclei controlling extrinsic eye muscles. Fibres also communicate with the cerebellum to fine tune movement.