Vestibular System and its functions

Vestibular apparatus is the part of labyrinth or inner ear. It plays an important role in maintaining posture and equilibrium through statokinetic reflexes. Other part of labyrinth is the cochlea, which is concerned with sensation of hearing.

LABYRINTH

Labyrinth (inner ear) consists of two structures:

1. Bony labyrinth

2. Membranous labyrinth.

BONY LABYRINTH

Bony labyrinth is a series of cavities or channels present in the petrous part of temporal bone. Membranous labyrinth is situated inside bony labyrinth. The space between bony labyrinth and membranous labyrinth is filled with a fluid called perilymph or periotic fluid. This fluid is similar to ECF in composition with large amount of sodium ions. Bony labyrinth encloses membranous labyrinth.

MEMBRANOUS LABYRINTH

Membranous labyrinth is formed by membranous tubules and sacs. It consists of two portions:

1. Cochlea, which is concerned with sensation of hearing

2. Vestibular apparatus, which is concerned with posture and equilibrium.

Membranous labyrinth is filled with a fluid called endolymph or otic fluid. Endolymph is similar to ICF in

composition. It has large quantity of potassium ions.

FUNCTIONAL ANATOMY OF VESTIBULAR APPARATUS

Vestibular apparatus is formed by three semicircular canals and otolith organ (vestibule).

SEMICIRCULAR CANALS

Semicircular canals are the tubular structures placed

at right angles to each other. Because of this type of

arrangement, semicircular canals represent the three axes of rotation, i.e. vertical, anteroposterior and trans verse axes. Semicircular canals are named according

to the situation as follows:

1. Anterior or superior canal

2. Posterior canal

3. Lateral or horizontal or external canal.

Anterior and posterior canals are situated vertically and the lateral canal is situated in horizontal plane.

When the head is tilted forward at an angle of 30°, lateral canals of both the sides are at horizontal plane

parallel to earth with the convexities directed outward and a little backward. Anterior canals are at vertical

plane and directed forward and outward at 45°. Posterior canals are also at vertical plane, but directed backward and outward at 45°. Therefore, the plane of position of anterior canal of one side is parallel to the plane of posterior canal of opposite side.

Ampulla

There are two ends for each semicircular canal. One end is narrow and the other end is enlarged. The

enlarged end is called ampulla. Ampulla contains the receptor organ of semicircular canals known as crista ampullaris. Ampulla of all the three canals and narrow end of horizontal canal open directly into the utricle. The narrow ends of anterior and posterior canals open into utricle jointly, by forming the common crus. Thus, all the three semicircular canals open into utricle by means of five openings. Utricle opens into saccule.

OTOLITH ORGAN OR VESTIBULE

Otolith organ or vestibule is formed by utricle and saccule. Often utricle and saccule are together called

otoliths. Utricle communicates with saccule through utri culosaccular duct. Saccule communicates with

cochlear duct through ductus reuniens. Another duct called endolymphatic duct arises from utriculosaccular duct. It ends in a bag-like structure called endolymphatic sac, which lies on the cranial surface of petrous bone.

RECEPTOR ORGAN IN VESTIBULAR APPARATUS

Receptor organ in semicircular canal is called crista ampullaris and that in otolith organ is called macula.

These receptor organs contain the proprioceptors. fluid is similar to ECF in composition with large amount of sodium ions. Bony labyrinth encloses membranous labyrinth.

MEMBRANOUS LABYRINTH

Membranous labyrinth is formed by membranous tubules and sacs. It consists of two portions:

1. Cochlea, which is concerned with sensation of hearing

2. Vestibular apparatus, which is concerned with posture and equilibrium.

Membranous labyrinth is filled with a fluid called endolymph or otic fluid. Endolymph is similar to ICF in

composition. It has large quantity of potassium ions.

FUNCTIONAL ANATOMY OF VESTIBULAR APPARATUS

Vestibular apparatus is formed by three semicircular canals and otolith organ (vestibule).

SEMICIRCULAR CANALS

Semicircular canals are the tubular structures placed at right angles to each other. Because of this type of

arrangement, semicircular canals represent the three

RECEPTOR ORGAN IN SEMICIRCULAR CANAL – CRISTA AMPULLARIS

Crista ampullaris is a crest-like structure situated inside the ampulla of semicircular canals. The crest

is formed by a receptor epithelium (neuroepithelium), which consists of hair cells, supporting cells and

secreting epithelial cells. The secreting epithelial cells secrete the ground substance, proteoglycan. These cells are arranged in planum semilunatum (group of epithelial cells) around hair cells .

Hair Cells

Hair cells are the receptor cells (proprioceptors) of crista ampullaris. There are two types of hair cells, type I and type II hair cells. Hair cells of semicircular canals, utricle and saccule receive both afferent and efferent nerve terminals.

Type I hair cells

Type I hair cells are flask shaped. Afferent nerve terminates in the form of a calyx that surrounds the

cell body. Efferent nerve terminal ends on the surface of calyx.

Type II hair cells

These cells have a cylindrical or test tube shape. Both afferent and efferent nerve fibers terminate on the

surface cell body without forming calyx.

Cilia of hair cells

Apex of each hair cell has a cuticular plate. From this plate, about 40 to 60 cilia arise, which are called

stereocilia. Each stereocilium is attached at its tip to the neighboring taller one by means of a fine process called tip link. Because of the tip links, all the stereocilia are held together. One of the cilia is very tall, which is named as kinocilium .

Cupula

From crista ampullaris, a dome-shaped gelatinous structure extends up to the roof of the ampulla. It is

known as cupula. Cilia of hair cells are projected into cupula.

 

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