Anatomy and physiology Of The vestbular system

The Vestibular System

The vestibular apparatus is the sensory organ for detecting sensations of equilibrium located in the petrous portion of the temporal bone. It is composed mainly of three semicircular canals; and two large chambers, the utricle and saccule each one of them is composed of bony labyrinth. Within this bony labyrinth is membranous labyrinth.
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The utricle and saccule

Located on the inside surface of each utricle and saccule, is a small sensory area called a macula. The macula of the utricle lies mainly in the horizontal plane (on the floor of the utricle) and plays an important role in determining orientation of the head when the head is upright. Conversely, the macula of the saccule is located mainly in a vertical plane (in the wall of the saccule) and signals head orientation when the person is lying down

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Each macula is covered by a gelatinous layer in which calcium carbonate crystals called statoconia (ear dust , otoconia) are embedded. Also in the macula are thousands of hair cells,The bases and sides of the hair cells synapse with sensory endings of the vestibular nerve.
Each hair cell has small cilia called stereocilia, plus one large cilium, the kinocilium .The weight of the statoconia bends the cilia in the direction of gravitational pull, bending of stereocilia in the direction of the kinocilium, This opens several hundred channels in the cell membrane which conduct large numbers of positive ions. Therefore, causing depolarization. Conversely,bending of stereocilia in the opposite direction (away from kinocilium) ; this closes the ion channels thus causing hyperpolarization.

The Function of the Utricle and Saccule

1-The Maintenance of Static Equilibrium
2- Detection of Linear Acceleration

Semicircular Ducts:

The three semicircular ducts, known as the anterior, posterior and lateral (or horizontal) they are arranged at right angles to one another so that they represent all three planes in space.
Each semicircular duct has an enlargement at one of its ends called the ampulla, and the ducts and ampulla are filled with endolymph.
in each ampulla there is a crista ampullaris. On top of this crista is a gelatinous mass, called the cupula
Into the cupula are projected hundreds of cilia from hair cells . bending the cupula in that direction of kinocilium causes depolarization of the hair cells,whereas bending it in the opposite direction hyperpolarizes the cells. Then, from the hair cells, signals are sent by way of the vestibular nerve to the central nervous system inform it about a change in rotation of the head.
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Function of semicircular ducts
Detection of Head Rotation:(called angular acceleration).
Nystagmus
During rotation we have two eye movements:
1-The first is slowly in opposite direction of rotation called slow component.
2-The second is fast and occurs when the slow movement reaches the end and it is in the direction of rotation.
These movement from reflexes transmitted from the semicircular canals to vestibular nuclei to the medial longitudinal fasculis to the ocular nuclei.

Neuronal Connections of the Vestibular Apparatus with the

Central Nervous System

The impulses travel in the vestibular portion of the VIII cranial nerve and enter the brain stem Most of these afferents project to one of the four vestibular nuclei (medial and inferior nuclei. The lateral and superior nuclei).
Fibers that end in vestibular nuclei synaps with second order neuron that also send fibers into
1-Cerebellum to:Most of these afferents innervate the flocculonodular lobes of the cerebellum are especially concerned with dynamic equilibrium signals from the semicircular ducts.
2-To vestibula spinal tracts (medial vestibule spinal tract and lateral vestibule spinal tract). This pathway helps us walk upright (controlling head and body position).
3-To medial longitudinal fasciculus to the motor nuclei of the 3rd ,4th and 6th cranial nerves for corrective movements of the eye every time the head rotates.
4- To the reticular formation of the brain.
Signals also pass to the cerebral cortex, terminating in a primary center for equilibrium located in the parietal lobe deep in the sylvian fissure .

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