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What Is a Vestibular System?
The ear is a sensory organ that functions primarily as a hearing device. However, it is also essential for us to maintain a sense of balance in our body. The organ of balance, known as the vestibular system, is located inside the inner ear. The vestibular system is made up of two otolith organs — the utricle and saccule — and semicircular canals. All these structures are filled with fluid.
The vestibular system, along with the cochlea, make up the inner labyrinth of the ear. The semicircular canals of the vestibular system indicate rotational movements and the otoliths indicate the linear accelerations. The vestibular system sends signals to the nerves that control eye movements for a clear vision and to the muscles which help in keeping the posture of the organism upright.
The brain also picks up information from the vestibular system to understand the proprioception and kinematics of the movement of the body.
Structure
Semicircular Canals
The three semicircular canals —- namely superior, horizontal and posterior — are located at almost right angles to each other. Each canal has an extended swollen end called the ampulla, and it opens into the vestibule. The ampulla of the superior and horizontal canals lie together but the ampulla of the posterior canal opens on the other side of the vestibule.
The other ends of the posterior and superior canals form a structure called crus which opens into the vestibule. Another end of the horizontal canal also opens up into the vestibule. All in all, the vestibule completes the circle of all the three semicircular canals.
The swollen ampulla has a ridge structure called crista that is covered by neuroepithelium and hair cells. Cupula is another structure that arises from the ridge and divides the ampulla into equal halves. Kinocilium and stereocilia arise from the cupula and occupy parallel channels. The tuft of cilia in the cupula moves in the direction of body movement and modulates the rate of nerve transmission by the vestibular nerve fibres to the brainstem.
The semicircular canals are arranged in such a manner that each canal has an equal counterpart on both the sides. The three canals work together in a push-pull system such that if one canal is stimulated, the counterpart on the other side is inhibited and vice versa.
A reflex eye movement, called vestibulo-ocular reflex (VOR), stabilises the image on the retina in a direction that is opposite to the head movement. For instance, when our head moves to the right, the eyes move in the left direction. The VOR reflex is crucial for producing stable images in the eye, those with an impaired VOR find difficulty in reading.
Otolithic Organs
The two otolith organs of the ear are utricle and saccule. They are also known as gravity receptors because they respond to gravitational forces. The utricle and saccule are located on each side and both consist of a patch of hair cells and macula which are supporting cells. The hair cell has around forty to seventy stereocilia and one true kinocilium. The ends of these cilia remain embedded in the otolithic membrane.
Refer: Difference between Cilia, Stereocilia and Microvilli
The otolithic membrane has granules of protein and calcium carbonate called otoconia that weighs down the membrane. These granules provide inertia to the membrane and help in sensing the gravity and motion.
When the head is in an erect position, the membrane is weighed down by the hair cells and the stimulation is minimum. When the head tilts in any specific direction, the otolithic membrane crumples, bends the stereocilia and the hair cells are stimulated. Thus, in case of any head movement, the utricle and saccule of the ears are stimulated, the brain interprets the movement of the head by comparing the two inputs — the inputs from the eyes and neck — and detects whether the head is tilted or the whole body is moving.
The otolithic organs are quick in detecting whether the head is tilting up or down, left or right or accelerating backward or forward. The utricles elicit the eye movement whereas the saccule focuses on the body posture.
Clinical Significance
The most common disease of the vestibular system is vertigo. It is accompanied by nausea and loss of balance. Vestibular neuritis, a form of labyrinthitis is inflammation in the vestibular nerve that sends signals to the brain about motion.
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