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In this article we will discuss about the subject-matter and functions of vestibular apparatus.
Subject-Matter of Vestibular Apparatus:
i. Vestibular apparatus is present in inner ear.
ii. The three important parts of vestibular apparatus are utricle, saccule and semicircular canals.
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iii. There are three semicircular canals on the both sides and function as pairs.
iv. The three semicircular canals are termed as:
a. Anterior canal (superior)
b. Posterior canal
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c. Horizontal canal (lateral)
v. The anterior semicircular canal of one side and posterior canal of the opposite side are placed in the same plane and similarly other two pairs also.
vi. Whether in utricle or saccule or in three semicircular canals, receptors are present. These receptors are mechanoceptors in nature.
vii. These receptors are termed as proprioceptors.
viii. Receptors are present in dilated portions of semicircular canals termed as ampulla.
ix. Vestibular division of VIII cranial nerve carries impulses from vestibular apparatus to brain (afferents).
Receptors:
Receptors get stimulated when there is:
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a. Linear acceleration or deceleration
b. Angular acceleration or deceleration
c. Rotation of head
When there is dorsiflexion or ventriflexion of the head, receptors present in utricle gets stimulated. When there is movement of head sideways, receptors present in saccule get stimulated. Rotation of head along long axis of body brings about stimulation of receptors present in semicircular canals.
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Crista ampullaris:
They are present in semicircular canals. The receptors are known as hair cells. They are present over a ridge. Hairs of these cells are embedded in gelatinous substances called cupula and together with hair cells is called cristae ampullaris (Fig. 9.47).
Otolith organ:
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In utricle and saccule, the receptors are present on a ridge. The hair cells are embedded in a gelatinous matrix namely otolith membrane. This membrane is rich in calcium salts. Otolith membrane with hair cells is known as otolith organ.
The hair cells present in vestibular apparatus have many hairs. There will be one kinocilium and direction of bending of this kinocilium determines whether receptor is stimulated or not. In case stereocilia move away from kinocilium, receptor is inhibited (Fig. 9.48). So there is differential stimulation of receptor depending on movement of cilia.
From receptors, afferent inputs are carried to central nervous system by vestibular division of vestibulocochlear nerve.
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Neural Communication:
Afferent inputs from vestibular apparatus are carried by vestibular nerve fibers. Cell bodies of these fibers are located in Scarpa’s ganglion. From here, impulses reach vestibular nucleus present in brainstem. Impulses are carried to cerebellum (to flocculonodular lobe or vestibulocerebellum) through vestibulocerebellar pathway.
From vestibular nuclei of brainstem, impulses are also sent to cranial nerve motor nuclei of 3rd, 4th and 6th nerves supplying extra- ocular muscles. In addition to this, vestibular nucleus in brainstem is also connected to lower motor neurons present in spinal cord through vestibulospinal tract which carry motor impulses.
Functions of Vestibular Apparatus:
1. Vestibular nucleus in brainstem is connected to flocculonodular lobe of cerebellum through both by afferent and efferent fibers as vestibulocerebellar and cerebellovestibular tracts, respectively. This tract has functional importance in maintenance of posture and equilibrium. Any postural imbalance is corrected in cerebellovestibular and vestibulospinal tracts.
2. Vestibular nucleus of brainstem is connected to 3rd, 4th, and 6th cranial nerves supplying extraocular muscles. Because of this, coordination of extraocular muscles is brought about while it is required for fixing gaze on a particular object. So if this nucleus is damaged, it can lead to nystagmus (inability to fix gaze).
3. Vestibular nucleus of brainstem is connected to lower motor neurons through vestibulospinal tract. These nuclei send motor impulses to lower motor neuron of spinal cord. And through this connection, it plays role in maintenance of muscle tone.
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Derangement of functioning of vestibular apparatus is seen in Meniere’s syndrome. Not only is vestibular apparatus affected, but cochlea function is also affected.
Features of Meniere’s syndrome are:
1. Vertigo is illusion of movement
2. Nystagmus
3. Ataxia
During motion sickness, there will be excessive stimulation of vestibular apparatus, e.g. when traveling across rough sea, in a plane, when taking a hair pin bend, during a train journey the person complains of vertigo, vomiting, nystagmus, etc.
