Vestibular Projections
Vestibular projections in the central nervous system consist of vestibular end organs which communicate with the brain stem and the cerebellum via the vestibular branch of the eighth cranial nerve. These nerves are also connected to the auditory fibers. Thus damage caused to these structures can cause both vestibular and auditory disturbances. The vital function of the vestibular system is to coordinate head and eye movements. The ability of the eye to stay focused on an object even while the head is moving is brought about by the vestibular system.
Vestibular Cochlear Nerve
The vestibulo cochlear nerve or the auditory nerve is a paired set of nerves located in the auditory canal of the human ear (Health Line, 2015). Its vital function is hearing, maintaining balance and relaying information from the inner ear to the brain. It is the eighth cranial nerve which determines equilibrium. It branches out into the cochlear nerve (arising from ventral and dorsal cochlear nuclei) and the vestibular nerve (arising from the vestibular nuclear complex in pons and medulla). The cochlear nerve transforms sound waves into electrical signals so that they may be interpreted by the brain, while the vestibular nerve detects head movements to maintain equilibrium (Health Line, 2015). Damage to the nerve can cause vomiting, motion sickness, hear loss etc.
Vestibular Afferents
The vestibular afferents belonging to the vestibular system is found in the internal auditory meatus with its axon travelling along the eighth cranial nerve and entering the brainstem where pons and medulla meet in the eighth ventricle of the brain. While most of the afferents enter into the medulla and pons, a few of them go into the cerebellum via the inferior cerebellar peduncle. These afferents are received by the vestibular nuclei which are the prime centers for signal reception and relay these signals using the vestibular pathway which are located in the vestibular ganglion called the Scarpa’s ganglion. The signals mainly consist of postural information of the head in relation to the body to maintain equilibrium.
Vestibular Nuclear Complex
The vestibular nuclear complex is located mainly in the pons but also extends into the lateral position of the medulla. It consists of four main kinds of nuclei: inferior, lateral, medial and superior vestibular nucleus and seven kinds of minor nuclei (Hain, 2014). The vestibular nucleus sends out signals to the spinal cord, cerebellum and the vestibular cortex. They are also extensively connected to the ocular motor nuclei and the brainstem reticular working systems in order to provide efferent signals to the vestibulo ocular reflex (VOR) and vestibulo spinal reflex (VSR) effector organs.
Deep Cerebellar Nuclei
Maintenance of equilibrium is a coordinated effort of several organs, the cerebellum being the most vital one. Input and output of the cerebellum are determined by its most vital part, the deep cerebellar nuclei. It consists of four kinds of nuclei: fastigial nucleus, interposed nucleus, dentate nucleus and the vestibular nuclei. Located medially it receives input from the vermis and cerebral afferents carrying secondary, auditory and visual information. Lateral to the fastigial nucleus, the interposed nuclei receive input from the intermediate zone and cerebellar afferents. Being the largest of the nuclei, the dentate nucleus receives input from the lateral hemisphere and from cerebral afferents of the cerebral cortex. Located outside the cerebellum, the vestibular nuclei perform the same function as the cerebellar nuclei. They receive input from the vestibular labyrinth and the flocculondular lobe. This lobe consists of vestibulo cerebellum which makes up the largest functional division of the cerebellum. It responds to vestibular reflexes such as vestibulo ocular reflex and helps in maintenance of posture.
Vestibulo Ocular Reflex
The vestibulo ocular reflex (VOR) is the reflex by which the eye is made to focus the image on the fovea precisely even while the head is moving (Augustine and Fitzpatrick, 2001). This implies the ability of the eye to focus on objects when the head is in motion. Without the VOR, images would appear blurred when the head is in motion. When the head moves in one direction the eye focuses on an image by moving with the same magnitude but in the opposite direction (Amin, 2016). This sudden and rapid movement of the eye requires muscles which work at a fast pace. The human eye makes use of six kind of ocular muscles for its rotation. Different eye motions can be seen with different head movements. When the head rotates, back and forth, or while nodding, there is an angular motion of the head (AVOR) which stimulates the r-VOR (rotational VOR), after being stimulated by the semicircular canals. When there is a translational movement of the head t-VOR (translational VOR), it is stimulated by the otolith organs to respond to the linear motion LVOR (Amin, 2016). Head movements with a combination of both kinds can also be seen. The mechanism of VOR can be ascertained by studying the 3-neuron arc. It is comprised of the vestibular ganglion, vestibular nuclei, and occulomotor nuclei. When the head rotates, inertia in the endolymphatic fluid of the semicircular canals shifts and causes the afferent nerves to carry electric impulses to the four major vestibular nuclei. These nuclei project into the occulomotor nuclei in cranial nerves III, IV, and VI. The signals cause contraction and relaxation of the ocular muscles. These signals are both excitatory and inhibitory in their function. Excitation of the superior canal causes an upward torsional eye movement while excitation of the posterior canals causes a downward torsional eye movement. When the lateral canal is excited, it causes a horizontal eye movement. Impulses are also sent to the vestibulo cerebellum which ensures fine eye movements.
Vestibulo Colic Reflex
The vestibulo colic reflex (VCR) acts on the neck muscles to keep the eye steady while the head is in motion. It works in coordination with the VOR while the head is moving. The VCR helps to stabilize motion of the head in space by instructing the eye to move in a direction opposite to the direction of movement of the head. Motion of the head stimulates the semicircular canal (SCC), which in turn causes stimulation of individual ampullary nerves. Different motions of the head stimulate different ampullary nerves. For example, upward head rotations activate the posterior ampullary nerves; head rotations to the right activate the right horizontal ampullary nerve etc (Goldberg and Cullen, 2011). Motion of the head may also be due to ambulation (walking). The receptors present in the saccule are stimulated during walking or running, due to which they transmit afferent signals to the vestibular nuclear complex present in the brainstem. This, it does, along the inferior vestibular nerve and ganglion. The efferent signals then travel through the medial vestibulo spinal tract and the spinal accessory nerve to the muscles of the neck. The sternocleidomastoid muscles are also included in it. This mechanism stabilizes the neck to promote better eye focus while the head is in motion. The VCR acts to oppose head movement. Normal motion of the head shows a suppression of the VCR. The vestibular only neurons belonging to the VCR pathway, project to the spinal cord. They are so called because they do not relay movements related to the eye. Their role in the mechanism is yet to be investigated. The neurotic pathway of VCR is trisynaptic, most of which pass through the medial vestibulo spinal tract. It follows a 3-neuron arc as the VOR (Goldberg and Cullen, 2011). Since the VCR acts on the neck muscles which have more than 30 muscles in it, the mechanism is much more complex than the VOR. Hence, movement of head is measured in terms of muscles rather than axes. Postural movements are suggestive of a sensory environment which needs to be fine tuned with growth and maturity. Sensory inputs in childhood are very immature and incomplete during infancy. Only with growth and age does the sensory environment mature to attain postural equilibrium.
References
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vestibulo colic reflex. Pub Med Central. [e-journal]. Available at
http://www.ncbi.nlm.nih.gov/pmcarticles/PMC4157641/#_ffn_sectitle [Accessed 5 July
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Hain, T.C., 2014. Brainstem structures related to dizziness and balance. Dizziness and
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http://www.dizziness-and-balance.com/anatomy/brainstem.htm [Accessed 6 July 2016].
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