Balance and Motion
The vestibular system of the inner ear contains various structures responsible to keep the person’s balance and posture
The balance system, otherwise called vestibular system, is a complex system with many structures. These are the bony labyrinth, the membranous labyrinth of the inner ear, the cerebellum, the eyes, the brain cortex, and the muscles and joints which connect to the brainstem known as the vestibular nuclei (Munir and Clarke 2013; Chang and Khana 2013).
The Vestibular System
The vestibular system contains two structures known as the macula and crista ampullaris; these are sensory neuroepithelium (Chang and Khana 2013). They consist of sensory mechanoreceptors called hair cells contained in a neuroepithelium membrane. Russel and Sellick (1978) are credited with being the first to study the mammalian inner cells. The job of these cells is to distribute the movements of the cochlea partition to the central nervous system for processing (Pickles 2012, p. 57; Chang and Khana 2013).
The basic structure of the hair cell is a single large kinocilium and about 100 stereocilia on its end (Chang and Khana 2013; see Pickles 2012, pp. 57–66). The structure of the kinocilium is that of a motile cilium with nine peripheral doublet microtubules and two central microtubules (Cuschieri 2009). The stereocilia are long microvilli organized in rows behind the kinocilium in a descending order. The polarity of the receptors is established by the kinocilium, and the stereocilia holds the sensory transduction. Deflections of the stereocilia in the direction of kinocilium opens ion channels that causes an influx of potassium and calcium ions and an efflux of sodium ions, causing depolarization of the receptor membrane. When the stereocilia are deflected in the opposite way away from the kinocilium, this action closes the channels and the receptor cells is hyperpolarized (Cuschieri 2009).
The Bony and Membranous Labyrinth
The bony and the membranous labyrinth is a part of the peripheral vestibular system. This complex balance organ of the inner ear contains the cochlea and the semicircular canals. It is filled with perilymph (Hain and Helminsky 2007), a substance that resembles the cerebral fluid, and it is used by the perilymphatic duct into the subarachnoid space (Chang and Khana 2013). The perilymph is contained in the space between the bony labyrinth and the membranous labyrinth (Pickles 2012). The membranous labyrinth contains sensory epithelium and surrounds the perilymph inside the bony labyrinth. There is a fluid called endolymph which is contained in the membranous labyrinth. This fluid is similar in composition to the intracellular fluid, and it is produced by the capillaries in the stria vascularis in the wall of the cochlear duct and is immersed by the endolymphatic sac (Chang and Khana 2013).
The membranous labyrinth also consists of the utricle, the saccule, and the lateral, superior, and posterior semicircular ducts. Each structure contains a sensory neuroepithelium called the macula. The macula’s physiological property is adaptation. Therefore, when the head performs minor movements, the bent hair cells and the depolarized membrane potentials return to normal. The function of the utricle is to sense horizontal movement and the function of the macula of the saccule is to sense vertical movement. Both the utricle and saccule respond to linear acceleration, forces of gravity, and the minor movements of the head (Chang and Khana 2013).
The semicircular canals partially embody the bony labyrinth (Munir and Clarke 2013). The structure of each semicircular canal is perpendicular to the other canals on the ipsilateral side and coplanar with the contralateral canals (Chandrasekhar 2013). The neurosensory elements of each canal are called the crista and the cupula and lie perpendicular to the plane of the canal (Chandrasekhar 2013).
The function of these three semicircular canals is to sense the turning motion of the head (Munir and Clarke 2013). The otolithic organs contained in the canals respond to the pull of gravity and provide mass to the hair cells of the balance system.
Another structure called the vestibular ganglion, also known as Scarpa’s Ganglion, is located in the lateral portion of the internal auditory meatus, and it is composed of approximately 20,000 bipolar cell bodies receiving afferent impulses from the hair cells of the crista ampullaris and the maculae (Chang and Khana 2013). According to Lorente de No (1926, 1931, as cited in Ballantyne and Engström 1969), the vestibular ganglion is consisted of five parts differentiated from each other on the size of their cells. These parts are called pars magnocellularis anterior; pars parvicellularis anterior; pars magnocellularis posterior; pars magnocellularis posterior; and pars parvicellularis posterior. The vestibular ganglion appears to be divided into a superior and inferior part united by an isthmus (Tascioglu 2005). The peripheral processes from the superior ganglion innervate the ampullary crests of the superior and lateral semicircular ducts and macula of the utricle, and the peripheral fibers from the inferior ganglion innervate the macula of the saccule (Tascioglu 2005). Also, the axons from the superior and inferior parts of the vestibular ganglion come together to form the vestibular nerve. In combination with the cochlear nerve, it becomes the vestibulocochlear nerve (Chang and Khana 2013).
Vestibular Nuclear Complex
Vestibular input is processed by the vestibular nuclear complex, which consists of four nuclei: medial, superior, lateral, and inferior (see Tascioglu 2005). This is the primary processor of any vestibular input.
It is noteworthy that the vestibular nuclei have four main connections: (1) connections with the ocular muscles; (2) connections with the Postural Back muscles; (3) cerebellar connections and; (4) connections with higher centers. These four connections constitute the Central Vestibular System (see Cuschieri 2009, for more info about these connections).
Moreover, the cerebellum is vital for the control of finer movement (Munir and Clarke 2013). The role of the cerebellum in the vestibular system is to function as an adaptive processor monitoring vestibular performance and to readjust vestibular input through inhibitory input as necessary (Chang and Khana 2013). The “vestibulocerebellum” (Chang and Khana 2013) is consisted of the flocculonodular lobe and the vermian cortex. Efferent information is received from the bilateral vestibular nuclei from the ipsilateral cerebellum. The projection fibers included in the cerebellum go directly to the ipsilateral vestibular nuclei and to the ipsilateral fastigial nucleus (Chang and Khana 2013). The axons from the fastigial nucleus project to the contralateral vestibular nucleus through the juxtarestiform body, an area that is significant in terms of postural reflexes and orientation (Chang and Khana 2013). The vestibulocerebellum controls body balance and posture and is linked to the ocular motor neurons (Cuschieri 2009).
The cortex is the part of the brain that is responsible for conscious awareness and the manipulation of space (Munir and Clarke 2013). However, specific information regarding complex cortical vestibular connections is not yet thoroughly explained nor is there a consensus of the location of a vestibular cortex (Chang and Khana 2013). Studies on humans have suggested that the main cortical processing region is most likely in or near the parietal or insular cortex (Chang and Khana 2013). Based on a conjunction analyses (see Lopez et al. 2012), it was found that the regions showing convergence between two stimulation methods were located in the median and posterior insula, parietal operculum, and retroinsular cortex. The right hemispheric parietal opercular area OP 2 was demonstrated to be the most consistent area of activation, based on a study from Zu Eulenburg et al. (2012). The authors further suggest that the cytoarchitectonic area OP 2 in the parietal operculum may represent the human homology to the vestibular area PIVC as proposed by Guldin and Grüsser (1998) in non-human primates.
Furthermore, signals from the cortex to the muscles and vice versa via the motor nerves are needed to maintain posture and balance. Additional visual stimuli from the eyes constantly inform the nervous system regarding motion in the environment (Munir and Clarke 2013). Specifically, the vestibulo-ocular reflex is at play, which helps to coordinate eye movement in order to stabilize retinal images while the head rotates (Chang and Khana 2013). The ocular muscles cause eye movement responding to stimuli from the brainstem (vestibular nuclei) as well as from the cortex in order to maintain focus while the head exercises movement. Information from all these structures is sent to the brainstem for processing regarding balance (Munir and Clarke 2013).
Alcohol and Balance
Alcohol intoxication has devastating results on the brain and the body. Postural sway is directly affected as well as the oculomotor system. Studies has shown that indeed alcohol interfere more with the synaptic transmission in the lateral vestibular nucleus (LVN) monosynaptic and polysynaptic II neurons (Nieschalk et al. 1999; Ikeda et al. 1980).
The deleterious effects of alcohol are further supported based on a research of Modig et al. (2012). In their study, they have found that, based on the dosage, postural control is affected, specifically latera motion rather than anteroposterior. They added that alcohol intoxication decreases the adaptation mechanisms used for stability in lateral movement.
Specifically, the cupula in the semicircular canals becomes lighter than the endolymph leading to the process called the buoyancy hypothesis (Fetter et al. 1999). Fetter et al. added that alcohol intoxication causes a vertical velocity offset, which may be toxic to the central vestibular pathways, producing a tone imbalance of the vertical vestibulo-ocular reflex. There is also more reliance in visual information when alcohol is digested in high levels, as well as a decompensation of small subclinical vestibular asymmetries and a dysfunction of the otoliths (Hafstrom et al. 2007).
The main structures of the balance system (vestibular system) were explained. These contain the bony and membranous labyrinth which contains the cochlea and the semicircular canals. There is also the vestibular ganglion, also known as Scarpa’s Ganglion, with its bipolar cells that contribute to keep the person’s equilibrium. The cerebellum and the cortex are responsible to the control of finer movement and the perception and awareness of space. The intoxicating results of alcohol on balance have also been briefly reviewed.
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