The superior, posterior, and lateral semicircular canals of the inner ear responsible for balance and the movement of the head
The semicircular canals are part of the structures of the inner ear, specifically the bony labyrinth. (Munir and Clarke 2013). The canals are filled with perilymph (Hain and Helminsky 2007). Their neurosensory elements are known as the crista and cupula, and the neurosensory elements of the canals’ otolithic organs are known as the macule and the striole. The semicircular canals are responsible for keeping the person’s balance and posture; they are also responsible for the movement of the head with the help of the otolithic organs.
Structure of Semicircular Canals
The semicircular canals are part of the bony labyrinth of the inner ear (Munir and Clarke 2013). The bony labyrinth consists not only of the semicircular canals but of the cochlea as well. The canals are filled with a fluid similar to the cerebral fluid, called the perilymph (Hain and Helminsky 2007), which is contained in the perilymphatic duct in the subarachnoid space (Chang and Khana 2013). There is perilymph also between the membranous and the bony labyrinth (Pickles 2012).
Each semicircular canal is perpendicular to the other canals on the ipsilateral side and coplanar with the contralateral canals (Chandrasekhar 2013). Each canal forms two thirds of a circle with a diameter of roughly 6.5 mm and a cross-sectional diameter of 0.4 mm (Lee et al. 2016). The neurosensory elements of each canal are called the crista and cupula and are perpendicular to the plane of the canal (Chandrasekhar 2013). Each otolithic organ has a curvilinear shape at right angles with each other. Their neurosensory elements are called the macule and the striola; the macule is located around a central dividing line, and the hair cells of the striola are located either toward the line in the utricle or away in the saccule (Chandrasekhar 2013).
The three semicircular canals of the inner ear (superior, posterior, and lateral) perceive the turning motion of the head (Munir and Clarke 2013). They are oriented at right angles to each other. Each canal is responsive to angular motion and is paired with a canal on the contralateral side so that what is an excitatory stimulus to the one, to be inhibitory to the other (Lee et al. 2016).
The utricle and saccule respond to the pull of gravity rather than the motion. These two otoliths provide mass to the balance system hair cells, and when the head is moved into new positions, gravity pulls on that mass and pressures the tips of the hair cells. Consequently, this sends a nerve signal to the brain which indicates movement (Munir and Clarke 2013). A structure called semicircular ducts has the same structure as the bony semicircular canals where they are contained (Chang and Khana 2013). They compose the kinetic labyrinth that senses the acceleration or rotation of the head. The semicircular ducts are attached to the utricle, and at the end of each duct there is a structure called the ampulla, which contains the sensory neuroepithelium on the crista ampullaris. This is covered by the cupula; a gelatinous substance entrenched with hair cells (Chang and Khana 2013). The cupula is displaced by the endolymph motion based on the rotational acceleration of the head, which bends the hair cells in the opposite direction of the rotation, subsequently opening the ion channels and depolarization of the hair cells, which in turn increases the firing of its afferent fibers (Chang and Khana 2013). Rotational deceleration of the movement of the head displaces the cupula in the same direction of the head movement, which subsequently closes the ion channels of the hair cell which causes it to become hyperpolarized, having the result of a reduction in afferent nerve firing (Chang and Khana 2013).
The structure and function of the semicircular canals were reviewed as well as a brief discussion of how the otolithic organs contribute to keep a person’s posture and balance in check was provided.
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