Table of Contents
Anatomy of Semicircular Canals
The bony labyrinth is composed of semicircular canals, vestibule, and the cochlea. These are substances present within the cavity of dense bone. These canals are lined by endosteum and contain a fluid, the perilymph, suspended in the membranous labyrinth.
Semicircular canals are part of the vestibular system, which collects information regarding the motion of head in order to maintain balance and coordination. These canals are composed of three fluid-filled tubes in the inner ear, each forming approximately two-thirds of a circle with a diameter of approximately 6.5mm and are proved to be effective in detecting the rotational acceleration of the head. are part of the vestibular system, it collects and uses information regarding rotational head movements in order to maintain balance.
The three semicircular canals are posterior, superior, and lateral open into the posterior part of the vestibule, each canal detects motion in a single plane.
Superior semicircular canal is arranged vertically and is at right angles to the petrous bone. its specialty is to detect rotations of the head around the lateral axis, or along the sagittal plane. This occurs, when we nod our head, and our head is tilted in alternating up and down arcs in the sagittal plane. The posterior canal is also vertical but is placed parallel with the long axis of the petrous bone, that detects rotation of the head around the antero-posterior axis, to be more precise, it is capable to detecting motion in coronal pane. This occurs, for example, when you perform a sideway rotator movement of the body or try to move your head in order to touch your shoulders.
The lateral canal is arranged in a horizontal manner, and it lies in the medial wall of the aditus to the mastoid antrum, above the facial nerve canal and it shortest of all three of them. Movement of fluid within the lateral canal detects the rotation of the head around a vertical axis. This occurs, for example, when you turn your head to the left- and right-hand sides while crossing the road.
These canals communicate with the vestibule of the bony labyrinth and lie posterosuperior to the vestibule where it opens. These canals are approximately 1.5mm in diameter except at one end where there is a swelling which is called as the bony ampulla. The canals occupy three planes in space and are arranged at right angles with the vestibule, and have total five openings in the vestibule, one of which is common to two of the canals. Lodged in the canals are semicircular ducts.
Each semicircular duct is elliptical and believed to be linked to sensory control of body movements. Semicircular duct is comprised of an ampulla containing a sensory area known as ampullary crest. The ampullary crests detects the movement within the endolymph present in the ampulla resulting from the movement or rotation of head. The hair cells of the ampullary crest have the capability of stimulating the primary sensory neurons, whose cell bodies are in vestibular ganglia.
Development of Semicircular Canals
During the fourth week of development, a thickening of surface ectoderm leads to the formation of otic placodes. Each otic placode invaginates into the surface ectoderm of the mesenchyme, which leads to the formation of otic pit. The formation of otic vesicle begins when the edges of otic pits are fused together. The otic vesicle further give rise to membranous labyrinth.
The otic vesicle soon detaches from the surface ectoderm. Two regions of otic vesicle can be recognized, and a diverticulum arises from the vesicle and grows into a diverticulum which elongates to form endolymphatic duct and sac.
• dorsal utricular parts: small endolymphatic ducts, utricles and semicircular ducts arises.
•ventral saccular parts: the saccules and cochlear ducts arise from this.
The utricular parts of the primordial membranous labyrinths give rise to three disc-like diverticula. The peripheral unfused part of the diverticula grows and become semicircular ducts, which are attached to the utricle and are later enclosed in the semicircular canals of the bony labyrinth. Localized dilatations, the ampullae, develop at one end of each semicircular ducts. Specialized receptor area (cristae ampullares) differentiates in the ampullae and the utricle and saccule.
Role of Semicircular Canals
The semicircular canals send impulses to the brain regarding the direction and speed of rotation of the head,for example, when nodding the head up and down or looking from right to left.
In addition to that, Semicircular canals react to head movements yet have a restricted unique reach. The otolith organs react proportionately to both gravity and linear acceleration. To help beat these weaknesses, the sensory system joins visual, vestibular, and different signals to yield rotations, interpretation, tilt, Directional selectivity, sensitivity, speed, and dynamic range which depend upon the biomechanics of this apparatus.
The semicircular canals can sense rotational acceleration or deceleration of our head, such as when turning the head, starting, or stopping spinning, or somersaulting.
To detect Acceleration and deceleration when head rotates in any direction causes endolymph movement within semicircular canals. As our head moves, the bony canal and ridge of hair cells in the cupula move with head. however, the fluid within the canal lags behind because of inertia and does not move in the direction of motion.
As our head begins to move in a circle, endolymph lags behind, while the movement of fluid shifts to the opposite side with respect to the movement. This fluid movement causes the cupula to lean in the opposite direction from the head movement causing the sensory hair to bend.
If the motion of head continues in the same direction at a uniform rate, the endolymph moves simultaneously with our head. This phenomenon is performed so that the hair can return back to an unbent state.
When your head stops and slows down, the reverse situation occurs. The endolymph continues to move in the direction of rotation while your head stops.
The hairs of vestibular hair cells consist of one cilium known as the kinocilium, along with a tuft of 20-50 microvilli; the stereocilia. The kinocilium is arranged in rows with stereocilia. The kinocilium is the tallest one while stereocilia are comparatively shorter. As in the auditory hair cells, the stereocilia are linked by tips. These tips can pull mechanical gated ion channels when stereocilia are deflected by the movement in endolymph. The hair cells are either polarized or depolarized depending upon the ion channels being mechanically opened or closed. The hair cell is oriented so that it depolarizes when its stereocilia are bent away from kinocilium. The hair cells form a chemically mediated synapse with terminal endings of afferent neurons whose axons join with those of other vestibular structure to form the vestibular nerve.
The release of neurotransmitter depends upon the Depolarization of hair cells thereby increasing the rate of firing which is carried by the afferent fibers.
However, hyperpolarization will decrease the release of neurotransmitter from the hair cells eventually decreasing the frequency of action potentials in the afferent fibers.
When the fluid stops moving, the hairs straighten again. Thus, the semicircular canals are able to sense changes in the rate of angular movement of head. No response is generated when our head is motionless or when it is moving in a circle at a uniform speed.
Disorders Related to Semicircular Canal
Disorders which change and affects the biomechanics of semicircular canals can lead to transmission of inappropriate and faulty signals to the central nervous system (CNS).
Vestibular disorders include benign paroxysmal positional nystagmus (BPPN), caloric nystagmus, positional direction-changing nystagmus, alcohol positional nystagmus and others. Here, the transduction of angular motion to central nervous system (CNS) and how it contributes to neural encoding by the semicircular canals is significant in both health and disease conditions.
Vertigo is a spectrum of motion, usually rotational, of the patient feeling off balance or the patient’s surroundings. extraordinary consideration is required in order to diagnose whether the vertigo is central or peripheral.
Utricle, semicircular canals, and saccules are the part of inner ear that faces irritation Along with the irritation of vestibulocochlear nerve due to an underlying infection, toxins or suspended foreign particle embedded in the tympanic membrane can help clinicians to diagnose peripheral vertigo.
The hair cells are aligned as a single tuft which projects into a gel like mass known as cupula. Together, these form the crista.
At the point when the head turns in the plane of the canal, the inertia of the endolymph makes it wash over the cupula, diverting the hair cells. A similar course of action is reflected on the two sides of the head. Every tuft of hair cell is polarized. Whenever diverted one way, it will be excited; whenever diverted the alternate way, it will be repressed or inhibited.
In this manner, the canals on one or the other side of the head work in a push–pull rhythm; when one is energized, the other is repressed. In particular, it is the trench towards the course of turn that is energized. Be that as it may, on the off chance that, for instance, the two sides push without a moment’s delay, at that point incapacitating vertigo and queasiness result. This is the reason diseases of the endolymph or harm to the inward ear can cause vertigo. In instances of extreme immovable vertigo, this can be assuaged by cutting one vestibular nerve with the goal that the cerebrum progressively becomes acclimated to accepting contribution from one side as it were.
Damaged or Regenerating Cupula
Hair cells within the semicircular canals comprising of kinocilium is bounded to the cupula leading to the displacement of sensory hair bundle which detects the angular displacement supposedly. This mechanism can be nonfunctional and will be lost if there is any damage to the cupula. The factors which can cause significant damage to the cupula includes mechanical injury, sudden compression of membranous duct. These events have the capability to detach the cupula from the ampulla at the area where it usually attaches to the epithelial cell lining inside the ampulla.
The separation of cupula brings about significantly diminished affectability of the trenches to precise movement upgrades since endolymph streams over the highest point of the cupula as opposed to diverting it. As the cupula detaches, it shows very little response to the same stimulus followed by a quick jerk at the beginning of the stimulus which is maintained to not completely relax in response to the pre stimulus.
The cupula eventually loses its sensitivity to angular motion including rotational acceleration and deceleration despite the afferent neurons and sensory hair cells being highly functional and capable of eliciting a response.
Light or Heavy Cupula
Under normal physiological conditions the specific gravity of the cupula is equal to the endolymph due to this leaning the head relative to gravity will not induce any buoyancy force in the cupula or neural response depending upon gravity. However, under some pathological conditions the match may vary. However, a light or heavy cupula generates nystagmus since cupula either floats or sinks within the endolymph. Nystagmus is a condition with repetitive, involuntary, and uncontrolled eye movements.
In most of the cases it is believed that dense calcium carbonate rich elements or particles cling to the cupula. This condition is called as cupulolithiasis. However, Factors other than this includes calcium precipitation inside the cupula. Cupulolithiasis generates persistent gravity dependent nystagmus.
Benign Paroxysmal Positional Vertigo
It is a condition characterized by sensitivity of semicircular canals due to an underlying pathological cause due to which semicircular canals cannot detect the direction of head orientation relative to gravity. This condition is most likely to occur when calcium carbonate crystals (otoconia) which are normally present within the gel inside the utricle are displaced and dislodged into either of the fluid filled semicircular canals, where they are not supposed to be. The presence of heavy particles inside the membranous labyrinth called canaliths. The identified risk factors for benign paroxysmal positional vertigo includes vitamin D deficiency, head injuries and low bone mineral density.
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