The three paired semicircular canals in the inner ear, equipped with mechanoreceptors, belong to the organs of equilibrium and are each nearly perpendicular to each other, providing one semicircular canal for each of the three main directions of rotation in three-dimensional space. The arcuates are sensitive to rotational accelerations, but not to uniform rotations. They are filled with endolymph, which, due to the principle of inertia, starts to move during rotational acceleration and bends small sensory hairs that deliver a corresponding electrical signal to the vestibulocochlear nerve.
What are the arcuate ducts?
The three arcuate ducts located in the petrous bone of the inner ear, together with the two otolith organs sacculus and utriculus, form the paired vestibular or equilibrium apparatus. The principle of operation of the semicircular canals is based on the inertia of the endolymph located in the semicircular canals. During rotational acceleration, which can also be caused by rapid rotation of the head, the endolymph of the arcuate duct, which is located in the plane of rotation, momentarily pauses. In the ampulla, the lower thickening of the arcuate duct, there is a mechanoreceptor with sensory hairs that are bent by the movement of the endolymph and give a corresponding signal to the vestibulocochlear nerve. Stopping a rotational movement is also perceived as acceleration, but as acceleration in the opposite direction. Due to their principle of action, the arcuates react extremely quickly to rotational accelerations. A disadvantage is that the endolymph “spins” briefly after each acceleration before coming to rest again in the initial position. During the settling phase, which can last up to one second after a pirouette, for example, an acceleration is subjectively felt although objectively none is present.
Anatomy and structure
The small tube-like arcades in the membranous labyrinth in the left and right inner ear all arise from the atrium (vestibule), with which the two otolith organs for sensory detection of linear accelerations are also connected. The semicircular canals each have a thickening, the crista ampullaris, at one end just above the vestibule, in which the end of the receptor cell is located. Above the crista ampullaris arches a small capsule, the cupula, which is filled with a jelly and into which the sensory hairs of the mechanoreceptor project. The crista with the cupula sitting on top practically closes the archway at the site. Because the endolymph, which fills all vestibular organs, moves momentarily with respect to the walls of the arcuate duct due to its inertia during rotational acceleration, “entraining” the cupula, the sensory hairs are bent and generate an electrical potential which they transmit to the vestibulocochlear nerve. The entire membranous labyrinth is surrounded by perilymph, which is distinguished from the endolymph in the vestibular organs by a reversal of its electrolyte ratios. The endolymph is high in potassium and low in sodium, whereas the perilymph, which is identical to the extracellular lymph of the rest of the body tissue, is low in potassium and high in sodium.
Function and Tasks
The most important task and function of the arcuate ducts is to maintain body balance in “cooperation” with the otolith organs, the proprioceptor system, also called the proprioceptor system, and the eyes, and to trigger certain eye reflexes. One important reflex is the vestibulo-ocular reflex (VOR), which enables the body to fixate an object firmly, even during very rapid head movements. The vestibular organs are directly connected to the eye muscles and trigger involuntary corrective movements of the eyes against the direction of acceleration, which can be much faster than voluntary eye movements. Another advantage of the VOR is to be able to effortlessly keep the stationary environment in view even during complex movements such as running and jumping. The effect is somewhat comparable to a gyro-stabilized camera in use on moving platforms. The acceleration messages from the arcades are very fast, much faster than central vision, because much less “processing power” must be provided by the brain for vestibular messages than for central vision. The interaction of several sensor systems for motion coordination has the advantage that one sensor can compensate for the failure of another, at least for a certain time.Thus, we can stand upright and walk even in complete darkness despite the loss of the sense of sight. Unfortunately, after each stop of a rotational acceleration, the arcuates briefly provide false messages because the endolymph lags a little due to inertia, so that the sensory hairs remain distracted for a short time and report “wrong” acceleration impressions. If there is a good view of the environment or reference surfaces at that moment, the brain accepts the visual impressions as “correct” and suppresses the “false” motion impressions within less than 100 milliseconds.
Diseases
The most common complaints associated with the arcades are so-called vertigo, which can be very unpleasant and can be due to various causes. Vertigo as a leading symptom – also in neurology – is understood as symptoms such as mistaken perception of a movement. The vertigo may be accompanied by headache and nausea to vomiting. In the frequency of causes of all vertigo symptoms, benign peripheral paroxysmal positional vertigo (BPPV) tops the frequency list with about 17%. It is a benign condition, but may be preceded by traumatic brain injury or inflammation of the vestibular nerve. BPPV is caused by the detachment of one or more calcite crystals from one of the two otolith organs and their transport to the posterior arch. This is possible because the endolymph is interconnected. Although the symptoms can be severe, the problem can be solved by appropriate body positions, because this allows the crystal granules to leave the arcade naturally. Besides some other causes of vertigo such as neurotoxins, alcohol and other toxins, Meniere’s disease is a relatively common cause of symptoms, accounting for about 10%. Meniere’s disease is due to an overpressure of the endolymph in the inner ear. Severe attacks of vertigo are usually accompanied by tinnitus and the onset of hearing loss.
