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Arch duct

Ductus semicircularis

Endolymph

The semicircular canals belong to the balance organ.

The task of the three semicircular canals is to, when moving the head (which is inevitable in a number of everyday contexts, e.g. when walking, standing up, etc.), provide the brain with information that enables the image that falls on the retina to "lie still" and not become blurred, i.e. enable coordinated eye movements that compensate for the movements of the head.

The semicircular canals (also the membrane sacs) contain a fluid, endolymph, and are suspended in the bone walls of the inner ear. If you turn your head, the membrane maze will immediately follow the movements of the head. The endolymph does not immediately catch up due to its "inertia". It falls behind for a short while; "lying around". This means that the endolymph relative to the membrane labyrinth "moves" in the opposite direction.

Inside each semicircular canal there is a group of specially arranged sensory cells that react when the endolymph pushes on from a certain direction.

The "semicircular canal information" is then distributed to the nuclei of the various external eye muscles using the vestibular nuclei and fasciculus longitudinalis medialis (FLM) as well as the core areas of the brainstem that connect to the FLM. Due to the strong link between the vestibular nuclei and the cerebellum, the cerebellum also has a decisive influence in this context.
This refined coordination of eye movements with head movements is called the vestibulo-ocular reflex.
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