Semicircular ducts
Ductus semicirculares
Definition
The semicircular ducts are the three thin, membranous, endolymph-filled tubes located within the semicircular canals of the vestibular system in the inner ear.
There are three semicircular ducts, duct corresponding to one of the three spatial planes—anterior, posterior, and lateral—and is oriented approximately orthogonally to the others, allowing detection of rotational movements in three planes.:
These ducts are essential for detecting angular acceleration and head rotation, as movement of endolymph within the ducts deflects the cupula in the ampullae, leading to activation of sensory hair cells and subsequent neural signaling for balance and spatial orientation.
Structurally, the semicircular ducts are distinguished from the surrounding bony semicircular canals by their delicate membranous walls and their role in housing the sensory apparatus for rotational movement detection. The ducts are interconnected and allow endolymph to flow between them, which is critical for their function in transducing mechanical stimuli into electrical signals for the central nervous system.
The semicircular ducts are about one-fourth of the diameter of the osseous canals, but in number, shape, and general form they are precisely similar, and each presents at one end an ampulla.
They open by five orifices into the utricle, one opening being common to the medial end of the superior and the upper end of the posterior duct.
In the ampullæ the wall is thickened, and projects into the cavity as a fiddle-shaped, transversely placed elevation, the septum transversum, in which the nerves end.
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