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The saccule is a small membranous sac, paired with the utricle, within the vestibule of the inner ear. It is part of the membranous labyrinth and has an important role in orientation and balance, particularly in vertical tilt 1.
The vestibule is located within the bony labyrinth (temporal bone) of the inner ear 2, inferior to the semicircular canals 1. The utricle and the saccule are co-located within the vestibule 3. The saccule lies in the medial wall of the vestibule, near the opening of the vestibular duct of the cochlea in the recessus sphaericus 1,5. It is smaller in size compared to the utricle and globular in appearance. The saccule is oriented in a vertical plane 2.
Within the anterior aspect of the saccule lies a projection known as the macula, the sensory organ of the saccule 2. Like the utricle, this sensory epithelium of the saccule is lined by vestibular cells 1. These cells project into the membranous labyrinth called the statoconial membrane, a gelatinous layer beyond containing otoliths (calcium carbonate particles) 1,2. Cilia from the hair cells are attached to these otoliths 5.
Any change in the orientation of the head results in stimulation of the saccule, via movement in the otolith-cilia-macula complex, releasing signals that the brain interprets 2,4. The saccule is sensitive to vertical acceleration in comparison to the utricle, which is more sensitive to horizontal acceleration 2,5. Unlike the utricle, the saccule also provides information about head position even when it is stationary.
The saccule receives its blood supply from two arteries. The anterior vestibular artery (second branch of the labyrinthine artery) and the posterior vestibular artery both supply the saccule 5.
The inferior vestibular nerve supplies the saccule. This nerve is a branch of the vestibular nerve, itself a division of the vestibulocochlear nerve (VIII) 5.
The saccule sits in the medial portion of the vestibule. On high-resolution, heavily T2-weighted MRI, the normal saccule appears as an oval high signal structure, which in the coronal plane is delimited superiorly by the hyposignal band of the utricular macula, inferiorly by the cochlear bone, and laterally by another hyposignal band that separates it from the perilymphatic fluid in the rest of the vestibule (cisterna perilymphatica of the vestibulum) 7,8.
Damage to the otolith-containing organs such as the saccule can result in a reduced ability to sense motion and orientation, leading to vestibular symptoms, e.g. imbalance 5.