Saccule

The saccule of the membranous labyrinth (or simply saccule, Latin: sacculus) is a part of the balancing apparatus of the inner ear lying within the vestibule. The receptors within the saccule respond to movements in the vertical plane.

Structure of saccule

The saccule is smaller than the utricle. The saccule is an elongated, globular-shaped membranous sac connected to the utricle and the endolymphatic duct (located in the posterior wall of the saccule) by the utriculosaccular duct and to the cochlea by the ductus reuniens. The endolymphatic duct extends from the utriculosaccular duct.

The endolymphatic duct goes into the vestibular aqueduct, then through the temporal bone, and stops as the endolymphatic sac at the posterior surface of the petrous portion of the temporal bone.

The saccule lies within the spherical recess on the medial wall of the vestibule near the place where the vestibular duct opens in the cochlea. It contains sensory epithelium - the macula, which is composed of hair cells and associated supporting cells.

The macula of the saccule is an oval thickening of the saccule, which lies in a vertical plane on the inner surface of the wall of the saccule and is formed by sensory epithelium containing receptor hair cells. The saccular filaments of the vestibular branch of the vestibulocochlear nerve are directed to the macula.

Because of its vertical orientation, the saccule is mainly sensitive to the linear acceleration of the head in the vertical plane. The information received here goes further through the saccular nerve, starting in the macula of the saccule. The saccule and the utricle provide information about acceleration. The only difference is that the saccule is responsible for the vertical acceleration, while the utricle - for horizontal acceleration.

The saccule serves as a gravitational sensor when the head is in an upright position and also is sensitive to movement along the anteroposterior axis. The saccule sends the information about the movements as neural impulses to the brain.