Saccule

Introduction
The human body is equipped with special receptor organs to help maintain a sense of balance, or equilibrium. These organs, the saccule, utricle, and semicircular canals, are collectively called the vestibular apparatus, and they are located in the inner ear. The saccule and utricle are located together in the vestibule of the inner ear.

Anatomy
The saccule, or sacculus, is the smaller of the two vestibular sacs. It is globular in form and lies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea. Its cavity does not directly communicate with that of the utricle. The anterior part of the saccule exhibits an oval thickening, the macula acustica sacculi, or macula, to which are distributed the saccular filaments of the vestibular branch of the vestibulocochlear nerve, also known as the acoustic nerve or cranial nerve VIII.

Within the macula are hair cells, each having a hair bundle on the apical aspect. The hair bundle is composed of a single kinocilium and many (at least 70) stereocilia. Stereocilia are connected to mechanically-gated ion channels in the hair cell plasma membrane via tip links. Supporting cells are interdigitate between hair cells and secrete the otolithic membrane, a thick, gelatinous layer of glycoprotein. Covering the surface of the otolithic membrane are otoliths, which are crystals of calcium carbonate. For this reason, the saccule is sometimes called an "otolithic organ."

From the posterior wall of the saccule is given off a canal, the ductus endolymphaticus. This duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater.

From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity.

Physiology
The saccule gathers sensory information to orient the body in space. It primarily gathers information about linear movement in the vertical plane. The structures that enable to saccule to gather this vestibular information are the hair cells. When the head tilts, the otolithic membrane slides over the hair cells in the direction of the tilt. This sliding bends the hair bundles of the hair cells, which causes stretching of the tip links and opening of the mechanically gated ion channels. Cations such as K+ rush into the hair cell cytosol, depolarizing it. Depolarization induces opening of voltage-gated Ca2+ channels at the basal aspect of the hair cell, which triggers exocytosis of neurotransmitter to the vestibular neurons. These impulses are communicated via the vestibular nerve to vestibular nuclei in the brainstem and medulla. Impulses are also carried to the cerebellum via the inferior cerebral peduncles.