Saccule
The saccule is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the head accelerates vertically, the sensory cells of the saccule are moved due to a combination of inertia and gravity. In response, the neurons connected to the saccule transmit electrical impulses that represent this movement to the brain. These impulses travel along the vestibulocochlear nerve to the vestibular nuclei in the brainstem.
The vestibular system is important for balance, or equilibrium. It includes the saccule, utricle, and the three semicircular canals. The vestibule is the name of the fluid-filled, membranous duct that contains these organs of balance and is in turn encased in the temporal bone of the skull as a part of the inner ear.
Structure
The saccule, or sacculus, is the smaller of the two vestibular sacs. It is globular in form and lies in the spherical recess near the opening of the vestibular duct 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 of saccule, or macula, to which are distributed the saccular filaments of the vestibular branch of the vestibulocochlear nerve, also known as the statoacoustic 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 at least 70 stereocilia. Stereocilia are connected to mechanically gated ion channels in the hair cell plasma membrane via tip links. Supporting cells 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 endolymphatic duct. This duct is joined by the ductus utriculosaccularis, and then passes along the vestibular aqueduct and ends in a blind pouch the endolymphatic sac 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 ductus reuniens, passes downward and opens into the cochlear duct near its vestibular extremity.
Both the utricle and the saccule provide information about acceleration. Ihe utricle is more sensitive to horizontal acceleration, and the saccule is more sensitive to vertical acceleration.
Function
The saccule gathers sensory information to orient the body in space. It primarily gathers information about linear movement in the vertical plane, including the force due to gravity. The saccule, like the utricle, provides information to the brain about head position when it is not moving. The structures that enable the saccule to gather this vestibular information are the hair cells. The 2 by 3 mm patch of hair cells and supporting cells are called a macula. Each hair cell of a macula has 40 to 70 stereocilia and one true cilium called a kinocilium. The stereocilia are oriented by the striola, a curved ridge that runs through the middle of the macula; in the saccule they are oriented away from the striola The tips of the stereocilia and kinocilium are embedded in a gelatinous otolithic membrane. This membrane is weighted with protein-calcium carbonate granules called otoliths, which add to the weight and inertia of the membrane and enhance the sense of gravity and motion.Not much is known of how this organ is used in other species. Research has shown, like songbirds, females in some species of fish show seasonal variation in
auditory processing and the sensitivity of the saccule of females peaks during the breeding season. This is due to an increase in the
density of saccular hair cells, partly resulting from reduced apoptosis. The increase the hair cells make also increase the sensitivity to male mating calls. An example of this is seen in Porichthys notatus, or plainfin midshipman fish.