Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
Organs of Hearing and Balance (Vestibulocochlear Organ)
Organ of Balance (Vestibular Apparatus)

The vestibular apparatus is responsible for perceiving body position in space and maintaining balance. Whenever THE POSITION OF the body (or HEAD) changes, the receptors of the vestibular apparatus are stimulated. These impulses are transmitted to the Brain, which then sends signals to the appropriate Muscles to correct body posture and movement.

The vestibular apparatus consists of two parts: the vestibule and the semicircular ducts (canals). The bony vestibule houses two enlargements of the membranous labyrinth: the elliptical utricle and the spherical saccule. The saccule lies closer to the cochlea and communicates with the membranous cochlear duct via a connecting duct. The openings of the three membranous semicircular ducts—anterior, posterior, and lateral, oriented in three mutually perpendicular planes—open into the elliptical utricle (utriculus). The anterior (or superior) semicircular duct lies in the frontal plane, the posterior in the sagittal plane, and the lateral (external) in the horizontal plane. One end of each duct is dilated, forming an ampulla. On the inner surfaces of the saccule, utricle, and the ampullae of the semicircular ducts, there are specialized regions containing sensory Hair Cells that detect body position in space and imbalances.

Adjacent to the utricle and saccule is a complex Structure known as the otolith apparatus, specifically referred to as the maculae. The maculae of the saccules consist of clusters of sensory hair cells. Resting On the surface of these hair-bearing sensory cells is a gelatinous otolithic membrane containing calcium carbonate crystals, known as otoliths or statoliths. The hairs of the receptor cells are embedded within this otolithic membrane. In the ampullae of the semicircular ducts, the receptor hair cells are located on crest-like folds within the ampullary crests (cristae ampullares). Covering the hair Cells of the cristae is a gelatinous, transparent cupola, whose shape has been compared to a solid, bell-like structure. Both the maculae of the saccules and the cristae of the ampullae serve as structures where sensory receptor hair cells are highly sensitive to any changes in head (and body) position in space. Whenever the head changes position, the receptor hair cells detect alterations in the state or movement of the gelatinous otolithic membrane with its otoliths (in the maculae) or the gelatinous cupola (in the ampullary cristae). Any such mechanical stimulation of the receptor hair cells triggers the generation of a Nerve Impulse.