Equilibrium, Organ of

in animals and man, an organ that perceives changes in the position of the body in space and the effects of accelerations and changes in gravitational forces on the body.

The organs of equilibrium of invertebrates are statocysts, or auditory vesicles, which vary in structure and location. In most invertebrates the statocysts are in the form of the ectodermal invaginations that communicate with the environment by means of a canal or that are pinched off, forming a closed vesicle. Inside the statocysts are special solid bodies, known as statoliths. The interior cavity of the statocyst is usually lined with ciliated sensory cells. The statolith generally has greater density than the fluid surrounding it and is supported by sensory hairs. If the statolith is surrounded on all sides by sensory hairs, the displaced statolith will stimulate the corresponding group of hairs whenever there is any change in the animal’s body position.

The hair cells of crustaceans consist of primary sensory neurons. The statocysts of medusae and sea urchins are small, flask-shaped protrusions of the body coverings, inside of which are statoliths. However, the ciliated cells are located outside the statocyst—among the epithelial cells that surround it—or in the outer wall of the statocyst proper. Scyphozoan coelenterates have eight statocysts located radially along the margin of the mantle.

True statocysts have not been discovered among insects. In some Cryptocerata, including the Notonectidae, the role of statocysts is performed by external body parts covered with sensory hairs, which confine the air vesicles. The organs of equilibrium of cephalopods are complex. Statocysts in the form of vesicles are located in a capsule of the cranial cartilage; however, even in octopuses their removal causes only slight disruption of orientation ability. Excitation of the sensory cells of statocysts is transmitted to the central sections of the nervous system. The mechanisms of responsive reactions of animals that lack a nervous system are less clear. Many equilibrium receptors give signals of two types—static ones, associated with body position, and dynamic ones, associated with acceleration.

In vertebrates, including man, the organs of equilibrium are represented by the vestibular apparatus, whose receptor is located in the inner ear. Signals that are associated with body position or acceleration arise from the equilibrium receptors when there is mechanical stimulation of the sensory hairs by movement of the otoliths, cupulae, or endolymph. The impulses that arise are transmitted along the vestibular nerve to the brain. The complex organization of the central vestibular mechanisms and their numerous connections with the cerebellum and the reticular formation ensure functional interconnections with other analyzers. The close interaction between the central vestibular and nervous mechanisms that effect deep “muscle senses” produces subtle regulation of muscle tone (seeKINESTHESIA). The aggregate of sensory signals from the labyrinths, the eyes, and the muscle, joint, and skin receptors produces statokinetic reflexes. These reflexes enable animals and man to maintain normal orientation in relation to the force of gravity and to counteract accelerations in all planes. These reflex reactions occur with participation of the spinal cord and the lower sections of the brain.

Disturbances of equilibrium in man are observed in a number of diseases of the nervous system (seeATAXIA) and with irritation or diseases of the vestibular apparatus (dizziness, Ménière’s disease, seasickness).