The organ of balance, or vestibular apparatus, is located in pairs in the right and left inner ear. The three arcades, each perpendicular to the other, report rotational accelerations, and the otolith organs (sacculus and utriculus) respond to translational accelerations. Because of the physical mode of action, brief disorientations may occur after an acceleration or deceleration phase.
What is the organ of balance?
The vestibular organ in its entirety, or better, the vestibular apparatus, consists of several vestibular organs in the inner ear. One arcuate duct each is available for reporting rotational accelerations for the pitch, roll, and elevation axes. The three arcades are perpendicular to each other. The two otolith or macula organs sacculus and utriculus are used to detect and report linear (translational) accelerations. Although they are only two organs, linear accelerations can be detected in the three possible directions: up/down, right/left, and forward/backward. The sensory system of all balance organs is based on the inertia of endolymph in the arcuates and tiny calcium carbonate crystals (otoliths) in the macular organs. The function of the vestibular organs is very important for the sense of balance, but not exclusively responsible for balance. Complementary – and partially corrective – is the sense of sight. The sense of balance is further supplemented by thousands of proprioceptors, which contribute information about muscle tension and about the flexion angle of individual joints.
Anatomy and structure
The three endolymph-filled arcuate ducts consist of small semicircular tubes that connect at the base. Each arcuate duct thickens at the base to form an ampulla into which fine sensory cilia (mechanoreceptors) extend. In case of a rotational acceleration in the plane of the corresponding arcuate duct, the endolymph remains in the initial position for a short time due to the mass inertia, so that it moves against the rotational acceleration. This deflects the sensory cilia in the ampulla and generates an electrical stimulus that is reported via the vestibulocochlear nerve to specific brain areas and to the visual center. A gelatinous membrane in the two macular organs contain tiny calcium carbonate crystal granules (otoliths or statoliths). In the case of linear acceleration, the statolith membrane is momentarily deflected in the direction opposite to the acceleration by the inertia of the tiny crystals. The sensory hairs projecting into the macula membrane bend, producing an electrical stimulus that is also transmitted by the vestibulocochlear nerve.
Function and tasks
The vestibular organs have the task of transmitting rotational and linear accelerations to specific brain areas. Rotational accelerations or decelerations in the tilt, roll, or vertical axes are reported from one arcuate ganglia each. The utriculus perceives linear accelerations in the two horizontal directions “front/back” and “right/left” and the utriculus responds to vertical accelerations. In the upright position, the sensory cilia of the saccule are constantly slightly deflected due to gravity, which represents a vertical linear acceleration towards the center of the earth. This creates a sense of the body’s position in space, which is extremely helpful for orientation in the dark and makes walking or standing without sight possible in the first place. Uniform movements are the result of previous accelerations, but are not perceived by the mechanoreceptors because they are not deflected. Decelerations of a linear or rotational movement are perceived as accelerations in the respective opposite direction. In addition to aiding orientation in space, reported accelerations have direct effects on involuntary eye movements. Each perceived rotational or linear acceleration causes an involuntary reflexive eye movement in the opposite direction in order to be able to keep the natural environment in the field of view. The eyes are stabilized, so to speak, while walking, running, and jumping, so that we can continue to see the environment sharply without “blurring” despite accelerations (comparable to a gyro-stabilized camera).
Diseases and ailments
When perceived accelerations do not match vision, the different sensations cause a sensory conflict (balance disorder).In these cases, vision always acts as the “master”, the vestibular acceleration sensation is manipulated and adapted to vision. This can be very helpful in cases where vestibular mismatches occur for a short time (1 to 3 seconds). If the discrepancies persist longer, discomfort, dizziness, or nausea to the point of vomiting may occur. Similar symptoms can also be caused by disorders and diseases in the inner ear at the vestibular organs. Temporary disorders can be caused by inflammatory processes in the inner ear or by injuries such as skull base fractures. After healing, the symptoms disappear by themselves. Unpleasant vertigo can have a variety of causes and can occur as a concomitant of circulatory disorders or be caused by nervous damage or particular exceptional mental states. In the case of prolonged and recurrent attacks of spinning vertigo (vertigo), Meniere’s disease may be the cause. This is a metabolic disorder of the inner ear, the cause of which has not been adequately clarified. Since direct causal treatment is not possible, drug therapies and treatments are aimed at alleviating the symptoms.
