Synonyms
Vestibular apparatus, vestibularis organ, vestibular organ, vestibular balance ability, movement coordination, dizziness, vestibular organ failure
Introduction
The human organ of equilibrium is located in the inner ear, in the so-called labyrinth. Several structures, fluids and sensory fields are involved, which measure rotational and linear accelerations in order to maintain the body’s equilibrium and enable spatial orientation by maintaining a constant field of vision.
Anatomy
The organ of equilibrium is located together with a part of the auditory organ in the inner ear, which is located in a section of the skull called the petrous bone. These structures are known as labyrinths, where bony labyrinths are distinguished from membranous labyrinths. Bony labyrinths are interconnected cavities embedded in the bone.
It begins with an atrium (vestibulum) which continues forward into the cochlea (part of the auditory organ) and backward into the semicircular canals (part of the organ of equilibrium). This bony labyrinth contains a water-clear liquid, called perilymph, in which the membranous labyrinth floats. The perilymph follows the structure of the bony labyrinth and thus represents its outlet.
It is also filled with a liquid, the viscous endolymph. Another division of the labyrinth is into vestibular and cochlear. The cochlear belongs to the hearing organ, whereas the vestibular forms the organ of balance and consists of several interconnected parts: The archways are perpendicular to each other.
In relation to the body axes, the upper 45 degrees deviates from the median plane (the mirror axis of the body running through the head and feet), the rear 45 degrees deviates from the frontal plane and the lateral 30 degrees deviates from the horizontal plane. The membranous labyrinth contains several sensory fields, so-called sensory epithelia, which are responsible for the acquisition of the balance parameters. In Sacculus and Utriculus these are the Macula sacculi and the Macula utriculi (Macula = spot), which are at right angles to each other.
In the archways, these are the 3 cristae ampullares (crista = groin). The information obtained through these sensory fields is passed on to the vestibular nerve, the vestibular nerve, with the help of the sensory cells and from there to its nerve nuclei, the vertibular nuclei in the brain stem. From there, there are connections to the brain (gyrus postcentralis), the spinal cord, other parts of the brain stem, the cerebellum, the muscles of the eyes and other parts of the muscles.
- Sacculus (small bag)
- Utriculus
- 3-arched canals = Ductus semicirculares (semicircular canals) à upper, rear and lateral
The structure of the different sensory epithelia is comparable except for small differences. There are always sensory cells, the hair cells, and supporting cells, in which the hair cells are embedded. Each hair cell has several cell extensions, namely a long (Kinozilium), and several short (Stereozilien).
These are connected by a tip on the left, which can be imagined as rope-like structures between the individual cilia (cilium = cilia). Above the hair and supporting cells there is a gelatinous mass, which has a different structure depending on its location. Above the maculae in sacculus and utriculus there is a so-called gelatinous statolith membrane, which gets its name from embedded calcium carbonate crystals (= statoliths).
The cell extensions of the hair cells protrude into this membrane. However, they are not directly immersed in the membrane, but are still surrounded by a narrow endolymph-containing space. The cristae of the archways, on the other hand, are covered by the cupula, also a gelatinous mass, into which the cell extensions also protrude.
In both the maculae and the cristae, the hair cells are coupled to the vestibular nerve via synaptic connections at the organ of equilibrium. The sensory epithelia are surrounded by other epithelium, but are taller and protrude above it. The fluids present in the labyrinth also have a special composition.
The perilymph that surrounds the membranous labyrinth contains an aqueous electrolyte-containing fluid similar to that of the interstitial fluid in the body.This means that the sodium content is high, but the potassium content is low. The mechanism of perilymph formation is not well understood; the connection with the subarachnoid space of the brain, which is located between the brain and the meninges, probably plays a role. The endolymph contained in the membranous labyrinth is also a fluid, but in contrast to the perilymph it contains little sodium and much potassium. The endolymph is produced by structures in both the vestibular labyrinth and the cochlear labyrinth (stria vascularis). The different content of electrolytes (= ions) is important for the excitation of the sensory cells, which can pass on information to the brain.
