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THE ORGAN OF EQUILIBRIUM

Дата публикации: 01 октября 2021
Автор(ы): Galina SOLNTSEVA
Публикатор: Научная библиотека Порталус
Рубрика: МЕДИЦИНА
Источник: (c) Science in Russia, №2, 2012, C.13-19
Номер публикации: №1633075403


Galina SOLNTSEVA, (c)

by Galina SOLNTSEVA, Dr. Sc. (Biol.), A. Severtsov Institute of Problems of Ecology and Evolution, Russian Academy of Sciences (Moscow)

 

The vestibular apparatus, due to which humans and animals perceive their position and movement in space, is well studied as regards its structure. These data are essential in many spheres of human activity. In biology the knowledge of morphology of the acoustic system and organ of equilibrium enable scientists to understand the routes of their evolution caused by environmental changes.

 

However, scientists still argue about the origin of the vestibular apparatus in mammals. According to the most prevalent hypothesis, animals "received" it from fishes, whose lateral line organs (located on the body surface on both sides--from gill cleft to tail), intended for perception of the movement and vibration of water, had transformed under changing habitation conditions. In mammals, a special structure of the organ of equilibrium corresponds to its new functions; it is located deep in the skull and has no contacts with the environment.

 
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In order to understand how the vestibular apparatus works in vertebrates, we should know the structure of their peripheral auditory system.* In the 1960s and 1970s, I, as a specialist in morphology at N. Koltsov Institute of Biology of Development, studied the characteristics of terrestrial, subterranean, aerial, semi-aqueous and aqueous mammals (these studies were financed due to the then special interest in the dolphin echolocation capacity). We analyzed the changes in the structure of auditory organs and the organ of equilibrium in representatives of various ecological groups in the course of their adaptation to the aqueous life style and eventually found morphological characters reflecting the evolution of animals; this work was highly appreciated by the scientific community. Our studies continued during the subsequent decades and even today attract the attention of specialists: it is not accidental that the name of the author of this paper is included in Volume 29 of Who Is Who in the World (2012).

 

ISOLATED

 

FROM THE ENVIRONMENT

 

Before comparing the characteristics of ecological groups of mammals we are interested in, let us dwell on modern concepts on the morphology of their auditory system. In mammals, the internal ear is located in the petrous bone. Due to its intricate structure, it is also called a labyrinth. In fact, there are two such struc-

 

See: G. Solntseva, "Worlds of Sounds and Anatomy of Hearing", Science in Russia, No. 2, 2009.--Ed.

 
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tures--osseous and membranous labyrinths. The former includes the auditory organ (cochlea) and vestibular structures: three semicircular canals of a complex structure with an oval cavity (vestibule), separating them from the cochlea. The membranous labyrinth is inside the osseous one and by shape usually repeats it, but is much smaller, with walls formed by compact connective tissue.

 

In the mammals the structure of the organ of equilibrium is a closed system of two membranous sacs: round (sacculus), oval (utriculus), and three semicircular canals located in mutually perpendicular planes and opening into the utriculus, widening at the entry and forming an "ampulla". The membranous labyrinth of semicircular canals (like the cochlea) is filled with liquid--endolymph--and washed by the perilymph from the outer side.

 

Now, let us describe the structure of the vestibular apparatus in more detail. There are special formations on the inner surface of the sacculus and utriculus--auditory spots (maculi) and auditory cristae with receptor (villous) cells connected with the vestibular nerve fibrils. The cristae are usually oriented perpendicularly to the semicircular canal plane and located at the site of its widening, thus are called ampullar cristae. Each of them is covered with the receptor epithelium enveloping its both slopes and the apex or concentrated in its middle part. In addition to the villous cells, there are supporting cells, between which nerve fibrils, stimulating the receptor cells, pass. A little bit above in the

 
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ampulla there is a gel-like membrane, the so-called cupula, with much moisture in it.

 

And now let us speak about the mechanisms of the vestibular apparatus. In the vertebrates its receptors, reacting to body movements, are located (as we have mentioned) in the cristae and maculi. That is why these structures are so important. As an individual turns its head, or the velocity of its movements increases or reduces--the endolymph pressure on the sensitive cells of the auditory maculi and cristae changes, which, in its turn, stimulates the vestibular nerve, then the resultant pulse is transmitted to the brain, and the sensation of body position in space emerges in the cortex.

 

Accordingly there takes place reflex changes of the tone of different groups of muscles. Due to this mechanism the mammals keep balance.

 

Two signal types are sent from balance receptors: static (caused by body position) and dynamic (associated with acceleration). Both emerge as a result of mechanical stimulation of sensitive hairs conditioned by displacement of gel-like cupula or calcareous formations of the endolymph--otoliths, usually more compact than the liquid around them. Due to different inertia of the endolymph and cupula, the latter shifts during the body acceleration, while friction resistance in fine canals serves as a muffler of the entire appara-

 
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tus. The oval sac, utriculus, plays a leading role in the body position perception and, presumably, is involved in the rotation sensation, for which it is called the gravitation organ. The round sac--sacculus--is necessary for vibration perception.

 

ADAPTATION

 

TO LIVING CONDITIONS

 

The brain "learns" about the vestibular apparatus receptor cell stimulation through the appropriate branch of the auditory nerve. The stimulation signal evokes a number of reflexes (regression or reinforcement of cervical body, or limb muscle tone), due to which the balance is maintained during shifts of the body position. Under these conditions some people develop signs of motion sickness, characteristic of sea sickness (vertigo, heart and respiratory rhythm disorders, nausea, vomiting). Frequent repeating of these unpleasant situations leads to gradual regression of the abovementioned reaction. Adaptation of this kind is the base of vestibular training, used in physical training of seamen, pilots, cosmonauts, etc. It includes movements stimulating the organ of equilibrium (bending, turns, jumps, special exercises on trampoline, etc.), and repeated exposure of the organism to angular and straight accelerations on a rotating device (centrifuge), swing, etc.

 

Science in Russia, No.2, 2012

 
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Studies of the vestibular apparatus of mammalian embryos of various ecological groups showed that the structures of this organ, irrespective of their shape, structure, and location, developed from one anlage. Hence, today, we make special preparations, examine them under a microscope, and can trace a long biological evolution of one of the most important organs of senses.

 

In addition, we have revealed interesting peculiarities of stages of differentiation of the sensitive epithelium of maculi and cristae into receptor and supporting cells. In terrestrial and semi-aquatic animals (sea lion, walrus), whose life is mainly spent on dry land, the organ of equilibrium is twice larger than the auditory organ (cochlea). Hence, the latter is more important for these animals.

 

By the way, a man with hearing disorders fairly well adapts to the environment by means of special devices and even without them. However, if the vestibular apparatus is seriously impaired, the gravitation principle fails and he becomes incapable of properly standing and walking. The matter is that the cell differentiation of sensitive epithelium starts in macula utriculi. In a word, the organ of gravitation is the main organ in vital activities of terrestrial and semi-aquatic mammals.

 

In Phocidae family, the cell differentiation of sensitive epithelium in macula sacculi and macula utriculi takes place simultaneously and the cochlea and vestibular apparatus are of similar size. Evidently, the organs of gravitation and vibration are equally essential for proper life of these animals. Each of these organs is

 
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adapted to functioning in the environment with certain physical characteristics.

 

In absolute hydrobionts (whales), the vestibular apparatus is twice smaller than the cochlea, while the initial cell differentiation in macula sacculi indicates that the organ of vibration performs a more important function than the organ of gravitation. The abovementioned proportion of sizes in echolocating mammals (dolphins, bats) reflects adaptation of the cochlea to perception of a wide range of frequencies, including ultrasounds. On the other hand, its enormous size in hydrobionts can be regarded as adaptation to life in water, as in this environment hearing is a predominating distant analyzer (reacting to stimuli released by distant objects), thus ensuring survival of the host.

 

Our results are interesting not only for science proper, but have useful practical application. People whose work involves alteration of environmental conditions (cosmonauts, divers, miners, speleologists) permanently use the adaptation potential of their vestibular apparatus. The knowledge of the work of the organ of equilibrium is essential for them. Understanding of the trend in changes of the structures of the auditory organ in representatives of different ecological groups of mammals can be useful in simulation of some processes in technology.

 

 

Опубликовано на Порталусе 01 октября 2021 года

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