The membranous labyrinth is a part of the inner ear that is responsible for converting mechanical signals into electrical ones and maintaining balance. It represents a system of interconnected cavities and channels having a connecting wall.
What is the inner ear
This part of the ear is a hollow bone formation, the part that includes the sensory organs of hearing and balance. The system of communicating bone channels within it is called the bone labyrinth. The membranous labyrinth is also a system of cavities and channels. This whole structure is immersed in a fluid - endolymph and perilymph.
The outlines of the bone and membranous labyrinths completely coincide. The latter is located inside the first. Three parts are distinguished in the bone labyrinth: vestibule, semicircular canals and cochlea. The webbed labyrinth is divided into parts:
- semicircular canals;
- two pouches of the vestibule, plumbing of the vestibule;
- a snail;
- cochlear canal, which is the only part of the inner ear that is the organ of hearing.
The structure of the membranous labyrinth
This labyrinth, despite the fact that its shape coincides with the bone, is much smaller and partially separated from the bone walls by a liquid - perilymph. In some places, it attaches to the walls of the cavity. The membranous labyrinth contains fluid, the endolymph, and branches of the acoustic nerve extend on its walls.
On the eve of the bone, it does not quite retain the shape of the bone cavity, but consists of two membrane bags, a duckbill and a succule (sac).
Semicircular ducts
They make up about one quarter of the diameter of the bone canals, but in terms of quantity and general shape they almost exactly coincide, and each has an ampoule at one end. They open with five holes in the walled, one hole is common to the medial end of the upper end of the posterior canal. In the ampoule, the wall is thickened and projected into the cavity in the form of a transverse elevation, a septum in which the nerves end.
Utricles, sacs and semicircular ducts are held in place by numerous fibrous strips that extend through the space between them and the bone walls.
Utrikl and Sakkula
The membranous vestibular labyrinth of the inner ear consists of three sacs on the threshold: the utricle (utriculus), sac (sacula) and the endolymphatic canal and sac, as well as three semicircular canals located in the bony canals. Utricle has an elongated shape and is located in the upper back of the vestibule, near the upper and horizontal ampoules of the channels. The sakkula is more round in shape and is located lower and anterior in the bony vestibule, closer to the cochlea.
The saccule is connected with the webbed labyrinth of the cochlea by a thin channel. Utricl and sac have small channels, utricular and saccular ducts, which merge to form an endolymphatic canal. This channel ends in a blind endolymphatic sac located under the dura mater. The endolymphatic canal and sac are extremely important for the regulatory, homeostatic and protective functions associated with the circulation of endolymph.
In the walls of the utril and saccule there are thickenings called utricular (macula acoustica utriculi) and saccular (macula acoustica sacculi) spots (macula), respectively. These thicker membranes of connective tissue support the sensory epithelium, which consists of supporting cells and sensory hair cells. Supporting cells extend from the base membrane to the apical surface of the macula, and their cell nuclei form one row next to the connective tissue. Sensory hair cells are located above the nuclei of the supporting cells.
Utricle and saccule are called otolith organs, they transduce translational (linear) accelerations acting on the head. The sensory epithelium is covered with a gelatinous otolith membrane, which, in turn, is covered with a layer of crystals called statoconium or otoliths. In mammals, the otoconia containing otoliths consists of a glycoprotein / proteoglycan core surrounded by a mineral sheath of thousands of calcium carbonate crystalloids embedded in a calcite lattice. The human otolithic membrane has a thickness of about 20 microns and demonstrates regional diversity. Below is a macula with a narrow central stripe called striola, where sensory hair cells exhibit various characteristics, morphology, orientation specificity, and connectivity. Otoliths are the thickest in the striolar region, here the polarity of the bundles of hair cells is reversed.
The endolymph drains from the saccule and flows into the endolymphatic canal. The canal passes through the vestibular aqueduct to the posterior region of the stony part of the temporal bone. Here the canal expands to the sac where the endolymph can be secreted and absorbed.
Structure
The walls of the duckbill, sacs and semicircular ducts consist of three layers:
- The outer layer is a loose and flocculant structure, consisting of ordinary fibrous tissue containing blood vessels and some pigment cells.
- The middle layer, thicker and more transparent, forms a homogeneous membrane propria and presents on its inner surface, especially in semicircular ducts, numerous papillary protrusions.
- The inner layer is formed by polygonal germinal epithelial cells.
In the maculae (spots) of the duckbill and saccule, as well as in the transverse septa of the ampulla of the semicircular ducts, the middle layer thickens and the epithelium is columnar and consists of supporting (supporting) cells and hair cells. The former are fusiform, their deep ends are attached to the membrane, and the free limbs are combined. Hair cells have the shape of a bulb; their rounded ends lie between supporting cells. The deep part of each contains a large core, and the surface part is granular and pigmented. Filaments of the acoustic nerve enter these parts and pass through the outer and middle layers.
Web snail
The cochlear duct consists of a spirally located tube enclosed in the bone channel of the cochlea and lying along its outer wall.
The bone spiral plate extends only for a part of the distance between the modiolus (bone shaft) and the outer wall of the cochlea, while the basilar membrane extends from its free edge to the outer wall of the cochlea. The second and more delicate vestibular membrane extends from the thickened periosteum covering the spiral bone plate to the outer wall of the cochlea, where it is attached at some distance above the outer edge of the basilar membrane. Thus, the top of the duct is formed by the vestibular membrane, the outer wall is the periosteum, lining the bone canal, and the bottom is the basilar membrane and the outer part of the spinal disc.
The vestibular membrane is thin and uniform, covered with a layer of epithelium. The periosteum, forming the outer wall of the duct, is greatly thickened and changed in nature.
The bone spiral plate of the membranous labyrinth of the ear divides the spiral channel into two parts.
Basement membrane
It extends from the tympanic lip of the bone spiral plate to the spiral crest and consists of two parts: internal and external. The inner part is thin, on it is the spiral organ of Corti.
Spiral organ corti
This part of the membranous labyrinth of the inner ear consists of a number of epithelial structures located on the inside of the basilar membrane. The central of these structures are two rows of fibers, internal and external, or Corti columns. The fiber bases are supported on the basement membrane, and the internal ones are at a certain distance from the external ones; two rows bend to each other and, touching above, form a triangular tunnel, the Corti tunnel between them and the basement membrane. On the inner side of the fibers there is one row of hair cells, and on the outer side there are three or four rows of similar cells, together with supporting cells called Deiters and Hensen cells. All this is the receptor department of the auditory analyzer.