The membranous labyrinth is the part of the inner ear responsible for converting mechanical signals into electrical signals and maintaining balance. It is a system of interconnected cavities and channels with a connecting wall.
What is the inner ear
This part of the ear is a hollow bone formation, the part that includes the senses of hearing and balance. The system of communicating bony canals within it is called the bony labyrinth. The membranous labyrinth is also a system of cavities and canals. This whole structure is immersed in a liquid - endolymph and perilymph.
The outlines of the bony and membranous labyrinths are exactly the same. The latter is located inside the former. In the bony labyrinth, three sections are distinguished: the vestibule, the semicircular canals and the cochlea. The membranous labyrinth is divided into parts:
- semicircular canals;
- two bags of vestibule, vestibule plumbing;
- snail;
- the cochlear canal, which is the only part of the inner ear that representsis the organ of hearing.
Structure of the membranous labyrinth
This labyrinth, despite the fact that its outlines coincide with the bone, is much smaller and partially separated from the bone walls by a liquid - perilymph. In some places, it is attached to the walls of the cavity. The membranous labyrinth contains fluid, the endolymph, and branches of the acoustic nerve extend along its walls.
In the bony vestibule, it does not quite retain the shape of the bone cavity, but consists of two membranous sacs, the utricle and the succulus (sac).
Semicircular ducts
They are about one quarter the diameter of the bony canals, but they almost exactly match in number and overall shape, and each has an ampulla at one end. They open with five holes in the utrikli, one hole is common to the medial end of the upper end of the posterior canal. In the ampulla, the wall is thickened and is 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 bands that extend through the space between them and the bony walls.
Utrickle and Sacculus
The membranous vestibular labyrinth of the inner ear consists of three sacs in the vestibule: utricle (utriculus), sac (saccule) and endolymphatic canal and sac, as well as three semicircular canals located in the bony canals. The utrikl has an oblong shape and is located in the upper backparts of the vestibule, near the upper and horizontal ampullae of the canals. The saccule is more round in shape and is located below and in front of the bony vestibule, closer to the cochlea.
The saccule is connected to the membranous labyrinth of the cochlea by a thin channel. The utrickle and sac have small canals, the utricular and saccular ducts, which merge to form the 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 endolymph circulation.
In the walls of the utricle and saccule there are thickenings called utricular (macula acoustica utriculi) and saccular (macula acoustica sacculi) spots (maculas), respectively. These thicker connective tissue membranes support the sensory epithelium, which is composed of supporting cells and sensory hair cells. The supporting cells extend from the basement membrane to the apical surface of the macula, and their cell nuclei form a single row next to the connective tissue. Sensory hair cells are located above the nuclei of supporting cells.
Utricles and saccules are called otolith organs, they transduce translational (linear) accelerations acting on the head. The sensory epithelium is covered by a gelatinous otolithic membrane, which in turn is covered by a layer of crystals called statoconia or otoliths. In mammals, the otoconium containing otoliths consists of a core of glycoprotein/proteoglycan surrounded by a mineral coat of thousandscalcium carbonate crystalloids embedded in the calcite lattice. The human otolithic membrane is about 20 µm thick and shows regional diversity. Below is the macula, which has a narrow central strip called striole, where sensory hair cells exhibit distinct features, morphology, orientation specificity, and connectivity. The otoliths are thickest in the striolar region, where the polarity of the hair cell bundles is reversed.
Endolymph drains from the saccule and flows into the endolymphatic canal. The canal passes through the vestibular aqueduct to the posterior region of the petrous part of the temporal bone. Here the channel expands to a sac where endolymph can be secreted and reabsorbed.
Structure
The walls of utricles, sacs and semicircular ducts consist of three layers:
- The outer layer is a loose and flocculent structure, consisting of a normal fibrous tissue containing blood vessels and some pigment cells.
- The middle layer, thicker and more transparent, forms a homogeneous membrane proprie and presents on its inner surface, especially in the semicircular ducts, numerous papillary protrusions.
- Inner layer formed by polygonal germinal epithelial cells.
In the maculae (spots) of the utricle 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 haircells. The first are spindle-shaped, their deep ends are attached to the membrane, and the free limbs are combined. The hair cells are flask-shaped, their rounded ends lie between the supporting cells. The deep part of each contains a large nucleus, and the surface part is granular and pigmented. Acoustic nerve filaments enter these parts and pass through the outer and middle layers.
Membranous snail
The cochlear duct consists of a spirally arranged tube enclosed in the bony canal of the cochlea and lying along its outer wall.
The bony spiral lamina extends only 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 bony spiral 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 formed by the periosteum lining the bone canal, and the bottom is formed by the basilar membrane and the outer part of the spinal disc.
The vestibular membrane is thin and homogeneous, covered with a layer of epithelium. The periosteum, which forms the outer wall of the duct, is strongly thickened and changed in character.
The bony spiral plate of the membranous labyrinth of the ear divides the spiral canal into two parts.
Basal Membrane
It extends from the tympanic lip of the bony spiral plate to the spiral crest and consists of two parts: internal and external. The inside is thin and contains the spiral organ of Corti.
Spiral Organ of Corti
This part of the membranous labyrinth of the inner ear consists of a series of epithelial structures located on the inside of the basilar membrane. Central among these structures are two rows of fibers, internal and external, or Korti pillars. The bases of the fibers are supported on the basement membrane, and the inner ones are at some distance from the outer ones; two rows lean towards each other and, touching at the top, form a triangular tunnel between them and the basement membrane, the tunnel of Corti. 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, along with supporting cells, which are called Deiters and Hansen cells. All this is the receptor department of the auditory analyzer.
