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Lecture notes, cheat sheets
Histology. The structure of the organ of hearing and balance
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 29. STRUCTURE OF THE ORGANS OF HEARING AND BALANCE Development of the organ of hearing and balance In a 22-day-old embryo at the level of the rhomboid brain, paired thickenings of the ectoderm appear - auditory placodes. By invagination and subsequent separation from the ectoderm, the auditory vesicle is formed. On the medial side, the rudimentary auditory ganglion is adjacent to the auditory vesicle, from which the ganglion of the vestibule and the ganglion of the cochlea subsequently differentiate. As it develops, two parts appear in the auditory vesicle - an elliptical sac (utriculus with semicircular canals) and a spherical sac (sacculus) with the rudiment of the cochlear canal. The structure of the organ of hearing The outer ear includes the auricle, the external auditory meatus and the tympanic membrane, which transmits sound vibrations to the auditory ossicles of the middle ear. The auricle is formed by elastic cartilage covered with thin skin. The external auditory meatus is lined with skin containing hair follicles, typical sebaceous glands, and ceruminous glands, modified sebaceous glands that produce earwax. The outer surface of the eardrum is covered with skin. From the inside, from the side of the tympanic cavity, the tympanic membrane is lined with a single-layer cubic epithelium, which is separated from the outer layer by a thin connective tissue plate. The middle ear contains the auditory ossicles - the hammer, anvil and stirrup, which transmit vibrations from the tympanic membrane to the membrane of the oval window. The tympanic cavity is lined with stratified epithelium, which turns into a single-layer cylindrical ciliated at the opening of the auditory tube. Between the epithelium and the bone is a layer of dense fibrous connective tissue. The bone of the medial wall of the tympanic cavity has two windows - oval and round, which separate the tympanic cavity from the bony labyrinth of the inner ear. The inner ear is formed by the bony labyrinth of the temporal bone, which contains a membranous labyrinth that repeats its relief. Bone labyrinth - a system of semicircular canals and a cavity that communicates with them - the vestibule. The membranous labyrinth is a system of thin-walled connective tissue tubes and sacs located inside the bony labyrinth. In the bone ampullae, the membranous canals expand. In the vestibule, the membranous labyrinth forms two interconnected sacs: the ulus (elliptical sac), into which the membranous canals open, and the sacculus (spherical sac). The membranous semicircular canals and sacs of the vestibule are filled with endolymph and communicate with the cochlea, as well as with the endolymphatic sac located in the cranial cavity, where the endolymph is resorbed. The epithelial lining of the endolymphatic sac contains cylindrical cells with dense cytoplasm and irregularly shaped nuclei, as well as cylindrical cells with light cytoplasm, high microvilli, numerous pinocytic vesicles and vacuoles. Macrophages and neutrophils are present in the lumen of the sac. The structure of the snail. The cochlea is a spirally twisted bony canal that developed as an outgrowth of the vestibule. The cochlea forms 2,5 whorls about 35 mm long. The basilar (basic) and vestibular membranes located inside the cochlear canal divide its cavity into three parts: the scala tympani, the scala vestibularis, and the membranous cochlear canal (the middle scala or cochlear duct). Endolymph fills the membranous canal of the cochlea, and perilymph fills the vestibular and tympanic scala. The scala tympani and the vestibular scala communicate at the top of the cochlea through an opening (helicotrema). In the membranous canal of the cochlea on the basilar scala there is a receptor apparatus - a spiral (or Corti) organ. The concentration of K+ in the endolymph is 100 times greater than in the perilymph; Na+ concentration in endolymph is 10 times less than in perilymph. Perilymph is close in chemical composition to blood plasma and siuid and occupies an intermediate position between them in terms of protein content. The structure of the organ of Corti. The organ of Corti contains several rows of hair cells associated with the tectorial (integumentary) membrane. There are inner and outer hair and supporting cells. Hair cells - receptor, form synaptic contacts with peripheral processes of sensory neurons of the spiral ganglion. Internal hair cells form one row, have an expanded base, 30-60 immobile microvilli (stereocilia) passing through the cuticle in the apical part. Stereocilia are located in a semicircle, open towards the external structures of the organ of Corti. Inner hair cells are primary sensory cells that are excited in response to a sound stimulus and transmit excitation to the afferent fibers of the auditory nerve. Displacement of the integumentary membrane causes deformation of stereocilia, in the membrane of which mechanosensitive ion channels open and depolarization occurs. In turn, depolarization promotes the opening of voltage-sensitive Ca2+ and K+ channels embedded in the basolateral membrane of the hair cell. The resulting increase in the concentration of Ca in the cytosol2+ initiates secretion (most likely glutamate) from synaptic vesicles with its subsequent action on the postsynaptic membrane as part of the afferent terminals of the auditory nerve. The outer hair cells are arranged in 3-5 rows, have a cylindrical shape and stereocilia. Myosin is distributed along the stereocilia of the fibrous cell. supporting cells. Supporting cells include inner phalangeal cells, inner pillar cells, outer phalanx cells of Deiters, outer pillar cells, Hensen cells, and Boettcher cells. The phalangeal cells come into contact with the hair cells on the basement membrane. The processes of the outer phalangeal cells run parallel to the outer hair cells, without touching them for a considerable length, and at the level of the apical part of the hair cells come into contact with them. Supporting cells are connected by gap junctions formed by the gap junction membrane protein connexin-26. Gap junctions are involved in restoring the level of K+ in the endolymph during trace reactions after excitation of hair cells. The way of transmission of auditory irritation The sound pressure transmission chain is as follows: the tympanic membrane, then the auditory ossicles - the hammer, anvil, stirrup, then - the oval window membrane, the perilymph basilar and tectorial membranes and the round window membrane. When the stirrup is displaced, the particles of relymph move along the vestibular scala and then through the helicotrema along the scala tympani to the round window. The fluid shifted by the displacement of the membrane of the foramen ovale creates excess pressure in the vestibular canal. Under the influence of this pressure, the basal part of the main membrane will be mixed towards the scala tympani. An oscillatory reaction in the form of a wave propagates from the basal part of the main membrane to the helicotrema. The displacement of the tectorial membrane relative to the hair cells under the action of sound causes their excitation. The displacement of the membrane relative to the sensory epithelium deflects the stereocilia of the hair cells, which opens mechanosensing channels in the cell membrane and leads to cell depolarization. The resulting electrical reaction, called the microphone effect, follows the shape of the audio signal in its form. The structure and functioning of the organ of balance In the ampullar extension of the semicircular canal are cristae (or scallops). The sensitive areas in the sacs are called patches. The composition of the epithelium of spots and cristae includes sensitive hair and supporting cells. In the epithelium of spots, kinocilia are distributed in a special way. Here the hair cells form groups of several hundred units. Within each group, the kinocilia are oriented in the same way, but the orientation of the groups themselves is different. The epithelium of the spots is covered with an otolithic membrane. Otoliths are crystals of calcium carbonate. The epithelium of the cristae is surrounded by a gelatinous transparent dome. Hair cells are present in each ampulla of the semicircular canals and in the maculae of the sacs of the vestibule. There are two types of hair cells. Type I cells are usually located in the center of the scallops, while type II cells are located at the periphery. Cells of both types in the apical part contain 40-110 immobile hairs (stereocilia) and one cilium (kinocilium) located on the periphery of the bundle of stereocilia. The longest stereocilia are located near the kinocilium, while the length of the rest decreases with distance from the kinocilium. Hair cells are sensitive to the direction of the stimulus (direction sensitivity). When the stimulus is directed from the stereocilium to the kinocilium, the hair cell is excited. With the opposite direction of the stimulus, the response is suppressed. Type I cells are amphora-shaped with a rounded bottom and housed in the goblet cavity of the afferent nerve ending. Efferent fibers form synaptic endings on afferent fibers associated with type I cells. Type II cells have the form of cylinders with a rounded base. A characteristic feature of these cells is their innervation: the nerve endings here can be both afferent (most) and efferent. With superthreshold sound stimulation (acoustic trauma) and under the action of certain ototoxic drugs (antibiotics streptomycin, gentamicin), hair cells die. The possibility of their regeneration from progenitor cells of the neurosensory epithelium is of great practical importance; it is considered established for birds and intensively studied in mammals. The vestibular nerve is formed by processes of bipolar neurons in the vestibular ganglion. The peripheral processes of these neurons approach the hair cells of each semicircular canal, utriculus and sacculus, and the central ones go to the vestibular nuclei of the medulla oblongata. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Organs of taste and smell >> Forward: Organs of hematopoiesis and immunological protection
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