Ear Anatomy PDF

II. EAR The ear is the organ responsible for the reception, transmission and transformation of acoustic waves towards the central nervous system where the cerebral auditory centres are located, as well as information about the animal spatial situation through the stimulation of a neuroepithelium. Fo...

II. EAR The ear is the organ responsible for the reception, transmission and transformation of acoustic waves towards the central nervous system where the cerebral auditory centres are located, as well as information about the animal spatial situation through the stimulation of a neuroepithelium. For both functions there are different specific structures integrated and distributed in three anatomical areas: the external (outer), middle and internal (inner) ear. A B C Figure 3. Inner ear diagram (A). Tympanic membrane (B). Malleus, incus and stapes (C). 1. External ear The external ear is made up of the auricle, the external auditory canal, and the tympanic membrane (also called eardrum). It captures sounds from outside through the auricle and transports them through the auditory canal as acoustic waves to the eardrum. The acoustic wave transforms into a mechanical wave upon impact with the tympanic membrane. Auricle The auricle or pinna is made up of a central body of elastic cartilage tissue, covered externally and internally by skin. On the concave or external face there are numerous sebaceous and merocrine tubular glands. The function of the pinna is to capture acoustic waves as a screen. External auditory canal The external auditory canal or external auditory meatus begins in the ear and extends to the tympanic membrane. The length of this structure depends on the animal species, as well as the sinuosity of the light, which is greater in the dog, while in the pig it is very uniform. It is lined by skin with small hair follicles, sebaceous glands and ceruminous glands (modified apocrine sweat glands). It is mostly surrounded by elastic cartilage. Cerumen is a combination of the secretions of ceruminous glands, sebum and sloughed epithelial cells. 7 Tympanic membrane The auditory canal is closed internally by the tympanic membrane, which connects the external ear with the middle ear. It is made up of three thin layers of connective tissue: two peripheral layers where the collagen fibers are oriented radially and a central layer where the collagen fibers are oriented concentrically. The dorsal part, known as flaccid part, only contains a single layer of loose connective tissue. The membrane is lined externally by a keratinized squamous stratified epithelium and internally by a simple squamous epithelium. The tympanic membrane vibrates thanks to the acoustic waves due to its three collagen membranes and this movement ceases when the vibration reaches the flaccid part. The vibrations are transmitted to the middle ear and specifically to the chain of ossicles, which rest on the tympanic face of the membrane. 2. Middle ear The middle ear is located in a bony chamber whose walls are the tympanic membrane, the anterior part of the internal ear and the temporal bone. This cavity is filled with air coming from the nasopharynx through the Eustachian or auditory tube, which maintains its pressure and is lined by a respiratory epithelium. In horse the tube presents a ventral expansion called guttural pouch. The walls of the cavity are lined by a simple squamous or ciliated columnar pseudostratified epithelium. The area related to the nasopharynx is usually free of glands and only in carnivores and sheep there are serous glands. The main elements of the middle ear are a chain of small ossicles, called: the malleus, the incus and the stapes. These bones are articulated with each other and in turn associated with muscle tissues. The bone chain is related through the malleus to the tympanic membrane, which collects the vibrations of said membrane and translates them into a movement, which passes through the incus and this in turn to the stapes. Finally, the stapes hits the membrane that lines the oval window of the internal ear. 3. Internal ear The inner ear is the portion where the auditory and spatial sensations are collected. For this, complex structures are differentiated, which are found in the so-called osseous labyrinth and membranous labyrinth. The osseous labyrinth is composed of a system of cavities and channels excavated in the petrous portion of the temporal bone, inside which there are a set of membranous ducts and cavities called the membranous labyrinth, of very similar morphology, where the sensory organs are located. Between the osseous labyrinth and the membranous is the perilymphatic space, which is filled with perilymph. The ducts and cavities of the membranous labyrinth are filled with 8 endolymph. The osseous labyrinth of the inner ear includes the vestibule, the semicircular canals and the cochlea. Cochlea (hearing) Inside it, and specifically in the spiral channel of the cochlea, the cochlear duct is housed. The spiral channel is formed between the external wall of the osseous cochlea (periostatic wall) and the modiolus, the modiolus being an internal bony axis or column that longitudinally crosses the entire cochlea and from which bony spiral laminae project laterally. The tympanic wall of the cochlear duct is attached to those spiral laminae, which form its floor, where the spiral organ or organ of Corti is located. The cochlear duct, which describes a coniform spiral around the modiolus, is protected within the spiral canal of the cochlea by two perilymph-filled ramps or scales, the scala vestibuli and the scala tympani. The scala vestibuli contacts the vestibular or oval window and the scala tympani contacts the round or cochlear window. Both ramps communicate through a bony opening called the helicotrema, located in the dome of the osseous cochlea coinciding with the end of the cochlear duct. The scala vestibuli has a spherical cap shape in cross section. It is lined by a simple cuboidal or squamous epithelium whose cells present numerous folds on the apical and basal surfaces. Under the epithelium there is a basement membrane and under it appears fibrous tissue that constitutes the so-called Reissner's membrane. This membrane constitutes the roof of the cochlear duct, which is lined by a simple squamous epithelium. The scala tympani is shaped like an inverted spherical cap and forms the floor of the cochlear duct. It is lined by a simple squamous epithelium. The section of the cochlear duct is shaped like a right triangle. Its acute angle contacts the modiolus and is called the tympanic lip of the spiral limbus. The epithelium that lines the lesser leg is called stria vascularis and is a stratified epithelium in which numerous capillaries are arranged. This structure participates in the production and metabolism of endolymph. The stria vascularis continues with the spiral prominence, constituted by an epithelium with two layers of cells with cubical morphology called Claudius cells and Böttcher cells. The spiral organ or organ of Corti is located on the floor of the cochlear duct and is made up of an epithelial complex with two types of cells: neuroepithelial cells and supporting cells. The supporting cells are represented by five types: cells of Hensen, Deiter or outer phalangeal cells, inner phalangeal cells, pillar cells and marginal cells. Neuroepithelial cells are of two types: outer hair cells that are arranged in three rows and inner hair cells of which there is only a single row. The cells of Hensen are tall columnar, rest on the basement membrane, have few organelles and the nucleus in the basal third. These cells will define the 9 outer tunnel along with the outer phalangeal cells. The outer phalangeal cells (Dieter cells) are very elongated, with a basal nucleus and abundant cytoplasmic organoids. They have an excavation in the apical area in which the corresponding hair cell is located. The inner phalangeal cells are very similar to the outer phalangeal cells and house the inner hair cells in the apical area. The cells of the pillars (outer and inner) will configure the inner tunnel. They are triangular cells with a wide base located on the basement membrane and a thin conical body. The cells of the outer pillar are attached to the hair cells. The marginal cells are columnar cells that form a row on the tympanic lip of the limbus. A B Figure 4. Scheme of cochlear scales or ramps (A). Organ of Corti (B). Stereocilia under a scanning electron microscope (C). C The neuroepithelial cells of the organ of Corti are the hair cells. They are transformed neurons. They present abundant stereocilia in their apical portion and are located embedded in the apical cavities of the phalangeal cells. In general, they are elongated columnar cells with an euchromatic nucleus in the basal zone and acidophilic cytoplasm. The stereocilia of the outer hair cells are distributed in a W manner and those of the inner ones in a V pattern. The stereocilia are coupled to the tectorial membrane and are housed in small notches in which small microfilaments that form a joint complex are differentiated. Laterally, the hair cells are attached by thick desmosomes to the phalangeal cells and in the apical zone the cytoplasm is very electrodense where the microfilaments rest. Immediately below are large clusters of mitochondria and numerous smooth endoplasmic reticulum cisterns arranged in the vicinity of the cytoplasmic membrane. The basal areas are associated with presynaptic bulbs, which terminate in ganglion cells. The tectorial membrane is formed from interdental cells and is made up of homogeneous, medium-density keratinized layers associated with each other. In the innermost layer they show a series of excavations to relate to the stereocilia. In close relationship with the bone tissues of the modiolus, large ganglion cells accumulate, which will form the fibers of the acoustic nerve. They are spherical cells surrounded by satellite cells. The excitation of the hair cells comes from the capture of acoustic waves and their action on the tympanic membrane producing vibrations. The vibrations 10 give rise to mechanical movements that are transmitted to the chain of ossicles of the middle ear, that hit the membrane of the oval window. The vibrations are then transmitted to the perilymph of the scala vestibuli that communicates with the scala tympani through the helicotrema, to terminate the vibratory wave by contacting the membrane that lines the round window. When the acoustic wave passes through the scala vestibuli, impinges on Reissner's membrane and is transmitted to the cochlear duct, hitting the external face of the tectorial membrane, which in turn ends up hitting the stereocilia of the Corti cells. The movement of the perilymph also affects the basement membrane of the organ of Corti in the scala tympani and, in this way, it would be the hair cells that would strike with their stereocilia on the tectorial membrane. Vestibule and semicircular canals (balance) Vestibule is a hollow carved in the petrous portion of the temporal bone where two cavities or membranous diverticula are housed: the utricle and the saccule. They have specialized regions of the epithelium that act as receptors, which are called maculae. The macula saculi is hook-shaped and oriented vertically when the head is in its normal position. The macula utriculi is kidneyshaped and its orientation is horizontal when the head is in its normal position. The maculae are made up of neuroepithelial cells, supporting cells and a gelatinous cupula to which calcified pieces (otoliths or statoconia) are attached. Neuroepithelial cells are of two types: ciliated type I and type II. Type I cells are distributed within the neuroepithelium in an isolated manner and have globose (bottle) shapes with a narrowing in the apical area, where highly developed type 92+2 cilia are located accompanied by abundant stereocilia, which gradually decrease in length. The cells of the type II have a more columnar morphology and at the apical portion also have cilia 92 +2 large proportions and many stereocilia of the same characteristics of the type I cell. Overlying the entire neuroepithelium it is present the gelatinous cupula formed by glycoproteins synthesized by the supporting cells. This substance is on top of the epithelium and immersed in it the cilia and stereocilia of the ciliated cells are arranged. Statoconia or otoliths are deposited in the apical area of the cupula, forming the statoconial membrane. Otoliths are polyhedral concretions of different sizes of calcium carbonate. Semicircular canals originate from the vestibule and host the membranous semicircular canals. These ducts, in their junction area with the utricle, present dilations called membranous ampullae. The neuroepithelia that appear at the level of the ampullae are called cristae ampullares. The cristae ampullares are located in a thickening of connective tissue and its structure is similar to that of maculae. The neuroepithelium is made up of hair cells and supporting cells (similar to those of maculae) on which a gelatinous cupula appears that crosses the ampulla and contacts the opposite wall. 11