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EARS: THE VESTIBULOAUDITORY SYSTEM The ear is a three-chambered sensory structure has two main systems; the auditory system, for the perception of sound, and the vestibular system for the maintenance of balance. The three divisions of the ear are the external, the middle, and the inner (internal) ear, within which the auditory and vestibular systems are located. The sensory receptors transduce the sound waves to electrical impulse in the first, and to maintain equilibrium in the second. EXTERNAL EAR The External Ear is composed of an Auricle and an External Auditory Meatus. The auricle (pinna) is oval-shaped, composed of elastic cartilage with tightly adhering thin skin with hair follicles, sebaceous glands, and sweat glands. The pinna is an essential component in sound localization and amplification. The external auditory meatus is an air-filled tubular space that follows slightly S-shaped course for about 25 mm to the tympanic membrane (eardrum). The wall of the lateral one third is cartilaginous and is continuous with the elastic cartilage of the auricle. The medial two thirds of the canal is contained within the temporal bone. The lateral part of the canal is lined by skin that contains hair follicles, sebaceous glands, and ceruminous glands. The secretion of the ceruminous glands mixes with the sebaceous glands and desquamated cells to form the cerumen or earwax. The cerumen is a brownish semisolid mixture of fats and waxes, lubricates the skin and impedes the entry of foreign particles. The tympanic membrane is composed of tough connective tissue contains collagen, elastic fibers, and fibroblasts. Its external surface is covered with a thin layer of epidermis, and its inner surface is covered with simple cuboidal epithelium. MIDDLE EAR The middle ear, or the tympanic cavity, is an air-filled space within the temporal bone between the tympanic membrane and the bony surface of the internal ear. It contains three small bones, the auditory ossicles, connected by two movable joints. These bones are o Malleus (hammer), attached to the tympanic membrane o Stapes (Stirrup), whose footplate fits into the oval window o Incus (anvil), linking the malleus to the stapes These ossicles cross the space in the middle ear and connect the tympanic membrane to the oval window. These bones convert sound waves in the air to mechanical (hydraulic) vibrations in tissues and fluid filled chambers. Anteriorly, the middle ear communicates with the pharynx via the auditory tube (Eustachian tube), posteriorly with the air-filled cavities of the mastoid process of the temporal bone. The middle ear is bound laterally by the tympanic membrane and medially by the bony wall of the internal ear which contains two membranecovered openings, the oval window and the round window. In the middle ear, two muscles, tensor tympani and stapedius, those are inserted into the stapes and malleus. The middle ear is lined with simple squamous epithelium, near the auditory tube and in its interior; the simple squamous is transformed gradually into ciliated pseudostratified columnar epithelium INTERNAL EAR The internal (inner) ear is located completely in the temporal bone, where the bony labyrinth (a complicated set of spaces) houses the smaller epithelium-lined, fluid filled spaces and tubes, the membranous labyrinth. The membranous labyrinth: lies within the bony labyrinth, it is divided into two major divisions Utricle Semicircular ducts The membranous labyrinth Saccule Cochlear duct The semicircular ducts originate from the utricle, while the cochlea containing the cochlear duct, connected to the saccule, and provides for hearing In each of these structures the epithelial lining contains large areas of columnar sensory mechanoreceptors called the hair cells in the following regions i. Two maculae ii. Three cristae ampullaris in the enlarged ampullary region of each semicircular duct Long spiral organ of Corti in the cochlear duct iii. in the utricle and saccule The bony labyrinth consists of spaces in the temporal bone, has the irregular central cavity, the vestibule within which the saccule and the utricle are enclosed. Behind this, three semicircular canals enclose the semicircular ducts; the anterolateral cochlea contains the cochlear duct Semicircular canals enclose the semicircular ducts vestibule, Cochlea containing the cochlear duct The cochlea (L.cochlea, snail, screw) about 35 mm in total length, makes two-andone-half turns around a bony core known as the modiolus. The modiolus has spaces containing blood vessels and the cell bodies and processes of the acoustic branch of the eighth cranial nerve (spiral ganglion). Extending laterally from the modiolus is a thin bony ridge, the osseous spiral lamina. The bony labyrinth is filled with perilymph, which is similar in ionic composition to cerebrospinal fluid and the extracellular fluids elsewhere but has very low protein content. The perilymph emerges from the blood vessels of the periosteum and drained by a perilymphatic duct into the subarachnoid space. The membranous labyrinth contains endolymph, which is characterized by its low sodium (16mM) and high potassium (150mM) content. The protein concentration in endolymph is low, similar to the intracellular fluid. Endolymph is generated by capillaries in the stria vascularis in the wall of the cochlear duct and is drained from the vestibule into the venous sinuses of the dura mater by the endolymphatic duct. SACCULE AND UTRICLE The saccule and the utricle are composed of a very thin connective tissue sheath lined with simple squamous epithelium. The two maculae in the wall of the saccule and utricle are small sensory areas of columnar neuroepithelial cells innervated by branches of the vestibular nerve. Both maculae are similar histologically and contain mechanoreceptor cells (hair cells) which use gravity and endolymph movement to detect the orientation of the stationary head and linear acceleration of the moving head. The macular wall is composed of hair cells, supporting cells, and endings of the vestibular branch of the eighth cranial nerve. The apical surface of the hair cells is covered by a gelatinous otolithic layer or membrane (contains otoliths which are crystalline structures). The hair cell contains numerous straight stereocilia and a longer single kinocilium, a modified cilium whose tip may be slightly enlarged. SEMICIRCULAR DUCTS The three semicircular ducts are parts of the membranous labyrinth having the same general form as the semicircular canals in the bony labyrinth. Each extends from and returns to the wall of the utricle. They lie in three different spatial planes at right angles to one another; their receptor areas are present in their ampullae and are called cristae ampullaris. Cristae are histologically similar to maculae, with hair cells, supporting cells, and nerve endings, with proteoglycan layer called the cupula. The cupula here is thick, and attached to the sensory cells' hair bundles. It does not have otoliths and extends completely across the ampulla. COCHLEAR DUCT AND AUDITORY FUNCTIONS The cochlear duct, connected to the saccule, is the specialized sound receptor. It is about 35mm long and is surrounded by the perilymphtic spaces. In histological sections, the cochlea contains three spaces: the scala vestibule, the middle cochlear duct (or scala media), and the scala tympani. The cochlear duct contains edolymph and ends at the apex of the cochlea. The other two spaces contain perilymph and they are considered one long tube, beginning in the oval window and ending in the round window. They communicate at the apex of the cochlea via the opening helicotrema. The cochlear duct is separated from the scala vestibule by the vestibular membrane. It consists of basement membrane with simple squamous epithelium on both sides. The epithelial cells have tight junctions to maintain the ionic gradient between perilymph and endolymph. In the lateral wall of the cochlear duct is the stria vascularis, an epithelium for the production and maintenance of the endolymph for the entire membranous labyrinth. Stria vascularis encloses a network of capillaries from which K ions and fluids are released. The spiral organ (organ of Corti) lies in the wall that separates the cochlear duct from the scala tympani. The spiral organ rests on a thick basal lamina- the basilar membrane. Two types of hair cells are present, outer hair cells OHC and inner hair cells IHC, both having stereocilia. No kinocilium is present in cochlear hair cells. The OHC are embedded in the tectorial membrane, an acellular layer extending over the spiral organ from the modiolus. The cell bodies of the afferent bipolar neuron are located in the bony core of the modiolus, the spiral ganglion. Two major types of columnar supporting cells, Pillar cells, and Phalangeal cells. The pillar cells contain large number of microtubules and keratin, they outline a triangular tunnel-like space between the outer and inner hair cells. Endolymphatic Duct and Sac The endolymphatic duct initially has a simple squamous epithelial lining. As it nears the endolymphatic sac, it gradually changes to tall columnar epithelium composed of two cell types; one of these cell types has microvilli on its apical surface and abundant pinocytotic vesicles and vacuoles. These cells may be responsible for the absorption of endolymph and for the endocytosis of foreign material and cellular remnants that may be present in endolymph.