REGIONS of the ear The ear has three distinct regions -- outer ear, middle ear, and inner ear. The outer ear includes the pinna (the visible ear, consisting mostly of skin and cartilage) and the ear canal. The latter is lined by keratinized stratified squamous epithelium. This lining differs from skin by the presence of specialized ceruminous (ear-wax) glands. The middle ear is basically a space, communicating via the eustacian tube with the oropharynx. It is lined by a very thin non-keratinized stratified squamous epithelium. Spanning the space of the middle ear are the three middle ear bones, the malleus (hammer), incus (anvil), and stapes (stirrup). The eardrum is a thin membrane separating the outer ear from the middle ear. It is sandwich of tissues, with keratinized stratified squamous epithelium facing the outer ear, non-keratinized stratified squamous epithelium facing the middle ear, and a very thin layer of connective tissue in between. The inner ear is the portion of the ear which contains sensory receptors. The remainder of this study guide describes the inner ear. Cochlear Fluids Laboratory, Washington University; used with permission. The inner ear, located within the bony labyrinth, contains sense organs serving both balance and hearing. Head position (i.e., gravity; also linear acceleration) is sensed by the otolith organs of the sacculeand utricle. Head rotation (i.e., angular acceleration) is sensed by the cristae ampularis of the semicircular canals. Hearing is sensed by the organ of Corti within the scala media of the cochlea. All of these several senses of the inner ear utilize the same mechanoreceptor cell type, epithelial hair cells. Hair cells are housed within an elaborately-shaped chamber called the membranous labyrinth. The membranous labyrinth is filled with a unique fluid called endolymph, secreted by cells of the stria vascularis. Endolymph differs substantially from all other fluids of the body and provides a special fluid environment for the hair cells The membranous labyrinth includes interconnection among the cochlea, saccule, utricle, and semicircular canals. The membranous labyrinth is housed within the bony labyrinth. Perilymph fills the space of the bony labyrinth around the membranous labyrinth. Cochlear Fluids Laboratory, Washington University; used with permission. The inner ear resides within a space called the bony labyrinth. The oval window forms a potential opening from the middle ear into the bony labyrinth. The stapes of the middle ear plugs this opening; but . . . The stapes is flexibly attached and can vibrate to transmit pressure waves to the fluid that fills the bony labyrinth. (Sound is carried from the eardrum across the middle ear by the three middle ear ossicles, ending with the stapes at the oval window.) Suspended within the bony labyrinth, and approximating its shape, is an interconnected set of membrane-lined chambers and passageways called the membranous labyrinth. In the diagram, the color orange occupies the space of the bony labyrinth, while the membranous labyrinth is blue. The name labyrinth suggests the complex shape of these chambers and passageways. The vestibule is (logically enough) the "entry room" into the deeper passageways. The vestibule of the bony labyrinth contains the saccule and utricle of the membranouslabyrinth; Three semicircular canals comprise looping tubules which leave and return to the vestibule. Within each semicircular canal of the bony labyrinth is a semicircular canal of the membranous labyrinth. The cochlea is shaped like a snail-shell which spirals away from the vestibule. A single coiled tunnel of the bony labyrinth is subdivided into three levels ("scalae") by membranes of the membranous labyrinth. The portion of the membranous labyrinth within the cochlea is called the scala media. Inside the membranous labyrinth is a unique fluid called endolymph. Surrounding the membranous labyrinth (i.e., filling the remaining space of the bony labyrinth) is a fluid called perilymph. endolymph, with a unique ionic composition. The process of sensory transduction in hair cells (i.e., the conversion of an external stimulus, in this case small movement, into neural activity) has been intensively investigated. For detailed information, consult your print resources (e.g., Kandel at al.) The mechanical disposition of the jelly in relation to the spaces of the membranous labyrinth determines how hair cells respond. Hair cells in the semicircular canals respond to angular acceleration (rotation). Hair cells in the in the otolith organs respond to linear acceleration. Hair cells in the organ of Corti of the cochlea respond to sound. The saccule and utricle contain patches of hair cells called maculae ("macula" means "spot" or "patch"). A small mass of jelly rests on top of the hair cells of the macula. In this jelly are numerous tiny mineral concretions, called otoliths ("earstones" or "earsand"). Clinical Note: If any otoliths break loose (e.g., due to head trauma), they may come to rest in an inappropriate place, stimulating the hair cells in a semicircular canal) and cause disturbance in balance (see benign positional vertigo). Hair cells of the macula are deflected by the weight or inertia of the otoliths. Together the two pairs of otolith organs (one of each in each ear) can sense head orientation (gravity) or linear acceleration in any direction. Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission. Each semicircular canal of the bony labyrinth is a hollow passageway looping out from and back to the vestibule. Within each of these passageways is asemicircular canal of the membranous labyrinth. At one end of each membranous semicircular canal is a small enlargement called the ampulla. Within each ampulla is a ridge or "crest" called the crista. The crista is covered with hair cells. A small mass of jelly, called the cupola ("cap") rests on top of the hair cells of the crista. Hair cells of the ampullae respond to angular acceleration (i.e., rotation of the head). There are three semicircular canals in each ear, oriented in three mutually-perpendicular planes. Rotation of the head in any direction will cause inertial fluid movement in one or more of the semicircular canals. Fluid motion in a semicircular canal pushes the the cupola like a swinging door. Movement of the cupola in turn deflects the projections of the hair cells. Clinical Note: Should loose otoliths enter a semicircular canal, they may stimulate the hair cells inappropriately and cause disturbance in balance (see benign positional vertigo). The planes of orientation of the semicircircular canals correspond to the planes of action of the extraocular muscles, allowing simple reflexes to coordinate eye movement with head rotation. Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission. COCHLEA The cochlea houses an elaborate configuration of membranous labyrinth and hair cells, called the organ of Corti, designed for auditory reception. The basic shape of the cochlea is that of a snail-shell, or tapering helix. The human cochlea is short and broad; micrographs at this website (and in many other references) show the cochlea of a laboratory rodent which is proportionately taller and narrower. The spiraling tunnel (blue, in image at right) that forms the cochlea of the bony labyrinth is divided into three distinct channels by portions of the membranous labyrinth attached to bony ridges. Each of these channels is called a "scala", meaning "ramp" or "incline" (think of a musical "scale"). The scala vestibuli ascends from the vestibule (hence vestibuli in the name) to the tip of the cochlea. The scala vestibuli contains perilymph. The scala tympani descends from the tip of the cochlea to the round window. (There is an elastic energy-dissipating membrane covering the round window (hence tympani in the name). The scala tympani, like the scala vestibuli, contains perilymph. At the tip of the cochlea, the scala vestibula and the scala tympani are connected through the helicotrema. Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission. The scala media, also called the cochlear duct, lies along the length of spiral cochlea, in a "medial" position between the scala vestibuli and scala tympani. The scala media contains endolymph. The organ of Corti lies within the scala media. The scala media is separated from the scala vestibuli by the very thin Reissner's membrane. The scala media and the scala tympani are separated by the basilar membrane. Clinical note: A cochlear implant is inserted into the scala tympani, where it lies close to the organ of Corti and can artificially stimulate axons of the auditory nerve. The central column (the modiolus) of the helical cochlea contains axons serving the organ of Corti on their way to the auditory nerve. A bony ridge, the spiral lamina, extends out from the modiolus and provides support for the organ of Corti. A tubular cavity within the spiral lamina contains the cell bodies of the axons of the auditory nerve. Because this collection of nerve cell bodies has a helical shape paralleling the cochlear scalae, it is called the spiral ganglion. Anatomy and pathology of organ of Corti (Bohne laboratory, Washington University). The organ of Corti is contained within the scala media. The organ of Corti is a long strip of tissue that extends the length of the scala media, from the base of the cochlea to its apex. The organ of Corti is usually illustrated in cross-section. Tissue sections of the cochlea typically contain several appearances of the organ of Corti, as the organ is sliced in each turn of the helix. The fluid environment for the organ of Corti is endolymph, which fills the scala media. (Endolymph is secreted by cells of the stria vascularis.) Within the complex strip of tissue that comprises the organ of Corti are specialized sensory hair cells. The entire complex (the whole organ of Corti) rests on the basilar membrane. This basilar membrane supports the basal ends of the hair cells in the organ of Corti. The apical ends of hair cells touch the tectorial membrane, a "shelf" of jelly that is supported immovably on the spiral lamina. When the basilar membrane flexes in respond to sound waves (i.e., pressure waves delivered to inner-ear fluid by the middle-ear ossicles), the organ of Corti, including its hair cells, also moves. Thus, when the basilar membrane is moved by pressure waves (i.e., sound), the hair cells move relative to the tectorial membrane, causing stimulatory deflection of the apical ends of the hair cells. Clinical note: A cochlear implant is inserted into the scala tympani, where it lies close to the organ of Corti and can artificially stimulate axons of the auditory nerve. (more) The organ of Corti is considerably more complex than this simple account implies, with, among other things, two functionally distinct classes of hair cells (inner andouter). Synapses from the inner hair cells apparently supply most of the sensory information that goes to the brain, while the outer hair cells (the ones which are most readily recognized by light microscopy) have a curious mechanical function (for more, see for example How the Ear Works). ENDOLYMPH and PERILYMPH The membranous labyrinth is filled with endolymph and surrounded by perilymph. Endolymph (blue, in image at right) is a unique fluid, with high K+ concentration and very low Na+ concentration. Thisendolymph provides the proper ionic environment for hair cell function. Endolymph is secreted by cells of the stria vascularis, along the scala media of the cochlea. The stria vascularis resembles a stratified cuboidal epithelium, but unlike any proper epithelium (and as the name vascularis indicates) this tissue contains capillaries among the cuboidal cells. Perilymph (orange, in image at right) is similar to ordinary interstitial fluid. Perilymph fills the spaces of the bony labyrinthsurrounding the membranous labyrinth. In the vestibular system (surrounding the saccule, utricle, and semicircular canals), perilymph simply provides a cushioning support for the membranous labyrinth. In the cochlea, perilymph of the ascending scala vestibuli and the descending scala tympani conveys pressure waves (sound) across the scala media. Pressure waves flex the basilar membrane and thereby stimulate hair cells of the organ of Corti. Source
