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The Ear(See Figure 16.25 and Labeled Ear) The ear is a three-chambered sensory structure that functions in the perception of sound (auditory system) and in the maintenance of balance (vestibular system). Each of the three divisions of the ear, the external ear, the middle ear, and the inner ear, is an essential part of the auditory system. The external and middle ear collect and conduct sound energy to the inner ear, where auditory sensory receptors transduce that energy into the electrical energy of nerve impulses. The sensory receptors of the vestibular system are also located in the inner ear. These receptors respond to gravity and movement of the head. EXTERNAL EAR: The external ear is composed of an auricle and an external auditory meatus. The auricle (pinna) is the appendage that projects from the lateral surface of the head, i.e., the "ear." The characteristic shape of the auricle is determined by an internal supporting structure of elastic cartilage. Thin skin with hair follicles, sweat glands, and sebaceous glands covers the auricle. The auricle is considered to be a nearly vestigial structure in humans, compared with its development and role in other animals. However, it is an essential component in sound localization and amplification.The external auditory canal (meatus) follows a slightly S-shaped course for about 25 mm to the tympanic membrane (eardrum). The lateral one-third of the canal is cartilage 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 (wax) glands. The coiled tubular apocrine ceruminous glands are modified sweat glands. Their secretion mixes with that of the sebaceous glands and with exfoliated cells to form cerumen or earwax. The cerumen lubricates the skin and coats hairs near the opening to impede the entry of foreign particles into the ear. Excessive accumulation of cerumen can plug the meatus, however, resulting in conductive hearing toss. The medial part of the canal, within the temporal bone, has thinner skin and fewer hairs and glands. MIDDLE EAR: The middle car is an air-filled mucus-membrane-lined space in the temporal bone, the tympanic cavity (Fig. 16.26). It is spanned by three small bones, the auditory ossicles, that are connected by two movable joints. The middle ear also contains the internal auditory canal (Eustachian canal) as well as the muscles that move the ossicles. The middle ear is bounded anteriorly by the auditory tube, posteriorly by the spongy bone of the mastoid process, laterally by the tympanic membrane, and medially by the bony wall of the inner ear.The primary function of the middle ear is to convert sound waves (air vibrations) arriving from the external auditory meatus into mechanical vibrations that are transmitted to the inner ear. Two openings in the medial wall of the middle ear, the vestibular (oval) window and the cochlear (round) window, are essential components in this conversion process. The tympanic membrane (Eardrum) separates the external auditory canal from the middle ear.It consists of a framework of collagen fibers covered by skin on the lateral side and mucosa on the side of the middle ear cavity. One of the auditory ossicles, the malleus, is attached to the tympanic membrane. Sound in the form of pressure waves causes the tympanic membrane to vibrate, and these vibrations are transmitted to the attached auditory ossicles that link the external ear to the inner ear. Perforation of the tympanic membrane may cause transient or permanent hearing impairment. The three small bones known as the ossicles, the malleus, the incus, and the stapes, cross the space of the middle ear in series and connect the tympanic membrane to the oval window. These bones help to convert sound waves, i.e., vibrations in air, to mechanical vibrations in tissues and fluid-filled chambers of the inner ear. In the process the strong vibrations are magnified and the weak ones are overshadowed. 1) Themalleus (hammer) is attached to the tympanic membrane, 2) The incus (anvil), links the malleus to the stapes, 3) The stapes (stirrup), whose footplate fits into the oval window leading to the inner ear. Muscles attach to the ossicles and affect their movement. The tendon of the tensor tympani inserts on the malleus. Contraction of this muscle increases tension on the tympanic membrane. The stapedius tendon inserts on the stapes. Contraction of the stapedius tends to dampen the movement of the stapes at the oval window. The stapedius, only a few millimeters in length, is the smallest of all the skeletal muscles. The two muscles of the middle ear are responsible for a protective reflex called the attenuation reflex centered in the inferior colliculi of the midbrain. Contraction of the muscles makes the chain of ossicles more rigid, thus reducing the transmission of vibrations to the inner ear. This protects the inner ear from the damaging effects of very loud sound. The internal auditory canal, commonly known as the eustachian canal, a narrow flattened channel lined with ciliated pseudostratified columnar epithelium, is approximately 3.5 cm long and connects to the nasopharynx. It allows pressure in the middle ear to equilibrate with atmospheric pressure. The walls of the tube are normally pressed together but separate during yawning and swallowing to allow equalization of pressure. It is common for infections to spread from the pharynx to the middle ear via the auditory tube (causing otitis media). A small mass of lymphatic tissue, the tubal tonsil. is often found at the pharyngeal opening of the auditory tube to help protect against infection. INNER EAR: (See Figure 16.27) The inner ear Consists of two compartments or labyrinths, one contained within the other. The bony (osseous) labyrinth is a complex system of interconnected cavities and canals in the temporal bone. The membranous labyrinth lies within the bony labyrinth and consists of a complex system of small sacs and tubules that also form a continuous space enclosed within a wall of epithelium and connective tissue. There are three fluids in the inner ear: 1) the endolymph, similar to intrcellular fluid, contained within the membranous labyrinth, 2) the perilymph, similar to extracellular fluid, lying between the bony labyrinth and the membranous labyrinth, 3) and a lesser known fluid space, the cortilymph lying within the organ of Corti. The three components of the inner ear are: 1) Semicircular canals, 2)Vestibule, 3)Cochlea. The vestibule is the central space of the bony labyrinth. The utricle and saccule of the membranous labyrinth lie in an elliptical and spherical recess, respectively. The semicircular canals extend from the vestibule posteriorly, and the cochlea extends from the vestibule anteriorly. The oval window into which the footplate of the stapes inserts lies in the lateral wall of the vestibule. Two groups of sense cells of the utricle and saccule sense the position of the head and linear movement. The Semicircular Canals, three narrow bony-walled tubes, each forming about three-quarters of a circle, lie at approximately right angles to each other in superior, posterior, and horizontal planes. At the lateral end of each semicircular canal, close to the vestibule, is an enlargement called an ampulla. Each inner ear has three ampullae. The three canals open into the vestibule through five openings, with the superior and posterior semicircular canals sharing a common ampulla medially. Three cristae ampularis located in the ampullae of the semicircular ducts respond to angular acceleration of the head, e.g. turning, flexing the neck, etc. The Cochlea is a conically shaped helix connected to the vestibule. The lumen of the cochlea, like that of the semicircular canals, is continuous with that of the vestibule. It connects to the vestibule on the side opposite the semicircular canals. Between its base and the apex, the cochlea makes about 2-3/4 turns around a central bony core called the modiolus. A sensory ganglion, the spiral ganglion, lies in the modiolus. One opening of the canal, the cochlear round window on its inferior surface near the base, is covered by a thin membrane (the secondary tympanic membrane) by which it absorbs or damps vibrations reaching it. The organ of Corti that projects into the endolymph of the cochlear duct is the sense organ for hearing. Despite having different specific functions, all receptors mentioned above share similar structural specializations and characteristics. The several different functions of the receptors of the inner ear are performed by hair cells that are remarkably similar in structure. They also have a common basis of function in the initiation of nerve impulses. Several important characteristics are common to these hair cells: 1) All are epithelial cells; 2) Each possesses numerous stereocilia, modified microvilli, called sensory "hairs"; 3) In the vestibular system, each hair cell possesses a single true cilium called a kinocilium; 4) In the auditory system, hair cells lose their cilium during development but do have a residual basal body. 5)All hair cells are associated with both afferent and efferent nerve endings; 6) All hair cells are transducers; i.e., they convert mechanical energy to electrical energy that can be transmitted via the vestibulocochlear nerve to the brain. All receptor (hair) cells of the inner ear appear to function by the bending or flexing of their stereocilia (sensory hairs). The means by which the stereocilia are bent varies from receptor to receptor. Stretching of the plasma membrane caused by the bending of the stereocilia generates trans-membrane potential changes in the receptor cell that are conveyed to the afferent nerve ending(s) associated with each hair cell. When a kinocilium is present, its location relative to the bending of the stereocilia is important. Stereocilia that are bent away from the kinocilium cause hyperpolarization of the receptor cell; stereocilia that are bent toward the kinocilium cause depolarization of the receptor cell and consequent generation of an action potential. In the crista ampullaris a gelatinous mass called the cupula adds inertia to the stereocilia making them bend slowly when movement of the head creates a differential between the walls of the semicircular canals and the endolymph inside. Bending of the stereocilia in the narrow space between the hair calls and the cupula leads to the generation of nerve impulses in the associated nerve endings. The maculae are innervated sensory thickenings of the epithelium facing the endolymph in the saccule and utricle of the vestibule. As in the cristae, each macula consists of hair cells and nerve endings associated with the hair cells. The maculae of the utricle and saccule are oriented at right angles to one another. When a person is standing, the macula utriculi is in a horizontal plane, and the macula sacculi is in a vertical plane. The gelatinous material that overlies the maculae is called the otolithic membrane. It contains 3-5 mm crystalline particles of calcium carbonate and protein, the otoliths, on its outer surface. This surface of the otolithic membrane lies opposite to the surface in which the stereocilia of the hair cells are embedded. The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista. Stereocilia of the hair cells are bent by gravity in the stationary individual when the otolithic membrane and its otolith pull on the stereocilia. They are also bent during linear movement when the individual is moving in a straight line and the otolithic membrane drags on the stereocilia because of inertia. The Organ of Corti is the sensor of sound vibrations. (See Figure 16.28) The cochlear duct divides the cochlear canal into three parallel canals or scalae: 1) Scala media, the middle compartment in the cochlear canal; 2) Scala vestibule or vestibular canal; 3) Scala tympani or tympanic canal. The cochlear duct, itself, is the scala media. The scala vestibuli and scala tympani are the spaces above and below, respectively. The scala media is an endolymph-containing space that is continuous with the lumen of the saccule and contains the Organ of Corti, which rests on its lower wall. The scala vestibule and the scala tympani are perilymph containing spaces and communicate with each other at the apex of the cochlea through a small channel called the helicotrema. The scala vestibule is described as beginning at the oval window, and the scala tympani is described as ending at the round window.The upper wall of the scala media, which separates it from the scala vestibuli, is the vestibular (Reissner's) membrane. The lower wall or floor of the scala media is the basilar membrane. The organ of Corti rests on the basilar membrane and is overlain by the tectorial membrane. The Organ of Corti is composed of hair cells and supporting cells. Sound waves striking the tympanic membrane are translated into simple mechanical vibrations. The ossicles of the middle ear convey these vibrations to the cochlea. Movement of the stapes in the oval window of the vestibule sets up vibrations or traveling waves in the perilymph of the vestibular canal. The vibrations are transmitted through the vestibular membrane to the scala media (cochlear duct), which contains endolymph, and are also propagated to the perilymph of the tympanic canal. Pressure changes in this closed system are reflected in movements of the membrane that covers the round window in the base of the cochlea. As a result of sound vibrations entering the inner ear, a traveling wave is set up in the basilar membrane. A sound of specified frequency causes displacement of a relatively long segment of the basilar membrane, but the region of maximal displacement is narrow. High-frequency sounds cause maximal vibration of the basilar membrane near the base of the cochlea; low-frequency sounds cause maximal displacement nearer the apex. The point of maximal displacement of the basilar membrane is specified for a given frequency of sound, and this is the basis of frequency discrimination. Perception of sound intensity or loudness depends on the degree of displacement of the basilar membrane at any given frequency range. Hair cells are attached, through other cells, to the basilar membrane, which vibrates during sound reception. The stereocilia of these hair cells are, in turn, attached to the tectorial membrane, which also vibrates. The shearing effect between the basilar membrane and the tectorial membrane distorts the stereocilia of the hair cells and this distortion generates membrane potentials that, when conveyed to the brain via the cochlear nerve (cochlear division of the vestibulocochlear nerve, cranial nerve VIII), are perceived as sound. THE END OF BIOL 237! What is the function of the saccule and utricle quizlet?- They are both organs for static equilibrium, which maintains the stability of the head and body when they are motionless or during linear (straight) movements. - Both the utricle and saccule have a small area of hair cells called the macula.
Which type of movement would the saccule detect?The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane.
Which situation would stimulate the utricle and saccule?There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and three semicircular canals. Together, they make up what's known as the vestibular labyrinth that is shown in Figure. The utricle and saccule respond to acceleration in a straight line, such as gravity.
Which term refers to the perception of the orientation of the head when the body is stationary multiple choice question?Static equilibrium. The perception of the orientation of the head when the body is stationary.
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