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16.3: Auditory and Equilibrium Anatomy

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    Auditory and Equilibrium Anatomy

    Diagram differentiating the outer, middle, and inner ear

    Above: The outer, middle, and inner ear.

    The ear is a complex organ which houses special structures that allow us to hear, balance and orientate ourselves. Sound waves are collected by the auricle (outer ear) and funneled into the external acoustic meatus. The ear is divided into three sections, the outer, middle, and inner ear. The outer ear consists of the auricle which extends through the external auditory canal and terminates at the tympanic membrane.

    Outer Ear

    The auricle or pinna (external ear) consists of a core of elastic cartilage covered by thin skin. Skeletal muscle fibers of the auricular muscle have little, but variable, function in humans. Numerous hair follicles with their accompanying sebaceous glands are also present.

    Thin skin covering the external acoustic meatus possesses small dermal papillae and epithelial pegs to anchor the epidermis with the dermis. The sebaceous glands are large. Additionally, ceruminous glands, specialized coiled, tubular, apocrine sweat glands, secret cerumen, a waxy secretion. Skeletal muscle is also present. Elastic cartilage and the temporal bone form the support for the auricle.

    Outer Ear Structure

    Location

    Function

    auricle (pinna)

    paired (right and left) external ear structure composed of elastic cartilage and skin

    collecting sound and funneling it into the external auditory canal

    ceruminous glands

    located under the skin along the external auditory canal

    produce and secrete cerumen (ear wax); cerumen lubricates the external auditory canal and is antibacterial

    external acoustic meatus

    paired (right and left) canal in the bone of the skull, specifically the temporal bone

    creates passage within the skull for the external auditory canal

    external auditory canal

    paired (right and left) skin-covered canal positioned within the external acoustic meatus (sometimes used synonymously with external acoustic meatus)

    passageway carrying sound collected by the auricle to the middle ear

    tympanic membrane

    paired (right and left) membranes in the external auditory canal separating the outer ear from the middle ear

    ear drum; sound causes vibration of the tympanic membrane; vibrations of the tympanic membrane are transferred to the middle ear and then ultimately the inner ear for detection of sound

    Middle Ear

    The main structures of the middle ear are the auditory ossicles, Eustachian tube (also known as the pharyngotympanic tube), oval window and round window. The auditory ossicles inward from the tympanic membrane, are the malleus, incus, and stapes. The base of the stapes covers the oval window which allows sound waves to pass from the tympanic membrane, into the cochlea of the inner ear.

    Middle Ear Structure

    Location

    Function

    Eustachian tube (pharyngotympanic tube)

    passage between the middle ear and the nasopharynx (paired, one right and one left)

    equalizes the pressure of the middle ear with external air pressure (ear popping)

    incus

    middle auditory ossicles, anvil-shaped bone

    receives vibrations from malleus and transmits them to stapes

    malleus

    between the tympanic membrane and incus, hammer-shaped bone

    receives vibrations from the tympanic membrane and transmits them to incus

    oval window

    between stapes and the cochlea, membrane-covered opening

    transfers vibrations from stapes to the cochlea

    round window

    inferior to the oval window on the exterior of the cochlea, membrane-covered opening

    creates space for the fluid within the cochlea to move with the vibrations of the auditory ossicles

    stapes

    between incus and the oval window, stirrup-shaped bone, smallest bone in the human body

    receives vibrations from stapes and transmits them to the oval window

    Diagram of the structures of the ear. Stapes (attached to oval window), Incus, Malleus, External auditory canal, tympanic cavity, Tympanic membrane, Semicircular canals, Vestibular nerve, Cochlear nerve, Cochlea, Eustachian tube, Round window

    Above: Structures of the outer, middle, and inner ear.

    The middle ear, or tympanic cavity, is an air-filled space in the temporal bone that is lined by a mucosa. Three auditory ossicles span the cavity between the tympanic membrane and an opening in the wall of the inner ear, the oval window. The middle ear communicates with the mastoid air cells posteriorly and with the pharynx anteriorly through the Eustachian tube.

    Vibrations of the vestibular membrane are transmitted through the malleus and incus to the stapes. The stapes is seated in the oval window between the middle ear and the vestibule of the inner ear. Inward movement of the stapes creates pressure on the perilymph filling the osseous labyrinth (inner ear), initiating responses in neural receptors of the cochlear duct.

    Inner Ear

    Diagram of the position of the osseous labyrinth embedded in the temporal bone and the passage of C.N. VIII through the internal acoustic meatus

    Above: Superior view of the cranial cavity (skull) showing where the inner ear is embedded in the temporal bone and how C.N. VIII vestibulocochlear nerve passes through the internal acoustic meatus into the cranial cavity.

    The inner ear, located in the petrous portion of the temporal bone (between the external acoustic meatus and the internal acoustic meatus), houses receptors that monitor changes in sound, gravity and movement. These receptors are located in a series of membranous tubes and sacs, collectively termed the membranous labyrinth, which are suspended in a series of bony spaces, the osseous labyrinth. The osseous labyrinth can be subdivided into three regions: the cochlea, vestibule, and semicircular canals. The cochlea, vestibule, and semicircular canals are responsible for hearing, static and dynamic equilibrium respectively.

    Illustrations of the osseous labyrinth and its parts

    Above: Osseous labyrinth of the inner ear. (Top) The bony labyrinth and its components, the cochlea (auditory sense), the vestibule (sense of gravity and acceleration), and the semicircular canals (sense of balance). (Bottom) A look inside the osseous labyrinth reveals passageways inside called the membranous labyrinth that is filled with endolymph, which moves with vibrations from auditory ossicles in the cochlea, or with movements of the head in the vestibule and semicircular canals. Changes sensed in the endolymph is transmitted to neurons in C.N. VIII. (Right) The utricle and saccule of the vestibule, specialized sensory regions.

    The osseous labyrinth is lined with periosteum and filled with a fluid, perilymph surrounding the membranous labyrinth. Within the membranous labyrinth is endolymph, specialized fluid, as well as epithelia and its underlying connective tissue, containing receptors for hearing and for static and kinetic senses. C.N. VIII, the vestibulocochlear nerve, innervates the receptors in the membranous labyrinth. The cochlear division of the nerve supplies the organ of Corti in the cochlear duct. The vestibulocochlear nerve passes into the cranial cavity through the internal acoustic meatus.

    Inner Ear Structure

    Location

    Function

    cochlea

    inner ear, spiral part of the osseous labyrinth

    contains receptor cells (hair cells)

    cochlear nerve

    nerve connected to the cochlea, a branch of C.N. VIII vestibulocochlear nerve

    carries auditory signals to the brain

    endolymph

    inside the membranous labyrinth, produced from perilymph

    this fluid moves in response to movements and vibrations; fluid movements are detected by hair cells

    hair cells

    inside the membranous labyrinth of the cochlea, vestibule, and ampullae of the semicircular canals

    receptor cells detect movements in the fluids inside the membranous labyrinth and transmit to sensory neurons of the vestibulocochlear nerve

    membranous labyrinth

    ducts located inside the osseous labyrinth of the inner ear (cochlea, vestibule, and semicircular canals)

    contains endolymph and hair cells for detecting vibrations and changes in position

    osseous labyrinth

    bony structure in the inner ear surrounding the membranous labyrinth, composed of the cochlea, vestibule, and semicircular canals

    contains the membranous labyrinth where auditory, gravitational, balance, and acceleration information is sensed

    perilymph

    fluid between the osseous labyrinth and the membranous labyrinth

    transfers vibrations to the endolymph

    saccule

    a region of the vestibule containing receptor cells (hair cells)

    senses head tilting and accelerations

    semicircular canals

    utricle

    a region of the vestibule containing receptor cells (hair cells)

    senses balance, orientation, and motion

    vestibular nerve

    nerve connected to the vestibule, a branch of C.N. VIII vestibulocochlear nerve

    carries balance, orientation, motion, head tilting, and acceleration sensory information to the brain

    vestibule

    central region of the osseous labyrinth

    senses balance, orientation, motion, head tilting, and acceleration

    vestibulocochlear nerve (C.N. VIII)

    between the inner ear and the brain, passes into the cranial cavity through the internal acoustic meatus

    carries special sensory signals to the brain including auditory, balance, equilibrium, and acceleration signals

    Anatomy within the cochlea. cochlea (uncoiled), oval window (underneath stapes), round window, vestibular membrane, scala vestibuli, spiral ganglion, scala media, tectorial membrane, organ of Corti, basilar membrane, scala tympani

    Above: The cochlear duct (scala media), vestibular duct (scala vestibuli), and tympanic duct (scala tympani) of the cochlea. Cross section of part of the cochlea to reveal the internal structure of the cochlea, the cochlear duct, including the organ of Corti, where endolymph fluid movements in the cochlea are detected and transferred to sensory neurons of C.N. VIII vestibulocochlear nerve.

    Cochlear Structure

    Location

    Function

    basilar membrane

    inside the cochlea, structure between scala media and scala tympani

    separates scala media and scala tympani

    cochlear duct / scala media

    a duct inside the cochlea filled with endolymph

    contains organ of Corti for sensing auditory information

    hair cells

    inside the membranous labyrinth of the cochlea, vestibule, and ampullae of the semicircular canals

    receptor cells detect movements in the fluids inside the membranous labyrinth and transmit to sensory neurons of the vestibulocochlear nerve

    organ of Corti (spiral organ)

    inside the cochlea, within scala media

    contains receptor cells (hair cells) that detect vibration movements in the endolymph as auditory signals

    scala tympani / tympanic duct

    inside the cochlea, a cavity inside the cochlea

    contains perilymph, transmits vibration information to endolymph in scala media

    scala vestibuli / vestibular duct

    inside the cochlea, a cavity inside the cochlea

    contains perilymph, transmits vibration information to endolymph in scala media

    spiral ganglion

    inside the cochlea, bundle of neuron cell bodies

    these neurons collect auditory information from the hair cells and transfer auditory sensory information

    tectorial membrane

    inside the cochlea, attached to hair cells in the cochlear duct

    transfers movements of the endolymph to the hair cells

    vestibular membrane (Reissner's membrane)

    inside the cochlea, between the cochlear duct and the vestibular duct

    separates the cochlear duct from the vestibular duct

    Cross section showing the ducts within the cochlea including the cochlear duct containing the organ of Corti

    Above: Structure of the organ of Corti with its hair cells attached to the tectorial membrane. Endolymph in the scala media (cochlear duct) moves with vibrations from the auditory ossicles and transfers motion to the tectorial membrane. These movements are sensed by the hair cells that transfer these sensory signals to cochlear nerve fibers.

    Attributions


    This page titled 16.3: Auditory and Equilibrium Anatomy is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

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