16.3: Auditory and Equilibrium Anatomy

Auditory and Equilibrium Anatomy

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

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

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.

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

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

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

• "Anatomy and Physiology Lab Reference" by Laird C Sheldahl, OpenOregonEducational Resources, Mt. Hood Community College is licensed under CC BY-SA 4.0
• "Anatomy of the Human Ear.svg" by Lars Chittka; Axel Brockmann is licensed under CC BY 2.5
• "Cochlea-crosssection.svg" by Fred the Oyster is licensed under CC BY-SA 3.0 / A derivative from the original work
• "Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs, Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0
• "Gray's Anatomy plates" by Henry Vandyke Carte is in the Public Domain
• "Human Anatomy Lab Manual" by Malgosia Wilk-Blaszczak, Mavs Open Press, University of Texas at Arlington is licensed under CC BY 4.0
• "Inner ear.png" by Sheldahl is licensed under CC BY-SA 4.0