13.3: Hearing and Balance

By the end of this section, you will be able to:

• Describe the gross and microscopic structures responsible for the special senses of hearing and balance
• Trace the pathway of auditory and equilibrium information from the inner ear to the brain

Audition (Hearing)

Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure \(\PageIndex{1}\)).

External Ear

The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear.

Middle Ear

The middle ear consists of a space, the tympanic cavity, spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes.  The stapes is then attached to the oval window, which is the entrance to the inner ear, where the sound waves will be transduced into a neural signal.  The tympanic cavity is connected to the pharynx through the auditory (or Eustachian) tube, which helps equilibrate air pressure across the tympanic membrane by providing and entrance/exit point for air. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning. This helps to equalize the pressure on both sides of the ear drum with change in altitude, such as driving up the mountain or in a plane.

Figure \(\PageIndex{1}\): Structures of the Ear. The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the auditory tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively. (Image credit: "External, Middle and Inner Ear" by Cenveo is licensed under CC BY 3.0.)

Inner Ear

The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone (Figure \(\PageIndex{2}\)). Within the bony labyrinth are membranes that separate tubes and spaces filled with liquid. This is called the membranous labyrinth.  The inner ear has three separate regions, the cochlea, the vestibule, and the semicircular canals. The cochlea is responsible for hearing while the vestibule and semicircular canals are important for balance. The neural signals from these regions are relayed to the brainstem through separate fiber bundles called the cochlear nerve and the vestibular nerve. However, these two distinct bundles travel together from the inner ear to the brainstem as the vestibulocochlear nerve (CN VIII).

Figure \(\PageIndex{2}\): Inner Ear. Within the bony labyrinth are membraneous tubes and spaces that are filled with fluid.  Fluid movement is the stimulus for both hearing and balance.  (Image Credit: "Inner Ear Anatomy - Cochlea and Vestibular Apparatus" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Blausen.com staff (2014), "Medical gallery of Blausen Medical", WikiJournal of Medicine.)

Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window. The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, traveling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time traveling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala.

A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure \(\PageIndex{2}\) and Figure \(\PageIndex{3}\)). The cochlear duct contains the spiral organ (organs of Corti), which transduce the wave motion of the two scala into neural signals. The spiral organ lies on top of the basilar membrane, which separates the cochlear duct and the scala tympani. Another membrane called the vestibular membrane separates the cochlear duct from the scala vestibuli.

Figure \(\PageIndex{3}\): Cross Section of the Cochlea. The membranous labyrinth separates the cochlea in three major regions. The scala vestibuli and scala tympani lie on either side of the cochlear duct. The roof of the cochlear duct is formed by the vestibular membrane while the base of it is formed by the basilar membrane. Within the cochlear duct, the organ of Corti sits atop the basilar membrane and its cells extend towards the tectorial membrane. (Image credit: “Section of the Cochlea" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, derivative of original work by OpenStax.)

Figure \(\PageIndex{4}\): Histology of the Cochlea. The membranous labyrinth of the cochlea is divided into three ducts: vestibular, cochlear, and tympanic (A).  The vestibular membrane forms the roof of the cochlear duct, and the floor is formed by the basilar membrane on which the spiral organ sits (B).  The receptors for sound are the hair cells that sit on the basilar membrane and extends towards the tectorial membrane (C).  (Image credit: “Histology of the Cochlea" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, slide provided by the Regents of the University of Michigan Medical School © 2022)

 The spiral organ contains hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surface (Figure \(\PageIndex{5}\)). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest.  The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the spiral organ.  Protein fibers tether adjacent stereocilia together within each array, such that relative shifts between the basilar membrane and the tectorial membrane bend the stereocilia, causing ion channels to open.  This converts the mechanical force of the fluid wave into an electrical signal.

Figure \(\PageIndex{5}\): Hair Cells of the Spiral Organ. The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia extend towards the tectorial membrane. The stereocilia are tethered together by proteins and bend in response to movements of the basilar membrane. (Image credit: “Cochlear Hair Cells" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, derivative of original work by OpenStax.)

The transmission and transduction of sound require the participation of the majority of the structures of the ear. Each sound wave has a specific frequency, which depends on its pitch, and amplitude, which depends on its loudness. Sound waves are funneled into the ear canal by the auricle and reach the tympanic membrane (Figure \(\PageIndex{6}\)). The vibration of the tympanic membrane is amplified across the ossicles. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves. The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot of the cochlea, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea. When the fluid waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array of stereocilia. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will electrically change the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. The relative movement of different arrays of stereocilia along the length of the basilar membrane allows the brain to perceive frequency, or pitch. The greater force of louder sounds causes an increase in the range of movement of the stereocilia of the hair cells, determining the loudness of a sound.

Figure \(\PageIndex{6}\): Transmission of Sound Waves to Cochlea. The transmission is represented in five steps. 1) Sound wave represents alternating areas of high and low pressure, whose frequency can be measured in Hz (cycles per second). 2) Tympanic membrane vibrates in response to sound wave. 3) Vibrations are amplified across the ossicles, in order malleus, incus, and stapes. 4) Vibrations against oval window set up standing wave in fluid of the scala vestibuli and scala tympani. The frequency of the standing wave is the same as the initial sound wave. 5) Pressure bends the membrane of the cochlear duct at a point of maximum vibration for a give frequency, causing hair cells of the spiral organ on the basilar membrane to vibrate. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear. (Image credit: "Cochlear function based on wave frequency" by Cenveo is licensed under CC BY 3.0.)

The initial afferent fibers are bipolar neurons.  The dendrites of these neurons receive the signal from the hair cell then carry the signal to the cell cell body located in the spiral ganglion.  Axons emerging from the spiral ganglion then carry neural signals through the cochlear nerve which travels within the vestibulocochlear nerve (CN VIII), terminating in the cochlear nuclei in the pons(Figure \(\PageIndex{7}\)). From the cochlear nuclei the auditory information travels along multiple pathways, but the primary auditory pathway goes through the inferior colliculus (auditory reflexes) and then through the thalamus and into the primary auditory cortex of the temporal lobe.

Figure \(\PageIndex{7}\): Auditory Pathways.  The primary pathway of auditory signals goes from the cochlea to the 1) cochelar nuclei, 2) inferior colliculi, 3) medial geniculate nucleus of the thalamus, and then to 4) the primary auditory cortex in the temporal lobe. (Image Credit: "Auditory Pathways" by Jonathan E. Peelle is licensed under CC BY 4.0, via Wikimedia Commons.)

Equilibrium (Balance)

Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located in the vestibule and semicircular canals within the inner ear. Vestibule and semicircular canals compose the vestibular system. The vestibule is the central part of the bony labyrinth, and is located posterior to the cochlea and anterior to the semicircular canals. Within the vestibule, the membranous labyrinth forms two chambers, the utricle and saccule, which contain endolymph. The utricle and saccule are interconnected by a narrow endolymphatic duct and their endolymph is confluent with that of the semicircular ducts and cochlear duct. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals.

The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by supporting cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure \(\PageIndex{8}\)). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the stereocilia, causing the hair cells to elicit electrical changes. The exact position of the head is interpreted by the brain based on the pattern of hair-cell activation.

Figure \(\PageIndex{8}\): Linear Acceleration Coding by Maculae. The maculae within the utricle and saccule of the vestibular apparatus are specialized structures lined with supporting cells and hair cells and filled with endolymph. The maculae sense linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration. (Image credit: "Cenveo - Drawing Structure of the Maculae - English labels" by Cenveo, license: CC BY.)

The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure \(\PageIndex{9}\)). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a gelatinous membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.

Figure \(\PageIndex{9}\): Rotational Coding by Semicircular Canals. Rotational movement of the head is encoded by the hair cells contained in ampullae at the base of the semicircular canals. The ampulla is filled with endolymph and contains hair cells that have stereocilia embedded in a gelatinous substance called the cupula. As one of the canals moves in an arc with the head, the endolymph moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction ("Cenveo - Drawing Structure and Function of the Semicircular Canals - English labels" by Cenveo, license: CC BY)

The neural signals generated in the vestibule and semicircular canals are transmitted through the vestibular nerve which travels within the vestibulocochlear nerve (CN VIII). The nerve carries the equilibrium information into the brain, through the thalamus, and to cerebral nuclei, brainstem and cerebellum.

Structure of the Inner Ear

Explore the structure of the inner ear in the 3D rendering of an inner ear model.

 

Inner Ear by AnatomyPhysiologySCC on Sketchfab

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Concept Review

Hearing and balance belong to the special senses with their receptors present in the inner ear.

The ear is divided in three regions: external, middle and inner ear. In the external ear, the auricle is the fleshy structure that convey sounds into the auditory canal. At the end of the canal is the tympanic membrane that vibrates and transforms sound waves into mechanical waves. The middle ear consists of a space spanned by three small bones called ossicles (malleus, incus, and stapes) that conduct the mechanical waves. The middle ear is connected to the pharynx through the Eustachian (or auditory) tube, which helps equilibrate air pressure across the tympanic membrane. The inner ear is made of a bony labyrinth lined with a membranous labyrinth that separate tubes and spaces. The space between the two labyrinths is filled with a fluid called perilymph. Inside the membranous labyrinth the space is filled with endolymph. The inner ear is responsible for transforming mechanical waves into electrical signals, which are then sent to the brain through the vestibulocochlear nerve (CN VIII).

The cochlea is a spiral-shaped tube, divided into three compartments: the scala vestibuli, scala tympani and cochlear duct. All compartments are filled with endolymph. The scala vestibuli starts at the oval window, curves over the top of the cochlear duct and becomes the scala tympani, that returns to the base of the cochlea, traveling under the cochlear duct and ends at the round window. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The cochlear duct contains several organs of Corti, which transduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array, which causes an electrical signal to be generated.

The vestibule and the semicircular canals are responsible for the sense of equilibrium. The vestibule is composed of the utricle and saccule. Hair cells are located in maculae of the utricle and saccule. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane, on top of which is a layer of calcium carbonate crystals, called otoliths. The vestibule senses the linear acceleration of the head and gravity. When the head moves, the otoliths move and the otolithic membrane bends the stereocilia of the hair cells. Hair cells are also located in ampullae at the base of the three semicircular canals. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. The semicircular canals sense the head rotation. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement, and bending the stereocilia of the hair cells.

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Review Questions

Q. What type of receptor cell is involved in the sensations of sound and balance?

 

A. photoreceptor

B. chemoreceptor

C. mechanoreceptor

D. nociceptor

Answer

C

Glossary

ampulla

in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head

audition

sense of hearing

auditory canal

passageway of the external ear that leads to the tympanic membrane; also known as ear canal

auditory tube

tube that connects the nasopharynx to the middle ear; also known as Eustachian tube

auricle

fleshy external structure of the ear

basilar membrane

in the ear, the floor of the cochlear duct on which the organ of Corti sits

bony labyrinth

complex cavities in the inner ear made by bones

brainstem

region of the adult brain that includes the midbrain, pons, and medulla oblongata and develops from the mesencephalon, metencephalon, and myelencephalon of the embryonic brain

cerebellum

region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord

cerebral nuclei

deep gray matter of the cerebrum

cochlea

auditory portion of the inner ear containing structures to transduce sound stimuli

cochlear duct

space within the auditory portion of the inner ear that contains the spiral organ and is adjacent to the scala tympani and scala vestibuli on either side

cochlear nerve

branch of the vestibulocochlear nerve projecting from the cochlea

cupula

specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid

ear canal

passageway of the external ear that leads to the tympanic membrane; also known as auditory canal

endolymph

fluid in the membranous labyrinth of the ear

equilibrium

sense of balance that includes sensations of position and movement of the head

Eustachian tube

tube that connects the nasopharynx to the middle ear; also known as auditory tube

external ear

structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane

hair cells

mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance

incus

(also, anvil) ossicle of the middle ear that connects the malleus to the stapes

inner ear

structure within the temporal bone that contains the sensory apparati of hearing and balance

macula

enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla

malleus

(also, hammer) ossicle that is directly attached to the tympanic membrane

membranous labyrinth

membranes of the the inner ear that line the bony labyrinth

middle ear

space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window

organ of Corti

structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals (also spiral organ)

ossicles

three small bones in the middle ear

otolith

layer of calcium carbonate crystals located on top of the otolithic membrane

otolithic membrane

gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded

oval window

membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli

perilymph

fluid between the membranous labyrinth of the ear and the bony labyrinth

primary auditory cortex

region of the cerebral cortex within the temporal lobe responsible for the perception of sounds

round window

membrane that marks the end of the scala tympani

saccule

structure of the inner ear responsible for transducing linear acceleration in the vertical plane

scala tympani

portion of the cochlea that extends from the apex to the round window

scala vestibuli

portion of the cochlea that extends from the oval window to the apex

semicircular canals

structures within the inner ear responsible for transducing rotational movement information

spiral ganglion

location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve

spiral organ

structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals (also organ of Corti)

stapes

(also, stirrup) ossicle of the middle ear that is attached to the inner ear

stereocilia

array of apical membrane extensions in a hair cell that transduce movements when they are bent

tectorial membrane

component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded

thalamus

major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery

tympanic membrane

ear drum

utricle

structure of the inner ear responsible for transducing linear acceleration in the horizontal plane

vestibular membrane

membrane separating the cochlear duct and scala vestibuli

vestibular nerve

branch of the vestibulocochlear nerve projecting from the vestibule

vestibular apparatus

system composed of the vestibule and semicircular canals to sense equilibrium

vestibule

in the ear, the portion of the inner ear responsible for the sense of equilibrium

vestibulocochlear nerve

eighth cranial nerve; responsible for the sensations of hearing and balance

Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology