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SPECIAL SENSES: THE EAR
Hearing and Equilibrium
The ear has sensory receptors that transform sound vibrations into electrical signals 1000 faster
than photoreceptors in the eye. The ability to perceive sound is called hearing. The ear is also responsible
for equilibrium which allows us to maintain our balance and awareness in space.
ANATOMY OF THE EAR
The three main regions of the ear are (1) the external ear, (2) the middle ear, and (3) the internal
ear. The external ear collects sound waves, the middle ear sends sound vibrations to the oval window, and
the hearing and equilibrium receptors are found in the internal ear.
External (Outer) Ear
The parts of the external (outer) ear include the auricle, external auditory canal, and tympanic
membrane. The auricle, also known as the pinna, is made up of elastic cartilage covered by skin and
consists of the helix (rim) and the lobule (inferior portion). It is attached to the head by ligaments and
muscles. Within the cartilage of the auricle and temporal bone lies the external auditory canal, which
connects the external ear to the tympanic membrane. The external auditory canal and middle ear is
separated by a thin, semitransparent partition called the eardrum or tympanic membrane. At the base of
the external acoustic meatus, the eardrum is attached to the temporal bone by a fibrocartilaginous ring and
stretches as a three-layered window across the opening. From outside to inside, the three layers of the
eardrum include an epidermis, dense connective tissue and simple cuboidal epithelium. The apex of the
eardrum is called the umbo, which is convex towards the middle ear cavity. The malleus, the first middle
ear ossicle, is attached to the superior inner surface of the tympanic membrane. Pressure from a cotton
swab, or a middle ear infection, can cause a perforated eardrum, which is a tear in the tympanic
membrane.
Ceruminous glands that secrete earwax or cerumen, and a few hair follicles can be found in the
external auditory canal. These protect the ear by preventing dust and foreign objects from entering. When
cerumen collects in this cavity, it can cause a muffling of sounds. Impacted cerumen is treated by periodic
ear irrigation or by using a blunt instrument to remove the wax.
Middle Ear
In the petrous portion of the temporal bone is
a small, air-filled cavity called the middle ear. The
tympanic membrane separates this area from the
external ear. A thin, bony partition that contains the
oval window and round window separates this cavity
from the inner ear. The three auditory ossicles, the
smallest bones in the body, extend across the middle
ear and are suspended in this area by ligaments.
Synovial joints connect these three bones to each
other. The auditory ossicles are the malleus
(hammer), incus (anvil) and stapes (stirrups). The
malleus articulates with the tympanic membrane and
the incus, which is attached to the head of the stapes.
The base of the stapes fits into the fenestra vestibuli,
also known as the oval window.
Two skeletal muscles attach to the ossicles: the
tensor tympani muscle and the stapedius muscle.
The tensor tympani muscle prevents damage to the
inner ear by limiting movement and increasing tension
on the eardrum. The smallest of all skeletal muscles in
the body, the stapedius muscle, decreases sensitivity
of hearing and protects the oval window by
dampening large vibrations of the stapes. The auditory (pharyngotympanic) tube, commonly known as the
eustachian tube, connects the middle ear to the nasopharynx. This tube contains both bone and elastic
cartilage and is usually closed at its medial end. In order to equalize the pressure in the middle ear to
atmospheric pressure during yawing or swallowing, this tube opens allowing air to enter or exit.
Internal (Inner) Ear
The internal (inner) ear, also known as the labyrinth, contains a series of complicated canals. It
contains an outer bony labyrinth and an inner membranous labyrinth. The bony labyrinth, which is lined
with periosteum and filled with perilymph, is divided into three areas: the semicircular canals, the
vestibule and the cochlea. Receptors for equilibrium and hearing are contained within a series of epithelial
sacs and tubes called the membranous labyrinth, which lie within the bony labyrinth. This inner series of
tubes contain a fluid called endolymph.
The oval central portion of the bony labyrinth is called the vestibule. The sac-like portions of the
membranous labyrinth found within the vestibule are the utricle and saccule. Three bony semicircular
canals project superiorly and posteriorly from the vestibule. The anterior, posterior and lateral semicircular
canals lie at approximately 90 degree angles from each other. Swollen enlargements called ampulla lie at
the ends of the canals. The semicircular ducts are the portions of the membranous labyrinth that lie inside
the bony semicircular canals.
The ampullary, utricular and saccular nerves of the vestibular branch of the vestibulocochlear (VIII)
nerve contain first-order sensory and motor neurons that synapse with equilibrium receptors. Sensory
information from the receptors travel along first-order sensory neurons and feedback signals that modify
their sensitivity travel back to the receptors via motor neurons. The vestibular ganglia are areas where cell
bodies of sensory neurons are located.
The bony spiral canal anterior to the vestibule is the cochlea. The central bony core surrounding
this snail shaped canal is called the modiolus. The cochlear ducts, the scala vestibuli and the scala tympani
are the three channels that make up this structure. Continuous with the membranous labyrinth, the
cochlear duct (scala media) contains endolymph. The scala vestibuli, lies above the cochlear duct and ends
at the oval window. The scala tympani is located below the scala vestibuli and ends at the round window.
Both scala vestibuli and scala tympani contain perilymph because they are part of the bony labyrinth of the
cochlea. These two structures are separated by the cochlear duct except at the helicotrema, the opening
at the apex of the cochlea.
The cochlear duct is separated from the scala vestibuli by the vestibular membrane. The basilar
membrane, on the other hand, separates the cochlear duct from the scala tympani. The basilar membrane
contains the spiral organ or organ of Corti, which is composed of a coiled sheet of epithelial cells,
supporting cells and 16,000 hair cells that transmit auditory information. The hair cells found here are
separated into two groups: inner hair cells (arranged in single row) and outer hair cells (arranged in three
rows). A hair bundle composed of 40-80 stereocilia and kinocilium (cilium) can be found at the apical tip of
each hair cell. These bundles extend into the cochlear duct’s endolymph.
At the basal ends of these hair cells, auditory information is transmitted to first-order sensory
neurons and motor neurons from the cochlear branch of the vestibulocochlear (VIII) nerve. The spiral
ganglion is the area where the cell bodies of these neurons can be found.
Inner hair cells relay auditory information to the brain by synapsing with 90-95 percent of the firstorder sensory neurons in the cochlear nerve. Outer hair cells, on the other hand, synapse with 90 percent
of the motor neurons in the cochlear nerve. Lying over the hair cells is a flexible gelatinous membrane
called the tectorial membrane. Embedded in this membrane are the ends of the sterocilia of the hair cells;
the hair cell bodies lie on the basilar membrane.
MECHANISM OF HEARING
A series of alternating high and low pressure regions that travel in the same direction are called sound
waves. Much like the way ripples in a pond spread over the surface, sound waves originate from a
vibrating object. In the human ear, the series of events involved in hearing is as follows:
1. Sound waves are directed by the auricle into the external auditory canal.
2. The sound waves hit the tympanic membrane,
causing it to vibrate back and forth. Low pitched
(low-frequency) sounds cause the tympanic
membrane to vibrate slowly, and high pitched
(high-frequency) sounds cause a rapid vibration.
3. The vibrations at the center of the tympanic
membrane cause the connected malleus to
vibrate. The vibration is then carried from
tympanic membrane, to malleus, to the incus, to
the stapes.
4. The oval-shaped footplate of the stapes then vibrates in the oval window. Due to its ability to
condense small vibrations over a large area into large vibrations over a small area, the vibrations at
the oval window are about 20 times more vigorous than on the tympanic membrane.
5. This movement at the oval window creates fluid pressure waves in the cochlear perilymph. When
the oval window bulges inward, it pushes on the perilymph of the scala vestibuli.
6. The pressure waves travel from the scala vestibuli to the scala tympani to the round window, which
then bulges outward and into the middle ear.
7. The pressure waves vibrate through the perilymph of the scala vestibuli and then the vestibular
membrane. Then they move into the endolymph inside the cochlear duct.
8. The pressure waves in the endolymph make the basilar membrane vibrate, which causes the hair
cells of the spiral organ to move against the tectorial membrane. This is what causes the bending of
the stereocilia and ultimately the nerve impulses in first-order neurons in cochlear nerve fibers.
Each segment of the basilar membrane is ‘tuned’ – sound waves of various frequencies (particular
pitch) cause different areas of the basilar membrane to vibrate more or less intensely. High pitched sounds
cause maximal vibrations near the stiffer base of the cochlea. Low pitched sounds cause maximal vibration
near the more flexible apex of the cochlea. Note these areas are described by their proximity to parts of
the cochlea, but the actual vibration takes place on the basilar membrane itself. High intensity sounds
(loudness) create large vibrations of the basilar membrane and thus cause a greater number of nerve
impulses sent to the brain.
THE AUDITORY PATHWAY
Sensory neurons are attached to the hair cells of the spiral organ, and so when the stereocilia of
those hair cells are bent, a neurotransmitter is released and a nerve impulse is generated. The spiral
ganglion is where all the cell bodies of those sensory neurons are located. The axons of the neurons form
the cochlear branch of the vestibulocochlear (VII) nerve. In the medulla oblongata on the same side, these
axons synapse with the neurons of the cochlear nuclei. Some axons then ascend in a tract called the lateral
lemniscus on the opposite side of the brain and end in the inferior colliculus of the midbrain. Others
terminate in the pons in the superior olivary nucleus. The slight difference in time of when a sound hits
both ears allows us to locate the sound. The nerve impulses finally arrive to the primary auditory area of
the cerebral cortex by way of (in correct order) the inferior colliculi, the medial genticulate nucleus of the
thalamus, and then the primary auditory area. Because some auditory axons can and do cross over in the
medulla, both the left and right primary auditory areas of the cerebral cortex receive information from
both ears.
MECHANISM OF EQUILIBRIUM
Balance is called equilibrium, and there are two types. Relative to the force of gravity, the
maintenance of body (and primarily head) position is called static equilibrium. Whenever you tilt your
head, or speed up or slow down in a car, you are stimulating your receptors for static equilibrium.
Maintaining body (head) position when making sudden movements, or rapid rotational acceleration or
deceleration of the head, is referred to as dynamic equilibrium. The vestibular apparatus is the collective
term for the receptor organs for equilibrium, which includes the saccule, utricle, and semicircular ducts.
Otolithic Organs: Saccule and Utricle
Both the utricle and saccule’s sense organs for static equilibrium are small, thickened regions called
maculae (singular is macula). The maculae are the receptors for static equilibrium and are perpendicular to
one another. They also contribute to some facets of dynamic equilibrium. Their perpendicular alignment
allows them to detect linear acceleration and deceleration via the position of the head in space, and thus
their sensory information is crucial to maintaining the necessary posture and balance.
Hair cells and supporting cells are the two cell types of the maculae. The hair cells have many
shorter stereocilia and one longer firmly rooted kinocilium – these two together are called a hair bundle.
The supporting cells are littered among the hair cells, and they secrete the gelatinous thick glycoprotein
layer (the otolithic membrane) that sits on the hair cells. Sitting above the otolithic membrane is a dense
layer of otoliths, calcium carbonate crystals.
When you lean your head forward or backward, the weight of the otolithic membrane and otoliths
respond to the gravitational change, thus bending the hair bundles and initiating the creation of a nerve
impulse. First-order sensory neurons of the vestibular branch of the vestibulocochlear (VII) nerve synapse
with the hair cells.
Semicircular Duct
The three semicircular ducts lie at right angles to one another on three different planes and function
in dynamic equilibrium. The lateral semicircular duct is the horizontal duct; the two vertical ducts are the
anterior and posterior semicircular ducts. By covering three planes, the ducts are able to detect rotational
acceleration or deceleration.
Each duct has a dilated portion called
the ampulla. The ampulla rests in a small
elevation called the crista. Within the cristae
is a group of supporting and hair cells
blanketed by a gelatinous material called the
cupula. When you move your head, you
move the attached semicircular ducts and
hair cells as well. The unattached endolymph
within the ampulla, however, drags behind
due to inertia. This causes the moving hair
cells to drag along the stationary fluid and the
hair bundles bend. Once again, this bending
of the hair bundles leads to the creation of a
nerve impulse that travels down the
ampullary nerve to the vestibulocochlear (VII)
nerve.
EQUILIBRIUM PATHWAYS
Bending the hair bundles of the hair cells in the utricle, saccule, or semicircular canals generates
nerve impulses in the attached sensory neurons. The cell bodies of the hair cells are in the vestibular
ganglia. After traveling down the vestibular branch of the vestibulocochlear (VII) nerve, most of the axons
synapse with sensory neurons in the vestibular nuclei. The vestibular nuclei are found in the medulla
oblongata and pons, and are the primary location for equilibrium integration. The vestibular nuclei also
receive positioning information about the rest of the body via proprioceptors of the neck and limbs, as well
as the eyes. The axons that don’t follow this path enter the cerebellum through the inferior cerebellar
peduncles. The cerebellum and vestibular nuclei are connected by bidirectional pathways.
Once the vestibular nuclei have integrated information from visual, vestibular and proprioceptors, it
sends commands to the following areas:
1. The help maintain visual focus, information is sent to the nuclei of cranial nerves III, IV and VI which
help control coupled movements of the eye.
2. The nuclei of cranial nerve (XI) help control neck and head movements, thus assisting in
equilibrium.
3. Impulses are sent down the spinal cord via the vestibulospinal tract to direct muscle tone in skeletal
muscles, helping to maintain equilibrium.
4. Conscious awareness of the position and movements of the head and limbs is made possible by
sending information to the ventral posterior nucleus in the thalamus which then communicates
with the vestibular area in the parietal lobe of the cerebral cortex.
The cerebellum plays a key role in equilibrium maintenance. Basically, it is constantly receiving
information from the utricle, saccule and semicircular canals regarding body positioning and equilibrium.
Thus, the cerebellum is able to continuously send input to the motor areas of the cerebrum, which can then
send out signals too the rest of the body.