Chapter 17 Module 2 Ears and Hearing


This is Chapter 17, Module
2– Ears and Hearing. The learning objectives
of this module are one, describe the structures
of the ear and their roles in the process of hearing and
two, describe the structures and processes involved in
maintaining equilibrium. In the external
anatomy of the ear, the auricle or pinna is the
outer fleshy and cartilaginous part of the ear. This area protects the
opening of the auditory canal and it also direct sound
waves into the auditory canal. The external acoustic
meatus or auditory canal is a passageway
to the middle ear. At the end of the
external acoustic meatus is the tympanic
membrane or eardrum. The tympanic membrane is
a thin, transparent sheath that separates the external
ear from the middle ear. It is very delicate. Ceruminous glands are
along the auditory canal and they secrete a waxy material
called cerumen or earwax. Cerumen helps to slow the
growth of microorganisms. The middle ear is an
air filled space deep to the tympanic membrane. Within the middle ear
is the auditory tube which is a passageway
leading from the middle ear to the nasopharynx and
the mastoid air cells. This auditory tube
equalizes pressure on either side of the eardrum. An infection in the middle
ear is called otitis media. In the middle ear are
three tiny ear bones called auditory ossicles. These ear bones connect
the tympanic membrane to the internal ear. The names of the bones are
the malleus, the incus, and the stapes. Sound waves will
cause the eardrum to vibrate, which in turn
causes the ossicles to vibrate, which in turn causes the
inner ear to vibrate. There are two small
muscles that protect the tympanic membrane and the
ossicles from loud noises. The tensor tympani originates
on the temporal bone and inserts into the malleus. This muscle will stiffen
the tympanic membrane. The stapedius muscle inserts
into the stapes bone. This muscle will reduce
the movement of the stapes at the oval window
of the inner ear. The internal ear is
deep to the middle ear. The bony structure in the inner
ear is the bony labyrinth. The walls of the
bony labyrinth are continuous with
the temporal bone. Inside the bony labyrinth
is a membranous labyrinth containing a membranous tube. The space in between
the bony labyrinth and the membranous
labyrinth is filled with a fluid called perilymph. The space inside the
membranous labyrinth is called the endolymph. The bony labyrinth
can be divided into the vestibule, the
three semicircular canals, and the cochlea. The vestibule consists of
a pair of membranous sacs. The saccule and
the utricle, which provide sensations of gravity
and linear acceleration, are found within the vestibule. The semicircular canals
enclose the membranous tubes called the semicircular ducts. Receptors in the
semicircular ducts are stimulated by
rotation of the head. The vestibule and the
semicircular canals together are called
the vestibular complex. The cochlea is a spiral
shaped bony chamber that contains the cochlear duct
of the membranous labyrinth. Receptors within
the cochlear duct provide the sense of hearing. The bony labyrinth
surrounding the cochlea consists of dense bone except at
two areas that are membranous. The round window
separates the perilymph of the cochlea from
the middle ear. The oval window is
found under the stapes. Equilibrium is a
sensation that is provided by the receptors
of the vestibular complex. The semicircular canals
provide information about rotation of the
head– nodding “yes” or “no” or laterally bending your head. The receptors in the canals
give information about how rapid the movement is and in what
direction the movement is. The saccule and utricle
convey information about your position
with respect to gravity. If you stand with your
head tilted to one side, these receptors report
the angle involved and whether your head
tilts forward or backward. The saccule and utricle are
also stimulated by acceleration and deceleration. When your car
accelerates from a stop, the receptors in the
saccule and utricle give you the sensation
of increasing speed. The semicircular canal
contains three separate ducts– the anterior, posterior, and
lateral semicircular ducts. Each semicircular duct
contains an ampulla, which is an expanded region
that contains the receptors. The wall of the canal where
the receptors are found is called the crista. Within the crista
are receptors called hair cells and external
force can bend the hair cells, which release chemical
transmitters when stimulated. Each crista is bound
to a cupula, which is a gelatinous
structure that extends the full width of the ampulla. When your head rotates,
the endolymph fluid in the semicircular ducts move
along the semicircular duct. When you shake your head
“no,” the hair cells of the lateral semicircular
duct are activated. When you nod your head
“yes,” the hair cells of the anterior semicircular
ducts are activated. And when you tilt your
head from side to side, the hair cells of the
posterior semicircular ducts are activated. The utricle and saccule
provide information on whether the body is moving
or not, as well as equilibrium. The hair cells of the
utricle and saccule are clustered in oval
structures called [? macula. ?] The hair cell
processes are embedded in a gelatinous material. The surface of this
gelatinous material contains dense crystals
called statoconia. When your head is in the
normal, upright position, the statoconia sit
atop the macula. The weight of the crystals
pushes the hair cells down rather than to the side. When you tilt your
head, the pull of gravity on the statoconia
shifts some to the side, therefore distorting
the hair cell processes. This stimulates the hair cell to
tell the central nervous system that your head is
no longer level. The sensory neurons that are
activated by the hair cells will send action
potentials to the brain. The sensory information
will reach the brain stem and then cross to
the cerebellum. The receptors for hearing
are found in the cochlea. The cochlea is a
spiral structure that is filled with
a perilymph fluid. The cochlea has a membranous
duct inside its bony labyrinth. This duct is called
the cochlear duct. The cochlear duct divides
the bony labyrinth into two ducts– the vestibular
duct known as the scala vestibule, to and
the tympanic duct, known as the scala tympani. The outer surfaces of these
ducts are the bony labyrinth. The vestibular
duct is continuous with the tympanic ducts at the
tip of the cochlear spiral. The vestibular duct
starts at the oval window and extends to the tip
of the cochlear spiral and then continues back
to the round window as the tympanic duct. The cochlear duct
is the structure the divides the vestibular
duct from the tympanic duct. The hair cells of
the cochlear ducts are located in the organ
of Corti– a structure also called the spiral organ. The organ of Corti sits
on the basilar membrane, which is a membrane that
separates the cochlear duct from the tympanic duct. The hair cell extensions
are in contact with the tectorial
membrane which lays over the top of the hair cells. The tectorial membrane is firmly
attached to the inner wall of the cochlear duct. When a portion of the basilar
membrane bounces up and down, the hair cell
extensions are pressed against the tectorial
membrane and become distorted. The basilar membrane
moves when fluctuations within the perilymph move it. The pressure fluctuations
within the perilymph are triggered by
sound waves that vibrate the tympanic membrane. Hearing is the
perception of sound. The pitch of sound is
our sensory response to its frequency
of a sound wave. Amplitude of sound is
our sensory response to the intensity
of a sound wave. In the hearing process,
the sound waves enter the external
acoustic meatus and travel toward the tympanic membrane. The tympanic membrane
vibrates from the sound waves and causes the displacement
of the auditory ossicles. That is, when the tympanic
membrane vibrates, so do the malleus, the
incus, and the stapes. The stapes is connected
to the oval window and when the stapes vibrates,
it causes the oval window to bulge. This action causes
pressure waves in the perilymph fluid on the
other side of the oval window in the vestibular duct. The pressure waves
that were created in the perilymph fluid
in the vestibular duct will distort the
basilar membrane as it comes in contact
with the cochlear duct. The pressure waves first
distort the vestibular membrane of the cochlear duct
and then the basilar membrane of the cochlear duct. The location of
the distortion is different for every frequency. If the frequency of the
sound is high pitched, it has a very short wavelength
and will distort the basilar membrane closer to the
base of the cochlear duct. If the frequency of
sound is low pitched, it has a long wavelength
and will distort the basilar membrane closer to the
tip of the cochlear duct. When the basilar membrane
becomes distorted, it causes a pressure wave
in the tympanic duct. This pressure will travel in the
tympanic duct toward the round window. The round window
will bulge out when the pressure wave
hits it and then helps to absorb that pressure. The vibration that occurs
in the cochlear duct will cause the basilar
membrane to vibrate. This vibration
moves the hair cells against the tectorial membrane. The extensions of the hair cells
bend as the hair cells move against the tectorial membrane. Stimulating the hair
cells by bending them will result in
the opening of sodium gated channels in
the hair cells. This causes depolarization
of the hair cells, leading to an action
potential in the axon and a release of
neurotransmitters. The release of
neurotransmitters will stimulate the sensory neuron. The sensory neurons are
bipolar sensory neurons. A very soft sound may stimulate
only a few hair cells. With more intensity, more
hair cells will become active. The number of hair
cells responding give information to the
intensity of the sound. The action potentials
in the sensory neuron will be relayed to the
central nervous system over the cochlear branch
of the cranial nerve VIII– vestibularcochlear nerve. Cranial nerve VIII then synapses
in the medulla oblongata onto a neuron that delivers the
information to the thalamus. At the thalamus,
another synapse occurs and the information
is then delivered to the auditory cortex and
other regions of the brain. This ends Chapter 17,
Module 2– Ears and Hearing.

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