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PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
15
The Special
Senses:
Part C
Copyright © 2010 Pearson Education, Inc.
The Ear: Hearing and Balance
• The three parts of the ear are the inner, outer, and
middle ear
• The outer and middle ear are involved with hearing
• The inner ear functions in both hearing and
equilibrium
• Receptors for hearing and balance:
• Respond to separate stimuli
• Are activated independently
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The Cochlea
• The cochlea is divided into three chambers:
• Scala vestibuli
• Scala media
• Scala tympani
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The Cochlea
• The scala tympani terminates at the round window
• The scalas tympani and vestibuli:
• Are filled with perilymph
• Are continuous with each other via the helicotrema*
• The scala media is filled with endolymph
The helicotrema is the part of the cochlear labyrinth where the scala typmani and the scala
vestibuli meet.
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http://www.ece.rice.edu/~dhj/cochlea.html
http://www.egms.de/figures/journals/cto/2005-4/cto000007.f3.png
Copyright © 2010 Pearson Education, Inc.
Properties of Sound
• Sound depends on elastic medium for its transition (can not be
transmitted in vacuum).
• Sound is:
• A pressure disturbance (alternating areas of high and low
pressure) originating from a vibrating object
• Composed of areas of rarefaction (less molecules) and
compression (more /compressed molecules)
• Represented by a sine wave in wavelength, frequency, and
amplitude
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Properties of Sound
• Frequency – the number of waves that pass a given point in
a given time
• Wavelength – the distance between 2 consecutive crests; it is
constant for a particular tone
• Pitch – perception of different frequencies (we hear from
20–20,000 Hz)
• The higher the frequency – the higher the pitch
• Amplitude – Height of the wave - loudness
• Loudness – subjective interpretation of sound intensity
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Transmission of Sound to the Inner Ear
• The route of sound to the inner ear follows this pathway:
• Outer ear – pinna, auditory canal, eardrum
• Middle ear – malleus, incus, and stapes to the oval
window
• Inner ear – scalas vestibuli and tympani to the cochlear
duct
• Stimulation of the organ of Corti
• Generation of impulses in the cochlear nerve
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Transmission of Sound to the Inner Ear
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Figure 15.31
http://www.britannica.com/eb/art-536?articleTypeId=1
Resonance of the Basilar Membrane
• As the stapes rocks back and forth against the oval window, it
moves the perilymph in the scala vestibuli into a similar backand-forth motion
• A pressure wave travels through the perilymph from the basal
end toward the helicotrema.
• Sounds of very low frequency (below 20 Hz) create pressure
waves that take the complete route through the cochlea
toward the round window through the scala tympani.
• Such sounds do not activate the spiral organ (are below the
threshold of hearing).
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http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/sound/u11l2d.html
Resonance of the Basilar Membrane
• Sound waves of low frequency (inaudible):
• Travel around the helicotrema
• Do not excite hair cells
• Audible sound waves:
• Penetrate through the cochlear duct
• Vibrate the basilar membrane
• Excite specific hair cells according to frequency of
the sound
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The Organ of Corti
• Is composed of supporting cells and outer and inner hair cells
• The hair cells are arranged in one row of inner hair cells and
three rows of outer hair cells - sandwiched between the tectorial
and basilar membranes.
• Afferent fibers of the cochlear nerve are in contact with the
bases of the hair cells.
• The hair cells have numerous stereocilia (actually long
microvilli) and a single kinocilium (a true cilium) project from
their apices.
• The “hairs” (stereocilia) of the hair cells are stiffened by actin
filaments and linked together by fine fibers called tip-links
• They project into the K+-rich endolymph, and the longest of
them are embedded in the overlying tectorial membrane
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Excitation of Hair Cells in the Organ of Corti
• Transduction of sound stimuli occurs after the stereocilia of the hair
cells are turn aside by movements of the basilar membrane.
• Bending the cilia toward the kinocilium puts tension on the tiplinks, which in turn opens cation channels in the adjacent shorter
stereocilia.
• This results in an inward K+ (and Ca2+) current and a graded
depolarization
• Depolarization increases intracellular Ca2+ and so increases the
hair cells’ release of neurotransmitter (glutamate), which causes
the afferent cochlear fibers to transmit a faster stream of
impulses to the brain for auditory interpretation.
• Bending the cilia away from the kinocilium relaxes the tip-links,
closes the mechanically gated ion channels, and allows
repolarization and even a graded hyperpolarization.
Copyright © 2010 Pearson Education, Inc.
http://www.keele.ac.uk/depts/co/auditory/pages/projects.htm
http://www.wadalab.mech.tohoku.ac.jp/corti-e.html
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Excitation of Hair Cells in the Organ of Corti
• The outer hair cells send little information to the brain. Instead, they
act on the basilar membrane itself.
• Most (90-95%) nerve fibers around the OHC are efferent (from the
brain to the ear)
• In response to sound, the OHC send signals to the medulla and the
pons sends immediately signals back
• In response, The OHC contract by about 15% of their height
• Because the OHC are attached to the basiliar membrane and the
tectorial membrane, contraction decrease the ability of the basiliar
membrane to vibrate.
• As a result, some areas of the duct send less signals to the brain
which allow the brain to distinguish between more and less active
hair cell.
• Give a more precise perception of different pitches
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Mechanisms of Equilibrium and Orientation
• Vestibular apparatus – equilibrium receptors in the
semicircular canals and vestibule
• Maintains our orientation and balance in space
• The position of the body with respect to gravity (static
equilibrium) – the vestibule
• The motion of the body (dynamic equilibrium) – the
semicircular canals
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Static equilibrium
• The receptors for static
equilibrium are the maculae –
one in the urticle and one in the
saccule
• The utricle is sensitive to a
change in horizontal movement,
• The saccule gives information
about vertical movement
Copyright © 2010 Pearson Education, Inc.
http://www.tchain.com/otoneurology/disorders/unilat/utricular.html
Anatomy of Maculae
• Maculae are the sensory receptors for
static equilibrium
• Contain supporting cells and hair
cells
• Each hair cell has stereocilia and
kinocilium embedded in the otolithic
membrane
• Otolithic membrane – jellylike mass
covered with tiny CaCO3 stones called
otoliths
• Utricular hairs respond to horizontal
movement
• Saccular hairs respond to vertical
movement
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Effect of Gravity on Receptor Cells
• When the head starts or stops moving in a linear direction, the otolithic
membrane slides backward or forward like a plate over the hair cells,
bending the hairs.
• The hair cells release neurotransmitter continuously but movement of
their hairs modifies the amount they release.
• When the hairs are bent toward the kinocilium, the hair cells depolarize,
increasing their pace of neurotransmitter release, and a faster stream of
impulses travels up the vestibular nerve to the brain
• When the hairs are bent in the opposite direction, the receptors
hyperpolarize, and neurotransmitter release and impulse generation
decline.
• In either case, the brain is informed of the changing position of the head
in space.
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Crista Ampullaris and Dynamic Equilibrium
• The crista ampullaris (or crista):
• Is the receptor for dynamic equilibrium
• Is located in the ampulla of each semicircular canal
• Responds to angular movements
• Each crista has support cells and hair cells that extend
into a gel-like mass called the cupula
• Dendrites of vestibular nerve fibers encircle the base
of the hair cells
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Activating Crista Ampullaris Receptors
• The cristae respond to changes in the velocity of rotation
movements of the head.
• the endolymph in the semicircular ducts moves briefly in the
direction opposite the body’s rotation, deforming the crista in the
duct.
• As the hairs are bent, the hair cells depolarize and impulses reach
the brain at a faster rate.
• Bending the cilia in the opposite direction
hyperpolarization and reduces impulse generation.
causes
• Because the axes of the hair cells in the complementary
semicircular ducts are opposite, rotation in a given direction
causes depolarization of the receptors in one ampulla of the pair,
and hyperpolarization of the receptors in the other
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Rotary Head Movement
Figure 15.37d
http://www.unmc.edu/Physiology/Mann/pix_9/left_mvt.gif
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Equilibrium Pathway to the Brain
• Pathways are complex and poorly traced
• Impulses travel to the vestibular nuclei in the brain stem or the
cerebellum, both of which receive other input
• Three modes of input for balance and orientation
• Vestibular receptors
• Visual receptors
• Somatic receptors
Copyright © 2010 Pearson Education, Inc.