Transcript 25. Organ of balance and hearing

Organ of balance and hearing
Ear
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
 Middle ear (Figure 15-11)
 Tiny, epithelium-lined cavity hollowed out of the
temporal bone
 Contains three auditory ossicles
 Malleus (hammer): attached to the inner surface of the
tympanic membrane
 Incus (anvil): attached to the malleus and stapes
 Stapes (stirrup): attached to the incus
 Openings into the middle ear cavity
 Opening from the external acoustic meatus covered with
tympanic membrane
 Oval window: opening into inner ear; stapes fits here
 Round window: opening into inner ear; covered by a
membrane
 Opening into the auditory (eustachian) tube
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Inner ear (Figure 15-12)
 Structure of the inner ear
 Bony labyrinth: composed of the vestibule, cochlea, and
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semicircular canals
Membranous labyrinth: composed of utricle and saccule inside the
vestibule, cochlear duct inside the cochlea, and membranous
semicircular ducts inside the bony semicircular canals
Vestibule and semicircular canal organs are involved with balance
Cochlea: involved with hearing
Endolymph: clear, potassium-rich fluid filling the membranous
labyrinth
Perilymph: similar to cerebrospinal fluid; surrounds the
membranous labyrinth, filling the space between the membranous
tunnel and its contents and the bony walls that surround it
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Cochlea and cochlear duct
 Cochlea: bony labyrinth
 Cochlear duct
 Lies inside the cochlea; only part of the internal ear
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concerned with hearing; contains endolymph
Shaped like a triangular tube
Divides the cochlea into the scala vestibuli, the upper
section, and the scala tympani, the lower section; both
sections filled with perilymph
Vestibular membrane: the roof of the cochlear duct
Basilar (spiral) membrane: floor of the cochlear duct
Organ of Corti: rests on the basilar membrane; consists of
supporting cells and hair cells; also called spiral organ
Axons of the neurons that begin around the organ of Corti
and extend in the cochlear nerve to the brain to produce the
sensation of hearing
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
 Sense of hearing
 Sound is created by vibrations
 Ability to hear sound waves depends on volume, pitch, and
other acoustic properties
 Sound waves must be of sufficient amplitude to move the
tympanic membrane and have a frequency capable of
stimulating the hair cells in the organ of Corti (spiral organ)
(Figure 15-13)
 Basilar membrane width and thickness varies throughout its
length
 High-frequency sound waves vibrate the narrow portion near the
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oval window
Low frequencies vibrate the wider, thicker portion near the apex
of the cochlea
Each frequency stimulates different hair cells and facilitates
perception of different pitches
Perception of loudness is determined by movement amplitude;
the greater the movement, the louder the perceived sound
Hearing results from stimulation of the auditory area of the
cerebral cortex
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Pathway of sound waves
 Enter external auditory canal
 Strike tympanic membrane, causing vibrations
 Tympanic vibrations move the malleus, which moves
the incus and then the stapes
 The stapes moves against the oval window, which
begins the fluid conduction of sound waves
 The perilymph in the scala vestibuli of the cochlea
begins a “ripple” that is transmitted through the
vestibular membrane to the endolymph inside the
duct, to the basilar membrane, and then to the organ
of Corti
 From the basilar membrane, the ripple is transmitted
through the perilymph in the scala tympani and then
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Neural pathway of hearing
 A movement of hair cells against the tectorial
membrane stimulates the dendrites that terminate
around the base of the hair cells and initiates
impulse conduction by the cochlear nerve to the
brainstem
 Impulses pass through “relay stations” in the nuclei
in the medulla, pons, midbrain, and thalamus
before reaching the auditory area of the temporal
lobe
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Vestibule and semicircular canals (Figure 1512)
 Vestibule: the central section of the bony
labyrinth; the utricle and saccule are the
membranous structures within the vestibule
 Three semicircular canals
 Each canal is at a right angle to the other
 Membranous semicircular ducts within the canals;
each contains endolymph and connects with the
utricle
 Each canal enlarges into an ampulla near junction
with utricle
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
 Sense of balance
 Static equilibrium: ability to sense head position relative to
gravity or acceleration/deceleration (Figure 15-14)
 Movements of the maculae, located in both the utricle and
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saccule, provide information related to head position or
acceleration
Otoliths are located within the matrix of the macula
Changing head position produces a change of pressure on the
otolith-weighted matrix, stimulating the hair cells that stimulate the
receptors of the vestibular nerve
Vestibular nerve fibers conduct impulses to the brain and sense
head position and a change in the pull of gravity
Righting reflexes: muscular responses to restore the body and its
parts to their normal position when displaced; caused by stimuli of
the macula and impulses from proprioceptors and from the eyes
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Dynamic equilibrium: needed to maintain balance when the head
or body is rotated or suddenly moved; able to detect changes in
direction and rate at which movement occurs (Figure 15-15)
 Depends on the functioning of the cristae ampullaris,
located in the ampulla of each semicircular duct
 Cupula: gelatinous cap where the hair cells of cristae are
embedded
 Does not respond to gravity
 Moves with the flow of endolymph in the semicircular ducts
 Semicircular ducts are arranged at nearly right angles to
each other to detect movement in all directions
 Hair cells bend as cupula moves, producing a receptor
potential followed by an action potential
 Action potential passes through the vestibular portion of the
eighth cranial nerve to the medulla oblongata
 Sent next to other areas of the brain and spinal cord for
interpretation, integration, and response
Lecture 16
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Basic Parts of the Ear
Fig. 19.20
 External ear: Hearing; terminates at eardrum
 Middle ear: Hearing; contains auditory ossicles
 Inner ear: Hearing and balance; interconnecting fluidfilled tunnels and chambers
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External Ear
 Auricle or pinna: elastic cartilage
• External auditory canal
 Tympanic membrane
External ear
Auricle
(pinna)
Inner ear
Middle ear
External auditory canal
Tympanic
membrane
Elastic
cartilage
Fig. 19.20
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Middle Ear
 Auditory or
eustachian tube
 Opens into
pharynx,
equalizes
pressure
 Ossicles:
malleus, incus,
stapes: transmit
vibrations
 Oval window
Fig. 19.2115-32
Labyrinth
 Bony
Inner Ear
 Cochlea: Hearing
 Vestibule: Balance
Lymphs
•
– In
membranous
labyrinth
 Semicircular canals: Balance
 Membranous
Endolymph
•
Perilymph
– Space
between
membranous
and bony
labyrinth
Fig. 19.22
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Structure of Cochlea
•
Membranous labyrinth of cochlea
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Scala vestibuli (perilymph)
Scala tympani (perilymph)
Cochlear duct (endolymph)
Fig. 19.27
Fig. 19.27
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Structure of Cochlea
•
Spiral organ (organ of Corti)
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•
Hair cells
• Stereocilia (microvilli)
Tectorial membrane
Cochlear nerve
Fig. 19.27
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Effect of Sound Waves on
Cochlear Structures
Fig.
19.28
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Balance
Two structural and functional components of balance in inner
ear
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Stationary Position and Linear Movement of Head
Evaluates position of head relative to gravity
Detects linear acceleration and deceleration
Utricle and saccule
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Maculae: Consist of hair cells embedded in statoconic membrane
containing otoliths
Fig. 19.23
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Vestibule in Maintaining
Balance
Fig. 19.24
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Balance
2. Rotational
Movements of Head
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–
Evaluates movements of
head (i.e. angular
acceleration)
3 semicircular canals
•
Ampulla
– Crista ampullaris
– Hair cells
– Cupula
Fig. 19.25
Vestibular nerve + Cochlear nerve = Vestibulocochlear nerve (VIII)
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Crista Ampullaris and Balance
Fig. 19.26
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Review Question
A person driving a car along a straight street suddenly sees an
animal dart in front of the car. He slams on the brakes and
manages to stop in time. The sensation of rapid deceleration is
generated by the
(a) Bending of the microvilli of the spiral organ
(b) Movement of perilymph fluid in the vestibule
(c) Movement of the gelatinous covering over the maculae
(d) Movement of endolymph fluid in the semicircular canals
(e) Movement of the cupula
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Points to Remember
 Inner ear functions for hearing and balance.
 Sound waves enter the external auditory canal,
impact tympanic membrane, vibrate middle ear
ossicles, strike oval window, create waves in
perilymph of scala vestibuli, increase pressure in
endolymph in cochlear duct, membrane
supporting hair cells vibrates, hair cells
stimulated, vibrations transferred to perilymph of
scala tympani, travel to round window and
dampened.
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Points to Remember
 Static balance is orientation of body relative to
pull of gravity - maculae of utricle and saccule
(static labyrinth) are sense organs of static
balance.
 Kinetic balance is maintenance of body position
in response to movement - crista ampullaris in
semicircular canals (kinetic labyrinth) are sense
organs of kinetic balance.
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