Transcript Special senses - hearing and equilibrium

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Hearing – allows us to detect and interpret sound waves
Equilibrium – inform us of the position of the head in
space
The receptors for both senses are mechanoreceptors called
hair cells.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Ear: Hearing and Balance
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The three parts of the ear are the inner, outer, and
middle ear
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The outer and middle ear are involved with hearing
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The inner ear functions in both hearing and
equilibrium
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Receptors for hearing and balance:
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Respond to separate stimuli
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Are activated independently
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Region
Part
Description and function
External ear Auricle/pinna
Surrounds and protects the ear canal
and funnels vibrations toward the
auditory canal and eardrum
External acoustic * contains ceruminous glands that
canal
secret cerumen – waxy material that
protects the ear
* contains small hairs that protect the
ear and increase sensitivity
Tympanic
membrane (ear
drum)
* Converts sound waves into
mechanical movements
* separate external and middle ear
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Region
Part
Description and function
Middle
Pharyngotympanic Connects between the middle ear and the
ear/tympanic tube / Eustachian
nasopharynx. Has a role in equalization
cavity
of pressure inside and outside the ear
tube
drum
Auditory ossicles
3 bones:
mallus – attached to the eardrum
incus – middle bone
stapes – attached to the oval window
When the eardrum moves it moves the
ossicles which in turn move the oval
window
Tensor tympani and
stapedius muscles
Protect the eardrum and ossicles from
violent movement under very noisy
conditions.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Inner ear
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The inner ear is housed within a bony labyrinth.
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Passageways within the bone are lined with membranous
labyrinth.
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Between bone and membrane is a fluid called perilymph
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Endolymph fills the chamber within the membranous
labyrinth.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Bony
labirinth
http://thalamus.wustl.edu/course/audvest.html
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Cochlea
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The cochlea is divided into three chambers:
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Scala vestibuli
Reissner’s
membrane
Scala media
Scala
vestibuli
Scala tympani
Scala
media
Scala
tympani
Tectorial membrane
Auditory
nerve
Outer
hair cell
Basilar
membrane
Inner hair cell
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Cochlea
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The scala tympani terminates at the round window
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The scalas tympani and vestibuli:
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Are filled with perilymph
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Are continuous with each other via the helicotrema
The scala media is filled with endolymph
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sound and Mechanisms of Hearing
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Sound vibrations beat against the eardrum
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The eardrum pushes against the ossicles, which presses
fluid in the inner ear against the oval and round
windows
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This movement sets up shearing forces that pull on
hair cells
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Moving hair cells stimulates the cochlear nerve that
sends impulses to the brain
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Transmission of Sound to the Inner Ear
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The route of sound to the inner ear follows this
pathway:
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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
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Stimulation of the organ of Corti
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Generation of impulses in the cochlear nerve
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Organ of Corti
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Is composed of supporting cells and outer and inner
hair cells
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Afferent fibers of the cochlear nerve attach to the base
of hair cells
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The stereocilia (hairs):
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project into the endolymph
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Touch the tectorial membrane
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Excitation of Hair Cells in the Organ of Corti
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Bending cilia:
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Opens mechanically gated ion channels
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Causes a graded potential and the release of a
neurotransmitter (probably glutamate)
The neurotransmitter causes cochlear fibers to transmit
impulses to the brain, where sound is perceived
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Mechanisms of Equilibrium and Orientation
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Vestibular apparatus – equilibrium receptors in the
semicircular canals and vestibule
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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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings