April 21, 2014 – Hearing and equilibrium
1. What is sound and how is it perceived?
2. Functional anatomy of the mammalian (human) ear.
3. Distinguishing volume and pitch
4. Other modes of animal hearing
5. Ears and equilibrium in mammals
6. Equilibrium in other critters
What is sound?
Captures and funnels sound
to tympanic membrane
Townsend's big-eared bat
(Corynorhinus townsendii)
Harbor seal
(Phoca vitulina)
Amplifies sound and transfers
energy the oval window
Hearing- Mammals (con’t)
• Pathway:
1. Sound waves in air
travel through outer ear
until they reach
tympanic membrane
2. Sound waves vibrate
membrane, which in
turn vibrates 3 bones of
middle ear:
Malleus
Incus
Stapes
(outin: “MIS”)
Hearing- Mammals (con’t)
• Pathway:
3. Stapes transmits sound
to the oval window
-membrane on the
surface of cochlea
Detects sound frequency
transmits signal to the
auditory nerve
Hearing- Mammals (con’t)
• Pathway:
3. Stapes transmits sound
to the oval window
-membrane on the
surface of cochlea
4. Vibration on oval
window causes
vibration of perilymph
(fluid) in the cochlea
Side view of uncurled Cochlea
Vestibular
canal
Cochlear
duct
Tectorial
membrane
Hair cells
Axons of sensory
neurons
Tympanic
canal
Hair cells
Hearing- Mammals (con’t)
• Two Components of Sound Wave:
There are two main components of the sound
wave that are detected and used by the auditory
system:
1. Volume- Amplitude
(height of wave)
2. Pitch or Frequency
(no. of waves per unit time)
Hearing- Mammals (con’t)
1. Amplitude
larger amplitude= louder sound
-larger amplitude results in stronger pressure on the
hair cells, more rapid firing of action potentials
Hearing- Mammals (con’t)
1. Amplitude
larger amplitude= louder sound
-larger amplitude results in stronger pressure on the
hair cells, more rapid firing of action potentials
2. Pitch
- basilar membrane varies in thickness and flexibility
-base= narrow and stiff; stimulated by higher frequencies
-tip (apex)= wider and more flexible; stimulated by lower
frequenices
Cochlear animation
• http://www.hhmi.org/biointeractive/cochlea
Age-related hearing loss is often associated with a reduced ability to hear high-pitched
sounds. Propose a mechanism to explain this.
Blackburnian Warbler
(Setophaga fusca)
Cochlear implants
Charles Limb TED talk:
http://www.ted.com/talks/charles_limb_building_the_musical_muscle.html
Hearing in other vertebrates
1. Fish- have inner ear, but no tympanic membrane and no
opening to outside
-pathway: sound waves  skull inner ear  brain
-inner ear has small calcium carbonate particles
called otoliths
-stimulating otolith stimulates connected hair
cells
2. Amphibians
-have tympanic membrane on outside surface of body
-pathway: sound waves  tympanic membrane
middle ear bone  inner ear  brain
-single middle ear bone
Evolution of the vertebrate ear
• Inner fish
– http://www.pbs.org/your-innerfish/interactives/explore-your-inner-animals/
Hearing- Invertebrates
• Two main ways to detect sounds:
1. Hairs on body
-vibrate in response to sound waves
-vary in stiffness and length
2. Tympanic Membrane
-thin sheet stretched across an internal air
space
-localized on different parts of the body
-pathway: sound waves  tympanic
membrane  connected nerves  brain
Equilibrium- Mammals
1. Organs to detect body position and maintain balance located
in inner ear
a. Utricle and Saccule (2 parts of same organ)
-located next to oval window
-detect which direction is up and detect body position and
accleration
Semicircular Canals
b. Semicircular Canals (3 canals in total)
-next to utricle
-detect angular movements
2. Pathway:
a. Utricle and saccule contain clusters of hair cells embedded in a
gel called a cupula
b. Cupula contains otoliths
c. Cupula (with otoliths) is heavier than the endolymph (fluid) in the
utricle and saccule, so gravity is pulling the cupula down on to the
hairs of the hair cells
2. Pathway (con’t):
d. Changes in angle of body (i.e. changes in position of head)
change the force on the hair cells
-causes stimulation of some cells that weren’t
stimulated before
-causes some to increase/decrease their signals
Equilibrium- Mammals
3. Semicircular Canals
a. 3 canals (“loops”)
-1 for each plane:
side-to-side
front and back
up and down
Semicircular Canals
b. same mechanism of stimulation as for utricle and saccule
(cupula with otoliths, hair cells, etc.)
Equilibrium- Aquatic Organisms
Lateral Line System
1. Fish: lateral line on both sides of body
a. series of mechanoreceptors called neuromasts on body just
under the epidermis
b. Small openings (pores) in epidermis allow for water to
enter into lateral line canals
Equilibrium- Lateral Line (con’t)
Lateral Line System
c. Water stimulates clusters of hair cells in the neuromasts by
bending the cupula (gelatinous cap over the hair cells)
d. Stimulation causes release of neurotransmitters,
sending signals through sensory nerves to brain
Equilibrium- Invertebrates
Most invertebrates have sensory “organ”
called statocysts
a. Parts of statocyst:
-layer of ciliated receptor cells
surrounding an open chamber
-inside chamber are 1+ grains of
dense material called statoliths
1.
2. Pathway:
a. Gravity causes statoliths to settle downward
b. Once reach bottom of chamber, stimulating cilia of
receptor cells
c. Stimulated cells release neurotransmitters, stimulating
connecting sensory nerve fibers