Download Aud & Equil

Mechanoreception –
Audition and Equilibrium
Hearing and Equilibrium
Figure 10.37
The Middle Ear
Inner ear
• Membranous labyrinth contains endolymph
• Bony labyrinth surrounds and protects
membranous labyrinth
– Vestibule
– Semicircular canals
– Cochlea
The Inner Ear
Components of the inner ear
• Vestibule contains the utricle and saccule
• Semicircular canals contain the semicircular ducts
• Cochlea contains the cochlear duct
Windows
• Round window separates the perilymph from the air
spaces of the middle ear
• Oval window connected to the base of the stapes
• Basic receptors of inner ear are hair cells
– Provide information about the direction and strength of
stimuli
Hearing
• Sounds are waves of compressed air traveling
through space
- sound intensity wave height
- pitch  wave frequency
Organ of hearing (and equilibrium) – inner
ear
• Cochlea
• Vestibular
apparatus
Hearing
1- The sound waves enter the
external auditory canal and
trigger vibrations of the tympanic
membrane
2- The tympanic membrane induces
a vibration of the ossicles
3- the last ossicle, the stapes,
transmits amplified vibrations to
the oval window
4- The vibrations induce waves in the
perilymph of the various inner
ear chambers
5- the round window absorbs excess
energy and prevent wave
reverberation
6- the fluid wave is transduced into
an electrical signal by the
auditory receptors, the organs of
Corti located on the basilar
membrane
Receptors for Sound: The Organ of Corti
• The hair cells of the organ
of Corti transduce fluid
wave into an electrical
signal
• The energy of the wave
causes the basilar and
vestibular membrane to
move, thus displacing the
cilia from the organ of Corti
The Organ of Corti
Hearing
• Cochlear duct lies between the vestibular duct and the
tympanic duct
• Hair cells of the cochlear duct lie within the Organ of Corti
• Intensity is the energy content of a sound
– Measured in decibels
The Cochlea
Pathway of Sound
• Sound waves travel toward tympanic membrane, which vibrates
• Auditory ossicles conduct the vibration into the inner ear
– Tensor tympani and stapedius muscles contract to reduce the
amount of movement when loud sounds arrive
• Movement at the oval window applies pressure to the perilymph
of the cochlear duct
• Pressure waves distort basilar membrane
• Hair cells of the Organ of Corti are pushed against the tectoral
membrane
Sound and Hearing
Sound and Hearing
Neural pathway
• Sensory neurons of hearing are located in the
spiral ganglion of the cochlea
• Afferent fibers form the cochlear branch of
cranial nerve VIII
– Synapse at the cochlear nucleus
Signal Transduction
•
Movements of the cilia open or close
potassium channels  changes in the
state of polarization of the hair cell
•
Changes in potassium leakage due to
cilia bending trigger changes in
neurotransmitters exocytosis
•
The neurotransmitters send an
electrical signal to an afferent neuron
of the cochlear nerve
•
The louder the sound, the more the
cilia bend, the more numerous are
the APs produced
Coding for Pitch
• The location of the organs
of Corti on the basilar
membrane codes for pitch
- Organs of Corti located near
the oval window are more
sensitive to high pitch
sounds while the ones
located toward the tip of
the cochlea respond more
readily to low pitch sound
Coding for sound intensity
Neural Pathway for Sounds
• Cochlear nerve  nucleus in medulla
oblongata  thalamus  auditory cortex in
the temporal lobe
• So, how do we perceive the direction from
which a sound is coming from?
Equilibrium
• Anterior, posterior and lateral semicircular ducts
are continuous with the utricle
– Each duct contains an ampulla with a gelatinous
cupula and associated sensory receptor
• Saccule and utricle connected by a passageway
continuous with the endolymphatic duct
– Terminates in the endolymphatic sac
– Saccule and utricle have hair cells clustered in
maculae
– Cilia contact the otolith (statoconia)
Equilibrium
• Ability to detect head
position and movement (or
acceleration)
- Change of speed = linear
acceleration (utricle and
saccule)
- Turning = rotational
acceleration (semi-circular
canals)
Utricle and saccule
• Sensory cells have cilia
extending into a gelatinous
material topped by otoliths
• Saccule detects backwardfrontward movement
• Utricle detects changes
relative to gravity
The Vestibular Complex
Semi-circular canals
• The receptors in the
ampulla are hair cells with
cilia extruding into a
gelatinous mass (cupula)
• When the head rotates, the
cupula moves  cilia pulled
APs (vestibular nerve 
cerebellum …)
The Vestibular Complex
The Vestibular Complex
Vestibular Neural Pathway
• Vestibular receptors activate sensory neurons of the
vestibular ganglia
• Axons form the vestibular branch of cranial nerve VII
• Synapses within the vestibular nuclei
Pathways for Equilibrium Sensation
• So why does a person become dizzy after
he/she stops spinning?