Transcript The Inner Ear

The Inner Ear
Cochlea
• Amplifies sound waves
• Converts sound waves to neural signals
• Separates complex waveforms to
simpler elements.
Cochlea, physical properties
• A coiled up tube
• Basal end of tube, the oval window and
the round window
• Two membranes
– Basillar membrane
– Tectoral membrane
Cochlea, physical properties
• Fluid filled chambers
– Scala vestibuli
– Scala tympani
– Scala media - within the cochlear partition
Cochlea
• The scala vestibuli and the scala
tympani meet at the heliocotrema.
• The fluid in the scala vestibuli and the
scala tympani is the perilymph
Cochlea
• Inward movement of the oval window
moves the fluid of the inner ear
– The round window bulges
– The cochlear partition moves.
– The basilar membrane vibrates.
Basilar membrane
• Specific sound frequencies vibrate
specific regions of the basilar
membrane.
• Neurons along the basilar membrane
discharge at specific frequencies.
Frequency tuning
The basilar membrane is more flexible at
the apex than at the basal end.
Motion starts at the stiff part (basal end).
The basilar mb vibrates maximally at
specific locations depending on the
sound frequency.
Cochlea traveling wave
• Location of maximal displacement
determined by sound frequency.
• Base responds to highest frequency
(stiff)
• At the apex, lowest frequencies
(flexible)
• --> tonotopy
Complex sounds
• Pattern of vibration breaks down to the
component sounds
Outer hair cells
• Shear the tectoral membrane as the
basilar membrane vibrates.
• Bending of stereocillia leads to receptor
potentials in the hair cells.
Hair cells
• Transform vibrational energy to
electrical signals
• Very rapid, very sensitive
• Stereocilia contain actin.
Hair cells
• Stereocilia are graded in height
• Displacement towards the tallest cilia
– --> hair cell depolarizes
– Displacement towards the small cilia
– --> hair cell hyperpolarizes.
Hair cells
• Membrane potential can change within
10 microseconds.
• Direct mechanically gated ion channels
Hair cell transduction
• Tip links between adjacent stereocilia
directly open cation-selective
transduction channels (K+ channels)
• K+ flows inside because the endolymph
is high in K+.
Hair cell transduction
• Resting potential is -45 to -60 mV
• Some transduction channels are open
at rest
• Tip links open more channels, more
depolarization, calcium induced
exocytosis.
• When stereocilia move in the opposite
direction, the transduction channels
close.
• --> sinusoidal receptor potential.
• K+ channels are used to both depolarize and
repolarize.
• Endolymph is rich in K+, poor in Na+
• Tight junctions separate the endolymph from
the perilymph (scala tympani).
• The perilymph is Na+ rich and K+ poor.
• The endolymph is 80 mV more positive
than the perilymph (endocochlear
potential).
• The inside of the hair cell
– 45 mV more negative than the perilymph
– 125 mV more electronegative that the
endolymph
– At the stereocilia the 125 mV electrical
gradient moves the K+ into the cell through
open cation channels (cells already have
high K+)
• Opening of somatic K+ channels
favours repolarization into the perilymph
(low K+ in the perilymph) at the
basolateral mb
Ca++ channels
• Stimulate synaptic neurotransmission
• Open calcium dependant K+ channels
on the basolateral mb, more K+ leaves
the cell