Hearing Part 2
Tuning Curve
• Sensitivity of a single sensory neuron to a
particular frequency of sound
• Two mechanisms for fine tuning of sensory
neurons,
– Mechanical=location along the basilar
membrane
– Electrical=natural oscillations in membrane
voltage
Where are the hairs cells and
Neurons located along the
Basilar membrane in the
Organ of Corti? That dictates
Their Tuning Curve.
Frequency of cilia movement
Matches the innate oscillation
In membrane potential
Frequency Tuning in Hair Cells
• Hair cells have oscillating waves of
depolarizations that are unique to each hair cell
• If the frequency of sound wave matches the innate
oscilliation of membrane depolarization, then the
response amplitude is large
• A high frequency tuned hair cell has strong
depolarization in response to high frequency
sound waves but small depolarization to low
frequency sound waves.
Oscillation In Membrane
Potential
• Due to voltage sensitive calcium channel
and calcium sensitive potassium channel
• Frequency of oscillation depends on time
delay between opening of calcium channels
and opening of potassium channels
• Density of ion channels and different
sensitivity of potassium channel to calcium
levels
Innate Characteristics
• High Frequency Hair
Cells
• Short delay between
opening of calcium
and potassium
channels
• Low Frequency Hair
Cells
• Long Delay between
opening of calcium
and potassium
channels
Efferent Synapses
• Brainstem neurons innervate outer hair cells
and reduces sensitivity of cochlea to sound
and inhibits sensory output to brain
• Ach acts on muscarinic receptors to open
potassium channels in hair cell
• Leads to hyperpolarization, inhibits
oscillation of membrane potential & inhibits
cilia movement
Attenuation-Efferent Synapse
• Muscles to middle ear bone and tympanic
membrane can contract and relax modulating
movement of basilar membrane
• Muscles on the ossicles:
– tensor tympani attached to maleus and bones of the ear
canal
– stapedius attached to stapes
• Muscle contraction makes the ossicles more rigid
and do not move the oval window as much (50100 msec delay in response to loud sound)
• Works best with low frequency sound
CN8 Connection to Brain
• Brainstem-medulla, pons, midbrain
• Thalamus=medial geniculate nucleus
• Cortex=primary and secondary auditory
cortex
Parallel Connections
• Central process of Spiral Ganglion Neurons
innervate cochlear nucleus and then the
cochlear neuron branches to innervate 3
additional cochlear nuclei in medulla
Sound Information Transduced
• Frequency
• Intensity
• Point of Origin
Each are transmitted separately to brain
Tonotopy
• Systematic representation of sound frequency
along the length of basilar membrane.
• This is maintained at all levels of ascending
information until after primary auditory cortex in
temporal lobe
• Individual neurons at each brain level respond to a
characteristic frequency innervate adjacent
neurons so there is a geometric representation of
the basilar membrane at each level of the auditory
pathway
Tonotopic Map
• In the cochlear nucleus in the medulla, there
is a map of the basilar membrane, ie
neurons are organized from low to high
frequency responses, so neighboring
neurons respond to similar frequencies.
Monaural and Binaural
• Information from 1 ear and from 2 ears.
• A neuron can receive monaural or binaural
inputs
• Interaural=between ears=measuring the
time difference it takes for the sound waves
to reach each ear. Can detect 10-700usec =
to 1 degree
Cochlear Nucleus
• First synapse in brain is cochlear nucleus in
medulla of brainstem
• Tonotopic map of cochlea is maintained so that
adjacent areas of cochlea are mapped to adjacent
areas of cochlear nucleus-allow frequency to be
decoded
• Some cochlear neurons fire at the beginning, end
our during sound. Keep track of PHASE
Superior Olivary Nucleus
• Give rise to Efferent neurons to cochlea
• Used to detect sound location; have binaural
inputs from each cochlear nucleus
• Detects time of the arrival of sound to detect
direction of sound. Sound to each ear reaches
there at different times if sound is not directly
ahead or behind you.
• Detects difference in intensity between sound
reaching each ear
Inferior Colliculus
• Major connection for ascending auditory
information in brainstem is the inferior
colliculus in the midbrain
• Receives bilateral direct input from cochlear
nucleus and indirect input from superior
olivary nucleus=parallel processing
Function of IC
• Contains (barn owl) a space diagram
• Neurons will respond only to sound coming
from specific areas of space
• Have preferred elevation and horizontal
location
• Also process temporal patterns of sound
important for behavior
Medial Geniculate Nucleus
• In thalamus, MGN is the gateway to
auditory cortex
• Ipsilateral connection but since input was
bilateral previously each MGN receives
binaural input
Function of MGN
• Responds to selective combination of
frequencies
• Responds to specific time intervals of sound
Auditory Cortex
• Located in superior part of temporal lobe
• Input is mapped to cortex in tonotopic
fashion
• Cortex has 2 functional columns, 1 for
frequency in which neighboring cells in
cortex have similar frequencies varying
from low to high across the cortex
Primary cortex
• Tonotopic and receives precisely mapped
information form MGN
• Process combination of frequencies
• Process modulations of amplitude or
frequency
Binaural Columns
• Summation columns, neurons respond
strongly if stimulus arrives simultaneously
to both ears
• Suppression column, responds best to sound
in 1 ear only
Secondary Auditory cortex
• Also referred to as belt areas
• Involved in understanding speech, ie
recognizing temporal organization of sound
• Wernicke’s area in secondary cortex when
damaged patients cannot understand speech
because the sounds are all out of order
Nice diagram-thought it
Might help
Sound Intensity
Encoded by rate of firing and # of active
neurons
The greater the amplitude of sound wave the
greater distance along basilar membrane
that moves—activate more hair cells and
more SGNs
Phase Locking
• Neurons fire in phase with some point of
the sound wave
• either at the peak, valley or in between but
it is constant for that neuron
• This firing rate will then tell you the sound
frequency
Volley Principal
• As the frequency of sound gets higher 1K4K Hz then groups of neurons fire in phase
• So one neuron will fire every 4th cycle in
phase
• Above 4 K only tonotopy is used to encode
frequency information
Sound Localization
• Horizontal Plane
• Vertical Plane: Based on reflection of sound
from pinna that delay arrival of sound as
compared to sound waves that do not reflect
off pinna
Interaural Time Delay
• Difference in time it takes for sound to
reach each ear if sound is not coming from
directly in front or behind you
• Head size 20 cm, sound delay 0.6 msec
• Use for detecting direction of a sudden
sound
• Used for frequencies of 20-2000Hz
Phase Detection
• Used to localize continuous sound
• Sound waves (200Hz) travel 172 cm
/second
• So the cycle of the sound wave will be 0.6
msec delayed from ear to ear and will be at
a different phase.
Interaural Intensity Differences
• Your head blocks sound so intensity is less
in the ear away from the sound
• Your brain detects and computes this
differences in intensity to localize sound
direction.
• Used at frequencies of 2000-20,000 Hz
Localization
• At frequencies below 3kz, interaural time
differences alone are used to localize sound
since you have exact representation of
frequency with sensory neuron firing.
• At higher frequencies changes in intensity
are used as well. Occurs in superior Olive