Download The central auditory pathways

Lecture 11
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The auditory nerve enters the brainstem at
the pons-medulla junction, whereupon the
first order neurons terminate on the cell
bodies of the second order neurons in the
cochlear nuclei.
 There are several nuclei within this complex
but three are important in audition:
1. Anterior ventral cochlear nuclei (AVCN)
2. Posterior ventral cochlear nuclei (PVCN).
3. Dorsal cochlear nuclei (DCN).
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http://neuroscience.uth.tmc.edu/s2/chapte
r13.html
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Each first order neuron gives off collaterals or
branches which have synapses with second order
nerve cells located in different regions of the
dorsal-ventral cochlear nuclei
From this point on, the major ascending pathway
traditionally is described as crossing the midline
of the brainstem to the superior olivary complex
on the opposite or contralateral side.
That is, the second order neurons within the
cochlear nuclei give off axons which cross over
via the trapezoid body to terminate on third
order neurons the cell bodies of which are
located in the superior olive.
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The central auditory pathways
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As in the case of the cochlear nuclei, the
superior olivary complex is compromised of
various nuclei. But only or two seem
importance for hearing.
The majority of fibres originating from the
cochlear nuclei terminate in the medial olive
in humans but in the lateral nucleus in
animals such as cats or rodents.
Superior olivary nuclei assume different
relative sizes in different species (e.g the
medial olive is largest in man)
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The cochlear nuclei is located in the upper
medulla, while the Superior olivary complex is
located in the lower pons, within the
hindbrain.
Third order neurons in the superior olive give
off axons which course centrally through the
lateral lemniscus to terminate at the midbrain
level in the inferior colliculus.
From here, the fourth order neurons ascend
to the medial geniculate body, one of several
nuclei of the thalamus.
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Axons of the fifth order neurons then
terminate in the cell bodies in the auditory
cortex.
As noted in the previous lecture, the main
area within which these cell bodies are found
is the primary auditory cortex located on the
superior surface of the superior temporal
gyrus.
This pathway is the most familiar.
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Other ascending pathways within the
brainstem are substantial:
 Of particular importance is the ipsilateral
(same side) pathway which develops from
second order neurons in the cochlear nuclei.
-Crossed or uncrossed, fibers arising from
the cochlear nuclei need not terminate in the
superior olive but may go directly to the
inferior colliculus.
-These fibers ascend through the lateral
lemniscus
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Not all fibers ascending in this tract go directly to the
inferior colliculus.
Some fibers or collaterals may terminate in the nuclei
of the lateral lemnisci.
Fibers can also cross over to the opposite side at this
level.
Similarly, the inferior colliculus is not always a point
of termination.
Some ascending fibers also bypass the inferior
colliculus (on either side) to terminate directly in the
medial geniculate.
The last stop along the ascending pathway in the
brainstem before connection is made with the cortex.
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Ascending auditory fibers also may give
collaterals to other areas than nuclei which are
direct constituents of the central auditory
system.
The auditory neurons in the brainstem may give
off collaterals which go to the cerebellum.
The cerebellum is involved intimately in the
control of locomotion. This connection
undoubtedly facilitate reflexive movements
signaled by the acoustic stimuli, therefore,
avoiding more time consuming pathways up to
the cortex and back to this motor center.
This direct input would facilitate the sudden
removal of oneself from the path of oncoming
car
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Collaterals also are given off to terminate in a
highly diffuse neural structure known as the
reticular formation.
It is through the reticular formation that an
indirect route of communication is provided
between various parts of the brain.
The reticular formation is implicated strongly
in the control of the level of consciousness or
arousal and it receive information from all
sensory systems.
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There is also corticocochlear efferent system
or descending pathway.
It is more or less parallels the ascending
pathway.
However, only the pathway from the levelof
the superior olive down is known well.
Therefore, much attention has been focused
on this more peripheral protion of the
descending system which is formed by the
olivocochlear bundles.
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These efferent fibers course from the olivary
region through the internal auditory meatus
to innervate the inner and outer hair cells of
the organ of Corti.
Thus the brain not only receives information
from the periphery, it also can exercise
control over the peripheral system.
The two temporal lobes can communicate via
fibers of the commissure, the two halves of
the cerebrum.
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Thus there are points all along the central
auditory pathways which allow for possible
interaction between the two sides of the
system.
In other words, it is difficult to envision the
right side of the auditory CNS doing
something without the left side knowing
about it.
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Tuning curves providing best or characteristic
frequencies (CFs) have been reported for all the
levels of the auditory system from the cochlear
nuclie through the auditory cortex.
One of the most interesting aspects of the auditory
pathways is the relatively systematic representation
of frequency at each level.
There is virtual mapping of the audible frequency
range within each nuclear mass-neurons most
sensitive to high frequencies are in one area, those
sensitive to low frequencies are in another part,
and those sensitive to intermediate frequencies are
located successively between them.
This orderly representation of frequency according
to place is called tonotopic organisation.
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High frequencies are represented basally in
the cochlea, tapering down to low frequencies
at the apex.
This tonotopic arrangement is continued in
the auditory nerve, where the apical fibers are
found toward the core of the nerve trunk and
basal fibers in the on the outside of the
inferior margin.
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Reaction of the BM to sound varies from base
to apex according to the frequency of
stimulation. Studies cited by Moore (1997)
have shown that in normal healthy ears, each
point on the BM is sharply tuned.
This indicates that the response at each point
is very sensitive to a limited range of
frequencies, and that it requires higher
intensities to produce a response if the signal
frequency is outside this range.
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The frequency that gives maximum response
at a particular point along the BM is known as
the characteristic frequency (CF) for that
place.
Pure tones produce patterns with single
maxima, and the position depends on the
frequency, there is a frequency to place
conversion.
The place frequency map which was first
described by Liberman (1982) is the
relationship between the tuning of the
location along the cochlear partition and
distance from base to apex. This is
dependent on the CF.
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Fibers with high CFs innervate the base of the
cochlea while low CFs innervate the apex,
tonotopic organisation is the maintenance of
this neural spatial representation throughout
the nuclei of the central auditory pathway.
The neurons with different CF may represent
the basis of a neural circuit designed to
process spectral information, the ability of a
single neuron to process auditory information
depends on how inputs to the neurons
interact., excitation or inhibition
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Showing the tonotopic organisation of the basilar
membrane when stimulated by a musical note, Image
courtesy from
http://www.drcameronent.com/images/hearing.jpg
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The two most obvious parameters of the
sound stimulus should be encoded somehow
in the central auditory system:
Frequency
Intensity
The first order neurons will be very important
in this process.
The intensity of the stimulus must be
translated into a certain rate of discharge in
neuron.
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By virtue of the travelling wave in the cochlea,
the spike rate also will be frequencydependant so it is necessary to determine the
relationship between the activity of a single
auditory neuron and events occurring in the
cochlea.
First order auditory neurons respond to a
wide range of frequencies although not
equally to all frequencies.
The single unit response is frequency
selective.
As the stimulus frequency deviates from the
one frequency to which the unit is more
sensitive (CF)
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Temporal (time) encoding theories as the
basis of pitch perception have existed for
many years.
Helmholtz in the 19th century worked on
proving that the tuned string like fibers along
the basilar membrane constituted the place
mechanism for the frequency encoding.
Rutherford was working on the theory of
temporal encoding.
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Rutherford theory was that the basilar membrane
acts much like the diaphragm of the microphone.
The frequency of vibration of the diaphragm was
presumed to be preserved in the frequency of
the neural discharge, as if the neural discharges
of the first order neurons followed or were
perfectly in synchrony with the movements of the
basilar membrane.
However, by the end of the 20th century
physiologists were able to demonstrate that all
neurons are limited in how fast they can fire, due
to the limitations imposed by refractory period.
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This fundamental fact thus made Rutherford’s
telephone or frequency theory invalid since
the discharge rate commensurate
(correspond) with the upper frequency limit
of hearing in most animals are impossible for
any neuron.
For example, assuming a maximal rate of
2000 spikes/second this would mean that the
upper frequency cutoff for hearing is 2000 Hz
which is about one-tenth of the upper
frequency limit of hearing in man
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Only modification of the telephone theory only
gained acceptance.
This modification took the limitation in discharge
rates of neurons into account and was called
volley theory.
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It was postulated that whenever the frequency of
the stimulus exceeds the discharge rates of
neuron it is still possible to encode frequency
into a time pattern of neuronal discharges by
virtue of the discharge pattern of a group
neurons.
Even though the fiber cannot carry the
information , a group can.
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A frequency can be encoded by three
neurons:
One fiber may discharge at one cycle of the
stimulus but not recover sufficiently to fire
again until several periods later.
However, another fiber, which may have been
refractory during the first fiber discharge
might be ready to fire again.
Later a third fiber may be able to discharge.
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The encoding of frequency is likely to be the
result of a combination of place and time
encoding as suggested by Wever in his placevolley theory nearly three decades ago.
Wever noted that the place theory is quite
adequate for distinguishing between different
tones for all but the low frequencies.
Below about 400 Hz the selectivity of the
travelling wave envelope does not appear to be
sufficiently sharp to account for ability of the
humans and other animals to discriminate one
frequency from the other
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Thus the periodicity of the neural discharges is
properly the key component of frequency
encoding and the temporal code predominates.
Above 5000 Hz the place mechanism becomes
totally necessary because the neurons cannot
follow the frequency vibration of the basilar
membrane in any fashion.
Within the transition region between these two
frequencies, both place and temporal encoding
are involved in varying degrees in encoding of
frequency and perception of pitch.
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