Ascending auditory pathway

THE ASCENDING
AUDITORY PATHWAY


The output of the cochlea travels along the auditory
nerve fibres for a short distance in the cochlear nerve
and then enters the brainstem.

There regions that participate in the afferent auditory
pathway between the cochlear nerve and auditory
cortex in ascending order are, the
– cochlear nuclear complex,
– superiory olivary complex,
– inferior colliculus,
– medial geniculate nucleus and then
– auditory cortex.

There are commissural connections at various points
and multiple collaterals that make the pathway very
intricate.


The cochlear nuclear complex is divided into
– dorsal cochlear nucleus (DCN)
– anteroventral cochlear nuclei (AVCN) and
– posteroventral cochlear nuclei (PVCN).

The central processes of type I spiral ganglion
neurones enter the cochlear nuclear complex and
immediately bifurcate, sending branches to the DCN or
PVCN and the AVCN.

Low frequency fibres divide ventrally, and high
frequency fibres dorsally.

The cochleotopic map of frequency, represented
anatomically by the distribution of fibres in the auditory
nerve, is maintained across the cochlear nuclei as a
tonotopic map of neurones responding to progressively
higher frequency from one side to the other.

The auditory nerve afferents in the AVCN terminate on the
spherical/bushy cells, they are the principal projection
neurones of the cochlear nuclear complex, and are called
because they have round cell bodies and bushy dendritic
fields.

The end of most auditory nerve fibres expands into a
single very large terminal, the end bulb of Held, which
cups around the soma of the spherical cell.

This large excitatory terminal contains large numbers of
round neurotransmitter vesicles typical of glutamatergic
terminals.

It also ensures rapid transmission of the signal from the
auditory nerve fibre and preserves the original frequency
selectivity and sensitivity of the cochlear response.

These cells have electro physiological responses to
sound that are called primary-like because they reflect
the primary input from Auditory nerve fibres.


In the PVCN are octopus cells, which have an
extended dendritic field which lie across a number of
auditory nerve fibres, so that they receive input
representing a range of different frequencies.

These cells respond rapidly and are responsible for
determining the precise time of arrival of sounds.

They also send signals to motor nuclei in the brain
stem and midbrain and are involved in acoustic startle
responses, (where loud or unexpected sounds evoke
movement).

The cochlear nuclei also contain interneurones and
receive inputs from higher up the auditory pathway that
produces inhibition and generates more complex
responses in some neurones.


The auditory pathway splits as it leaves the cochlear
nuclear complex.

The dorsal pathway projects directly to the inferior
colliculus, the ventral pathway divides further and projects
to both the ipsilateral and contralateral superior olivary
complex.

Superior olivary complex is the first part of the ascending
auditory pathway where major binaural comparisons can
be made.

The superior olives receive binaural information from
spherical/bushy cells.

This arises from collaterals from the output fibres of the
cochlear nuclei on the same side that then cross over to the
opposite superior olivary complex.

This enables the superior olives to function in sound
localization.


Each superior olivary complex contains an S-shaped
lateral olivary nucleus, a disc-shaped medial olivary
nucleus and the medial nucleus of the trapezoid body
together with smaller periolivary nuclei.

Within the medial olivary nucleus, there are neurones
that use the binaural inputs to compare the time of arrival
of sounds to each ear.

Sound localization at higher frequencies may be carried
out by comparing sound intensities.

Neurones that detect differences in sound intensity are
located in the lateral superior olives.

Most of these neurones receive an excitatory input from
the ipsilateral cochlear nucleus and an inhibitory one
from the contralateral cochlear nucleus.


There are four elevations on the surface of the midbrain,
which together form the corpora quadrigemina.

These are composed of the two superior and two
inferior colliculi.

The inferior colliculi receive direct input from the
brainstem auditory nuclei via a tract called the lateral
lemniscus.

Each of the inferior colliculi consists of a
– central nucleus which receives the major auditory
input, and an
– outer region composed of a dorsal cortex and an
external lateral cortex.

The external portions of the inferior colliculus receive
connections from cerebral cortex and from mutimodal
sources respectively.


The central nucleus is layered into isofrequency bands.

Along each band, the cells have flattened dendritic fields
and respond best to approximately the same frequency.

The higher frequency bands are located towards the
midline of the brain, low frequency bands more laterally,
producing a tonotopic map.

Superimposed on each band is another map that relates
to intensity.

The cells in the centre of the disc have low thresholds,
whilst to the periphery of the disk there are concentric
areas in which the threshold of the neurones increases.

These intersecting maps in the inferior colliculi are thus
able to extract complex features of sounds thus
recognizing the patterns in sound.

MEDIAL GENICULATE NUCLEI
AND AUDITORY CORTEX


The thalamus contains three regions where auditory
influence occurs,
– the medial geniculate body,
– the posterior nucleus and
– part of the reticular nucleus of the thalamus.

The geniculate nuclei have three major divisions each
receiving a separate, parallel pathway from the inferior
colliculus.

The ventral division is organized tonotopically into
isofrequency layers, receiving its input from the central
nucleus of the inferior colliculus.

The dorsal division recieves the diffuse pathway and is not
tonotopically organized, arising from the dorsal cortex of the
inferior colliculus.

The medial division receives multimodal inputs from
external lateral cortex of the inferior colliculus.


The main projection to the primary auditory cortex arises
from the ventral division of the medial geniculate nucleus
and terminates in area A1, corresponding to Brodmann's
area 41, within the lateral fissure of the temporal lobe.

The dorsal division of the medial geniculate nucleus
projects to the non-primary auditory areas around A1.

The medial division projects diffusely to the whole region
and to surrounding cortical fields.

A1 is also organized into isofrequency layers arranged
tonotopically from low frequency in the rostral end to high
frequency in the caudal end.

Most cells within A1 respond to binaural stimulation.


There are two main types of response:
– neurones that summate excitatory responses
from both ears and
– neurones that receive excitatory stimulation from
one ear and inhibitory stimulation from the other.

Bands of cells displaying excitation-excitation and
excitation-inhibition responses run alternately across the
isofrequency layers.

The main function of these cells is sound localization.


There are descending projections from each of the
stations of the ascending auditory pathway, down as far
as the cochlear nuclei and from the superior olivary
complex to the cochlea.

The olivo-cochlear feedback loop is a major
descending projection.

Medial efferent system:

Originates adjacent to the contralateral medial superior
olives, and crosses the midline. Its fibres are myelinated,
and constitute the crossed olivocochlear bundle.

They contribute largely to the efferent projection to the
outer hair cells.

They function to suppress outer hair cell motility to make
the cells less sensitive, providing protection from very
loud sounds.

Lateral efferent system

A smaller number of unmyelinated efferents originate
from the lateral superior olive ipsilaterally and contribute
mainly to the efferent projection synapsing with the
peripheral processes of type I spiral ganglion neurones
beneath the inner hair cells.

They may be useful in maintaining accurate binaural
comparisons.

The pinna increases the pressure at the tympanic membrane
in a frequency sensitive way, thus emphasizing certain
frequencies in the input.
Second, it increases the pressure in a way that depends on
the direction of the sound source, and can therefore be
used as an aid to sound localization.

The gain in sound pressure at the tympanic
membrane
The pinna-concha system itself can act like an ear trumpet,
catching sound over a large area and concentrating it in
the smaller area of the external meatus. Thus the total
energy available to the tympanic membrane is increased.
A resonance in the external auditory meatus changes the
sound pressure at the tympanic membrane in a frequency-
selective way.
If a tube is one-quarter of a wavelength long, and one end is
open while the other is blocked with a hard termination, the
pressure will be low at the open end and high at the closed
end when the tube is placed in a sound field.
This phenomenon is seen in the human external meatus at a
frequency of approximately 3 kHz.
Here, the resonance adds 10-12 dB at the tympanic
membrane, over the mid-concha position.

Other resonances increase the sound pressure at other
frequencies.
The most important is a broad resonance, adding approximately
10 dB around 5 kHz, arising in the concha.
The two main resonances are therefore complementary, and
increase the sound pressure relatively uniformly over the
range from 2 to 7 kHz.
The total effect of reflections from the head, and pinna, and the
various external ear resonances is to add 15-20 dB to the
sound pressure, over the frequency range from 2 to 7 kHz.

Ascending auditory pathway

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