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.

COCHLEAR NUCLEAR COMPLEX

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.

SUPERIOR OLIVARY COMPLEX

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.

INFERIOR COLLICULUS

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.

DESCENDING PATHWAYS

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.

PHYSIOLOGY OF HEARING

THE EXTERNAL EAR

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 tympanicmembrane

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.