Embryological development of the auditory tract

The auditory tract follows a course from the ear to the auditory cortex which is located in the temporal lobe. Sound is coming towards us as invisable waves.

These waves cause the eardrum to vibrate. These vibrations are transmitted via the middle earbones to the cochlea where the sound is split up in it's different frequency components.

Every frequency is transferred via separate nerve tracts to the brainstem and from there to the auditory cortex. This means that at any stage of the auditory trajectory a tonotopic structure is present.

In the auditory cortex low frequencies are located superficially and high frequencies more medially along the super temporal gyrus. (blue low frequency, yellow high frequency). In the brain different frequencies are assembled to recreate the original sound.

Exactly how this integration of different frequencies happens is still subject to debate. In the brain certain every brain cell has a very specific function. Certain neurons will only react to certain frequencies, for example certain neurons will only react to high frequency tones while others will only react to low frequency tones.
At about day 24 post-conception, neural crest cells migrate to form ganglia of bipolar neurons. 4 days later these bipolar cells will form a nerve trajectory going into the brainstem on one side and migrating out to the otic vesicle on the other side. This otic vesicle will later become the cochlea.
Around day 36 postconception the auditory nerve is formed.



Central Nervous System of the embryo at 36 days
At this stage the auditory nerve is not functional yet. Only when nerve becomes myelinated (isolated) in the 5the fetal month it becomes functional. The myelinisation starts later than the myelinisation of most motor nerves (3th month).

At this stage the auditory nerve is ready to transfer auditory information but tonotopy has not developed yet. This tonotopy will arise depending on the auditory input it is exposed to. As mentioned before cochlea, auditory nerve, auditory tract in the brain stem, and the auditory cortex are tonotopically organized. This means that the different sound frequencies are transferred in organized ways, as such that the higher frequencies are more located more superficially than the lower frequencies.

When we inject antibiotics in the cochlea of animals the high frequency haircells become afunctional. Almost immediately afterwards a reorganization arises in the auditory tract up to the auditory cortex. In the brain,the braincells that are genetically determined to process mid frequencies are now moving into high frequency processing areas. Clinical evidence however suggests it might actually be the opposite. Braincells that are genetically determined to process high frequency information become unemployed after a toxic dose of antibiotics. A braincell without function dies (Darwinistic principle - survival of the fittest). In order to prevent this from happening those high frequency processing brain cells will go and look for information. The unemployed cells will grow into the adjacent area where information from mid frequencies arrives. In other words braincells normally processing high frequency information will now be activated by mid frequency auditory input.

This can be demonstrated by PET-scanning or magnetoencephalography. This reorganisation probably arises through activation of dormant synapses (connections between nerve cells), the sprouting of new dendrites in the adjacent brain area or the sensitisation of certain brain cells.

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