How do neural signals travel from hair cells in the Organ of Corti to the primary auditory cortex? Well, let’s examine the ascending auditory pathway. Why ascending you ask? Well, ascending means info goes TO the brain, while descending means info goes from the brain elsewhere.
Inner hair cells in the Organ of Corti are connected to type I spiral ganglion neurons (whose axons represent 95% of the cochlear nerve). So firstly, signals travel along the cochlear nerve to the cochlear nuclei in the brainstem on the ipsilateral side. Ipsilateral means same side, contralateral means opposite side. Kay, so here is where is starts to get a little complicated. Most auditory information crosses over, however, each cerebral hemisphere processes stimuli from both the ipsi and contralateral sides. This is advantageous for two reasons.
A) If you get brain damage in one hemisphere, your sense of hearing goes “meh, whatever!”. B) we as humans process some pretty complicated sounds – like those involved in speech, and getting input from both ears to both hemispheres allows for more processing potential.
Anyway, back to the ascending auditory pathway. Our information is at its first stop – the cochlear nucleus. From here, most of the neurons cross over to the contralateral side. This is the primary pathway that the information takes. However, there is also a secondary pathway, in which some neurons stay on the ipsilateral side. In both cases, the neurons synapse in the superior olivary complex, which is also in the brainstem.p The signal continues to be relayed along the lateral lemniscus to the inferior colliculus in the midbrain.
From the inferior colliculus, the information is relayed to the medial geniculate nucleus of the thalamus. Most of the neurons taking this trip stay on the ipsilateral side. However, some cross over. Finally, the information continues into the auditory cortex.
The auditory cortex is tucked into the lateral sulcus. The auditory core region contains the primary auditory cortex, or A1, which is organized tonotopically – in other words, it’s arranged by frequency. There are ACTUALLY tuned neurons that respond only to specific frequencies, and they are arranged in a tonotopic map – how cool is that?
Brain model by:
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