Auditory Pathway Crossover?

Question

Hi everyone, I've read both the Kaplan and Examcrackers explanations on the auditory pathways and I'm not impressed. It appears there's SOME\* crossover of information to the different hemispheres and that sound received from the left ear is predominantly registered in the left hemisphere and sound received from the right ear is predominantly registered in the right hemisphere. ​ Can anyone with a FIRM understanding of auditory pathways please confirm my understanding? Thank you.

Answer 1

Well the short answer is its complicated. This is a bit more in the weeds than you need for MCAT and even a bit in the weeds for Med School but there is a simple explanation. So the auditory pathway is basically a 2 neuron chain

Neuron 1 goes from cells of of inner ear (specifically the spiral ganglion of inner ear) The dendrites of these neurons contact the hair cells in the organ of corti. You get the axons of these neurons and they will converge and form the cochlear part of cranial nerve VIII. If you follow your neuroanatomy you'll know that the CN VIII enters the brainstem at the pontomedullary junction.

Neuron 2 is located at dorsal and ventral cochlear nuclei of the medulla at the pontomedullary junction

No where it gets weird is that MOST axons of the dorsal and ventral cochlear nucleus cross over at the lower pons and will ravel to the contralateral side. They will eventually ascend through other nuclei in the lateral lemniscus and to the inferior colliculus in the caudal midbrain.

Basically at the level of the inferior colliculus you have several nuclei that contribute to the axons of the Lateral Lemniscus. This leads to having some of the auditory information crossing back to the ipsilateral side. This characteristic of the auditory pathways is sometimes described as being a multisynaptic and bilateral system

TLDR: There is some crossing back at the level of inferior colliculus. So yes each auditory cortex ends up receiving info from both ears, however there will be greater input from the contralateral ear.

If you interested in the clinical relevance for this it means that if you get a unilateral lesion in the brainstem beyond the cochlear nuclei - you don't get complete loss of hearing of either ear. You end up having more trouble hearing from the contralateral ear.