Pathways to the Brain

How does the information about audition and balance get to the brain? Where are the neuronal cell bodies that carry this information to the brain? Where do they take it within the brain?

First of all, the neurons that pick up the information about audition and position are located (their cell bodies, that is) within the temporal bone, slightly deep to the inner ear. The auditory neurons are in the spiral ganglion. It is called the spiral ganglion because it is spread out to run around and around the cochlea on the inner-side (deep to it). The cell bodies send their dendrites to the hair cells within the Organ of Corti, and then send their axons back toward the brain.

The cell bodies of the vestibular neurons are in two vestibular ganglia, just deep to the vestibule and semicircular canals. These neurons send their dendrites in to the hair cells in either the maculae of the vestibule or the crista ampullaris of the semicircular canals. The vestibular neurons send their axons back toward the brain.

As the auditory neurons from the spiral ganglion send their axons back toward the brain, the nerve they create joins with the nerve containing the axons of the vestibular neurons from the vestibular ganglia. When all these axons come together, at the edge of the confines of the temporal bone, the nerve they form is called the vestibulocochlear nerve, and it is also known as cranial nerve VIII.

The auditory information goes from CN VIII to the midbrain, where it synapses in the inferior colliculus. You remember the inferior colliculus, right? It has to do with orientation to auditory stimuli. The auditory information continues on to the thalamus, where it synapses in the MGN (the medial geniculate nucleus). Remember that visual information synapsed in the LGN (the lateral geniculate nucleus audpath.jpg (25771 bytes) ) of the thalamus. From there, the auditory information continues to arrive at the primary auditory cortex on the superior gyrus of the temporal lobe.

Your book has a figure that illustrates this, and the last edition version of it is shown to the right. Note one more interesting thing... you should remember that our somatosensation is all mapped to the opposite side of the brain. Also, the visual information about the right half of our visual world is mapped to the left side of the brain...

Well, auditory information isn't discretely separated in a similar way. Most of the auditory information from our right ear goes to the left side of our brain, but some also goes to the right side of our brain. There is a lot more mixing up of our auditory information.

There is a reason for that... you even investigated it a bit in lab. Remember how you tried to localize sound in lab? You put the ticking clock all around your lab partner's head while her eyes were closed and made her tell you where the clock was? She was able to tell you where it was by comparing what she heard in her right and left ears. If the ticking is closer to her right ear, it will hear it before her left ear. We don't recognize any time difference between the two ears consciously, but we do recognize that the sound is near our right ear. In order to make such comparisons, we have to send information from both ears all around in our brains.

How about our vestibular pathways? Well, our book doesn't go into it. So I won't either. Let's just talk about some places where vestibular information should end up. We need vestibular information to go to our cerebellum, basal ganglia, and motor cortices in order to help us know how to adjust our movements. We also need vestibular information to get to our midbrain to assist with our eye movements (remember how vision and vestibular sense are linked from your lab exercises?). That's enough for our purposes.