Today we move on in our tour of the cranial nerves to another nerve that is basically responsible for an entire system. In fact, it's an overachiever...and does TWO. We've had the olfactory and the optic, and now, we welcome the vestibulocochlear, which does both auditory AND your sense of balance.
The auditory system is one of my favorites (I've always had a soft spot for the trochlear nerve, I don't know why, but this is the other favorite), because the interior of the ear is just so PRETTY. It's full of caverns and arches, delicate hairs and membranes. It really is very pretty.
I have to warn you, this is a really large and complicated system, right up there with the eye. So we're going to start with just the auditory portion of the vestibulocochlear nerve, and we'll cover the vestibular portion next time.
Being a sensory system, we're going to start from the outside and work our way in. And we'll start with the pinna.
The pinna is what you usually think of as your ear, which is coupled to your ear canal inside. Your ear is shaped kind of funny (you know, the way it is) because it's basically a complicated shaped funnel, made to conduct sound inside.
Yeah, like that.
Soundwaves will be conducted in, and vibrate your tympanic membrane, otherwise known as your eardrum. From there, the vibrations are passed along the three smallest bones in your body, the malleus, the incus, and the stapes. The malleus is connected to the tympanic membrane, the incus is connected to the malleus, and the stapes is connected to the incus (and the hip bone's connected to the...thigh bone, the thigh bone's connected to the...knee bones...the knee bones' connected to the...shin bones, and THAT'S the way it goes). The stapes ends at a little hole in the side of your skull called the oval window, and this is where the air filled part of the ear ends and the fluid part of the ear begins.
Now, sound as we know it travels through air, right? But your inner ear is filled with FLUID. And fluid is a heck of a lot harder to move than air. Those three little bones in sequences actually confer a mechanical advantage here, intensifying the vibration a little as it moves along, and making it easier to move the fluid once you get to the inner ear. But even so, it's a very little vibration, and the sensitivity of your ear is just stunning. The hair cells of your ear (which we're about to get to) deflect in response to a stimulus by 0.003 degrees. The common comparison is that if you deflected the Empire State Building that much, the very tip of it would move less than an inch.
But this is enough for us to hear a very wide range of sound. In fact, the range of sound we can hear is SO wide that we actually have a logarithmic scale for it, with 0 decibels being the hearing threshold, and 140 being the threshold of pain (this presumably puts the Disaster Area sound stage at roughly 5,000,000 decibels)
Passing into the fluid filled part of the inner ear brings up to the cochlea. The cochlea is that little bit in the first picture that looks like a little snail shell (isn't it cute?!). Inside that little bone snail shell is a series of fluid filled chambers, holding the main sensory organ of the auditory system, the Organ of Corti. There are tons of these inside the cochlea, all working together.
This is a slice through the cochlea, taken kind of like if you laid the snail shell on its side, and then sliced it like a hamburger bun. Each of those little visible flaps is an organ of Corti, and you can see the axons of the neurons in each organ coming out, bulging in a spiral ganglion, and then all joining together to become the cochlear branch of cranial nerve VIII.
And what IS this organ of Corti? Each organ of Corti contains a basal membrane, on which everything is built, outer and inner hair cells, and a tectoral membrane. These hair cells are exactly what they sound like, little cells with little cilia sticking out of the top, which can be mechanically deflected.
BUT, you can't just have these cilia all hangin out. So instead, the hair cells are embedded in the tectoral membrane. But even this is actually not WHAT MOVES. Nope. Remember that basal membrane that the cells are embedded in? THAT'S what moves in response to the vibrations transmitted to your cochlea. That moves, it deflects the hair cells accordingly.
So if it's just movement, how do your hear the world in all its varied sounds? Well, a high sound vs a low sound will activate different PARTS of the organs of Corti. Low frequencies, where the distance between each wave in the vibration is longer, will head further into the cochlea, and the regions that are innermost thus detect low frequency sound. The middle does middle frequencies, and the outer does the small, tight vibrations of higher frequencies. As for how LOUD something is, well, that's just a matter of the strength of the stimulus. Finally, WHERE the sound is coming from is actually a matter of comparing the signals coming in from each ear, and that will get processed later.
So, we've got the vibrations coming in, stimulating the hair cells in the organ of Corti. But hair cells are NOT themselves neurons! Nope, instead these hair cells, when mechanically stimulated by the movement of the basal membrane (I know, that sounds so backward. Ah, evolution, causing neuroanatomy students memorization fits since the beginning of time), create something called a receptor potential, and release neurotransmitter (so kind of like a neuron, only not quite), onto a specialized neuron. THAT neuron then fires, and the now electrical signal heads off along the axon as part of cranial nerve VIII.
And it's the start of a very long ad convoluted trail.
We start in the organ of Corti, with the first synapse from the hair cells to the neurons that make up the cochlear branch of the vestibulocochlear nerve. The axons of these neurons bunch together and head into the brain, stopping first at the ventral cochlear nucleus, which you can see up there are the first synapse coming in from the organ of Corti. The ventral cochlear nulceus is located in the junction between the pons and the medulla, which you can see on the underside of the brain here.
The horizontal stripey thing in the middle is the pons, just below it is the medulla, and at the junction is the area containing the cochlear nucleus. From there about half the neurons involved will decussate, or cross (but you say decussate if you want to sound very sciencey), heading toward the superior olivary nucleus where they synapse again (yes, there is an inferior olivary nucleus as well, though as you can see from the big picture of the pathway above, neither looks particularly...olive-y). From here, the new axons (lot of connections in this here pathway), join the lateral lemniscus pathway (a large tract in the brainstem, like a nice thick highway of axons), and make their next synpase in the inferior colliculus. The inferior colliculus doesn't look like much in slices of the brain, but one of the things I like about it is that you can SEE it on the OUTSIDE! Observe:
See those cute little bumps? You can get a good shot of them whenever you cut a brain in half like a hotdog down the central divide (cause I bet you get to do that most days). Those are your colliculi and the inferior is the lower one (the superior colliculus which is higher up is more vision oriented). From there they head up to the thalamus, where they hit the medial geniculate nucleus, which is one of the thalamic nuclei that kind of hangs off the back of the thalamus, like a little pair of testicles (goes with the lateral geniculate).
One of the things I LOVE about the auditory and the visual systems is how well they parallel each other. The visual system has the superior colliculus (which is organized really nicely for each point in the visual field), and the auditory system as the inferior colliculus (which is organized by frequency from low to high). The visual system has the lateral geniculate, the auditory system has the medial geniculate. And after passing through the thalamus, the visual system fans out in these gorgeous optic radiations, and the AUDITORY system fans out in auditory radiations!!! The visual and the auditory system, the brain's BFFs.
Sadly, the auditory radiations are kind of small and wimpy compared to the big fan of the optics, but there you have it. The auditory radiations head to the temporal lobe. While you're think that something as big a deal as sound would maybe take up a lot of space, you'd be wrong. The primary auditory processing takes place just on the top of the temporal lobe, in an area called the superior temporal gyrus (aka Brodman's areas 41 and 42).
What's particularly cool here is that the mapping from the cochlea remains the same! Low frequencies (think low tones) which start on the inside of the cochlea here are processed on the outside of the gyrus, and as you go in on the gyrus you get to higher and higher sounds. It just blows my mind some times how the neurons can do all these synapses and all these twists and turns and loops, and still remain in the same positions relative to each other. Talented, those neurons.
And that's the (very) basics of the auditory system. Next time we shall tackle the other half of the vestibulocochlear nerve, the vestibular system. Be careful not to lose your balance.