Tri-2, Wk10 Day 105

Got my bell rung today in Gross II. Quiz didn't go very well. I didn't plan well enough to work around my birthday celebration with my family. Kind of fried & very tired.

Eric, "How do we normally get carnitine?" to which Dr. Guitweiler replied, "We make it, it's an exhaustively methylated primary amine." ...I have my television turned down low and my biochemistry DVD at regular volume and that was the last question asked on the DVD. What the heck is an exhaustively methylated primary amine? I guess it's carnitine but, I don't think I'll ever have my teacher's appreciation for what that means.

Speaking of hearing ...I was reading about the vestibulocochlear cranial nerve earlier tonight ...I keep understanding things at deeper levels but it is slow going. The cranial nerves may be sensory, motor or both. The vestibulocochlear cranial nerve is sensory which means we are dealing with "afferent" nerves or nerves that travel from the sensory organ to the brain. I'm wondering if a cranial nerve includes everything along the path from the sensory organ to the brain - I'm not exactly sure.

In the inner part of the ear concerned with hearing is called the cochlea (look at today's pic for an example of one). It kind of looks like a snail's shell. The inside of the cochlea is divided down the middle and in the middle part is something called the cochlear duct which contains tiny hair cells which help the brain to register sound. From what I've read - the hair cells interpret volume and the basilar membrane which is directly below the hair cells help interpret frequency.

When sound waves first enter the cochlea the basilar membrane is narrow and as it travels through the cochlea the membrane gets wider. The narrow part of the membrane interprets high frequency sounds and the wider part of the basilar membrane responds to low frequency's.

What's amazingly interesting, and a bit complex, are those hair cells. Those cells run along the basilar membrane and seem to come in sets of two - as in a single hair cell along with a triplet of hair cells.

This is where things get interesting. Those hair cells are capable of transduction! That's what telephones do - at least, I know that's how the older telephones used to work (not sure about cell phones). Transduction is the conversion of energy from mechanical to electrical forms - it's how the mechanical sound waves we hear are changed into an electrical action potential capable of traveling over nerves to get to the part of the brain that makes sense of that electrical input.

Now, those hair cells I was talking about come in sets of three - the single hair cell has three hair cells and the triplets of hair cells each have three distinct hair cells. ...I guess we need to be a little more specific than "hair cells" they may look like hair cells but are actually called steriocillium which ...are just little things that look like hair :)

The cells have three sets of steriocillium which come in short, medium and tall. And all three steriocillium are connected to one another. when those little steriocillium are erect and not being swayed by sound then the tips of the steriocillium have little gates which are about 15% open. Those gates are specifically designed to let in Potassium ions (K+). When the hair cells get pushed towards the taller steriocillium then those gates open up and let in greater than normal amounts of K+ which in turn, causes the base of the steriocillium to let in greater than normal amounts of Calcium ions (Ca+). The Ca+ in turn cause a greater than normal release of neurotransmitters from their synaptic vesicles. This in turn causes increased impulses along the nerves traveling to the brain (afferent neurons) and the brain interprets this as an increase in sound intensity or how loud something is. This whole process in the inner ear is referred to has depolarization.

How about when things get quiet? That is known as hyperpolarization and occurs when the hair cells ...opps - I mean steriocillium bend towards the short hairs. This has an effect of closing the potassium gates which reduces the amount of potassium let into the cells which in turn reduces the amount of calcium released which reduces the amount of neurotransmitters to the cranial nerves which go to the brain ...it's how we interpret a decrease in sound intensity.

That's *part* of the vestibulocochlear cranial nerve (CN VIII). the other part is the vestibulo part which hooks up to the semicircular canals in the inner ear. I think the vestibular part of the nerve will be a bit more complex because that nerve also has innervations with the abducens, trochlear and oculomotor nerve to help keep us balanced. :)