Anatomy and Physiology: Don't Be So Touchy!
Don't Be So Touchy!
If you are still troubled by the idea of seven senses, how about this idea: It can be argued that touch is more than one sense. In addition to touch in general, what about the perception of levels of pressure, which has its own separate receptor, so necessary when holding delicate objects? How about temperature, which requires different receptors for the sensation of hot and for the sensation of cold? What about perceiving pain, with yet another receptor?
Splitting this up into multiple senses sounds like a bit much. After all, vision has separate receptors for black and white (rods) and for color (cones). Perhaps it is best to just think of touch as a complex sense involving multiple receptor types, similar to taste.
Knowing Which Way Is Up
In the inner ear the cranial nerve that goes to the inner ear is called the vestibulocochlear nerve (N VIII). This name illustrates the dual nature of this sensory nerve. Unlike mixed nerves with both sensory and motor functions, this one is all sensory. The cochlear branch goes to the cochlea, which, as I discussed earlier, is where the sounds we hear stimulate the nerve. The vestibular branch, however, has nothing to do with sound. This branch sends messages to our brains about balance, from a place that should have a familiar look to it!
As I mentioned earlier, we need the three axes (X, Y, and Z) to understand the areas of the body and body position. Mathematicians probably felt pretty good about themselves when they realized the need for three axes, but nature had us beat by hundreds of millions of years. The three axes are beautifully represented by the three semicircular canals, which make up the vestibular system.
Looking like some sort of weird alien snail head, the three semicircular canals (anterior, posterior, and lateral), which are about 90 degrees from one another, stick out of the wide end of the cochlea, one for each of the three axes. Let's try a little experiment. Sit upright and close your eyes. Now point straight up and open your eyes. (Perhaps you ought to memorize these instructions first, since your eyes will be closed!) Did you point in the right direction? Close your eyes again and lean all the way left, right, forward, and back. Were you always able to point up? That's what your vestibular sense does for you!
The sense of balance or equilibrium can be divided into two parts: static equilibrium, which mainly has to do with the head's position in terms of gravity, and dynamic equilibrium, which responds to rapid movement, such as spinning, accelerating, or braking to a halt. The key to all this is the movement of either fluid or crystals (yup, crystals) as perceived by hair cells. As I mentioned earlier when I discussed hearing, hair cells are not really nerve cells but epithelial cells that open ion channels to stimulate neurons.
Flex Your Muscles
If you really want to get a sense of your semicircular canals, stand up and spin around 10 times very fast and then stop! Dizzying, isn't it! Didn't you ever do this as a child? Now imagine the endolymph flowing around and around with you as you spin. When you stop, inertia will keep the endolymph sloshing around until friction and gravity finally slows it down! Think about what you see when you are dizzy: the room doesn't look like it spins all the way around, but only part way, and then the image snaps back, as your brain tries to reconcile messages from your retina and your hair cells!
For static equilibrium, there are two areas where the semicircular canals join the cochlea, called the utricle and the saccule. In both miniature organs are calcium carbonate crystals (CaCO3) called otoliths, or otoconia, that rest on a layer of glycoproteins called the otolithic membrane. Embedded in the base of that gelatinous membrane, from the surface of every hair cell, are microvilli called steriocilia, and a longer single cilium called a kinocilium. Between each of the hair cells is a supporting epithelial cell. The upper surface of the otolithic membrane is slippery, and tilting the head causes the otoliths to slide around, changing the pressure on the various hair cells, which in turn stimulate the neurons and alerts the brain to general changes in static equilibrium.
If you tilt your head slowly, the brain can still sense the change in direction (the otoliths still move), but if you are turned around very slowly (with eyes closed), or you accelerate in an airplane very slowly, you are not likely to perceive it. Rather than using otoliths, the semicircular canals use the movement of a fluid called endolymph to stimulate neurons as a result of a change in dynamic equilibrium. Imagine carrying a mug of hot coffee that is very full. If you don't want to spill it and scald yourself, don't you move slowly? If you move quickly, the coffee will spill. The reason for this is inertia. The coffee tends to stay at rest, but the acceleration of the mug slams into the coffee at the back of the mug, generating a wave from friction that causes the coffee to slop over the side.
In this way, the endolymph in the semicircular canals moves only to sudden changes in motion, hence the word “dynamic.” The base of each canal has an arched shape, and the neurons attach to the underside of the arch. Within the fluid is a structure called a christa, which is similar to the structures used in static equilibrium, except without otoliths. There are hair cells between supporting cells, and a gelatinous layer of glycoprotein, this time called a cupola. Sudden changes cause the endolymph to shift the cupola, like an ocean wave will shift kelp, moving the hairs, thus stimulating the neurons. Pretty cool!
Proprioception, or Where Are My Feet?
People always seem to take this sense for granted, which I always think a shame, because it is one of my favorites. I am talking about proprioception, or the ability to perceive body position, or more specifically the position of your muscles, even without using your eyes. It doesn't sound like much, but think about the last time you were walking upstairs talking to someone. Did you have to look at your feet in order to reach the next step? When Mozart was a child prodigy being paraded around before European royalty, his father used to have him perform one or more parlor tricks, such as playing harpsichord with a cloth covering the keys, or even playing while blindfolded.
I called those parlor tricks because any good piano player can do that without thinking. Can you type without looking at the keys? If so, you can thank proprioception. This sense is thrown into sharp relief on those rare occasions when you are deprived of it. I remember when I was in seventh-grade I broke clean through both the radius and ulna of my right arm. When the doctor was about to set my arm it was tied up in a tourniquet, and I was injected with a “horse syringe” full of Novocain. I remember looking to the left while answering the nurse's questions, thinking my right arm was hanging loose over the edge of the table. When I looked back I was shocked to find that it was being held with my hand up in the air! That shows how much we take proprioception for granted. One last thing. Say “Happy birthday.” Think you know how your tongue knows how to move without you being able to see it? That's right—proprioception!
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.