Move through the external ear, go deeper inside, past the eardrum and three middle ear bones, and you end up deep inside the skull. Here you will find the inner ear—tubes and some gel-filled sacs. In humans, as in other mammals, the bony tubes take the snail-shell shape that is so strikingly apparent in the anatomy lab.
The inner ear has different parts dedicated to different functions. One part is used in hearing, another in telling us which way our head is tilted, and still another in recording how fast our head is accelerating or stopping. In carrying out each of these functions, the inner ear works in roughly the same way.
The several parts of the inner ear are filled with a gel that can move. Specialized nerve cells send hairlike projections into this gel. When the gel moves, the hairs on the ends of the nerve cells bend. When these hairs bend, the nerve cells send an electrical impulse to the brain, where it is recorded as sound, position, or acceleration.
Each time you tilt your head, the tiny rocks on the fluid-filled sacs move. In doing so, they bend nerve endings inside the sacs and cause an impulse to be sent to your brain saying "Your head is tilted."
To envision the structure that tells us where our head is in space, imagine a Statue of Liberty snow globe. The snow globe is made of plastic and filled with gel. When you shake it, the gel moves and the "snow" falls on Lady Liberty. Now imagine a snow globe made of a flexible membrane. Pick it up and tilt it, and the whole thing will flop about, causing the gel inside to swish around. This, on a much smaller scale, is what we have inside our ears. When we bend our heads, these contraptions flop around, causing the usual chain of events: the gel inside swishes, the hair projections on the nerves bend, and an impulse is sent back to our brains.
In us, this whole system is made even more sensitive by the presence of tiny rock-like structures on top of the membrane. As we bend our heads, the rocks accentuate the flopping of the membrane, causing the gel to move even more. This increases the sensitivity of the system, enabling us to perceive small differences in position. Tilt your head, and little rocks inside your skull move.
You can probably imagine how tough it would be to live in outer space. Our sensors are tuned to work in the earth's gravity, not in a gravity-free space capsule. Floating around, our eyes recording one version of up and down, our inner ear sensors totally confused, it is all too easy to get sick. Space sickness has been a real problem for these very reasons.
The way we perceive acceleration is based on yet another part of our inner ear, connected to the previous two. There are three gel-filled tubes inside the ear; each time we accelerate or stop, the gel inside the tubes moves, causing the nerve cells to bend and stimulate a current.
The whole system we use to perceive position and acceleration is connected to our eye muscles. The motion of our eyes is controlled by eight small muscles attached to the side walls of the eyeball. The muscles contract to move the eye up, down, left, and right. We can move our eyes voluntarily by contracting these muscles each time we decide to look in a new direction; but some of the most fascinating properties of these muscles relate to their involuntary action. They move our eyes all the time, without our even thinking about it.
To appreciate the sensitivity of this eye-muscle link, move your head back and forth while looking at the page. Keep your eyes fixed in one place as you move your head.
What happened during this experiment? Your eyes stayed fixed on a single point while your head moved. This motion is so commonplace that we take it for granted, but it is incredibly complex. Each of the eight muscles in both eyes is responding to the movement of the head. Sensors in your head, which I'll describe in the next section, record the direction and velocity of your head's movement. These signals are carried to the brain, which then sends out signals telling your eye muscles to fire. Think about that the next time you fix your gaze as your head is moving. This system can misfire, and misfires have much to tell us about our general well-being.
Every time we accelerate, fluid in the inner ear swishes. The swish is transformed into a nerve impulse that is sent to the brain.
An easy way to understand the inner ear-eye connection is to interfere with it. One way humans do this is to imbibe too much alcohol. Drinking too much ethanol leads us to do silly things because our inhibitions are lowered. Drinking way too much gives us the spins. And the spins often predict a lousy morning ahead, hungover, with more spins, nausea, and headache.
When we drink too much, we are putting lots of ethanol into our bloodstream, but the fluid inside our ear tubes initially contains very little. As time passes, however, the alcohol diffuses from our blood into the gel of the inner ear. Alcohol is lighter than the gel, so the result of the diffusion is like the result of pouring alcohol into a glass of olive oil. Just as the oil moves around in the glass as the alcohol enters, so the gel inside our ear swirls. The convection wreaks havoc on the intemperate among us. Our hair cells are stimulated and our brain thinks we are moving. But we are not moving; we are slumped in a corner or hunched on a barstool. Our brain has been tricked.
The problem extends to our eyes. Our brain thinks we are spinning, and it passes this information to our eye muscles. The eyes twitch in one direction (usually to the right) as we try to track an object moving from side to side. If you prop open the eyes of someone who is stone drunk, you might see this stereotypical twitch, called nystagmus. Police know this well, and often look for nystagmus in people whom they have stopped for driving erratically.
Massive hangovers involve a slightly different response. The day after the binge, your liver has done a remarkably efficient job of removing the alcohol from your bloodstream Too efficient, for we still have alcohol in the tubes in our ears. That alcohol then diffuses from the gel back into the bloodstream, and in doing so it once more sets the gel in motion: the spins again. Take the same heavy drinker whose eyes you saw twitch to the right the night before and look at him during the hangover. His eyes might still twitch, but in the opposite direction.
We can thank our shared history with sharks and fish for this. If you have ever tried to catch a trout, then you have come up against an organ that is likely an antecedent to our inner ear. As every fisherman knows, trout hold only in certain parts of a stream, typically spots where they can get the best meal while avoiding predators. Often such places are in the shade and in the eddies of the stream's current. Great places for big fish to hold are behind big rocks or fallen logs. Trout, like all fish, have a mechanism that allows them to sense the current and the motion of the water around them, almost like a sense of touch.
Within the skin and bones of the fish, arranged in lines that run the length of the body and head, are small organs with sensory receptors. These receptors lie in small bundles from which they send small hair-like projections into a jelly-filled sac called a neuromast organ. It helps to think of the snow globe Statue of Liberty again. A neuromast organ is like a tiny one of these, with nerves projecting inside. When the water flows around the fish, it deforms this small sac, thereby bending the hair-like projections of the nerve. Much like the whole system in our ears, this apparatus then sends a signal back to the brain and gives the fish a sense of what the water is doing around them. Sharks and fish can discern the direction in which the water is flowing, and some sharks can even detect distortions of the water, such as are produced by other fish swimming near them. We used a version of this system when we moved our head with a fixed gaze, and we saw it go awry when we propped open the eyes of the inebriated individual at the start of this section. If the ancestor we have in common with sharks and fish had used some other kind of inner ear gel, say one that does not swirl when alcohol is added, we would never spin when drunk.
If you think of our inner ears and neuromast organs as versions of the same thing, you would not be far off. Both come from the same sort of tissue during development, and they share a similar structure. But which came first: neuromasts or inner ears? Here the evidence gets sketchy. If you look at some of the earliest fossils with heads, creatures about 500 million years old, you'll find little pits in their external armor that suggest they had neuromast organs. Unfortunately, we do not know much about the inner ears of these creatures because the preservation of that area of the head is wanting. Until more evidence rolls in, we are left with one of two alternatives: either our inner ears arose from neuromast organs or the other way around. Both scenarios, at their core, reflect a principle we've seen at work in other parts of the body. Organs can come about for one function, only to be repurposed over time for any number of new uses.
In our own ears, there occurred an expansion of the inner ear. The part of our inner ear devoted to hearing is, as in other mammals, huge and coiled. More primitive creatures, such as amphibians and reptiles, have a simple uncoiled inner ear. Clearly, our mammalian forebears obtained a new and better type of hearing. The same is true for the structures that perceive acceleration. We have three canals to record acceleration because we perceive space in three dimensions. The earliest known fish with these canals, a kind of jawless fish like a hagfish, has only one. Then, in other primitive fish, we see two. Finally, most modern fish, and other vertebrates, have three, like us.
We have seen that our inner ear has a history that can be traced to the earliest fish. Remarkably, the neurons inside the gel of our ears have an even more ancient history.
These neurons, called hair cells, have special features that are seen in no other neuron. With fine hair-like projections, consisting of one long "hair" and a series of smaller ones, these neurons lie with a fixed orientation in our inner ear and in a fish's neuromast organ. Recently, people have searched for these cells in other creatures, and have found them not only in animals that do not have sense organs like ours at all but also in animals that have no heads. They are seen in creatures like Amphioxus, which we met in Chapter 5, that have no ears, eyes, heads, or skulls. Hair cells, then, were around doing other things before our sense organs even hit the scene.
A primitive version of part of our inner ear is embedded in the skin of fish. Small sacs—the neuromasts—are distributed around the body. When they bend, they give the fish information about how the flow of water is changing.
All this is recorded in our genes, of course. If humans or mice have a mutation that knocks out a gene called Pax 2, the inner ear fails to form properly. Pax 2 is active in the ear region and appears to start a chain reaction of gene activity that leads to the development of the inner ear. Go fishing for this gene in more primitive animals and we find Pax 2 active in the head and, lo and behold, in the neuromasts. The spinning drunk and the fish's water-sensing organs have common genes: evidence of a common history.
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