listeninggravity

Listening to gravity

by Paul Doherty

Cross-country skiing by starlight in the Yosemite

backcountry was eerie, but fun. The snow glowed so dimly

and uniformly that my eyes could not make out detailsQI

had no depth perception. I was glad that I had scouted

the terrain by daylight and knew the location of nearby

cliffs. The hiss of my skis on the snow changed as I

picked up speed moving down an incline.

I was aware of the sights and sounds of the world

around meQand unconscious of other, far more important

signals that were continuously being sent from my inner

ears to my brain, signals that were crucial to my ability to

remain balanced over my skinny skis. As I telemark-

turned down the gentle slope, the sensors of my inner ear

provided my brain with information on my orientation with

respect to gravity, my acceleration, and my continuously

changing rotation.

Most of us remain blissfully unaware of the balance

systems of our inner ears. These systems are essential to

any activity requiring balance, whether it's skiing, riding a

bicycle, walking a tightrope, or even simply standing on

your own two feet. I had learned about my own inner ear

when an ear infection called labyrinthitis scrambled my

balance signals so that skiing became impossible and even

standing up was difficult.

Under most circumstances, I use three different

systems to sense my orientation: my vision, my body's

sense of the tensions and shapes of my muscles and the

pressures on my skin (a sense called proprioception), and

the signals from my inner ear. The inner ear includes an

array of sensor-hair-studded, fluid-filled tubes and

containers of rocks packed in gelatin. Let me share with

you what I found out about how your ear uses these hairs,

rocks, and tubes to detect the position and motion of your

body.

The tangle of tubes in your inner ear forms a

plumbing system so complicated that early researchers

called it the auditory labyrinth. This labyrinth includes a

snail-shell-shaped, fluid-filled tube called the cochlea in

which sounds are converted into nerve impulses. It also

includes two bags called the utricle and the sacule and

three mutually perpendicular semi-circular tubes.

Rocks in your head

You have rocks in your earsQand they help you keep

your balance. The rocks are called ear dust or otoliths

(from the Greek words for "ear" and "stone"). The otoliths

are packed in gelatin inside the utricle and saccule. They

are made of calcium carbonateQ just like limestone Q and

are about three times denser than water. Each otoliths is

tiny, about a tenth the diameter of a human hair across

(that's about 10 micrometers). (photo) In one of the bags,

a set of rocks is packed in a horizontal layer of gelatinous

material, like pebbles sprinkled onto a Jello-covered

cookie sheet. If you tilt the cookie sheet down, gravity will

drag the pebbles in the direction of the tilt. When you tilt

your head forward, gravity pulls the otoliths toward the

front of your head and your vestibular system detects the

change. Your vestibular system is so sensitive to the

motion of the otoliths that it can detect a tilt as slight as

one degree from the verticalQa rotation of just 1/360th of

a circle.

The rocks in our heads are sometimes called our

Rgravity sensors.S When you are standing still or moving at

a constant speed, the otoliths do function as gravity

sensors. However when you are acceleratingQchanging

your speed or direction of motionQthe otoliths sense a

combination of gravity and acceleration. This can lead to

sensory illusions. Next time you have an aisle seat near the

tail of a large jet aircraft, look forward along the aisle as

the plane begins to accelerate for its take-off. You will

sense that the aisle is tilting uphill away from you. Look out

the window and you will see that the plane is still level

relative to the horizon. As the plane accelerates forward,

the inertia of the rocks causes them to move back relative

to your inner earQjust as they would move if your seat and

the aisle tilted uphill.

Dancing hairs

Electron microscopes have only recently revealed

how the tiny motions of otoliths and gelatin are converted

into nerve impulses. (photo) Hair cells grow into the

gelatin from below. The hair cells cluster together(photo

or drawing) in bundles with short hairs on one side of the

bundle and long hairs on the opposite side. The bundles

remind me of lines of highrise buildings which are taller

toward the city center. The top of each hair is connected

to a neighboring hair by a very thin filament.

When the hairs are standing straight up, nerves at

the base of the cluster of hairs fire about 100 times each

second. When gravity or acceleration moves the otoliths so

that they pull on the bundles of sensor hairs toward the

tallest hair, the connecting filaments pull open channels

which allow ions to leak into and out of each hair cell.

This causes the nerve connected to the bundle to triple its

firing rate. When the hairs are bent the other way, toward

the shorter hairs, the nerve stops firing. The clusters of

hair cells are arranged so that the pattern of nerve signals

encodes the direction of tilt of the head.

Your body responds to these nerve signals

automatically. When your body tips forward, for example,

the nerves signal the tilt and the muscles at the back of

your neck tighten to keep your head from falling forward.

Since all this happens automatically, how can you tell

that you have sensors in your ears? As long as your

vestibular system is working properly, you'll never be

aware of it. This sense works at a subliminal level, outside

your conscious control. People who have been buried in

snow by an avalanche report that they cannot tell which

way is up. They are blinded and pressed upon from all

directions by snow and so cannot use light or pressure to

sense orientation. Though their vestibular apparatus is

still working, but they cannot consciously hear its

messages.

I performed an experiment to test these reports. I

went into a swimming pool, rolled into a ball like a pill

bug, covered my eyes with my hands, breathed out a little

air so I wouldnUt float, and had a friend spin me around.

When I stopped rotating, I had a weird feeling in my

stomach that told me I was not right-side-up. Even so, I

could not point toward the surface of the pool.

Under normal conditions, you remain unaware of the

workings of your vestibular system. But if it breaks down,

youUll notice instantly. When I had my case of labyrinthitis I

felt the world spinning around me and had great difficulty

standing and walking.

Sensing the direction we call RdownS is so

important that all animals have a system to detect it. In

animals, the detector is called a statocyst and is not always

connected to an Rear.S The sea horse's statocyst is made of

a grain of sand inside a hair-lined cavity. Gravity pulls the

sand down, the stone bends hairs, and the bent hairs send

nerve impulses which the sea horse uses to orient itself

right-side-up. If you raise sea horses in a tank of magnetic

sand, then bring a magnet next to the tank, the sea horses

will tilt to odd angles as they line up with the combination

of magnetic forces and gravity. An octopus uses its

statocyst to keep its slit pupil horizontal. (Photo of an

octopus. sci am book on animal navigation).

Canals in your ear

Have you ever gotten off a carnival ride and felt like

the world was still spinning. You owe that dizzying

sensation to the second part of your vestibular apparatus:

your semicircular canals.

You have three semicircular canals. (illustration).

Each canal is filled with a fluid called endolymph. Where

each of the canals attaches to the body of the ear there is a

bulge containing a blob of hair-cell-filled gelatin. When the

fluid flows around the canal and over the gelatin, it pushes

the gelatin to the side, bending the hair cells and changing

the firing rate of nerves. By detecting the acceleration as

you start to fall or spin, your semicircular canals provide

the brain with an early warning of a fall. This gives your

muscles time to react.

To understand how your semicircular canals detect

changes in rotational motion, imagine a glass of water with

inside walls covered with thick fuzzy moss. Picture the

glass in the center of a lazy susan. When the water is at

rest, the moss grows straight out from the walls. When you

start to rotate the glass, the water doesnUt move at first.

The base of the moss moves with the glass and the tips of

the moss stay behind in the fluid. The moss bendsQjust

like the hair cells in your semicircular canals. The hair

cells send the message to the brain that rotation has

begun. If you keep rotating the glass long enough, the

water starts to rotate too, and the moss straightens out.

The same thing happens in your semicircular canals. Spin

long enough, and the fluid starts to spin and the hairs

straighten. When you stop spinning, the fluid will keep

moving, bending the hairs the opposite direction. ThatUs

why you feel like you are rotating after you stop a

prolonged rotationQthe fluid in your inner ear keeps on

moving afterh you've stopped.

Another way to cause currents to flow in your

semicircular canals is to cause the fluid to undergo

convection. If a doctor squirts cold water in your outer ear

convection currents will be set up in your semicircular

canals. Your vestibular apparatus will sense these currents

and you will become dizzy. That's why doctors are careful

to use body temperature water to flush your ears.

The semicircular canal that detects right-left

rotation also provides nerve impulses which control your

eyes. The next time you see a friend who has been

spinning around for ten seconds or so (an ice skater or a

ballerina or just a friend who doesn't mind spinning),

watch his or her eyes. Just after your friend stops

spinning, his or her eyes will move to the side and then

jump back over and over again. The eye motion is called

nystigmus and is caused by signals from the fluids of the

inner ears. Dancers and ice skaters practice and learn to

live with these signals. In some people, the spinning and

its associated eye movements can lead to motion sickness.

We know that people often get motion sick when the

signals from their eyes and their inner ears disagree and

people have surmised that this disagreement leads to the

nausea of motion sickness. But no one know exactly why

this disagreement leads to a feeling of nausea. One

hypothesis is that food poisoning disrupts the signals from

the vestibular system. This causes a disagreement between

the signals received from your eyes and your vestibular

system. A life-saving response to food poisoning is for the

body to rid itself of the poison. Thus a feeling of nausea

accompanies crossed signals between different sets of

sensory apparatus.

Gravity makes itself heard

As I skied at night, signals flowed from my inner ear

to my brain continuouslyQlistening to gravity, to

acceleration, to rotations. After years of practice, I had

learned to use this unconscious flow of information to help

me remain balanced over my skis, and I was confident in

my own abilities.

Then suddenly, I was fallingQI had skied off an

invisible three-foot cliff. For a fraction of a second, rocks

jiggled, fluids sloshed, and hair cells danced inside my

head as I briefly entered freefall then crashed into a heap

in deep snow. As I struggled to my feet, brushing snow

from my body, I knew that a few more years of learning to

balance on skis wouldn't hurt.

Try This

Experience Formication

You hear sound and sense acceleration because hair cells

bend and change the firing of nerves. The short hairs on

your skin allow you to experiment with a similar mechano-

receptor system.

To do and Notice

Find a partner and a toothpick.

Close your eyes.

Have your partner use the toothpick to gently bend one of

the body hairs on your arm.

Notice the feeling produced by the bending hair.

WhatUs Going On

The hairs on your arm are mechano-receptorsQthey detect

mechanical deformation. When the hairs are bent, nerves

near the base of the hair send signals to the brain. This is

similar in operation to the sensory hair cells in your ear

which generate signals when they are deformed.

Formication is the name of the sensation of insects

crawling over your skin, bumping into and deforming

hairs. It comes from the same root as formic acid, which is

produced by ants.