Five senses, windows to the brain
Your brain sits inside your skull in complete darkness and complete silence. It has never seen the world for itself. Everything you know about the world arrives through your senses: eyes, ears, nose, tongue and skin turn light, sound, smell, taste and touch into tiny electrical signals for the brain to read. This lesson has plenty of experiments you can try yourself.
Every sense is turned into electricity first
Light, sound, smell, taste and pressure are completely different things. But by the time they reach the brain they have all become the same thing: tiny electrical pulses running along nerves. The brain works out whether something is light or sound from theroutethe signal came in by, not from what the signal looks like.
How much faster does myelin make it?
In a fibre wrapped in myelin, the signal jumps from one gap to the next, reaching about 100 metres per second. Thin fibres with no sheath crawl along at about 1 metre per second. In multiple sclerosis this sheath is damaged, so signals slow down or get lost.
The eye, a living camera
Light bounces off things around you, comes into the eye, passes through the lens and lands on the screen at the back called the retina. Tap a spot on the picture to see what each part does.
The iris changes the size of the opening
Experiment: find your own blind spot
The dot disappears because the place where the optic nerve leaves the eye has no light-sensing cells at all. Each eye has one blind spot. You never notice it, because your two eyes and your brain fill the gap in for you.
Experiment: afterimages
Stare at the black dot in the middle of the coloured picture for 15 seconds without moving your eyes. Then the picture turns white. See what appears.
The colour cells that have been working hard get tired. When the screen turns white, the other group of cells stands out, so you see the picture in the opposite colours.
Experiment: mixing three colours of light
This screen has only red, green and blue light. Your retina happens to have three kinds of cone cell tuned to those same three colours. Your brain mixes the proportions into every colour you see. Have a go.
The ear, a vibration catcher
Sound is air moving back and forth. The ear catches that vibration, passes it on through the three smallest bones in the body, into a coiled tube full of fluid, and only then does it become an electrical signal.
Experiment: high notes and low notes
A high note is air vibrating quickly, a low note is air vibrating slowly. We count the vibrations per second, in units called hertz. Press play and drag the slider. Keep the volume low and take your headphones off for this one.
How loud is dangerous?
Loudness is measured in decibels. Sounds above about 85 decibels, heard for long stretches, damage the hair cells in the cochlea, and human hair cells do not grow back. Hearing lost to loud noise does not come back.
The nose, a molecule detector
Smell is not a wave like light or sound. It is actual tiny molecules floating through the air. When you smell bread, molecules from that bread really have floated up and landed on the smell cells in the roof of your nose.
Try at home: taste or smell?
Take two foods with different flavours, say grape and apple. Close your eyes, pinch your nose tightly shut, and let someone feed you one at a time. Try to guess which is which. Then let go of your nose, and the flavour rushes in at once. What we call flavour mostly comes from smell drifting up the back of the nose. That is also why nothing tastes good when you have a blocked nose.
The tongue, the food inspector
Your tongue has thousands of taste buds, each with taste cells inside. There are 5 basic tastes. Tap each one to see what it tells your body.
Skin, the largest sense organ
Skin does more than feel a touch. It has several kinds of receptor buried at different depths. Tap a spot on the picture to see who does what.
Experiment: where is skin most detailed?
If two toothpick tips touch your skin at the same time, close together, some places feel like a single touch. The smallest gap you can still tell apart as two points shows how densely packed the receptors are there. Drag the slider and compare with the table below.
The gap on screen depends on your screen size, so it is only an illustration. To do it for real, use two toothpicks, close your eyes and have a friend do the touching.
Experiment: measure your reaction time
Press start and wait. The moment the box turns green, tap it. The time you get is how long light takes to reach your eye, travel to the brain, be decided on, and be sent as an order to the muscles of your hand.
Most people get about 200 to 300 milliseconds. Nerve signals travel up to about 100 metres per second, but the slowest part is the brain working out what it is seeing and deciding what to do.
We see with the eyes, but we look with the brain
The eyes send millions of coloured dots, nothing more. The brain guesses, fills in and decides what that is. Most of the time it guesses right, but sometimes it can be fooled, and that is a sign the brain is using a shortcut.
Which orange circle is bigger?
Which line is longer?
Senses work as a team
Eyes and ears work together all the time. If you see lips making one sound while your ears hear another, the brain combines them and you hear a third sound entirely. That is why watching someone's face helps you follow what they say in a noisy room.
The sixth sense we really do have
Close your eyes and touch the tip of your nose with a finger. You can do it, even though you cannot see a thing, because your muscles and joints have receptors that constantly tell the brain where your arms and legs are. It is called body position sense. Balance, meanwhile, comes from the semicircular canals in the inner ear.
We actually have more than five senses
The idea of five senses comes from Aristotle, more than two thousand years ago. Today we know the body has several other kinds of receptor that fit into none of those five.
Why does spinning make you dizzy?
The inner ear has three semicircular canals set at right angles to each other, with fluid inside. When you turn your head, the fluid lags slightly behind, so the receptor sitting in it tilts and tells the brain you are turning. The moment you stop spinning, the fluid keeps flowing for a while so the brain is still being told you are turning, while your eyes say you have stopped. That disagreement is exactly what dizziness is.
Travel sickness comes from the same clash. The inner ear says you are moving, while eyes fixed on a book in the car say you are still, and the brain gets confused.
Pain in two waves
Think about stubbing your toe on a table leg. There is a sharp pain first, and a duller ache arrives a moment later. That is not one feeling, but two sets of signals travelling at different speeds. Press to watch.
Hot and cold that are neither hot nor cold
The temperature receptors in your skin are tiny protein channels. The one called TRPV1 opens when it meets heat above about 43 degrees. Butthe capsaicin in chilli opens that same channel so the brain gets exactly the message it gets from something hot, even though your tongue has not warmed up at all.
The other way round, menthol in mint opens TRPM8, which is the cold channel, so you feel a cool tingle with no change in temperature. That is why cold water helps a chilli-burnt mouth for a moment, but milk works better: capsaicin dissolves in fat, not in water.
Knowing where your arms and legs are without looking
You can touch your nose with your eyes closed because your muscles contain receptors called muscle spindles that measure how far the muscle is stretched, and your tendons have receptors that measure pull. The brain uses all of it to keep a live map of your body.
People who lose this sense cannot walk in the dark, even with perfectly strong muscles, because they have to watch their own legs the whole time instead.
Senses from inside your own body
Hunger, thirst, a full bladder, a pounding heart, being short of breath, feeling sick: these are all senses too, known together as sensing the inside of the body. The receptors sit in the stomach, the bladder, blood vessels and the heart.
The body also has receptors measuring how concentrated your blood is and how much oxygen it carries, all day long, without you noticing for even a second.
A world we cannot sense, but animals can
What we call the real world is really just the small slice our receptors happen to pick up. Other animals pick up different slices, so their world looks nothing like ours.
Bats see with sound
A bat calls at frequencies far too high for human ears, then listens to the echo. Its brain works out how far away the prey is from how long the echo takes to return: the closer it is, the faster it comes back. The bat calls faster and faster as it closes in, like a camera raising its frames per second.
Snakes see heat
Some snakes have small pits between eye and nostril that pick up infrared given off by warm-blooded animals. They can hunt in total darkness, as if they had a thermal camera built into their face.
Sharks sense electricity
Every animal's muscles give off a faint electrical signal as they contract. Sharks have electricity-sensing organs on the snout, so they can find a fish hidden under the sand without seeing or smelling it.
Birds read the Earth's magnetic field
Migrating birds cross continents without getting lost because they can sense the Earth's magnetic field. Scientists still argue about exactly how. One idea is a protein in the eye that is sensitive to magnetism.
Bees see light we cannot
Many flowers have patterns that reflect ultraviolet light, like landing lights pointing to the nectar. Bees see those patterns clearly. Human eyes have no cells for that range at all, so we just see a plain-coloured flower.
Look after your senses
Eyes
Read in good light, rest your eyes from screens about every 20 minutes by looking far away for 20 seconds, and play outdoors every day, because natural light slows down short-sightedness in children. Wear sunglasses in bright sun, and never stare at the sun.
Ears
Use the 60 by 60 rule with headphones, stay away from big speakers at loud events, andnever poke a cotton bud or anything else into your ear canal Earwax works its way out by itself. Poking only pushes it deeper and can burst the eardrum.
Nose and tongue
Wash your hands before eating, brush your teeth and gently brush your tongue, be careful with very hot food because it can scald your taste buds, and if you suddenly cannot smell or taste, tell a grown-up, because it is a sign of several illnesses.
Skin and feeling
Pain is not the enemy. It is a warning system that saves lives. People who cannot feel pain, such as those with nerve damage from diabetes, often get wounds on their feet without noticing. If your hands or feet go numb or tingle unusually often, tell a grown-up too.
Which sense is working?
Read the situation on the card, then quickly pick which sense is being used. You have 45 seconds, and a streak of right answers earns a bonus.
A right answer scores 10 points, and a run of right answers in a row keeps adding a bigger bonus.
Test yourself
Fancy a game on this lesson?
Every question from this lesson is in the Body 101 arena. Race the clock, duel someone at home, play as a team, beat the bosses, or practise with no timer at all.