How Does a Touch Reach Your Brain?

You use your sense organs every day to observe the world: your eyes, ears, nose, tongue and skin Grade 8: Grade 8: Observation. But how does an eye or a fingertip actually notice anything?

Each sense organ has receptors: special cells, or the very ends of nerve cells, that respond to one kind of change around them. A change that a receptor responds to is a stimulus (plural stimuli). When a stimulus reaches a receptor, the receptor starts a message, a tiny electrical signal, in a nerve.

Stimulus	Receptors	Where
light	rods and cones (about 120 million rods and 6 million cones in each eye)	the back of the eye (retina)
sound	hair cells (about 15,000 in each ear)	the inner ear
chemicals in food	taste cells, in taste buds	the tongue and mouth
chemicals in the air	smell cells (about 400 different kinds)	high up inside the nose
touch, pressure, vibration	touch receptors	the skin
heat and cold	temperature receptors	the skin, mouth and tongue
damage	pain receptors	almost everywhere

You may have learned that there are five senses. There are more. Close your eyes and touch your nose with one finger: you can, because stretch receptors in your muscles and joints tell your brain where every part of your body is. Receptors in your inner ear tell you which way is up, so you keep your balance on a bus. Other receptors inside you notice when your stomach is full, your bladder needs emptying, or there is too much carbon dioxide in your blood (that is what makes you gasp after holding your breath).

Inside a fingertip

Your skin is full of receptors, and they are not all the same. Some are bare nerve endings. Others are wrapped in tiny capsules that make them respond to one kind of touch. Look at Figure 14.1: it is a slice of fingertip, less than 3 mm deep, about as thick as two coins stacked.

Each kind of receptor answers only its own stimulus well. Light shining on your skin doesn’t make you see. Sound doesn’t make your tongue taste anything. And here is the strange part: your brain knows what a message means by where it comes from, not by what started it.

Chilli and mint

Bite into a green khursani and your mouth feels as if it is burning. Chew pudina, mint, and it feels cool. Neither is hot or cold. What is happening?

Chilli contains a chemical called capsaicin. It switches on the same receptors in your mouth that respond to real heat, above about 43 °C. Those receptors send exactly the message they send for hot tea, so your brain feels heat. Mint contains menthol, which switches on the receptors that respond to coolness. Your mouth is the same temperature all the time; only the messages change.

Receptors notice changes

Put on a shirt, and you feel it on your skin. A minute later, you don’t, though it is still there. Many receptors respond strongly when something changes, and then send fewer and fewer messages if it stays the same. This is why you notice the smell of a kitchen when you walk in, but the cook doesn’t. It is also why you can be fooled.

More receptors, finer touch

Your fingertips are packed with touch receptors. The skin of your back has far fewer. You can measure the difference.

Part of the body	Two-point threshold (roughly)
fingertip	2–3 mm
upper lip	5 mm
palm	10 mm
forehead	15 mm
foot	20 mm
forearm	35 mm
back	39 mm
calf	45 mm

These are the average results of a study by the American scientist Sidney Weinstein, published in 1968. Your own results will be a little different.

A receptor has noticed something. Now the message has to travel. You learned in Grade 8 that nerve cells are long and thin, like threads Grade 8: Shape and size of cells; in Grade 9 you will meet their parts, the dendrites, cell body and axon Grade 9: Neuron, and the sensory nerves that carry messages in from the receptors to the spinal cord and brain Grade 9: Nerve fibers. A nerve is a bundle of many of these long fibers, like the wires in a cable.

So the path of a touch on your fingertip is:

receptor in the skin → sensory nerve up the arm → spinal cord in the neck → up the spinal cord → brain

Only when the message reaches the brain do you feel anything. Until then, nothing has happened that you know about.

How fast?

Nerve messages are fast, but not instant, and they are not all equally fast. Thick nerve fibers, wrapped in a fatty coat, carry messages faster than thin ones.

Kind of message	Fibers	Speed
touch and pressure	thick, coated	about 35–75 m/s
sharp pain, cold	thin, lightly coated	about 5–30 m/s
slow, burning pain, warmth	thinnest, no coat	about 0.5–2 m/s

Even the fastest is much slower than the electricity in a wire, which carries a signal at more than half the speed of light. For comparison, a fast bus on the highway goes about 25 m/s (90 km/h).

What happens to a message once it arrives? Sometimes the body must act at once. Your hand pulls away from the hot iron; your lips jerk back from the tea; you blink when dust flies at your eye.

The very fastest of these, the reflex actions, don’t even wait for the brain to decide. The message reaches the spinal cord (or, for the head, the brainstem), and goes straight back out along a motor nerve to a muscle. You will meet this short path, the reflex arc, in Grade 9 Grade 9: Reflex action. The message to the brain arrives a moment later, and that is when you feel “hot!”, after your hand has already moved.

Most of what you do is not a reflex. When a ball is thrown at you, a bus horn sounds behind you, or your name is called in class, the message goes to the brain, which works out what it means and decides what to do; then it sends orders out to your muscles. All of this takes time: your reaction time.

Measure your reaction time

How can a distance be a time? A falling ruler speeds up in a way that is always the same, because gravity pulls on everything the same way. So the distance it falls tells you exactly how long it was falling: the time between your partner letting go and your fingers closing.

Caught at	Reaction time
5 cm	0.10 s
10 cm	0.14 s
15 cm	0.17 s
20 cm	0.20 s
25 cm	0.23 s
30 cm	0.25 s

Notice that the ruler falls only 5 cm in the first tenth of a second, but the 10 cm from 20 to 30 cm take less than a twentieth of a second. It keeps speeding up, so twice the distance does not mean twice the time.

What makes reaction time longer or shorter?

Your reaction time is the time for your eyes to notice, the messages to travel, your brain to decide, and your muscles to move. Anything that slows one of these slows you down. You can test this yourself, as a scientist would.

Scientists have measured reaction times for different senses. Many studies have found that people react a little faster to a sound or a touch (about 0.14–0.16 s) than to a light (about 0.18–0.20 s), for the simplest kind of task, when you only have to press a button.

Not every message leads to action. Most of what your senses pick up, your brain ignores: the color of the wall, the feel of your chair. Some of it is stored, so that you can use it later. Stored information is memory.

The hot tea is stored. So is the way to your school, your friend’s face, the smell of your grandmother’s kitchen at Dashain, how to ride a bicycle, and the multiplication table of 7. When you learn something, connections between nerve cells in your brain change: some grow stronger, and new ones form. Your brain is physically a little different after you have learned something.

Two kinds of memory

Someone tells you a phone number: 98 41 5 3 6 2 7 0. You repeat it to yourself until you find a pen. If someone talks to you, it is gone. That was your short-term memory: it holds a few things, for less than half a minute unless you keep repeating them.

Things you go over, use and connect to what you already know pass into your long-term memory, which can keep them for years: facts and events (what you ate yesterday, the capital of Nepal), and skills (writing, swimming, riding a bicycle).

In 1956 the American psychologist George Miller found that most people’s short-term memory holds about seven things, give or take two. But a “thing” can be a single digit, or a group. A Nepali mobile number has ten digits, too many to hold one by one. Split into three groups, “984 – 153 – 6270”, it becomes easier. Grouping things into bigger pieces like this is called chunking.

Why we forget

In the 1880s the German psychologist Hermann Ebbinghaus tested his own memory. He learned lists of made-up syllables, like “DAX” and “BUP”, which meant nothing, so that what he already knew couldn’t help him. After a wait, he learned each list again, and measured how much less time it took the second time. If he had remembered everything, relearning would take no time: 100 % of the work saved. If he had forgotten everything, it would take as long as the first time: nothing saved.

Look at Figure 14.4. After only 20 minutes, almost half the work of learning was lost; after a day, two-thirds. But after a month, he still saved about a fifth: something had stayed. Scientists who repeated his experiment in 2015 got very similar results.

What you can do about it is what scientists have found works best:

• a.Test yourself instead of rereading: cover the page and try to remember, then check. Each time you pull a memory out, it gets stronger.
• b.Space it out. Go over something again after a day, then a few days later, then a week later, rather than all at once the night before. The site’s flash cards, like those in Science and Math in Nepali, bring back the cards you don’t know yet.
• c.Sleep. While you sleep, your brain goes over the day and strengthens what it has stored.
• d.Connect it to what you already know, or to a picture or a story. Things that matter to you, or that surprise or frighten you, are remembered best: one sip of hot tea was enough.

Now you can answer the question in the title, and follow the hot tea all the way.

• What Happens to the Dal Bhat You Eat? Your nerves and brain need a steady supply of sugar and oxygen. How your body breaks food down and delivers it to every cell.
• Why Do Birds Sing and Flowers Smell Sweet? Animals’ senses at work: songs, colors and scents, and how animals and plants use them to find a partner or a pollinator.
• Why Is Speeding So Dangerous? Reaction time on the road: how far a car goes before the driver even starts to brake.