What you need:
- A willing volunteer
- Two toothpicks
How to:
- Get your volunteer to close their eyes.
- Press two toothpicks, point downwards, lightly on the
volunteer's arm - make sure they're around 5 cm apart. Don't press
hard - that would hurt! Ask the volunteer how many toothpicks they
can feel.
- Move the toothpicks closer together and keep asking the
volunteer how many toothpicks they can feel.
- At a certain distance apart, the volunteer will only be able to
feel one toothpick - even though there are two!
- When this happens, measure the distance between the two
toothpicks. Record your results in a table like this:
| Area of the body |
Distance between toothpicks |
| Arm |
|
| Hand |
|
| Index finger |
|
| Feet |
|
| Knee |
|
Repeat the experiment on different parts of the body - you could
try the back of the hand, index finger and feet - if they're not
too ticklish!
What did you find?
On some parts of the body, such as the index finger, the
distance between the two toothpicks should be much smaller. This
means that the volunteer has more sensory
receptors in that part of their body. Sensory receptors
are specialised cells. They detect changes in the environment,
called stimuli, and turn them into electrical
impulses. In this case, the sensory receptors are detecting
pressure. Sensory neurones - nerve cells - carry these electrical
impulses to the spinal cord and brain.
To be really sensitive, each sensory receptor must connect to a
different nerve cell.
To find out more about nerves, go to: What's in a
Nerve?