Touch a hot pan, and you pull your hand back before you even feel the pain. The nervous system is the body's fast message network. It is built from nerve cells, an example of cells building organs and systems.
Key idea
Sense receptors (in the eyes, ears, nose, tongue and skin) detect stimuli. Nerves carry signals to the brain and spinal cord, which interpret them and decide. Nerves carry signals back out to muscles. A reflex is a fast, automatic response routed through the spinal cord.
Why it works
A signal takes time to travel. A short path, from the skin to the spinal cord and straight back, is faster than going all the way up to the brain and back. For dangers like heat, that saves precious time. For everything else, such as reading or catching a ball, the brain does the work: it compares signals with memory and decides.
Worked example: Detect or decide?
Light from a page enters your eye, and you read the word "cat". Which part detects, and which part understands?
- Receptor cells at the back of the eye detect the light and send signals along the optic nerve.
- The brain interprets the pattern as letters and a word.
Answer: The eye detects; the brain interprets.
Common mistake
Students think the eyes or hands "feel" or decide on their own. Receptors only detect and send signals; the brain interprets them. That is why the brain can be fooled: in an optical illusion the eyes send correct signals, but the brain reads them wrongly.