Suppose you are running barefoot and step on a thorn.

Instantly the tiny nerve bulbs in the skin of the sole of your foot are stimulated, or set in vibration, and they send these vibrations up the sciatic nerve, into and up the whole length of the spinal cord, through the medulla, which switches them over to the other side of the brain up through the _brain stalk_, and out to the part of the surface (cortex) of the brain which controls the movements of the foot.

All this takes only a fraction of a second, but it is not until the message reaches the brain-surface that you feel pain.

If you were to cut the sciatic nerve, or even tie a string tightly around it, you could prick or burn the sole of your foot as much as you pleased, and you would not feel any pain at all.
As soon as the surface of the brain has recognized the pain and where it comes from, it promptly sends a return message back down the same cable, though by different nerve-wires, to the muscles of the foot and leg, saying, "Jerk that foot away!" As a matter of fact, this message will arrive too late, for the centres in the spinal cord will already have attended to this part of the matter, often almost before you know that you are hurt.
However, there is plenty of other work for the brain to do; and its next step, quicker than you can think, is to wake up a dozen muscles all over the body with the order, "Sit down!" And you promptly sit down.

At the same time, the brain "central" has ordered the muscles of your arms and hands to reach down and pick up the foot, partly to protect it from any further scratch, and partly to pull the thorn out of it.