A reflex like pulling your hand away happens before the brain registers pain fully.

The longer version

A withdrawal reflex is wired to be fast, and speed comes from skipping the long route to the brain. Sensory fibres from the skin enter the spinal cord and connect directly to motor neurons through one or more interneurons, so the muscles that pull the limb away fire almost immediately.

Only afterwards does a separate signal travel up to the brain, where it is registered as pain. The delay between the two signals is small in absolute terms but large compared with the reflex itself. This arrangement exists because damaged tissue is worse than a sudden movement. Other reflexes use the same shortcut principle, including the knee jerk and the pupil's response to bright light.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.