In short
A reflex action is a quick, automatic response to a stimulus that does not need thinking. It follows a short path called the reflex arc, usually through the spinal cord.
The arc has five parts: the receptor detects the stimulus, a sensory neuron carries the message in, a relay neuron in the spinal cord passes it on, a motor neuron carries it out, and an effector such as a muscle responds. The brain is told afterwards. Because the message does not travel to the brain first, the response is fast, and it protects the body from harm.
You touch a hot pan and your hand pulls away before you feel the pain. The reflex arc worked through the spinal cord, and the brain learned of it only afterwards.
Also searched as: reflex actions.
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