The shortest path
A hand touches a hot pan. It pulls back before you think.
- A hot pan touches the skin.
- A sensory neuron carries the message inward.
- It reaches the spinal cord, a relay.
- A motor neuron carries the order outward.
- The hand pulls back. No thinking yet.
- The brain is told a moment later.
Tap the stations in order. Wrong taps are not accepted.
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A reflex is a fast, automatic response. Automatic means you do not choose it. You do not think first.
A receptor notices the heat. A sensory neuron carries the message inward to the spinal cord. The cord is a long bundle of nerve cells inside the backbone. A relay neuron there passes the message to a motor neuron. The motor neuron sends the order out to the muscle. The muscle is the effector. Here the order starts in the cord, not in the brain.
This path is the reflex arc. Thinking takes more time, because many neurons work together. The arc skips that delay.
The message also travels on to the brain. The brain learns about the pan a moment later.
The rule
A reflex is a fast, automatic response. Its arc often closes in the spinal cord. The muscle acts without waiting for the thinking part of the brain.
For example, a doctor taps below the knee. The leg kicks out. That short arc closes in the spinal cord. The knee jerk has no relay neuron.