Reflex actions

A response without thinking

You touch a hot pan, and your hand jerks away. You did not decide to move it. It moved before you even felt the pain. Such a quick, automatic response is called a reflex action.

Reflexes protect the body. They act faster than thinking, so they save us from harm. Other reflexes are:

• blinking when something comes close to the eye;
• sneezing when dust gets into the nose;
• coughing when food goes the wrong way;
• the knee jerk, when the doctor taps just below the knee;
• the pupil getting smaller in bright light.

The reflex arc

The path that the message follows in a reflex is called a reflex arc.

1. Receptor. Heat receptors in the skin pick up the stimulus.
2. Sensory neuron. It carries the message to the spinal cord.
3. Relay neuron. Inside the spinal cord, a short relay neuron passes the message straight on.
4. Motor neuron. It carries the message out to the effector.
5. Effector. The arm muscle contracts, and the hand is pulled away.

The spinal cord is made of nerves that bring in messages and nerves that take messages out. In a reflex, it connects them directly.

Why through the spinal cord?

Thinking happens in the brain. To think about the hot pan, the message would go all the way up to the brain, the brain would decide what to do, and a message would come back down. That takes time.

In a reflex, the spinal cord answers at once. The path is shorter, there are fewer synapses, and no time is spent deciding. So a reflex is much faster.

The brain is not left out. A message also goes on from the spinal cord to the brain. That is why you feel the pain a moment after your hand has already moved away.

With the starting values, the reflex takes about $30$ ms and the thought-out response about $208$ ms, nearly seven times as long.

Measuring a reaction time

A thought-out response is slower, and you can measure it with a ruler. A friend holds a ruler, and you hold your fingers open at its zero mark. The friend lets go without warning, and you catch it. The distance $d$ that the ruler fell gives your reaction time $t$:

Catching the ruler is not a reflex: your eyes see it fall, and your brain decides to close your fingers.

Try it in Python

The program turns five catches into reaction times.

You should see times between about $160$ ms and $189$ ms, with $12.5$ cm the best catch.

Practise with new numbers

Common mistakes

• Saying the brain controls every reflex. Many reflexes are handled by the spinal cord.
• Thinking you feel the pain first. The hand moves first; the feeling comes a moment later.
• Calling catching a ruler a reflex. It needs the eyes and a decision by the brain.
• Leaving out the relay neuron. In the spinal cord, a relay neuron links the sensory and motor neurons.

Remember these

• A reflex is a fast, automatic response, done without thinking.
• The reflex arc: receptor, sensory neuron, relay neuron (spinal cord), motor neuron, effector.
• The spinal cord answers at once, so a reflex is fast; a message still reaches the brain.
• A thought-out response goes through the brain and takes longer.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Put the parts of the reflex arc in order: motor neuron, receptor, effector, sensory neuron, relay neuron.

2. Where is the relay neuron of a reflex like the knee jerk?

3. Why do you feel the pain of a pin prick only after you have pulled your finger away?

4. A ruler falls $4.9$ cm before you catch it. What is your reaction time?

5. Is blinking when a fly comes near your eye a reflex? Give a reason.