Coaching Centre
Reflexes and reaction time
Voluntary and reflex responses, the reflex arc, why reflexes are faster than decisions, examples of human reflexes, and how to measure reaction time with a falling ruler.
Elite sprinters are disqualified if they leave the blocks less than 0.1 seconds after the gun. The rule exists because nobody can hear the gun, decide to go and move their legs that fast. Anyone who does must have guessed.
Two kinds of response
Most of what you do involves a choice. When you see a friend and wave, the message from your eyes goes to your brain, which works out who it is, decides to wave and sends instructions out to your arm. This is a voluntary response. It is flexible, but it takes time.
A reflex is different. It is a fast, automatic response to a stimulus that you don't have to think about and usually can't stop. Reflexes mostly protect you from harm, so speed matters more than flexibility. Pulling your hand off a hot pan, blinking when something flies at your eye and coughing when food goes down the wrong way are all reflexes.
The reflex arc
The path a message takes in a reflex is called a reflex arc. In many reflexes, the decision is made in the spinal cord rather than the brain, which saves time.
- Receptor. Pain and heat receptors in the skin of your finger detect the flame.
- Sensory neuron. An impulse travels up your arm and into the spinal cord.
- Interneuron. Inside the spinal cord, the sensory neuron passes the message straight to an interneuron, which passes it to a motor neuron. There is no thinking involved.
- Motor neuron. The impulse travels back out to your arm.
- Effector. Your biceps contracts and pulls your hand away.
At the same moment, another interneuron sends a message up the spinal cord to the brain. That is why you feel the pain a fraction of a second after your hand has already moved. The brain also remembers what happened, so you learn to be careful next time.
The fewer synapses in the path, the faster the reflex. The knee-jerk reflex your doctor tests with a little hammer has just one synapse in the spinal cord, with no interneuron at all, and takes only about 30–50 milliseconds.
Reflexes you have
| Reflex | Stimulus | Response | How it helps |
|---|---|---|---|
| Withdrawal | Pain, heat | Limb pulled away | Limits injury |
| Blink | Object close to eye, puff of air | Eyelids close | Protects the cornea |
| Pupil reflex | Bright light | Pupil gets smaller | Stops too much light damaging the retina |
| Cough and sneeze | Irritation in airways or nose | Sudden blast of air | Clears the airways |
| Knee jerk | Tendon below kneecap stretched | Thigh muscle contracts | Helps keep you upright |
| Grasp (babies) | Something touches the palm | Fingers close tightly | Disappears after a few months |
The pupil reflex is one that doctors and paramedics check after a head injury. They shine a torch in each eye. If a pupil doesn't shrink, it suggests damage or swelling in the brain stem, where this reflex is controlled.
Reaction time
Reaction time is the time between a stimulus and your response to it. For a voluntary response, such as catching a dropped ruler, it includes the time for your eyes to detect the movement, the brain to recognise it and decide, and the muscles to act. Typical reaction times to something you see are about 0.2–0.25 seconds. Reactions to sounds tend to be a little faster.
You can measure reaction time without a stopwatch, because a dropped ruler falls by a predictable amount. The longer you take to react, the further it falls before you catch it.
time (s) = √(2 × distance fallen (m) ÷ 9.8)
A student catches a dropped ruler after it has fallen 18 cm. What is their reaction time?
- Press "Get ready", then catch the ruler as soon as it starts to fall. Do at least five catches and read your average reaction time.
- Switch to "Listen for the click". The ruler is hidden, so you must react to the sound. Is your average faster or slower than with your eyes?
- Look at the spread of your results. Why is it better to take an average of several trials than to trust one catch?
- Try pressing before the ruler drops. Why would a real experiment need to stop people guessing?
Test yourself
Why are reflexes faster than voluntary actions?
Fewer neurons and synapses, and no time spent thinking.
In the hand-withdrawal reflex, what is the effector?
The muscle carries out the response.
Which of these is not a reflex?
Answering a question is a voluntary, thought-out response.
A ruler falls 8 cm before a student catches it. Their reaction time is closest to
√(2 × 0.08 ÷ 9.8) = √0.0163 = 0.13 s.
Label the reflex arc.
Put the steps of the withdrawal reflex in order.
A reflex is fast and . The pathway it follows is called a reflex . In many reflexes the decision is made in the . The time between a stimulus and a response is the time. Shining a torch in the eye tests the reflex.
A student measures her reaction time with a falling ruler. Her five results are 15 cm, 17 cm, 31 cm, 16 cm and 14 cm. Identify the outlier, suggest a cause, and explain how she should process her results to find her reaction time.
31 cm is an outlier because it is about twice the others. She may have been distracted or blinked as the ruler was dropped. She should leave this result out, or redo that trial, and average the other four: (15 + 17 + 16 + 14) ÷ 4 = 15.5 cm = 0.155 m. Her reaction time is √(2 × 0.155 ÷ 9.8) = √0.0316 ≈ 0.18 s. Doing more trials would make the average more reliable.