Are reflexes really that rapid?

written by Nia Davies, Science Made Simple work experience student

Have you ever jumped away from something hot, blinked in a bright light, or blushed with embarrassment? We don’t decide to do these things, they just happen without us thinking! So if we don’t control them, what does?

These automatic reactions are a type of neural activity called reflexes.

What are reflexes? Why do they happen?

Reflexes are a rapid movement caused in response to a stimulus. Often they do not involve the brain and are therefore an involuntary response. A reflex arc makes the fast, automatic response of a reflex action possible.

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Demonstration of a reflex arc (Marta Aguayo, own work, CCBYSA)


Imagine that you have touched a hot surface. Before you know what’s happening you have pulled your hand away.  The impulse caused by the heat travels up your arm to your spinal cord, and then is relayed to the muscle, which contracts.

The reflex arc makes this happen. It contains just three neurons, which are specialised cells that transmit electrical nerve impulses.  It begins with the stimulus – in this case touching the hot surface. This causes the temperature receptors in the skin of the hand to create a nerve impulse in a sensory neuron.  The sensory neuron then passes the impulse to the spinal cord where a relay neuron links the sensory neuron to the motor neuron.  The motor neuron carries the nerve impulse from the spinal cord to the muscle in the upper arm whereby an effector produces the effect – the muscle in the upper arm contracts and you pull your arm away.

This all happens in about 0.15 seconds, which is the same amount of time as it takes for light (which is travelling at 300 million metres per second) to travel 45 million metres!

This type of reflex arc is called a somatic reflex arc because it affects your muscles. An autonomic reflex arc affects your inner organs.

Are reflexes REALLY that fast?

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Examples of neurons and electrical impulses travelling along them (Dr Jana, CCBY4.0)

Reflexes are caused by electrical impulses travelling from the receptor, to the spinal cord and then to the muscle.

When we think of electrical signalling, we usually think of those going on in our phones and laptops. This type of electrical signalling travels at around 669 million miles per hour.

However, the electrical signalling in our nervous system is a much slower process and only travels at about 150 miles per hour. This is more than four-million times slower.

So in fact, no, compared to electrical impulses our reflexes aren’t that fast at all!


A good analogy is imagining a giant were lying with its head in Baltimore and its feet dangling off the coast of South Africa.  If its toe got bitten by a shark on Monday at noon, it wouldn’t feel the sensation of pain until Wednesday at noon, and wouldn’t be able to respond by pulling its foot away until early Saturday. [David J. Linden, Touch: The Science of Hand, Heart, and Mind]

Can we make our reflexes faster?

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Example of the Jendrassik Manoeuvre by interlocking hands together in a hook like form (Arthur Van Gehuchten, public domain)

Although a reflex is involuntary and automatic, it can change over time.  If a stimulus is continuously repeated the reflex undergoes sensitization and the response is increased.  So the more you do it, the faster the reflex will be!

However, it is also possible to decrease or even eliminate a reflex altogether trough the same process. A bit like hearing a sudden loud noise constantly until it stops making you jump.

We can also use something called the Jendrassik Manoeuvre to increase reflexes.  This is different from making your reflex quicker, instead it is making the action of your reflect bigger. It happens when you are distracted by something else when the stimulus happens.

For example, if you clench your jaw, or grip a book tightly when your knee is hit with a reflex hammer your leg would do more of a kick than a twitch. The theory is the distraction prevents you from consciously inhibiting the response.

So, next time you hear a loud noise and jump, or start blinking when something is in your eye, think of the process happening, from the stimulus to the effector, and how it could be happening four million times faster!

Here at Science Made Simple we are keen to inspire not only the next generation of scientists but also the next generation of science communicators and share our love of what we do! We hope you enjoy this blog post from one of our latest work experience students, Nia. We loved having her and wish her every success for the future.

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Posted in Biology, Curriculum, Exploring Science