Think Fast! What’s a Reflex?

Written by: Joseph Flores Edited by: Kevin Nhan, Carine Medellus

Have you ever jumped after accidentally touching something really hot? Or maybe someone threw a ball at you, and you couldn't move out of the way fast enough? These are examples of how your body reacts (or doesn’t) to the world around you. Reactions like these are called reflexes. They’re fast, automatic actions your body makes without thinking.  But how can your body move without you deciding?  And why is it that sometimes you can’t move quickly enough?   Here, we’re going to use neuroscience, the study of the brain, to answer your rapid-fire questions.  

To explore this idea, you can test your reflexes with a 2-player game called Hot Hands. It’s a great way to test your reflexes and reaction speed, but it’ll also help us uncover what’s going on inside your body.

How to Play Hot Hands:

1. Both players face each other, spaced roughly at arm’s length. One player (the defense) extends their hands forward, palms up. The other player (the offense) hovers their hands just above the defense’s hands, palms down.

2. The offense (player with palms upward) should try to slap the back of the defense’s hands.  At the same time, the defense (player with palms downward) should quickly pull their hands back to dodge.

3. If the offense (the player with palms upward) successfully slaps the defense’s hands (player with palms downward), players should swap roles and play again.  You can add new rules to make the game more interesting; for example, try closing your eyes or using your fingers to tickle and distract the other player. 

After you’re done playing, ask yourself a few questions: did you get hit? Did you try to react but couldn’t move in time? What made it hard or easy to dodge? Did you move a bit without thinking?  It’s likely that sometimes you reacted quickly and dodged the slap, while other times you couldn’t move fast enough.

When you see your opponent's hand move, your eyes create a chemical signal. These signals travel all over your body through wires called nerves (think of them like roads, allowing travel back and forth). Here, signals from your eyes travel to your brain, an organ in your head that takes this information and decides on a response.  Here, your brain will decide it needs to move your arms.  To get this message to your arms, these signals travel through your spinal cord, a big bundle of nerves along your back (think of it like a massive freeway with tons of roads, connecting your brain to the rest of your body).  Here, these signals tell your muscles to tighten, pulling your hands away… if you’re fast enough!  When you react to your surroundings, your eyes, brain and spinal cord are constantly working together to tell your body to walk, talk, and play. 

Even though your brain is super quick, it takes time to send the “MOVE!” signal to your muscles. That’s why sometimes you still get hit: it depends how quickly your body can move chemical signals sent from the brain. In some special cases, your brain doesn’t even need to get involved. For example, if you accidentally touch something hot, your body will probably respond as soon as you feel the pain.  This is because of an emergency shortcut called a reflex arc.  In this case, the pain creates a chemical signal that skips the brain: information goes straight from the spinal cord and to your muscles, making you pull back almost instantly. Reflex arcs are the reason you can sometimes act faster than you think.  This amazing ability is so quick, humans can react faster than a quarter of a second (the time it takes to blink your eyes).  But this speed isn’t just for games: without fast reflexes, we could be too slow to react to danger (like a wild animal) or prevent harm (like skin damage from fire).  

So, what’s a reflex?  Reflexes are your body’s automatic reactions to the world around you. When you usually react to the world around you, signals need to travel from your eyes, then the brain, and then the rest of your body.  But occasionally, they can skip the brain to save time. Using simple games and experiments, we can explore how our brain, nerves, and muscles work together.  This is all part of neuroscience, the study of the brain and how it communicates with the body. Learning about reflexes shows us just how amazing and efficient our bodies are.  

References:

Reflexes. (n.d.). Physiopedia. Retrieved April 8, 2025 from https://www.physio-pedia.com/Reflexes.

WhiteRhin0h. (April 2012). Anyone else play hot hands as a kid?  Reddit.  Retrieved April 22, 2025 from https://www.reddit.com/r/nostalgia/comments/1r3qxp/anyone_else_play_hot_hands_as_a_kid/

Understand the structure and functions of the central nervous system. (n.d.).  Britannica.  Retrieved April 22, 2025, from https://www.britannica.com/video/brain-cord-central-nervous-system/-153049.