What it does
When you see something and react, a signal goes from your eye to your brain, the brain decides what to do, and an order runs down the spinal cord and out to your muscles. This page lets you time that whole chain on your own phone or computer: tap when the box turns green. It then shows how little of that time is spent by the signal actually travelling along the nerves. Most of it is the brain seeing and deciding. A reflex, like pulling your hand off a hot pan, skips the deciding: the spinal cord connects the incoming nerve straight to the outgoing one, so it is much faster. The page draws that reflex arc and runs a pulse round it, and turns the classic falling-ruler test into a reaction time. To see what the nerve signal itself is, an electrical spike that runs along the neuron, try the nerve impulse lab; the ruler’s fall uses the same formula as the equations of motion.
Using it
- Time yourself. Tap the big box to start, wait for green, then tap again as fast as you can. Tapping early shows “Too soon”. Take five or more tries; the page gives your average, best and median.
- Compare with the nerve signal. Set the path length L and the signal speed v. The bars show your average, the travel time L ÷ v, and a reflex.
- Fire the reflex arc. Press Touch the hot pan and watch the pulse go from the skin to the spinal cord and back out to the arm muscle, with each step named.
- The ruler test. Type how far a real ruler fell before you caught it. The page works out t = √(2d ÷ g).
- Class 11 adds the two synapses of the arc and their small delay.
Your screen time includes a little delay from the screen and the touch sensor, often 20 to 50 ms, so it will be a bit slower than a ruler catch.
Worked example
A signal travelling 1 m at 60 m/s takes 1 ÷ 60 = 0.0167 s, or 16.7 ms. A typical tap time is around 250 ms, about 15 times longer, so nearly all of it is the brain at work. A pull-away reflex with about 1.6 m of nerve takes 1.6 ÷ 60 = 26.7 ms of travel. If a ruler falls 15 cm before you catch it, your reaction time is √(2 × 0.15 ÷ 9.807) = 0.175 s, or 175 ms.
Where this fits in school
Class 10 science (control and coordination) covers neurons, reflex actions and the reflex arc, and the roles of the brain and spinal cord. Class 11 biology (neural control and coordination) adds the nerve impulse, synapses and the reflex arc in more detail. The ruler formula comes from Class 9 and 11 physics (motion). This page is an independent learning aid. It is not made by or connected with NCERT, and it does not copy any textbook.
Questions
Why is my time different each try? Attention, tiredness and how long you waited all change it. That is why scientists take many tries and use the average or median.
Is anything saved? No. Your times are kept only while the page is open and go when you leave or press Clear my times. Nothing is sent anywhere.
Is this a medical test? No. It is a classroom activity.
Where the numbers come from
- The reflex arc (receptor, sensory neuron, relay neuron, motor neuron, effector): standard school biology, written by fibub in its own words.
- Nerve speeds (about 60 m/s for a myelinated nerve, about 1 m/s without myelin), path lengths and the 0.5 ms synapse delay are round teaching values.
- Falling ruler: d = ½ g t² with standard gravity g = 9.80665 m/s² (BIPM/CODATA conventional value).
- Checked in the tests by hand (15 cm → 0.175 s; 1 m at 60 m/s → 16.7 ms). Your tap times stay on the page and are never stored or sent.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.