Reflex Arc & Nervous System Simulator
Trigger a stimulus and watch it race through the reflex arc, then compare a fast one-synapse knee-jerk reflex with a slower multi-synapse withdrawal reflex.
Press Stimulate and watch the signal travel receptor → sensory neuron → spinal cord → motor neuron → muscle. Compare a one-synapse knee-jerk reflex with a multi-synapse withdrawal reflex.
About the Reflex Arc & Nervous System Simulator
Free reflex arc & nervous system simulator. Trigger a stimulus and watch it race through the reflex arc, then compare a fast one-synapse knee-jerk reflex with a slower multi-synapse withdrawal reflex. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for biology, the reflex arc & nervous system simulator runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Trigger a stimulus and watch it race through the reflex arc, then compare a fast one-synapse knee-jerk reflex with a slower multi-synapse withdrawal reflex. Use it to explore biology ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Reflex Arc & Nervous System Simulator
- Use the controls to change Preset, Path distance, Conduction velocity, Number of synapses, Delay per synapse. The simulation reacts instantly.
- Pick an option such as Monosynaptic (patellar/knee-jerk), Polysynaptic (withdrawal from pain) to switch modes or load an example.
- Press "Stimulate", "Reset to defaults", "Check reaction time", "Check total reaction time" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Load the monosynaptic preset and read the reaction time.
- Load the polysynaptic preset and compare - predict which will be faster before checking.
- Raise the number of synapses one at a time and watch reaction time climb in steps.
- Lower conduction velocity to see how much slower an unmyelinated fiber makes the whole reflex.
Key ideas you can learn
- A reflex arc runs receptor → sensory neuron → spinal cord → motor neuron → muscle, usually without waiting for the brain.
- Reaction time = conduction time (distance ÷ velocity) plus synaptic delay time (delay × number of synapses).
- A monosynaptic reflex like the patellar (knee-jerk) reflex has only one synapse and is the fastest possible reflex.
- A polysynaptic reflex, like withdrawing from pain, uses more synapses and often slower fibers, trading speed for a more complex, adjustable response.
Where this is used in the real world
Doctors test reflexes like the patellar reflex for exactly this reason - an abnormally slow or absent reflex can point to nerve or spinal cord problems.
Who is this simulation for?
Biology students and teachers who want to explore living systems, populations and genetics interactively and safely.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why is the knee-jerk reflex so fast?
It uses only one synapse and fast, thick sensory and motor fibers, so almost no time is lost at synapses.
Why does pulling your hand from a hot stove involve more synapses than the knee-jerk reflex?
A withdrawal reflex coordinates several muscles and often crosses to the opposite leg for balance, which needs extra interneurons and synapses.
Is the Reflex Arc & Nervous System Simulator free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Reflex Arc & Nervous System Simulator work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.