CNS and Neuron Simulator: How the Nervous System Works in 3D
Move a hand, feel a touch, tap a knee, see an apple or think, and watch every signal race through a 3D brain, spinal cord and nerves at its real speed. Then zoom in to one neuron firing and one synapse.
Timing and speed
Lab notebook
Every scenario you run, every neuron you fire and every measurement is written here with its timing. Export a CSV, or turn the session into a PDF lab report.
| # | What | Settings | Result |
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Concept library
Click "New problem" to begin.
Click "Start quiz" for 5 questions on the nervous system, reflexes, neurons, the action potential and synapses.
1 Overview
The simulator has four zoom levels, switched at the top left of the view. Whole system is a body with its real skeleton, brain, spinal cord and nerves, in 3D (desktop) or as a 2D diagram (phones). Neuron fires one motor neuron, Synapse shows one synapse at work and Reflex arc shows the reflex in a section of the spinal cord. On the right, Signal path lists every step of the running scenario with its time, Graph shows the timeline, the membrane potential or conduction speed, and Part describes whatever you click. Experiment has six practicals with a lab report; Quiz has practice questions and a 5-question quiz.
2 Scenarios
Pick a scenario on the bar at the bottom of the view (or in the Scenario panel): Move your hand, Feel a touch, Knee-jerk reflex, Touch a hot stove, See an apple, Think 7 × 8, Catch a ball. Press ▶. Each signal is a glowing comet that travels its real path at its real conduction speed, slowed down so you can follow it; the clock shows real milliseconds and how much it is slowed. After the key moment the clock speeds up (⏩) through the slow tail, such as the burning second pain. Drag the time bar to scrub. Follow the signal (View panel) moves the camera with it; drag the view to look yourself.
3 Conditions and sides
Choose the right or left hand and leg, and a condition: multiple sclerosis, a spinal cord injury at C5 or T10, a stroke in the left internal capsule, a local anaesthetic at the wrist, diabetic neuropathy, a cold hand or alcohol. A blocked signal stops with a red ✗ where it fails and the Signal path says why; a slowed one arrives later. This is an educational model, not medical advice.
4 One neuron, one synapse, the reflex arc
Neuron: set the stimulus and press ⚡ Stimulate. Below threshold nothing happens; above it a full action potential starts at the axon hillock and runs to the muscle (all-or-none). The magnifier shows Na⁺ and K⁺ channels at work; the Graph shows the membrane potential. Change the diameter, myelin, temperature or add tetrodotoxin in the Neuron panel. Synapse: choose the synapse and a drug, then send one spike, a train, a second input (spatial summation) or an inhibitory input. Reflex arc: press ▶ to send a reflex through the cord section.
5 Experiment mode and the lab report
Six labs: reflex vs reaction time, a nerve conduction study, threshold and all-or-none, myelin and speed, identify a drug at the synapse, and find a spinal cord injury. Each shows its aim, what you need and a safety note; the method ticks off as you work and a conclusion question ends it. Then create a PDF lab report.
6 Quiz and units
The practice quiz covers the CNS and PNS, reflexes, pathways, the parts of a neuron, the action potential, synapses and drugs, and speed and timing calculations, each with a worked solution. SI | US switches conduction speeds between m/s and mph, lengths between cm/m and in/ft and temperature between °C and °F. Times stay in milliseconds and voltages in millivolts.
7 Keyboard
- Space plays or pauses; 1–4 pick Whole system, Neuron, Synapse or Reflex arc; S stimulates the neuron or sends a spike; P, G, A pick Signal path, Graph or Part; L shows or hides labels; O starts or stops the orbit (a click or touch on the 3D view also stops it); N starts again. Right-click a view to save it as an image.
The CNS and neurons: an interactive 3D nervous system simulator
Your nervous system is the body's fastest messaging network. This free CNS and neuron simulator builds it on a real skeleton: the brain in the skull, the spinal cord running down the vertebral canal, 31 pairs of spinal nerves and the nerves of the arms and legs. Choose a scenario and every signal travels its real path at its real speed, slowed down so you can follow it, from the moment you decide to move to the instant your hand closes. Then zoom in to the scale of a single cell and watch one neuron fire and one synapse pass the message on.
- Scenarios
- Move a hand, feel a touch, knee-jerk reflex, hot stove, see, think, catch a ball
- Close-ups
- One neuron (Hodgkin–Huxley), one synapse with ten drugs, the reflex arc in the cord
- Conditions
- 9, from multiple sclerosis and spinal cord injury to a stroke and a local anaesthetic
- Modes
- Simulate, Experiment (6 practicals with a PDF report) and Quiz
- Price
- Free, no account or download; works on Chromebooks, tablets and phones
How the nervous system works
Every response follows the same chain: stimulus → receptor → sensory neuron → coordinator (the CNS) → motor neuron → effector → response. Receptors in the skin, eyes and muscles turn a stimulus into electrical impulses, sensory neurons carry them to the spinal cord and brain, and motor neurons carry the decision out to muscles and glands. In the simulator the times are real: a touch on your fingertip reaches your cortex in about 25 milliseconds, the knee-jerk muscle fires about 20 ms after the tap, and a deliberate movement such as catching a ball takes about 200 ms because the brain has to see, judge and plan first.
| Scenario | Path | Time in the model |
|---|---|---|
| Feel a touch | Fingertip → median nerve → dorsal column → thalamus → somatosensory cortex | ≈ 25 ms to the cortex |
| Knee-jerk reflex | Muscle spindle → femoral nerve → L3 (one synapse) → quadriceps | ≈ 18 ms to the muscle, kick at ≈ 45 ms |
| Hot stove | Pain receptor → Aδ fibre → relay neuron → biceps; pain to the brain later | Hand pulled at ≈ 85 ms; burning pain ≈ 1 s |
| See an apple | Retina → optic nerve → chiasm → thalamus → visual cortex → temporal lobe | V1 ≈ 60 ms, recognised ≈ 130 ms |
| Catch a ball | Eyes → parietal → premotor → motor cortex → corticospinal tract → hand | Grip at ≈ 215 ms |
The central and peripheral nervous system (CNS vs PNS)
The central nervous system is the brain and spinal cord, protected by bone, three meninges and cerebrospinal fluid. The peripheral nervous system is every nerve outside them: 12 pairs of cranial nerves and 31 pairs of spinal nerves. Its somatic division controls skeletal muscles and carries the senses; its autonomic division (sympathetic and parasympathetic) runs the heart, gut and glands without you thinking about it.
The spinal cord and spinal nerves
The adult spinal cord is about 45 cm long and only as thick as a finger. It ends at the level of the first or second lumbar vertebra; below that, the roots of the lower nerves run down the canal as the cauda equina, which is why a lumbar puncture is done low in the back. Because the cord is shorter than the column, its segments sit higher than the vertebrae they are named after: the L3 segment that runs the knee jerk lies behind the T11 vertebra. Each segment has a dorsal (sensory) root with a ganglion and a ventral (motor) root; they join into a spinal nerve that leaves through the gap between two vertebrae. The cord is thicker in the neck and the lower back (the cervical and lumbar enlargements), where the nerves to the arms and legs come from; those nerves mix in the brachial and lumbosacral plexuses.
The reflex arc
A reflex is a fast, automatic response that does not wait for the brain. In the knee-jerk (stretch) reflex a tap stretches the quadriceps, its muscle spindles fire, and the Ia sensory fibre synapses directly on a motor neuron in the spinal cord: a monosynaptic arc. In the withdrawal reflex a pain receptor's sensory neuron synapses on a relay neuron (interneuron), which excites the motor neuron to the biceps. The hand moves before the pain signal reaches the cortex, so you feel the burn after you have pulled away.
Sensory and motor pathways: why the left brain moves the right hand
The corticospinal tract runs from the motor cortex through the internal capsule and brainstem, crosses to the other side at the base of the medulla (the decussation of the pyramids) and runs down the cord to the motor neurons. Touch climbs the dorsal column on the same side and crosses in the medulla; pain crosses as soon as it enters the cord. So each half of the brain senses and moves the opposite half of the body, which is why a stroke in the left hemisphere weakens the right side.
The parts of a neuron
A motor neuron has branching dendrites that receive signals, a cell body with the nucleus, an axon hillock where impulses start, a long axon wrapped in a myelin sheath made by Schwann cells with gaps called nodes of Ranvier, and axon terminals that form synapses. Sensory neurons have their cell body in a ganglion beside the cord; relay neurons are short and lie inside the CNS.
The action potential
At rest the inside of the axon is about −70 mV, kept there by the sodium–potassium pump and potassium leaking out. A stimulus that reaches the threshold (about −55 mV) opens voltage-gated Na⁺ channels: Na⁺ rushes in and the inside becomes about +40 mV (depolarisation). Na⁺ channels close and K⁺ channels open, K⁺ flows out (repolarisation) and the membrane dips below rest (hyperpolarisation). Spikes are all-or-none: a stronger stimulus makes them more frequent, not bigger. The refractory period keeps them apart and one-way. Myelin makes the impulse jump from node to node (saltatory conduction), about 6 m/s for every micrometre of diameter, up to 120 m/s; a thin bare C fibre manages only about 1 m/s.
How a synapse works, and how drugs act on it
When the impulse reaches the axon terminal, voltage-gated Ca²⁺ channels open, Ca²⁺ makes vesicles fuse with the membrane and release a neurotransmitter into the 20–40 nm synaptic cleft. It diffuses across, binds to receptors and opens their ion channels: an excitatory transmitter such as acetylcholine or glutamate depolarises the next cell (an EPSP), an inhibitory one such as GABA hyperpolarises it (an IPSP). Small potentials add up over time (temporal summation) or from several inputs (spatial summation) until the threshold is reached. The transmitter is then broken down by an enzyme (acetylcholinesterase) or taken back up. Drugs act at each step: agonists such as nicotine mimic the transmitter, antagonists such as curare block its receptor, botulinum toxin stops release, and SSRIs and cocaine block reuptake.
When the system fails
In multiple sclerosis the immune system strips myelin from axons in the brain and cord, slowing or blocking signals. A spinal cord injury stops every message that has to pass the injured segment, while reflexes below it still work. Diabetic neuropathy slows the longest nerves first, and a local anaesthetic blocks the sodium channels of one nerve. This is an educational model with average values: it is not medical advice.
Experiments and course links
| Lab | What you find out | Course links |
|---|---|---|
| Reflex vs reaction time | Why a reflex beats a decision | AQA GCSE 4.5.2 + RP; IGCSE 14.1; NGSS MS-LS1-8 |
| Nerve conduction study | Speed = distance ÷ time on a real nerve | A-level 3.6.2; IB C2.2; A&P |
| Threshold and all-or-none | Below threshold, nothing; above it, a full spike | A-level 3.6.2; IB C2.2; AP Psychology 1.3 |
| Myelin and speed | Diameter, myelin and saltatory conduction | A-level 3.6.2; IB C2.2 (HL) |
| Drugs at the synapse | Agonist, antagonist, release and reuptake | AP Psychology; A-level; IB C2.2 (HL) |
| Find the injury | Spinal segments, tracts and reflexes | A&P; health science; NCERT 11 |
Who uses this simulator?
Middle and high school students meeting the nervous system and the reflex arc (NGSS, GCSE, IGCSE, NCERT), AP Psychology students learning neural firing and neurotransmitters, A-level and IB students studying the action potential and synaptic transmission, first-year anatomy and physiology, nursing and physiotherapy students, and teachers who want a free, no-login model for a classroom screen or homework.
Frequently asked questions
What is the difference between the central and peripheral nervous system?
The central nervous system (CNS) is the brain and the spinal cord: it receives information, processes it and decides what to do. It is protected by the skull and the vertebral column and wrapped in the meninges and cerebrospinal fluid. The peripheral nervous system (PNS) is everything outside it: the 12 pairs of cranial nerves, the 31 pairs of spinal nerves and their branches, which carry sensory signals in from receptors and motor signals out to muscles and glands. The PNS has a somatic part (voluntary muscles) and an autonomic part (heart, gut, glands: sympathetic and parasympathetic).
How does a reflex arc work?
A reflex arc is the shortest route from a stimulus to a response. A receptor detects the stimulus, a sensory neuron carries the impulse to the spinal cord, it passes across one or more synapses (in the withdrawal reflex through a relay neuron, in the knee-jerk reflex straight to the motor neuron) and a motor neuron carries it out to an effector, a muscle, which contracts. The brain is not involved in the decision, so a reflex is fast and automatic: the knee jerk takes about 20 milliseconds from the tap to the muscle, while a deliberate reaction takes about 200 milliseconds. The brain is told afterwards.
How does an action potential travel along a neuron and cross a synapse?
At rest the inside of a neuron is about 70 millivolts negative. When a stimulus raises it to the threshold, about −55 mV, voltage-gated sodium channels open, sodium ions rush in and the inside becomes about +40 mV (depolarisation). Potassium channels then open and potassium leaves, so the membrane repolarises and briefly overshoots below rest; during this refractory period another spike cannot start, so impulses travel one way. The depolarisation spreads to the next patch of membrane; on a myelinated axon it jumps from one node of Ranvier to the next (saltatory conduction), which is much faster. At the axon terminal the impulse opens calcium channels, vesicles release a neurotransmitter into the synaptic cleft, it binds to receptors on the next cell and opens its ion channels, and then it is broken down by an enzyme or taken back up.
Explore related simulators
Lift each lobe out of the brain and see what it does in the Interactive 3D Brain Model, which also has the ruler-drop reaction time test. Follow light into the retina with the Human Eye Simulator, sound into the cochlea with the Human Ear Simulator, and touch, pain and temperature receptors in the Interactive Skin Diagram. The vertebrae and every other bone are in the Interactive 3D Skeleton, and the hormones that work alongside nerves are in the Endocrine System Simulator.
Sources: OpenStax Anatomy and Physiology 2e, chapters 12–16; OpenStax Biology 2e, chapter 35; Hodgkin & Huxley, J. Physiol. 117 (1952); Hursh, Am. J. Physiol. 127 (1939); Purves et al., Neuroscience 6th edition; Kandel et al., Principles of Neural Science 6th edition; Dodge & Rahamimoff, J. Physiol. 193 (1967); Thorpe, Fize & Marlot, Nature 381 (1996); Preston & Shapiro, Electromyography and Neuromuscular Disorders 4th edition. The skeleton is BodyParts3D, © The Database Center for Life Science, licensed under CC BY 4.0; the brain is this site's own anatomical model. An educational model: not medical advice.
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