Choose an activity — start with “Touch something hot” — and follow the nerve impulse step by step. Blue dots are sensory impulses travelling towards the spinal cord or brain, orange dots are motor commands going out to muscles, and green dots are autonomic signals to the organs.
Click a nerve, the brain or the spinal cord to see what it does, or click a colour in the key to light up a whole division. The brain and spinal cord form the central nervous system (CNS); all the nerves that branch out from them form the peripheral nervous system (PNS).
The 3D model above shows where the nerves run in the body; this flat diagram is the reflex arc you draw in an exam, with the spinal cord cut across. Follow the impulse: sensory neuron → relay neuron → motor neuron. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
The nervous system lets your body react to changes. A change in the surroundings is a stimulus. Receptors (in the skin, eyes, ears, muscles) detect it; sensory neurons carry nerve impulses to the central nervous system; the brain or spinal cord coordinates a response; motor neurons carry impulses to effectors — muscles that contract or glands that release substances. In short: stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response.
The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is everything else: the cranial and spinal nerves and their branches. The PNS has a somatic part (senses and the skeletal muscles you control) and an autonomic part that runs your organs automatically. The autonomic part has two divisions that work against each other: sympathetic (“fight or flight”) and parasympathetic (“rest and digest”). A nerve is a bundle of many neuron fibres wrapped together, like a cable of wires.
A reflex is a fast, automatic response that protects the body. Its pathway is the reflex arc: receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → effector. Because the decision is made in the spinal cord, without waiting for the brain, it is much faster than a voluntary action, and you usually feel the pain only after your hand has started to move. The knee-jerk reflex is even simpler: the sensory neuron connects straight to the motor neuron with no relay neuron in between.
Press Touch something hot and follow the blue sensory impulse up the arm, the flash in the spinal cord, and the orange motor impulse back to the biceps — then notice that the message to the brain comes last. Compare it with Decide to kick, where the command starts in the brain. Try Knee-jerk reflex from the Side view, switch off Colour CNS / PNS to see the nerves in a natural colour, and run Fight or flight then Rest & digest while watching the heart-rate readout.
| Division | Made of | Main job |
|---|---|---|
| Central nervous system (CNS) | Brain and spinal cord | Receives information, processes it, decides, stores memories, sends commands |
| Peripheral nervous system (PNS) | 12 pairs of cranial nerves, 31 pairs of spinal nerves and their branches | Connects the CNS to receptors, muscles and organs |
| Somatic nervous system | Sensory nerves from skin, muscles and joints; motor nerves to skeletal muscles | Senses and voluntary movement; simple reflexes |
| Autonomic nervous system | Sympathetic and parasympathetic nerves to the heart, gut, glands and blood vessels | Automatic control of the organs |
| Sympathetic division | Nerves from the thoracic and lumbar spinal cord via the sympathetic chain | “Fight or flight”: faster heartbeat, wider airways and pupils, less digestion |
| Parasympathetic division | Mainly the vagus nerve, plus some cranial and sacral nerves | “Rest and digest”: slower heartbeat, more digestion |
| Step | Part | Example: hand on a hot pan |
|---|---|---|
| 1 | Stimulus → receptor | Heat and pain receptors in the fingertip skin detect the hot surface |
| 2 | Sensory neuron | Carries impulses up the arm to the spinal cord, entering through the dorsal root; its cell body sits in the dorsal root ganglion |
| 3 | Relay neuron (interneuron) | In the grey matter of the spinal cord, passes the impulse to a motor neuron |
| 4 | Motor neuron | Leaves through the ventral root and carries impulses to the arm muscle |
| 5 | Effector → response | The biceps contracts and the hand is pulled away |
| Reflex action | Voluntary action | |
|---|---|---|
| Example | Pulling your hand off a hot pan; knee jerk; blinking | Kicking a ball; writing; talking |
| Decided by | Spinal cord (or brainstem); no conscious thought | Brain, after thinking |
| Speed | Very fast: tens of milliseconds to about $0.1\,\mathrm{s}$ | Slower: a simple reaction takes about $0.2$–$0.3\,\mathrm{s}$ |
| Same every time? | Yes, automatic and the same each time | No, you can choose to change it |
| Purpose | Protection from harm | Doing what you intend |
A neuron is a single nerve cell with a cell body, short dendrites and one long axon. A nerve is a bundle of the axons of many neurons, wrapped in connective tissue, together with blood vessels. Most spinal nerves are mixed nerves: they contain both sensory and motor fibres, which separate just before the spinal cord into the dorsal root (sensory, in) and the ventral root (motor, out).
The nervous system has two main parts. The central nervous system (CNS) is the brain and the spinal cord. The peripheral nervous system (PNS) is all the nerves that branch out from them: 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS is divided into the somatic nervous system (senses and voluntary movement) and the autonomic nervous system (automatic control of organs).
Key takeaway: CNS = brain + spinal cord; PNS = all the other nerves.The CNS (brain and spinal cord) processes information and makes decisions, and it is protected by the skull and backbone. The PNS (cranial and spinal nerves) carries information between the CNS and the rest of the body: sensory signals in from receptors, and motor signals out to muscles and glands.
Key takeaway: the CNS decides; the PNS carries messages to and from it.A reflex arc is the pathway of a reflex. A receptor detects the stimulus, a sensory neuron carries impulses to the spinal cord, a relay neuron passes them to a motor neuron, and the motor neuron carries them to an effector, such as a muscle, which produces the response. For example, heat receptors in your fingers trigger your biceps to pull your hand away from a hot pan.
Key takeaway: receptor → sensory neuron → relay neuron → motor neuron → effector.In a reflex the response is decided in the spinal cord, so the impulse travels a short path across only one or two synapses and nobody has to think. In a voluntary action the signal must reach the brain, be processed and then travel back down, which takes several times longer: about 0.2 to 0.3 seconds for a simple reaction, compared with tens of milliseconds to about a tenth of a second for a reflex.
Key takeaway: reflexes take a short cut through the spinal cord and skip conscious thinking.Tapping the tendon below the kneecap briefly stretches the quadriceps muscle. Stretch receptors in the muscle send impulses along sensory neurons in the femoral nerve to the spinal cord, where they connect directly to motor neurons. The motor neurons make the quadriceps contract, so the lower leg kicks forward. With only one synapse it is one of the fastest reflexes, and doctors use it to test the nerves and the spinal cord.
Key takeaway: stretch receptor → sensory neuron → motor neuron → quadriceps; only one synapse.Both are parts of the autonomic nervous system and control the organs automatically. The sympathetic division prepares the body for action (fight or flight): it speeds up the heart, widens the airways and pupils and slows digestion. The parasympathetic division, mainly through the vagus nerve, calms the body (rest and digest): it slows the heart and boosts digestion. Most organs receive both, and the balance between them sets how they work.
Key takeaway: sympathetic = fight or flight; parasympathetic = rest and digest.The spinal cord carries sensory information up to the brain and motor commands down to the body, and it runs simple reflexes on its own. It is about 45 cm long, protected by the backbone, and has grey matter (cell bodies, butterfly-shaped in cross-section) in the middle surrounded by white matter (bundles of axons). It ends near the first or second lumbar vertebra; below that a bundle of nerve roots, the cauda equina, continues down.
Key takeaway: a two-way motorway for messages and a reflex centre.The sciatic nerve is the longest and widest nerve in the human body. It starts in the lower back from spinal nerves L4 to S3, runs down the back of the thigh and splits into the tibial and common fibular nerves, which continue to the foot. At its start it is about as wide as a thumb.
Key takeaway: the sciatic nerve, from the lower back to the foot.