In this model the hand hangs at the side and does not bend; the 2D figure shows the arm pulling back. Drag to turn, pinch or the +/− buttons to zoom. The nerve paths are simplified: real nerves branch much more.
3D anatomy: Z-Anatomy (CC BY-SA 4.0), based on BodyParts3D © The Database Center for Life Science (CC BY-SA 2.1 Japan). The spinal cord and the nerves are modelled by SciSim along the real skeleton (simplified; real nerves branch much more) and the model files are shared under CC BY-SA 4.0. The 2D figure, the neuron and synapse animations are drawn by SciSim, far larger than true scale and in slow motion. All times and speeds are typical or illustrative values, not measurements of one person.
| Feature | Reflex (pulling the hand from heat) | Voluntary action (picking up a pencil) |
|---|---|---|
| Who decides? | The spinal cord (the brain is told at the same time) | The brain (cerebral cortex) |
| Speed | Very fast: tens of ms up to about 0.1 s | Slower: roughly 0.2–0.4 s or more to react |
| Can you choose it? | No: automatic and involuntary | Yes: you choose to do it |
| Pathway | Receptor → sensory → relay → motor → muscle | Brain → spinal cord → motor neuron → muscle |
| Purpose | Protects the body quickly | Reaches a goal you have chosen |
Resting and firing. A resting neuron is about −70 mV inside compared with outside. When a stimulus pushes the membrane to its threshold (roughly −55 mV), voltage-gated Na+ channels open and Na+ rushes in (depolarisation, up to about +30 to +40 mV). Then Na+ channels close and K+ channels open: K+ flows out (repolarisation). The Na+/K+ pump slowly restores the ion balance. These numbers are approximate and differ between neuron types.
All or none. If the threshold is reached, the action potential always has the same size; a stronger stimulus gives more impulses per second, not bigger ones. The impulse is regenerated along the axon, so it does not fade. For a short time after firing the membrane cannot fire again (refractory period), which makes the impulse travel in one direction.
Myelin and saltatory conduction. In the peripheral nervous system Schwann cells wrap the axon in a fatty myelin sheath. The ion channels are packed at the gaps, the nodes of Ranvier. The current spreads quickly inside the axon from node to node, so the action potential seems to jump (Latin saltare, to jump). This is faster and saves energy.
Different fibres, different jobs. Large, myelinated A-beta fibres carry light touch (roughly 30–70 m/s). Thin, lightly myelinated A-delta fibres give the quick, sharp “first pain” (roughly 5–30 m/s). Unmyelinated C fibres give the slower, dull, burning “second pain” (roughly 0.5–2 m/s). That is why a burn can hurt twice: sharp first, burning a moment later.
The synapse in detail. The impulse opens voltage-gated Ca2+ channels in the axon terminal. Ca2+ entering makes vesicles fuse with the membrane and release neurotransmitter (for example glutamate) into the synaptic cleft (about 20 nm wide). It diffuses across and binds receptors on the next neuron, opening ion channels. If enough channels open, the next neuron reaches threshold and fires. The transmitter is then removed (re-uptake or enzymes).
Touch and pain take different roads. The pain and temperature route shown here crosses to the opposite side in the spinal cord and climbs in the anterolateral (spinothalamic) tract. Fine touch uses another route (the dorsal column pathway) that crosses in the lower brainstem. Both end in the opposite somatosensory cortex.
Pick one answer for each question, then press Check my answers to see the explanations.
A 15–20 minute lesson on the reflex arc for Class 8–10. (1) Ask first: when you touch something hot, what happens first, the movement or the pain? Let students guess. (2) Project the simulator and press Touch the flame with Next step; pause at step 3 and ask which two routes the message takes. (3) Switch Reflex OFF and compare the two numbers on the timeline. (4) Students do the two Order the steps games in pairs, then the 10-question check. (5) Optional: Go deeper for older students (ion channels, myelin, synapse chemistry).
Say clearly that the times are typical, illustrative values (they differ between people and measurements), that the animation is slow motion and not to scale, and that the reflex does not skip the nervous system: it uses the spinal cord, and the brain is told as well. Safety: use no real fire in class. If someone really burns a finger or hand, cool it under cool running water for about 20 minutes and tell an adult or a doctor; do not use ice or creams.
No. A nerve impulse (action potential) is a brief signal made by ions such as sodium (Na+) and potassium (K+) moving across the membrane of a neuron through special channels. It is electrical in the sense that the voltage across the membrane changes, but nothing flows along the axon like current in a copper wire. It is also much slower: roughly 0.5 to 100 metres per second, compared with nearly the speed of light for electricity in a wire. At a synapse the signal becomes chemical: a neurotransmitter crosses the gap.
Key takeaway: an impulse is a travelling change of membrane voltage made by ions, not a current in a wire.The reflex loop is short: receptor, sensory neuron, a relay neuron in the spinal cord, a motor neuron and the muscle. It needs only a few neurons and a short path, so the command reaches the muscle quickly. The signal that goes to the brain has a longer road (spinal cord, brainstem, thalamus, cortex) and the brain then needs time to process it before you consciously feel the pain. In typical illustrative values the hand starts to move after about 0.1 s while the pain is felt after roughly 0.2 s; real values vary between people and situations.
Key takeaway: the short reflex loop acts first; the longer route to the brain makes you feel the pain a little later.A spinal reflex is started by the spinal cord and does not need the brain to begin the movement, but it is not outside the nervous system: the spinal cord is part of the central nervous system. The brain is not uninvolved either. The signal reaches the brain at the same time, so you feel the pain and can then decide what to do, and the brain can also change the strength of a reflex.
Key takeaway: the spinal cord starts the reflex; the brain is told at the same time and creates the feeling.It depends on the fibre. Thick, myelinated fibres that carry touch can conduct at roughly 30 to 70 m/s (up to about 100 m/s for some motor fibres). Thin, lightly myelinated A-delta fibres, which carry the quick, sharp first pain, conduct at roughly 5 to 30 m/s. Unmyelinated C fibres, which carry the slow, dull, burning second pain, conduct at roughly 0.5 to 2 m/s. The numbers on this page use one illustrative set of typical values.
Key takeaway: speed depends on myelin and axon thickness; typical range about 0.5 to 100 m/s.Neurons that carry pain and temperature signals from the right hand cross to the opposite side of the spinal cord and climb in the pathway to the thalamus. The thalamus then sends the signal to the touch area (somatosensory cortex) on the opposite side, so the right hand is mapped mainly in the left cortex. Fine touch uses a slightly different route that crosses higher up, in the brainstem, but the result is the same: the opposite side of the brain.
Key takeaway: the pathways cross, so each side of the brain mostly serves the opposite side of the body.Cool the burn under cool running water for about 20 minutes as soon as you can, take off rings or tight things near it, and tell an adult. Ask a doctor or call the local emergency number if the burn is large, deep, blistered, on the face, hands or joints, or if you are not sure. Do not use ice, butter or creams. This page is for learning and is not medical advice.
Key takeaway: cool running water for about 20 minutes, then ask an adult or a doctor.