On the right is a single motor neuron. At the left are the branched dendrites and the cell body, which hold the nucleus. From there a long axon stretches across, wrapped in a fatty myelin sheath made by Schwann cells, with tiny gaps — the nodes of Ranvier — along it. ▶ Play to send an impulse travelling out to the muscle.
Watch closely as the impulse moves along the axon. It cannot form under the insulating myelin — it only "happens" at the bare nodes of Ranvier, so it appears to jump from node to node. This is saltatory conduction, and it makes a myelinated nerve travel far faster (up to ~120 m/s). ⏭ Step to advance the impulse one node at a time.
Switch to the Action potential tab. The graph is membrane potential (mV) against time. It starts flat at the resting potential, −70 mV, held there by the Na⁺/K⁺ pump. ▶ Play and watch a stimulus push it up to threshold — then Na⁺ floods in and the line shoots up to about +40 mV: depolarisation, the action potential.
Keep watching the same curve. As soon as it peaks, the sodium channels close and potassium channels open, so K⁺ flows out and the line drops back down — this is repolarisation, returning toward −70 mV. It dips slightly too far, then settles. During that recovery the neuron cannot fire again: the refractory period, which keeps the impulse moving one way. ↻ Reset and ▶ Play to redraw the whole spike.
Open the Reflex arc tab. A hand touches something hot — the stimulus. ▶ Play to follow the impulse: skin receptor → sensory neuron → up to the spinal cord, where an interneuron relays it across → motor neuron → the arm muscle (effector) contracts and jerks the hand back. All of this happens before the brain even feels the pain.
Notice the signal turns straight around at the spinal cord — it does not climb all the way to the brain to "decide". That short cut is what makes a reflex involuntary and lightning-fast, protecting you from harm. The fixed route is the reflex arc: stimulus → receptor → sensory neuron → CNS → motor neuron → effector. ↻ Reset and ▶ Play to run the whole loop again, and revisit the brain regions in the Lecture.