Reflex arc sim
Drag a model hand onto a hot plate, step the impulse from receptor to sensory, relay and motor neurone to the biceps, name each stage as it glows, watch the transmitter cross a synapse, and compare the reflex time with a voluntary response.
The reflex arc: receptor, sensory neurone, relay neurone in the spinal cord, motor neurone and effector; synapses and chemical transmitter; why a reflex is fast and automatic; the brain informed by a separate neurone · covers the O-Level, IGCSE, GCSE topic
What the sim shows
The reflex arc: a model arm on a lab bench, with its fingertip dragged onto a hot plate. The impulse travels in this order: receptor (in the skin), sensory neurone, relay neurone (in the grey matter of the spinal cord), motor neurone, effector (the biceps muscle, which contracts and pulls the hand away). At each of the two synapses in the spinal cord a chemical transmitter diffuses across the gap. The response is fast and automatic: it goes through the spinal cord and does not wait for the brain to decide. The brain is informed by a separate neurone up the spinal cord, so the pain is felt slightly after the hand has moved. In this model the reflex takes 37.1 ms, about 5 times faster than a voluntary response of about 0.19 s. Simplified on purpose: illustrative lengths, speed, synapse delay and times (≈); one impulse speed (50 m/s) for every neurone, although real speeds differ between neurones; pain fibres are slower, so a real withdrawal reflex is often longer; one of each neurone, although a real reflex uses many; the arm is schematic, and the section through the spinal cord is not to scale; one muscle (the biceps) is shown, although other muscles help; the receptor stands for the heat and pain receptors in the skin; the times for the receptor to start an impulse and for the muscle to start contracting are illustrative; the junction between the motor neurone and the muscle (a neuromuscular junction, a kind of synapse) is not counted among the two synapses between neurones; its short delay is included in the 10 ms for the muscle; the brain path is shown dashed, with no time given; the model arm is a teaching model, and no injury is drawn; the close-up arm keeps one pose and shows the withdrawal with an arrow; the hand stays on the plate while the impulse is stepped; the stages are stepped one click at a time; at real speed the whole reflex lasts about 37 ms; the comparison with the ruler drop is rough, because the ruler drop starts with seeing, not touch.
- Glow: where the impulse is now
- Close-up: the reflex arc and one synapse (schematic, not to scale)
- Dashed: a separate neurone up the spinal cord to the brain
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