A reflex arc is the short path a nerve message takes in a reflex, here from a sensor in the skin to the spinal cord and straight back to a muscle, so you move before you think. Touch something hot and your hand is gone before you know it hurts. That is hard to explain with a still diagram and three arrows. Put this reflex arc animation on the board and the class can watch the message go: in along the nerve, round in the spinal cord, back out to the arm, and only then up to the brain.
Step through it: one sentence per step, with that part of the nerve lit up so the class looks at one thing. Press a number on the model to jump to that step. Slow it down to follow one message. Hide the labels and ask the class to name the parts. Turn the model by dragging. Share copies a link that opens it at the step you are on.
In exam order, a reflex arc runs from stimulus to receptor, sensory neurone, relay neurone, motor neurone and effector, and ends with the response. On Simple the board uses plain words, so here is how they match. The hot thing is the stimulus. The sensors in the fingertip are the receptors. The message travels in along a sensory neurone, crosses a relay neurone in the spinal cord and goes back out along a motor neurone. The arm muscles are the effector, and pulling the hand away is the response, which is why this one is called the withdrawal reflex. Press More detail, top right, and the board names each stage on the model and in the steps: receptors, sensory neurone, relay neurone in the spinal cord (the central nervous system), motor neurone and effector. Neurone is the UK exam spelling. In the United States it is neuron, and the relay neurone is usually called an interneuron.
Involuntary. A reflex happens without you choosing it: in this one the spinal cord sends the reply before the brain has had time to decide. A reaction you choose, such as catching a ball, goes through the brain and is slower. Other reflexes are just as automatic: blinking when something flies at your face, the knee jerk when a doctor taps below the kneecap, and your pupils shrinking in bright light, which you can watch in the eye simulation. Blinking and the pupils are run by the base of the brain, not the spinal cord, but you still do not choose them. They are fast because the path is short, and they are there to protect you.
The words are pitched at ages 9 to 14: Years 5 to 9 in England, 4th to 8th grade in the United States. In England the reflex arc is GCSE biology (AQA 4.5.2.1: sensory neurone, relay neurone, motor neurone), and the arm muscles link to antagonistic muscles at Key Stage 3. In the United States it supports NGSS 4-LS1-2 and MS-LS1-8, on how senses, the brain and the body respond. In Australia it fits Year 9 science, on how the body responds to a stimulus. A safety note for the class: watch it here, never test it with anything hot.
Why do you pull your hand away before you feel pain?
Because the message does not have to go to the brain first. It runs along a nerve to the spinal cord, and the spinal cord sends a message straight back to the arm muscles. The same message also carries on up to the brain, but by the time you feel the pain your hand is already moving.
What is a reflex arc?
The path a message takes in a reflex: from a sensor in the skin, along a sensory nerve cell to the spinal cord, across a relay nerve cell, and out along a motor nerve cell to a muscle. It is short, which is why a reflex is fast.
Is the brain involved in a reflex?
The brain does not have to decide. The spinal cord sends the pull away message on its own. The brain is still told, a moment later, which is when you feel the pain.
What does the simulation show?
A 3D model of the head, chest and one arm, cut open to show the brain, the spinal cord and the nerve running down to the fingertip. A bright blue message runs in from the finger, a pink one runs back out to the arm muscles, the arm pulls away and then the brain lights up.
What has been simplified?
Three things, on purpose. The model shows one relay nerve cell where a real arm uses several. The spinal cord is shown at the front so the class can see it, where it really sits inside the backbone. And one nerve is drawn for both directions: in a real spinal cord the message in enters at the back and the message out leaves at the front.
What age is it for?
The words are pitched at ages 9 to 14. It also works as a starter for the reflex arc at GCSE.
Is it free?
Free to try. Opening Science Simulations uses one token for the whole day, however many times you come back to it. Every account gets 20 free tokens, and Pro gives 50 a day.
The neuron 3D model zooms in on one nerve cell on this route, and the interactive brain model shows where the message ends up. The arm muscles that pull the hand away are on the muscular system model. All twelve are on the Science Simulations page. For models the class can spin, from skeletons to dinosaurs, try 3D Explorer.