GCSE · Biology · Edexcel · Spec 1BI0

Reflex arc

Ever jerked your hand off something sharp and only realised a moment later? That's no accident. Your body has a built-in shortcut, and you're about to trace it.

Follow one impulse

1Sharp pin23456789

Stage 1 of 9: Sharp pin. paused

Sharp pin · 1/9scrub the impulse →

The stimulus: a sharp pin presses into your fingertip. Nothing has decided anything yet. Watch where the message goes.

Drag the impulse from the pin to the muscle. Count the neurones as you go, and keep an eye on when the brain finds out.

Where everything sits

The reflex arc, mapped
Spinal cordto the brainReceptorSensory neuronesynapse 1Relay neuronesynapse 2Motor neuroneEffector (muscle)

View: Name the parts. Showing 2 layers: Reflex arc, Names

View

Explore

switch the view ↔ then tap a part

Three neurones and two synapses: that is the whole reflex arc.

Switch between the two views, then tap any part to see where it sits.

Why reflexes keep you safe

?

Reason it through

Why does a reflex protect your body from harm?

Link 1 of 4

First link · your turn

Think about the route you just traced. How long is it, and does anything along it have to decide what to do?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

What do you think?

So why is a reflex so fast?

You now know the route and you've seen when the brain finds out. Four students try to explain the speed.

Which is closest to what you think right now?
How sure are you?

Predict, then check

Reflexes are automatic. So what happens here?

You're carrying a very hot tray in oven gloves, and the heat is starting to come through. Does your reflex force you to drop it?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A reflex runs on a short, fixed three-neurone pathway through the spinal cord, so nothing has to be decided.

What you need to know

  • A reflex is fast and involuntary: it happens automatically, before you're aware of it.
  • The reflex arc runs receptor → sensory neurone → relay neurone → motor neurone → effector.
  • The relay neurone is in the CNS (the spinal cord). The sensory and motor neurones are in the PNS. A synapse joins each neurone to the next.
  • It's fast because the pathway is short and fixed, and no decision is made. The brain is told, but only afterwards.
  • Reflexes protect you from harm, and the brain can override some of them.

The big picture

A reflex is a fast, involuntary response. It runs along a short, fixed pathway of three neurones (sensory, relay and motor) joined by two synapses, with the relay neurone in the spinal cord. Nothing has to be decided, so the response is fast and automatic. The brain gets the message too, but only after you've already moved, and for some reflexes the brain can override the response.

Key points

1Reflexes are fast and involuntary: you respond before you're consciously aware of it.
2Three neurones in a fixed order: sensory → relay → motor. The effector is usually a muscle, which contracts.
3Two synapses: sensory to relay, and relay to motor.
4Relay neurone: CNS (e.g. spinal cord). Sensory and motor neurones: PNS.
5The brain is bypassed in the first response but still receives the message, after the response has happened.
6A fast response means less time exposed to harm, so less damage. Many animals have reflexes, e.g. woodlice curling into a ball.
7The brain can override some reflexes by sending impulses to the motor neurone, stopping the muscle contracting.

Worked example

Problem

Your arm brushes against a hot saucepan and you pull it away before you've realised it's hot. Use the reflex arc to explain how this happens.

⚠ Watch out

Saying the brain quickly decides to move your hand. In the first response nothing is decided at all: the relay neurone in the spinal cord passes the impulse straight to the motor neurone. The brain does get the message, just after you've already moved.

🧠

Memory hook

Red-hot Saucepan? Retreat, Move, Escape! → Receptor, Sensory, Relay, Motor, Effector. And the brain? It reads the news after the story's over.

✓

Check yourself

Cover the page. Can you name the three neurones in order, say which one is in the central nervous system, point to the two synapses, and explain why you move before you realise?

Flashcards

(11)
What makes a response a reflex?
It's fast and involuntary: it happens automatically, before you're consciously aware of it.
What job does the relay neurone do in a reflex arc?
It passes the impulse from the sensory neurone straight on to the motor neurone, with no decision made, and also sends the message on to the brain.
Which neurones of a reflex arc are in the central nervous system, and which are in the peripheral nervous system?
Central (CNS): only the relay neurone, e.g. in the spinal cord. Peripheral (PNS): the sensory and motor neurones.
How many synapses does the impulse cross in a reflex arc, and where?
Two: where the sensory neurone meets the relay neurone, and where the relay neurone meets the motor neurone.
What does the receptor do in a reflex arc?
It detects the stimulus (e.g. a sharp pin) and the detection is sent along the sensory neurone as a nerve impulse.
What is the effector in a reflex, and what does it do?
Usually a muscle. It contracts to produce the response, e.g. moving the hand away.
Why is a reflex response fast?
The pathway is short (three neurones over a short distance) and fixed, so no decision has to be made.
Does the message in a reflex ever reach the brain?
Yes. The relay neurone also sends it to the brain, but it arrives after the response has already happened.
Why are reflexes a survival mechanism?
Reacting fast to something harmful means less time exposed to it, so less damage to the body.
Give an example of a reflex in another animal.
Woodlice curl into a ball when they're disturbed.
How can the brain override a reflex?
It sends impulses along a neurone to the motor neurone of the arc, stopping the effector muscle contracting (e.g. keeping hold of a hot tray). Only some reflexes can be overridden.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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