👉 1. Receptor
Receptors in the skin of the finger detect the stimulus, which is the heat. They start an electrical impulse.
Co-ordination and Response » The Simple Reflex Arc
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.90
- What a reflex action and a reflex arc are
- The path of a reflex arc, from receptor to effector
- How the arc works when you pull your finger away from a hot object
- Why reflexes are automatic, fast and protective
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseReflexes: actions without thinking
Imagine you pick up a pan handle and it is much hotter than you expected. Your hand jerks away before you have even thought "ouch". You did not decide to move it. Your body did it for you, and it did it fast.
This is a reflex. Reflexes are the quickest responses your body makes, and they are built into your nervous system. In the lessons Stimulus, Response and Homeostasis and The Central Nervous System and Nerve Impulses you met receptors, sensory neurones, motor neurones and effectors. A reflex uses all of them in a short, fixed pathway.
Key terms:
- Reflex action: a fast, automatic response to a stimulus that happens without you having to think about it.
- Reflex arc: the pathway of neurones that carries the impulse during a reflex action, from the receptor to the effector.
- Relay neurone: a neurone inside the spinal cord that connects a sensory neurone to a motor neurone.
The parts of a simple reflex arc
A simple reflex arc has five parts, always in the same order. Here they are for the finger touching a hot object.
⚡ 2. Sensory neurone
The impulse travels along a sensory neurone from the skin towards the spinal cord.
🔄 3. Relay neurone
In the spinal cord, the impulse passes to a relay neurone. The relay neurone passes it on to a motor neurone.
🚀 4. Motor neurone
The impulse travels along a motor neurone, out of the spinal cord towards the muscle.
💪 5. Effector
The effector is a muscle in the arm. It contracts and pulls the finger away from the hot object.
🔗 Synapses
The neurones do not touch. There is a synapse between the sensory neurone and the relay neurone, and another between the relay neurone and the motor neurone. Neurotransmitter carries the message across each one.
Learn this: the pathway
stimulus → receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (muscle) → response
Following the message
Let us follow the finger and the hot object from start to finish.
- The heat is the stimulus. Receptors in the skin of the finger detect it.
- The receptors start impulses in a sensory neurone. The impulses travel along it to the spinal cord.
- At a synapse, the impulse passes to a relay neurone in the spinal cord.
- At a second synapse, the impulse passes from the relay neurone to a motor neurone.
- The impulse travels along the motor neurone to a muscle in the arm, the effector.
- The muscle contracts. The finger is pulled away from the hot object.
Notice what is missing. The impulse goes through the spinal cord, but it does not have to go up to the brain before the muscle moves. That is the secret of why reflexes are so quick.
Why reflexes are automatic, fast and protective
🤖 Automatic
You do not have to think or decide. The response happens by itself, in the same way each time.
⏱ Fast
The pathway is short, with only three neurones and two synapses. The message does not have to be worked out by the brain first, so the response is very rapid.
🛡 Protective
Reflexes happen before damage gets worse. Pulling your finger away quickly means less of your skin is burned.
You still notice the pain a moment later, because impulses also pass up to the brain. But by then your finger has already moved.
Worked example: the hot object
Question: Explain why you pull your finger away from a hot object before you feel pain.
The impulse passes through a short reflex arc. It goes from the receptor along the sensory neurone to the relay neurone in the spinal cord, then along the motor neurone to the muscle. The response is automatic, so it does not wait for you to think. Because the pathway is short, the muscle contracts very quickly and the finger moves away before more damage is done.
Drawing and labelling a reflex arc
In the exam you may be asked to label a diagram of a reflex arc. The spinal cord is shown in the middle, with the sensory neurone coming in from the skin on one side and the motor neurone leaving towards the muscle. The relay neurone sits in the spinal cord, joining the two. Remember that the sensory neurone carries the impulse into the spinal cord and the motor neurone carries it out. If you follow the arrows from the receptor to the effector, you will get the order right.
Common mistakes
- Putting the relay neurone outside the spinal cord. It is in the spinal cord.
- Mixing up sensory and motor neurones. Sensory goes towards the spinal cord, motor goes away from it.
- Saying the brain decides to move the finger in a reflex. A reflex is automatic, so you do not have to think about it.
- Calling the skin the effector. The skin has the receptors. The effector is the muscle.
- Writing "the reflex is quick" and stopping. Say why: the pathway is short and no thinking is needed.
- Missing out the relay neurone when listing the parts of the arc.
Exam-style question
A person touches a hot object and pulls their finger away.
(a) Name the type of response shown. [1 mark]
(b) Put the following parts of the pathway in the correct order: effector, motor neurone, receptor, relay neurone, sensory neurone. [2 marks]
(c) Explain why this response protects the body. [2 marks]
Model answer
(a) A reflex (action). (1)
(b) Receptor, sensory neurone, relay neurone, motor neurone, effector. (2 marks for all five in the correct order; 1 mark if only one pair is swapped)
(c) It is very fast / happens automatically without thinking (1). So the hand is moved away from the hot object before it is badly damaged / burned (1).
Exam tip
For an order question, picture the message travelling from the stimulus to the response. Receptor comes first, effector comes last, and the relay neurone is always in the middle of the sensory and motor neurones.