Order all five parts of a simple reflex arc
Include receptor sensory relay motor and effector in the correct sequence.
Section B13.1 · Coordination and response
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall cell membranes, diffusion, sensitivity and the difference between cells, tissues and organs.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which part detects the stimulus in the required arc?
What you will learn
- A reflex arc connects input with response
- Give the complete five-part route
Order all five parts of a simple reflex arc
| Feature | Meaning |
|---|---|
| 1 | Receptor detects stimulus |
| 2 | Sensory neurone carries input |
| 3 | Relay neurone connects within CNS |
| 4 and 5 | Motor neurone → effector response |
A reflex arc connects input with response
The required simple reflex arc is receptor → sensory neurone → relay neurone → motor neurone → effector. The receptor detects a stimulus and sensory communication carries information towards CNS. A central relay links incoming and outgoing neurones, and motor communication reaches the tissue that produces the response.
A stimulus and a response can be written before and after the five-part arc, but they are not substitutes for its named structures. For a withdrawal model, a detected stimulus begins receptor activity and a muscle effector contracts to move a limb. If the answer gives only skin → brain → arm, the specialised communication stages are missing.
Give the complete five-part route
In the supplied spinal model, the relay lies within the spinal cord, which is part of CNS. The incoming sensory and outgoing motor routes reach peripheral body structures. The diagram’s central boundary helps organise the stages; an arrow crossing that boundary does not turn a receptor or a muscle into the spinal cord itself.
Arrows describe the direction of information through the supplied pathway. They do not mean every stage has exactly the same electrical or chemical mechanism: impulses travel along neurones, while the Supplement account later describes chemical transmission across synapses. The simple arc names the whole route without requiring all microscopic details at Core level.
Sources for this lesson
Pause and explain
Which sequence includes all five required parts?
Worked example
Write the complete required simple reflex arc.
Show the worked solution
- Begin with receptor detection and incoming sensory communication.
- Include central relay, outgoing motor and effector.
Answer Receptor → sensory neurone → relay neurone → motor neurone → effector.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the five named parts in order.
Give me a hint
Do not omit detection or responding tissue.
Compare my reasoning
- Receptor → sensory neurone → relay neurone.
- Then motor neurone → effector.
Receptor → sensory neurone → relay neurone → motor neurone → effector.
Look for these in your work
- Name all five
- Use the correct order
Repair: receptor → sensory neurone → motor neurone → effector for the specified three-neurone arc.
Give me a hint
One central connecting role is missing.
Compare my reasoning
- The incoming sensory route connects via relay.
- Relay then connects with motor.
Receptor → sensory neurone → relay neurone → motor neurone → effector; the missing part is the relay neurone.
Look for these in your work
- Identify the missing relay
- Give the repaired route
A response is limb movement. Explain why response cannot replace effector in the structural sequence.
Give me a hint
An action differs from the tissue producing it.
Compare my reasoning
- Muscle is the responding effector.
- Its contraction produces the movement response.
The effector is the muscle tissue; limb movement is the resulting response. The action does not replace the named effector structure in the arc.
Look for these in your work
- Separate structure and action
- Give the muscle example
Common mistakes
- Omitting relay or receptor.
- Replacing all named cells with skin → brain → arm.
What can you explain now?
Which sequence includes all five required parts?
Compare with the explanation
Receptor → sensory → relay → motor → effector
It preserves detection, communication and response structure.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Retrieve the complete five-part arc without replacing structures with actions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.