Trace sensory and motor neurones
Relate sensory and motor neurone structure to impulse direction and identify the connecting intermediate neurone.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which neurone carries impulses from the CNS to a muscle?
What you will learn
- Describe the structure and function of sensory and motor neurones
- State that intermediate neurones connect sensory neurones and motor neurones
Sensory and motor neurone route
| Feature | Meaning |
|---|---|
| Sensory neurone | Carries impulses from a receptor towards the central nervous system |
| Intermediate neurone | Connects sensory and motor neurones in the central nervous system |
| Motor neurone | Carries impulses towards an effector such as a muscle |
| Direction | Receptor → sensory → intermediate → motor → effector |
Build the mechanism
A sensory neurone carries impulses from a sensory receptor towards the central nervous system. Its cell body lies along the route, with a long process from the receptor and a process entering the CNS. A motor neurone carries impulses from the CNS to an effector; its cell body is in the CNS and its axon extends towards the muscle or gland. Intermediate neurones connect sensory and motor neurones within the CNS.
Keep the sequence precise
Diagrams vary in orientation and scale. Identify a neurone by where its impulse begins and ends, not by whether its cell body is drawn to the left or right.
Pause and explain
A muscle contracts after an impulse travels down a long neurone from the CNS. Explain the neurone’s role.
Worked example
In a pathway from a skin receptor to a muscle, put the neurones in order.
Show the worked solution
- The sensory neurone carries the receptor signal towards the CNS.
- An intermediate neurone connects to a motor neurone, which carries an impulse to the effector.
Answer Receptor → sensory neurone → intermediate neurone in the CNS → motor neurone → muscle.
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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.
State the role of an intermediate neurone in a sensory-to-motor pathway.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Place it in the central nervous system.
- Describe which neurones it connects.
An intermediate neurone lies within the CNS and connects a sensory neurone with a motor neurone.
Look for these in your work
- Mechanism stated
- Steps linked in order
A labelled axon carries impulses from a receptor towards the spinal cord. Identify the neurone type and direction.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- The impulse begins at a receptor.
- It travels towards the CNS.
It is a sensory neurone carrying an impulse from the receptor towards the CNS.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A muscle contracts after an impulse travels down a long neurone from the CNS. Explain the neurone’s role.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- The signal begins in the CNS.
- The target is an effector muscle.
The neurone is motor: it carries impulses from the CNS towards a muscle effector, where transmission triggers a response.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Reversing sensory and motor directions
- Treating a receptor or muscle as a neurone
What can you explain now?
Which neurone carries impulses from the CNS to a muscle?
Compare with the explanation
Motor neurone
A motor neurone carries impulses from the CNS towards an effector.
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Make it stick
In which direction do sensory and motor neurones carry impulses?
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