A reflex arc is the fixed pathway for an automatic response. Trace it as stimulus, receptor, sensory neurone, spinal cord, motor neurone, effector and response.
This lesson opens the SPM Biology coordination and response chapter. The next lesson compares nerves and hormones.
What are the seven steps?
- Stimulus: the change that is detected.
- Receptor: the cell that detects the stimulus.
- Sensory neurone: carries the impulse towards the spinal cord.
- Spinal cord: the relay neurone passes the impulse on.
- Motor neurone: carries the impulse to the effector.
- Effector: the muscle or gland that acts.
- Response: the action that results.
Worked example: stepping on a thorn
A student steps on a thorn and pulls the foot up at once.
| Step | In this scenario |
|---|---|
| Stimulus | The sharp thorn pressing on the skin |
| Receptor | Pain receptors in the skin of the foot |
| Sensory neurone | Carries the impulse to the spinal cord |
| Spinal cord | Relay neurone passes the impulse to a motor neurone |
| Motor neurone | Carries the impulse to the leg muscle |
| Effector | The leg muscle |
| Response | The leg muscle contracts and the foot lifts |
The brain is told about the pain after the foot has moved, which is why the foot moves before you feel it.
The mistake that costs marks
The common slip is to put the brain in the middle of the reflex and to write the motor neurone before the sensory neurone.
| Wrong | Right | |
|---|---|---|
| Decision | The brain decides to move the foot | The spinal cord passes the impulse on automatically |
| Order | Motor neurone, then sensory neurone | Sensory neurone first, motor neurone second |
| Effector | The foot | The leg muscle (the foot lifts as a result) |
The sensory neurone always comes first, because it starts at the receptor.
How does a reflex compare with a voluntary action?
| Reflex | Voluntary action | |
|---|---|---|
| Control | Automatic, no thinking | Controlled by the brain |
| Speed | Very fast | Slower |
| Purpose | Protects the body | Carries out a chosen action |
| Example | Pulling the hand from a hot pan | Writing your name |
Check yourself
A bright torch shines into your eye and your pupil gets smaller. Name the stimulus, the effector and the response.
Answer
The stimulus is the bright light. The effector is the muscle in the iris.
The response is that the pupil gets smaller, which protects the retina from too much light.
What to study next
Move on to comparing nervous and endocrine coordination. Practise data and scenario questions with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to hear you trace a reflex, see online one-to-one Biology tuition.