A reflex is a fast, automatic response to a stimulus, and it does not need you to think about it. To trace one, name each part in the order the impulse travels: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response.
This skill sits at the start of coordination and responses. Questions usually give you a short scenario and ask you to complete a flow diagram or explain the sequence.
What does each part of the pathway do?
- Stimulus: the change that is detected, such as heat or a sharp point.
- Receptor: a cell or organ that detects the stimulus and starts an impulse.
- Sensory neurone: carries the impulse from the receptor to the central nervous system (CNS).
- Relay neurone: inside the CNS (the spinal cord here), it passes the impulse from the sensory neurone to the motor neurone.
- Motor neurone: carries the impulse from the CNS to the effector.
- Effector: a muscle or a gland that produces the response.
Use the direction words “to” and “from” carefully. The sensory neurone carries the impulse to the CNS and the motor neurone carries it from the CNS.
Worked example
A student touches the side of a hot kettle with a fingertip and her hand pulls away before she feels pain. Trace the pathway and explain why this helps.
Step 1, stimulus: the heat from the kettle.
Step 2, receptor: temperature receptors in the skin of the fingertip detect the heat and start impulses.
Step 3, sensory neurone: the impulses travel along a sensory neurone to the spinal cord.
Step 4, relay neurone: in the spinal cord, a relay neurone passes the impulses to a motor neurone.
Step 5, motor neurone: the impulses travel along a motor neurone to the arm.
Step 6, effector and response: muscles in the arm contract, which pulls the hand away.
Why it helps: the path is short and automatic, so the response is fast and the skin is burned less.
The mistake to watch for
A common answer is: “The receptor sends the impulse to the brain, the brain decides, and the muscle moves.” It sounds sensible, but it describes a voluntary action, not a reflex.
Mistaken answer: receptor, sensory neurone, brain, motor neurone, muscle.
The brain is placed in the middle, and the relay neurone is missing.
The correction is to ask, “Is this automatic?” If yes, the coordinator is the spinal cord (CNS) and the relay neurone does the connecting. The brain receives information afterwards, which is why you feel the pain a moment later.
Check yourself
Answer on paper first, then open each answer.
1. Put these in order for a reflex: effector, sensory neurone, stimulus, relay neurone, receptor, motor neurone.
Show answer
Stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector.
2. A bright light shines into the eye and the pupil gets smaller. Name the stimulus and the effector.
Show answer
The stimulus is the bright light. The effector is a muscle in the iris, which changes the size of the pupil. This is also a reflex.
3. Give one reason why a reflex protects the body.
Show answer
The response is fast and automatic, so the body moves away from a harmful stimulus before damage becomes serious.
Where this leads next
The next step is to zoom in on the junction between two neurones: see the role of a synapse. The hormone side of coordination is compared with nerves in nervous and hormonal signalling.
Some students can recite the parts but still misplace one in a sequence. Our teachers often see that pattern in online one-to-one Biology tuition, and it usually takes only a few guided examples to fix.