By the end of this lesson
- Trace a simple reflex arc.
- Explain why reflexes are rapid and protective.
- Distinguish reflex action from conscious awareness.
BIG QUESTION
How can a finger begin withdrawing before pain is consciously processed?
A short spinal pathway produces a rapid, automatic protective response while information also reaches the brain.
Heat or pain receptors detect danger
A receptor in finger skin detects damaging heat and generates impulses in a sensory neurone. The impulses enter the spinal cord through a sensory pathway.
The response does not wait for a conscious decision.
A relay pathway activates the motor output
In the spinal cord, neurotransmitters transmit across synapses from the sensory neurone to a relay neurone and then to a motor neurone.
The motor neurone carries impulses to an arm flexor muscle, which contracts and withdraws the finger.
Reflexes protect while the brain is informed
A reflexA rapid, automatic and stereotyped response to a stimulus that does not involve the conscious part of the brain, protecting the body from harm. is rapid, automatic and stereotyped. A relatively short pathway reduces delay and limits injury.
Other impulses travel to the brain, producing conscious pain awareness after or alongside the withdrawal response.
Withdrawal reflex sequence
Each component has a distinct role.
| Component | Event | Direction/function |
|---|---|---|
| Heat/pain receptor | Stimulus is transduced. | Begins sensory signal. |
| Sensory neurone | Impulse travels. | Skin → spinal cord. |
| Relay neurone/synapses | Signal crosses within CNS. | Links input to output. |
| Motor neurone | Impulse travels. | Spinal cord → muscle. |
| Flexor muscle | Contracts. | Finger withdraws. |
| Brain pathway | Information ascends. | Conscious awareness. |
Language that earns precision
- reflex arc
- Neural pathway producing a rapid automatic response.
- relay neurone
- CNS neurone linking other neurones.
- effector
- Muscle or gland carrying out a response.
- automatic
- Occurring without a conscious decision.
Common misconceptions
Claim: The brain is not involved at all in a reflex.
Correction: The spinal cord can initiate withdrawal, while information is also sent to the brain.
Claim: Reflexes are learned choices.
Correction: A simple withdrawal reflex is automatic and protective.
Claim: The sensory neurone connects directly to the muscle.
Correction: Motor neurones carry output to muscle after CNS relay/synapses.
Match the answer to the command word
Receptor → sensory → relay → motor → effector.
Rapid and automatic should be linked to protection from damage.
Recall, apply, then justify
Describe the reflex arc when a finger touches a hot object.
Hint
Begin with the receptor and end with withdrawal.
Suggested marking points
- A heat/pain receptor in skin is stimulated.
- An impulse travels along a sensory neurone.
- It enters the spinal cord.
- A relay neurone and synapses transmit the signal.
- A motor neurone carries an impulse to a muscle.
- The muscle contracts and withdraws the finger.
Explain why the withdrawal reflex is protective.
Hint
Speed, automatic action and injury.
Suggested marking points
- It occurs rapidly through a short spinal pathway.
- It does not require a conscious decision before action.
- The body part is removed quickly, limiting tissue damage.
Predict effects of damage separately to the sensory neurone, motor neurone and flexor muscle.
Planning hint
Test detection, transmission and action in turn.
What a strong design includes
- Sensory-neurone damage can prevent the spinal cord receiving the heat signal.
- The withdrawal reflex may not begin and sensation may be reduced.
- Motor-neurone damage can allow sensation but prevent output reaching muscle.
- Flexor-muscle damage can allow normal impulses but prevent effective contraction.
- All three can abolish withdrawal through different mechanisms.
- Testing sensation and motor output can help distinguish the site.