Reflex actions (AQA GCSE Biology Combined Science): Revision Notes
Reflex actions
What are reflex actions?
Reflex actions are automatic responses that happen without you thinking about them. They are controlled by your central nervous system (CNS), which includes your brain and spinal cord.
Key characteristics of reflex actions:
- They are automatic - you don't have to think about doing them
- They are rapid - they happen very quickly
- They are innate - you are born with them, not learned
- They protect you from danger and harm
- They control basic functions like breathing
How the nervous system coordinates responses
Your CNS coordinates responses by following this pathway:
Stimulus → Receptor → Sensory neurone → CNS coordinator → Motor neurone → Effector → Response
Understanding each component:
- Stimulus: A change in your environment (like heat or light)
- Receptor: Detects the stimulus (like heat receptors in your skin)
- Sensory neurone: Carries the message to your CNS
- Motor neurone: Carries the message from your CNS to muscles or glands
- Effector: The muscle or gland that creates the response
- Response: The action that happens (like pulling your hand away)
The reflex arc
A reflex arc is the complete pathway that a nerve impulse follows during a reflex action. It works like this:
Stimulus → Receptor → Sensory neurone → Relay neurone → Motor neurone → Effector → Response
The most important thing about reflex arcs is that they bypass the conscious brain. The message goes straight to your spinal cord and back out again. This makes reflexes much faster than normal responses.
Synapses - how nerve messages cross gaps
A synapse is the tiny gap between two neurones. Electrical nerve impulses cannot jump across this gap, so the message has to be passed using chemicals called neurotransmitters.
Here's how synaptic transmission works:
- An electrical impulse reaches the end of the first neurone
- This causes chemical neurotransmitters to be released into the gap
- The neurotransmitters cross the gap
- They cause a new electrical impulse to start in the next neurone
Critical facts about synapses:
- They work in one direction only
- They allow one impulse to trigger several others
- They are slower than direct nerve impulses
Example of a reflex action
Worked Example: Touching a Hot Object
When you touch something hot, like a flame:
Step 1: Heat receptors in your skin detect the high temperature
Step 2: Sensory neurones carry this message to your spinal cord
Step 3: A relay neurone in your spinal cord passes the message to a motor neurone
Step 4: The motor neurone tells your arm muscles to contract
Step 5: Your hand pulls away quickly before you can get badly burned
This happens without conscious thought and protects you from serious injury.
Why reflexes are important
Reflexes help you survive by:
- Protecting you from harm - like pulling away from hot objects
- Controlling vital functions - like breathing and heart rate
- Responding quickly - faster than if you had to think about it first
The speed is crucial because thinking takes time, and in dangerous situations, every split second counts.
Key Points to Remember:
- Reflex actions are automatic, rapid and innate - you don't learn them and don't think about them
- The reflex arc bypasses the conscious brain - making responses much faster
- Synapses use neurotransmitters to pass messages between neurones in one direction only
- Reflexes protect you from danger and control basic body functions like breathing
- The pathway is: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response