Reflex actions (AQA GCSE Biology): Revision Notes
Reflex actions
What is the central nervous system?
The central nervous system (CNS) is made up of your brain and spinal cord. It acts like the control centre for your body.
The CNS controls how your body responds to changes around you. It does this by coordinating different parts called effectors.
Effectors are muscles or glands that carry out the body's response to a stimulus. They're the "action parts" that actually make something happen in your body.
How responses work
When something changes in your environment, your body follows this pathway:
Stimulus → Receptor → CNS coordinator → Motor neurone → Effector → Response
- Stimulus: A change in your surroundings (like heat or light)
- Receptor: Detects the change
- Effector: A muscle or gland that creates the response
This pathway ensures that your body can detect changes in the environment and respond appropriately. The CNS acts as the decision-maker, determining what response is needed.
What makes reflex actions special
Reflex actions are automatic responses that happen without you thinking about them. They have three key features:
- Automatic: You don't need to think - they just happen
- Rapid: They work very quickly
- Innate: You're born with them - they're not learned
These features are really important because reflexes:
- Control basic body functions like breathing
- Help you avoid danger and harm quickly
Why Reflexes Are Crucial
Without reflexes, you would have to consciously think about every response to danger. By the time your brain processes "this is hot, I should move my hand," you could already be seriously burned. Reflexes bypass conscious thought to keep you safe.
How a reflex arc works
A reflex arc is the pathway that nerve signals follow during a reflex. Here's what happens:
Stimulus → Receptor → Sensory neurone → Relay neurone → Motor neurone → Effector → Response
Worked Example: Touching Something Hot
Let's trace through what happens when you accidentally touch a hot surface:
Step 1: Heat (stimulus) is detected by receptors in your skin
Step 2: Sensory neurones carry the signal to your spinal cord
Step 3: Relay neurones in the spinal cord pass the message on
Step 4: Motor neurones carry signals to your arm muscles
Step 5: Your muscles (effectors) contract to pull your hand away
Step 6: Your hand moves away from the heat (response)
Synapses - how nerve signals cross gaps
Neurones don't actually touch each other. The tiny gap where they meet is called a synapse.
When a nerve signal reaches the end of a neurone:
- It triggers the release of chemical messengers called neurotransmitters
- These chemicals cross the gap
- They cause a new electrical signal to start in the next neurone
Key Facts About Synapses:
- Signals can only travel in one direction
- One nerve signal can trigger signals in several other neurones
- This process is slower than normal nerve impulses
The one-way nature of synapses is crucial - it prevents signals from going backwards and causing confusion in your nervous system.
Real-life example
Real-Life Example: Touching a Hot Flame
When you accidentally touch a hot flame:
- Heat receptors in your skin detect the high temperature
- Sensory neurones send signals through your spinal cord
- Relay neurones quickly pass the message to motor neurones
- Motor neurones make your bicep muscle contract
- Your hand pulls away from the flame before you even think about it
This happens so fast that you avoid serious burns. Your conscious brain only realises what happened after your hand has already moved!
Key Points to Remember:
- Reflex actions are automatic, rapid and innate - you don't learn them and don't think about them
- The CNS (brain and spinal cord) coordinates all responses in your body
- Reflex arcs follow a set pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response
- Synapses are gaps between neurones where chemicals called neurotransmitters carry signals across
- Reflexes help keep you safe by making you respond quickly to danger