The Nervous System and Reflex Arcs — AQA GCSE Biology
The nervous system lets us detect and respond to changes in our surroundings using fast electrical impulses.
The nervous system
Information from receptors passes as electrical impulses along neurones to the central nervous system (CNS) — the brain and spinal cord — which coordinates a response carried out by effectors (muscles or glands).
The general pathway is: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response
Reflex actions
Reflexes are automatic and rapid responses that do not involve the conscious part of the brain. This makes them important for protection (e.g. pulling your hand away from something hot, or the pupil reflex).
The reflex arc
In a reflex arc, the impulse travels:
- Receptor detects the stimulus.
- Sensory neurone carries the impulse to the spinal cord.
- Across a synapse to a relay neurone in the CNS.
- Across another synapse to a motor neurone.
- To the effector, which carries out the response.
Synapses
At a synapse (a gap between two neurones), the electrical impulse triggers the release of chemicals (neurotransmitters) that diffuse across the gap and start a new impulse in the next neurone.
Exam tips
- Learn the response pathway in order.
- Reflexes are fast, automatic and not conscious — explain why this is protective.
- Get the reflex arc order right: sensory → relay → motor neurones with synapses between.
- Describe how a synapse works (chemicals diffuse across the gap).