This memory aid covers Memory Aids within Reflex Arc for GCSE Biology. Topic 3: Reflex Arc It is section 12 of 15 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Memory Aids
Reflex arc pathway in order — "Really Sensible Relay Moves Effectors":
- Receptor detects stimulus
- Sensory neurone carries impulse to spinal cord
- Relay neurone in spinal cord connects pathways
- Motor neurone carries impulse to effector
- Effector (muscle) responds
Synapse = "chemical bridge": At every synapse, the electrical signal converts to a chemical signal (neurotransmitter), crosses the gap, then converts back to electrical. Remember: electrical → chemical → electrical.
Why reflexes are fast — "Short Cut via Spinal Cord": The reflex arc is shorter than the full brain pathway. Shorter distance = less time for the impulse to travel = faster response = better protection from harm.
Involuntary vs voluntary: Reflex = Involuntary (automatic, no conscious control). Voluntary action (e.g. picking up a pen) = conscious decision via brain → long pathway.
Quick Check: Using your knowledge of the reflex arc, explain why a reflex response is faster than a voluntary response to the same stimulus.
In a reflex arc, the impulse travels from the receptor along a sensory neurone to the spinal cord, through a relay neurone, and directly to a motor neurone — the response is coordinated in the spinal cord. In a voluntary response, the impulse must travel all the way from the spinal cord up to the brain, be processed by the cerebral cortex, and then a signal must travel back down to the motor neurone. This is a much longer pathway, so it takes more time. The reflex arc's shorter pathway means the time between stimulus and response (latency) is significantly shorter, making it more effective for protective responses such as withdrawing from pain.
Quick Check: A student describes a reflex arc as: "receptor → sensory neurone → motor neurone → effector." Identify the error and explain why the missing component is important.
The student has omitted the relay neurone. The correct pathway is: receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector. The relay neurone is important because it is located within the CNS (spinal cord) and connects the sensory and motor pathways. It allows the reflex to be coordinated within the spinal cord rather than requiring the signal to travel to the brain. The relay neurone also allows the signal to be sent to multiple motor neurones simultaneously (e.g. to both contract one muscle and relax the opposing muscle in a withdrawal response).
Quick Check: Explain why the presence of synapses in the reflex arc means that impulses can only travel in one direction around the arc.
Synapses are directional: neurotransmitters are only released from the pre-synaptic membrane (the end of the sending neurone) and bind only to receptor proteins on the post-synaptic membrane (the start of the receiving neurone). There are no neurotransmitter vesicles at the post-synaptic end and no receptors at the pre-synaptic end. Therefore, a chemical signal can only be transmitted from the sensory neurone toward the relay neurone, and from the relay toward the motor neurone — never in reverse. This ensures impulses always travel from receptor to effector, not backwards through the arc.
Practice questions for Reflex Arc
Which word best describes a reflex action?
State the correct order of the reflex arc pathway from stimulus to response. Name each component.