AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The protective effect of a simple reflex, exemplified by a three- neurone…
Full specification wording
Survival and response (A-level only): The protective effect of a simple reflex, exemplified by a three- neurone simple reflex. Details of spinal cord and dorsal and ventral roots are not required.
A simple reflex is a rapid, automatic, involuntary response to a stimulus, and the three-neurone arc is the route it takes: receptor, sensory neurone, relay neurone, motor neurone, effector.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A simple reflex is a rapid, automatic, involuntary response to a stimulus, and the three-neurone arc is the route it takes: receptor, sensory neurone, relay neurone, motor neurone, effector. A painful stimulus such as heat on the skin is detected by a receptor, which produces a generator potential and, if the threshold is reached, an action potential in the sensory neurone. The sensory neurone synapses with a relay neurone in the central nervous system, the relay neurone synapses with a motor neurone, and the motor neurone carries impulses to the effector, usually a skeletal muscle, which contracts to withdraw the body part.
What earns the marks
- The pathway receptor, sensory neurone, relay neurone, motor neurone, effector in the correct order.
- The response being rapid, automatic and involuntary.
- The pathway not involving conscious regions of the brain, so no decision is needed.
- Few synapses giving a short response time, since synapses are the slowest step.
- The protective outcome, such as withdrawal from the stimulus limiting tissue damage.
Common mistakes and how to put them right
Including the brain as a stage in the reflex arc, when the reflex does not require conscious processing.
Listing only two neurones, so the relay neurone is missing from a three-neurone arc.
Calling the effector a reaction rather than naming a muscle or a gland.
Saying impulses travel in both directions across a synapse, when neurotransmitter is released only from the presynaptic neurone.
Spending answer space on the dorsal and ventral roots, which the specification explicitly excludes.
Exam tips
- Write the five-part pathway as a labelled sequence; it earns marks even in a suggest question.
- When asked why a reflex is fast, the mark is for few synapses and no conscious brain involvement, not simply that the neurones are fast.
Try the lesson
The start of the Survival and response (A-level only): The protective effect of a simple reflex, exemplified by a three- neurone simple reflex. Details of spinal cord and dorsal and ventral roots are not required. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the protective effect of a simple reflex?
Listen
A simple reflex is a rapid, automatic, involuntary response to a stimulus, and the three-neurone arc is the route it takes: receptor, sensory neurone, relay neurone, motor neurone, effector. A painful stimulus such as heat on the skin is detected by a receptor, which produces a generator potential and, if the threshold is reached, an action potential in the sensory neurone. The sensory neurone synapses with a relay neurone in the central nervous system, the relay neurone synapses with a motor neurone, and the motor neurone carries impulses to the effector, usually a skeletal muscle, which contracts to withdraw the body part. A strong answer earns marks for these points. the pathway receptor, sensory neurone, relay neurone, motor neurone, effector in the correct order. the response being rapid, automatic and involuntary. the pathway not involving conscious regions of the brain, so no decision is needed. few synapses giving a short response time, since synapses are the slowest step. Exam tip: Write the five-part pathway as a labelled sequence; it earns marks even in a suggest question.
See it
Put it in order
Put these steps in the order they happen.
- 1. Sensory neurone
- 2. Relay neurone
- 3. Receptor
- 4. Effector
- 5. Motor neurone
Key words
Say what each word means before you turn the card.
simple reflex
Try answering aloud or in your head before turning the card.
three-neurone arc
Try answering aloud or in your head before turning the card.
relay neurone
Try answering aloud or in your head before turning the card.
See every definition
- simple reflex
- A rapid, automatic, involuntary response to a stimulus, which has a protective effect.
- three-neurone arc
- The route from receptor to sensory neurone, relay neurone, motor neurone and effector, in that order.
- relay neurone
- The neurone in the central nervous system with which the sensory neurone synapses, and which synapses with a motor neurone.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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