5.3 Synapses and reflex arcs
7 exam questions worth 18 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What crosses the gap at a synapse?
Choose one answer.
Correct Not quite: the answer is A neurotransmitter
Mark scheme
B (a neurotransmitter)
Model answer
B: a neurotransmitter, a chemical that diffuses across the gap.
Describe how an impulse passes from one neurone to the next across a synapse.
Mark scheme
- (impulse arriving) causes release of a chemical / neurotransmitter (1)
- (chemical) diffuses across the gap (1)
- binds to receptors (on the next neurone) (1)
- (and) starts an (electrical) impulse in the next neurone (1)
Max 3
Model answer
When the impulse reaches the end of the first neurone, a neurotransmitter is released into the gap. It diffuses across the gap and binds to receptor molecules on the next neurone, which starts a new electrical impulse.
Explain why impulses can only travel in one direction across a synapse.
Mark scheme
- neurotransmitter is only released from one side / from the axon ending (1)
- receptors are only on the other side / on the next neurone (1)
Model answer
The neurotransmitter is only released by the neurone before the synapse, and the receptor molecules it binds to are only found on the neurone after it.
Put these parts of a reflex arc in the order an impulse passes through them:
motor neurone, receptor, relay neurone, effector, sensory neurone
Mark scheme
receptor → sensory neurone → relay neurone → motor neurone → effector
- all correct (2)
- one pair swapped (1)
Model answer
Receptor, sensory neurone, relay neurone, motor neurone, effector.
Give two reasons why a reflex action is faster than a voluntary action.
Mark scheme
- does not involve the conscious part of the brain / no decision needed (1)
- passes through fewer neurones / fewer synapses (1)
- allow: impulse goes through the spinal cord (rather than the brain)
Model answer
It does not involve the conscious part of the brain, so no time is spent deciding what to do, and the impulse passes through only a few neurones and synapses.
A person accidentally touches a hot pan and quickly moves their hand away.
Describe how the nervous system brings about this response.
Mark scheme
Level 3 (5-6 marks): a full, ordered description of the reflex arc from receptor to response, including what happens at synapses. Level 2 (3-4 marks): most stages in the correct order, with some detail of neurones or synapses. Level 1 (1-2 marks): some relevant points, e.g. names some neurones or says a muscle contracts.
Indicative content:
- receptors (in the skin) detect heat / the stimulus
- impulse travels along a sensory neurone
- to the spinal cord / CNS
- at a synapse a neurotransmitter is released
- neurotransmitter diffuses across the gap and binds to receptors
- impulse passes to a relay neurone (in the spinal cord)
- then (across a synapse) to a motor neurone
- impulse travels to the effector / muscle
- muscle contracts, moving the hand away
- reflex is automatic / does not involve the conscious brain, so it is fast
Model answer
Temperature receptors in the skin of the hand detect the heat. An electrical impulse travels along a sensory neurone to the spinal cord. At a synapse, the sensory neurone releases a neurotransmitter, which diffuses across the gap and binds to receptors on a relay neurone, starting an impulse in it. The relay neurone passes the impulse across another synapse to a motor neurone. The impulse travels along the motor neurone to the effector, a muscle in the arm, which contracts and pulls the hand away. This is a reflex, so it is automatic and fast because it does not involve the conscious part of the brain.
Explain why reflex actions are important to the body.
Mark scheme
- they are fast / automatic (1)
- so they protect the body from harm / damage (1)
Model answer
Reflexes are automatic and very fast, so they protect the body from damage, for example by moving a hand away from heat before it is badly burned.
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