Fig. 4.1 shows a synapse, the junction between two neurones, labelled X and Y, in the spinal cord. Where they meet the two neurones do not quite touch, and ...
Question 1 Report
Fig. 4.1 shows a synapse, the junction between two neurones, labelled X and Y, in the spinal cord. Where they meet the two neurones do not quite touch, and a very small gap, labelled G, separates the end of neurone X from the start of neurone Y. When a nerve impulse travelling along neurone X reaches this point, it cannot simply jump across the gap as an electrical signal. Instead the signal is carried across by a chemical released from neurone X. Fig. 4.1 shows the ending of neurone X, the gap G and the membrane of neurone Y beyond it.
(a) Name the small gap labelled G between the two neurones. [1]
(b) Name the chemical that is released to carry the signal across gap G. [1]
(c) Describe how an impulse is passed from neurone X to neurone Y across the synapse. [4]
(d) Explain why an impulse can cross this synapse in one direction only. [2]
(e) Suggest one reason why some drugs change a person's mood by acting at synapses. [1]
Answer Details
(a) The small gap G is the synaptic gap (synaptic cleft). [1]
(b) The chemical released to carry the signal across the gap is a neurotransmitter. [1]
(c) Four linked steps (1 mark each): the impulse reaches the end of neurone X; the vesicles release a neurotransmitter into the gap; the neurotransmitter diffuses across the gap; and it binds to receptors on neurone Y, triggering a new impulse. [4]
(d) The neurotransmitter is released only from neurone X [1], and the receptors are only on neurone Y [1], so the signal can pass from X to Y but never the other way. [2]
(e) Many mood-changing drugs work because they add to, imitate or block the neurotransmitter (or its receptors) at the synapse, altering the impulses that reach the brain. [1]
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