A-LevelBiologyControl and coordination9700/42 · Feb/Mar 2018 · Q210 Marks

Motor neurones are cells within the nervous system.

(a)(i)Name the structures labelled A, B and C on Fig. 2.1.[3]
(a)(ii)Describe the function of a motor neurone in a reflex arc.[2]
(b)(i)With reference to Fig. 2.1, explain the fast transmission of impulses along this type of motor neurone.[4]
(b)(ii)Suggest why fast transmission of nerve impulses is particularly important in the blue whale.[1]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

[3] A node of Ranvier ;
B axon;
C cell body; A soma
[2] transmit, impulses / action potentials, from, sensory / relay / intermediate, neurones ;
to, effectors / muscle / glands ;
[4] any four from:
1 Schwann cells wrap around, the axon / B ;
2 (to form) myelin sheath; A axon myelinated
3 insulate the axon / ref. to lack of movement of ions;
4 depolarisation / action potentials, can only occur at, nodes (of Ranvier) / A ;
5 ref. longer local circuits / (nodes are) 1–3 mm apart ;
6 action potentials, move by saltatory conduction / jump from node to node;
7 AVP; e.g. (speed of transmission) 100 m s⁻¹ / approx. 50× faster
[1] ref. to, large size / long neurones, and need to, respond / avoid danger, (quickly) ;

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