10 (a) Describe the roles of the neuromuscular junction, transverse system tubules (T-tubules) and the sarcoplasmic reticulum in stimulating contraction in striated muscle. (b) Outline the effects of mutant alleles on the phenotype in Huntington's disease.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1. [1] action potential / depolarisation / impulse, at pre-synaptic membrane ;
2. [1] (voltage-gated) calcium ion channels open / calcium ions enter (cell / cytoplasm / (motor) neurone / pre-synaptic knob) ;
3. [1] vesicles fuse with pre-synaptic membrane ;
4. [1] acetylcholine / ACh, released, by exocytosis / into synaptic cleft ;
5. [1] (ACh) binds to receptors on, muscle cell membrane / sarcolemma / motor end plate ;
6. [1] sodium ion channels open / sodium ions enter (muscle cell / sarcoplasm) ;
7. [1] depolarisation of, (muscle) cell surface membrane / sarcolemma ;
8. [1] (depolarisation) spreads / transmitted, to / down / via, T-tubules ;
9. [1] depolarisation of (adjacent) sarcoplasmic reticulum (membrane) ;
10. [1] (voltage-gated) calcium ion channels open ;
11. [1] calcium ions, move / diffuse, out of SR / out of cisterna(e) ;
12. [1] calcium ions, move / diffuse, into, sarcoplasm / cytoplasm ;
13. [1] calcium ions, start contraction / bind to troponin ;
14. [1] mutation / allele / gene, on chromosome 4 / autosome ;
15. [1] dominant ;
16. [1] normal / recessive, allele has 10–35 repeats of CAG ;
17. [1] HD / dominant / mutant, allele has, more / extra, repeats of CAG ;
18. [1] larger number of repeats gives earlier onset ;
19. [1] usual onset, after 28 / in middle age / before 65 ;
20. [1] onset, in babies / from 1 year old, if very numerous repeats ;
21. [1] ref. extra glutamine / polyglutamine ;
22. [1] mis-folded, protein / huntingtin ;
23. [1] neurological condition / brain problem OR (brain) neurones, die / destroyed ;
24. [1] motor control uninhibited / involuntary movements / chorea ;
25. [1] cognitive / mood, changes ;
26. [1] AVP ; e.g. GABA producing neurones lost ref. to basal ganglia / striatum, affected (first)