13.10 Sliding Filament Model

Question 1

Describe the changes that occur to the structure of the sarcomere during muscle contraction:

Answer 1

• light band becomes narrower
  
  • light band is area of actin only
  • since overlap of actin & myosin increases, area of myosin only (light band) decreases
• Z-lines move closer
  
  • sarcomere shortens
• H-zone becomes narrower
  
  • H-zone is area of myosin without actin
  • greater overlap of actin & myosin >> decreased myosin-only (H-zone) area

Question 2

Describe the structure of myosin:

Answer 2

globular heads

can change angle

& contain binding sites for actin & ATP

Question 3

Describe the structure of actin:

Answer 3

actin filaments + tropomyosin

troponin protein holds tropomyosin to actin

(prevents myosin from binding to actin binding site)

Question 4

What is a neuromuscular junction?

Answer 4

synapse

between motor neurone & muscle fibre(s)

Question 5

Describe how depolarisation occurs through a neuromuscular junction:

Answer 5

1. Action potential reaches end of motor neurone, which acts as presynaptic terminal
2. Depolarisation causes calcium ion channels to open
3. Ca²⁺ diffuse into synaptic knob
4. Causes vesicles containing acetylcholine to fuse with presynaptic membrane
5. Releases acetylcholine into synaptic cleft by exocytosis
6. Acetylcholine diffuses across synapse & binds to receptors on sarcolemma/postsynaptic membrane
7. Causes sodium ion channels to open, causing depolarisation in sarcolemma
8. Acetylcholine ⟶ choline + ethanoic acid by acetylcholinesterase
   
   1. breakdown of neurotransmitter afterwards ensures muscles are not overstimulated

Question 6

Outline the process which occurs in the sliding filament model to contract muscle:

Answer 6

1. At rest, tropomyosin molecule blocks myosin binding site on actin, preventing binding
2. Ca²⁺ ions released from sarcoplasmic reticulum after receiving action potential
3. Ca²⁺ binds to troponin >> shifts tropomyosin to expose binding sites on actin
4. Myosin head attaches to binding site
5. Myosin heads change angle >> moves actin filament along & releases ADP in the process
6. ATP binds to myosin head >> head detaches from binding site
7. Myosin head hydrolyses ATP to return to original angle, with ADP attached
8. Myosin head attaches to binding site further along actin filament, cycle is repeated.

Question 7

How is creatinine phosphate used during muscle contraction?

Answer 7

creatinine phosphate stored in muscle as reserve phosphate store

for short bursts of energy, used to phosphorylate ADP ⟶ ATP, since glycolysis would take longer

generated ATP rapidly, but creatinine phosphate store used up quickly

creatinine phosphate store replenished using phosphate from ATP

Question 8

Why is creatinine phosphate used to generate ATP instead of glycolysis?

Answer 8

short bursts of energy require rapid formation of ATP

glycolysis takes longer to form ATP than phosphorylation of ADP by creatinine phosphate, which is readily available

creatinine phosphate rapidly forms ATP, & can be replenished afterwards by ATP formed by glycolysis

Question 9

After a person’s death, they can no longer produce ATP. Explain why their muscles cannot relax:

Answer 9

ATP required to detach myosin heads from actin binding site

myosin heads remain attached to actin binding site

therefore actin & myosin remain largely overlapped since they cannot slide back, relaxing the muscle

Question 10

What is the role of calcium ions in muscle contraction?

Answer 10

• in neuromuscular junction - exocytosis of neurotransmitter
  
  • Ca²⁺ required for fusion of vesicles to presynaptic membrane, releasing neurotransmitter by exocytosis
• in muscle fibre - binds to troponin
  
  • calcium ions released by sarcoplasmic reticulum into sarcoplasm bind to troponin, shifting tropomyosin
  • determines whether myosin heads can bind to actin