6C: Nervous Coordination

Question 1

Stages in muscle contraction

Answer 1

• happens through sliding filament mechanism

Overall:
1. At rest, tropomyosin blocks the actin-myosin binding site meaning they cannot slide.
2. Action potential causes depolarisation in sarcolemma, T tubules and to sarcoplasmic reticulum.
3. Sarcoplasmic reticulum releases stored calcium ions into the sarcoplasm as channels open.
4. Calcium binds to protein on tropomyosin, protein changes shape and moves tropomyosin out of the binding site
5. The bulbous heads of the myosin filaments bind to the actin filament in the binding site
6. This forms cross-bridges with the actin filaments by attaching themselves to binding sites on the actin filaments
7.ATP → ADP + Pi by ATP hydrolase, energy released causes myosin heads to bend
8. This pulls the actin filaments along the myosin filaments.
9. They then detach and, using ATP, return to their original angle and re-attach themselves further along actin filament.
10. When stopped being stimulated, calcium ions actively transported back into sarcoplasmic reticulum
11. Tropomyosin blocks actin-myosin binding site so cross bridges cannot form so actin slides back and sarcomere lengthens and muscle is relaxed

Question 2

Muscle stimulation

Answer 2

1. An action potential from a motor neurone reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse
   into the synaptic knob.
2. The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft.
3. Acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell surface membrane, causing the sarcolemma to depolarise.

Question 3

Muscle relaxation

Answer 3

1. When nervous stimulation ceases, calcium ions are actively transported back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP.
2. This reabsorption of the calcium ions allows tropomyosin to block the actin-myosin binding sites again.
3. Myosin heads are now unable to bind to actin filaments so there are no cross bridges.
4. The actin filaments slide back to their relaxed position which lengthens the sarcomere