Sacroplasm

Question 1

What does the depolarisation of the sacrolemma result in?

Answer 1

• the depolarisation of the sacrolemma travels deep into the muscle fibre by spreading through the T-tubules
• these are in contact with the sacroplasmic reticulum

Question 2

What does the sacroplasmic reticulum contain?

Answer 2

• contrains stored calcium ions which it actively absorbs from the sacroplasm

Question 3

What happens when the action potential reaches the sacroplasmic reticulum?

Answer 3

• it stimulates calcium ion channels to open
• the calcium ions diffuse down their concentration gradient flooding the sacroplasm with calcium ions

Question 4

WHat is the function of the release of calcium ions>

Answer 4

• calcium ions bind to troponin, causing it to change shape
• this pulls on the tropomysin, moving it away from the actin-myosin binding sites on the actin filament
• now that the binding sites have been exposed, the myosin head binds to the actin filament forming an actin-myosin cross-bridge

Question 5

What happens once the myosin head attaches to the actin filament?

Answer 5

• the myosin head flexes
  
  • pulling the actin filament along
    *

Question 6

How does the myosin head return to its original position?

Answer 6

• the molecule of ADP bound to the myosin head is released
• an ATP molecule can now bind to the myosin head
• this causes the head to detach from the actin filament

Question 7

How is the ATPase active of the myosin increased?

Answer 7

• the calcium ions present in the sacroplasm also activate the ATPase activity of the myosin

Question 8

Why is ATPase acitivty increased?

Answer 8

• ATPase hydrolyses the ATP to ADP and phosphate, releasing energy which the myosin head uses to return to its original position

Question 9

Why does the myosin head return to its original position?

Answer 9

• the head can not attach itself to another actin-myosin binding site further along the actin filament and the cycle is repeated
• the cycle continues as long as the muscle remain stimulated
• during the period of stimulation many actin-myosin bridges form and break rapids, pulling the actin filament along
  
  • this shortens the sacromere and causes the muscle to contract