8 Muscles (Cross Bridge Cycling)

Question 1

what is cross bridge cycling?

Answer 1

the process of myosin heads binding to actin filaments, and drawing them towards the centre of the sarcomere in a ‘power stroke’, shortening the sarcomere and hence creating muscular contraction

Question 2

what initiates cross bridge cycling?

Answer 2

the release of calcium ions from terminal cisternae into the sarcomere (triggered by the arrival of an action potential at the T tubules)

Question 3

what does calcium bind to in the sarcomere that allows cross bridge cycling to occur?

Answer 3

calcium ions bind to troponin on actin filaments. troponin pulls tropomyosin away from myosin binding sites of actin, exposing the binding sites and allowing cross bridge cycling to occur

Question 4

what are the 9 steps of cross bridge cycling (beginning with the end plate potential?)

Answer 4

1. end plate potential at neuromuscular junction ignites an AP which travels along muscle cell sarcolemma
2. AP travels into T tubules
3. AP in T tubules stimulates release of calcium ions from terminal cisternae into cytoplasm of sarcomere
4. calcium ions bind to troponin on actin filaments
5. troponin pulls tropomyosin away from myosin binding sites on actin filaments
6. an ATP molecule attaches to the myosin head. the head is energized and ‘cocked’ when the ATP molecule is hydrolyzed into ADP and P
7. the cocked myosin head attaches to the actin binding site, creating a cross bridge
8. the myosin draws the actin towards the centre of the sarcomere in a power stroke, creating a muscular contraction
9. new ATP molecule attaches to myosin and cross bridge breaks

Question 5

what is the role of ATP in cross bridge cycling?

Answer 5

An ATP molecule binds to the myosin head.

When the ATP is hydrolyzed (releasing ADP and P), this energizes the myosin head into a ‘cocked’ position.

the energized myosin head then binds with an actin molecule in a cross bridge, and performs a power stroke to create muscular contraction.

when a new ATP molecule attaches to the myosin head, the cross bridge detaches