Cross-Bridge Cycling

Cross Bridge Cycling

• Cycle asynchronously

• The repeated process of myosin heads attaching to and detaching from actin filaments in muscle cells, causing muscle contraction

Step 1

ADP-bound myosin head is ‘cocked’ and ready to bind to actin

i. No Ca²+

ii. No A-M bridge

iii. Head is at rest; 90 degrees

Step 2

In presence of Ca²+, Ca²+ binds to troponin, exposing binding sites for myosin

i. Ca²+ present via Ryanodine Receptors

ii. Crossbridge permit

Step 3

The bound myosin rotates its head, producing a ‘power stroke’

i. Lowest energy conformation is now 45 degree myosin

ii. ADP + Pi fall off

Locked actomyosin complex (No ATP, rigor mortis!)

Step 4 + 5

ATP binds to myosin head. Actin and myosin detach.

i. ATP bind to myosin

ii. Break A-M complex (not touching)

iii. Rest at 90 degrees