1. Muscle Fiber Contraction
Chapter 9
Section

2. Muscle Contraction
When muscles contract they are physically shortening.
They contract because of nerve impulses sent from our brain.

3. Remember… Skeletal Muscle Organization
Different types of Myofilaments
1. Thick Filaments
Made of bundles of protein called MYOSIN
2. Thin Filaments
Made of bundles of protein called ACTIN
Actin and Myosin bind to each other, causing muscle contraction

4. Actin Myofilament -THIN
Made up of 2 strands of protein (ACTIN) molecules
Like 2 strings of beads loosely wound together

5. Myosin Myofilament - THICK
Made up of 2 strands of protein (MYOSIN) molecules
10 x larger than thin myofilament
2 ends
Long rod
Double-headed
globular region

6. How Myofilaments Contract…
Myosin head attaches to actin.

7. How Myofilaments Contract…
Myosin head moves.
Heads bend backwards, like flexing hand or wrist
Because head is chemically bonded to actin, it makes actin get pulled along with the myosin head as it flexes

8. How Myofilaments Contract…
Result: Actin myofilament SLIDES past myosin myofilament in the direction of flex
One myosin head flexes, then another and then another, and essentially “walks” along actin

9. How Myofilaments Contract…
Takes energy from ATP to release myosin head from actin
This repositions myosin head for each new flex
To “lift foot from actin” regarding walking analogy

10. Check it out…
o/quiz/ch10/myofilament_contraction.swf

11. Sliding Filament Model
Z line
Muscle tissue that is anchored at one end of EACH actin myofilament
Because its anchored, when the actin moves, it drags the z line along with it
Area between Z lines = sarcomere

13. When Muscle is Relaxed…
Myosin heads are raised and ready to be attached.
But they are unable to bind to actin.
They are unable to bind because the myosin head’s attachment site on actin is PHYSICALLY BLOCKED by a protein called tropomyosin.

14. So Logically…
Tropomyosin needs to get out of the way for muscle to contract.
To do this, we require a protein called TROPONIN.

15. Regulating Muscle Contraction
Muscle contraction is regulated by calcium ions (Ca2+) in the sacroplasm.
Sarcoplasm - muscle fibre cytoplasm.
The calcium ion concentration regulates the troponin - tropomyosin complex.

16. Regulating Muscle Contraction
LOW concentration causes tropomyosin to inhibit myosin binding.
HIGH concentration causes tropomyosin to get out of the way so binding can occur.

17. Where does this Ca2+ come from?
Sarcoplasmic Reticulum
When muscle is stimulated to
contract, SR releases Ca2+
into myofilament.
When muscle is stimulated to relax, and stop contracting, Ca2+ returns to SR via ACTIVE TRANSPORT.

19. Sliding Filament Theory Video4w