Muscle Fiber Contraction Chapter 9 Section 9 4

Muscle Fiber Contraction Chapter 9 Section 9. 4 -2

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

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

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

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

How Myofilaments Contract… 1. Myosin head attaches to actin.

How Myofilaments Contract… 2. 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

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

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

Check it out… • http: //www. mcgrawhill. ca/school/applets/abbi o/quiz/ch 10/myofilament_contraction. swf

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

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.

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

Regulating Muscle Contraction • Muscle contraction is regulated by calcium ions (Ca 2+) in the sacroplasm. • Sarcoplasm - muscle fibre cytoplasm. • The calcium ion concentration regulates the troponin - tropomyosin complex.

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.

Where does this Ca 2+ come from? • Sarcoplasmic Reticulum ▫ When muscle is stimulated to contract, SR releases Ca 2+ into myofilament. ▫ When muscle is stimulated to relax, and stop contracting, Ca 2+ returns to SR via ACTIVE TRANSPORT.

Sliding Filament Theory Video • http: //www. youtube. com/watch? v=0 k. Fmbr. RJq 4 w