1. Internal structure of skeletal muscle Nucleus: Contain the genetic material of the cell Sarcolemma: Plasma membrane of muscle cell T Tubule: Invagination of the Sarcolemma project deep into Muscle cells interior Terminal cisternae: Serve as specialized reservoirs for Ca ions Myofibril: Bundle of contractile filaments

2. The myofibrils composed of individual contractile proteins called myofilaments There are two types of myofilaments: Thin filaments composed mainly of protein Actins The thick filaments is composed mainly of protein myosin Structure of Myofibrils

3. Organization levels of skeletal muscle Myofilaments: Smallest building blocks. Compose of thick and thin Myofibril: Many myofilaments bundle together making a single myofibril Muscle cell: Many myofibril are contained within the muscle cell Fascicle: Many muscle cells are packed into a fascicle Muscle: Many fascicle make up the muscle

4. The Sliding Filament Sliding filament occurs as a thin filaments slide past the thick filament. This involve the activity of: Myocin Actin Tropomyosin Troponin ATP Ca ions

5. The Thick Filament (Myosin) Consists of the protein called myosin. A myosin molecule is shaped a bit like a golf club, but with 2 heads. The heads stick out to form the cross bridge Many of these myosin molecules stick together to form a thick filament Each head contains two binding sites, one for actin and one for ATP.

6. Thin Filament (Actin) The thin filament consists of a protein called actin. It compose of actin subunit twisted into double helical chain. Actin has specific binding site to which the myosin head binds The thin filament also contains tropomyosin. The position of tropomyosin cover the binding sites on the actin during unstimulated muscle The third component is troponin. Attached along the tropomyosin strand.Which expose the binding site of actin to myosin

7. Arrangement of Myofilament The arrangement of thick and thin myofilaments forms light and dark alternating bands (striation). In the middle of the light band is the Z-line The repeating unit from one Z-line to the next is called the sarcomere

8. I Band = actin filaments

9. Ca Ion After action potential, Ca ions released from the T-Tubules and bind to troponin. This causes change in conformation of the troponin – tropomyosin complex and thus dragging tropomyosin strands off the binding site

10. Sliding-Filament Mechanism

11. Sliding-Filament Mechanism

12. Sliding-Filament Mechanism

13. Sliding-Filament Mechanism

14. Sliding-Filament Mechanism

15. Steps of cross bridge cycle The influx of Ca ion, triggering the exposure of binding sites on actin The binding of myosin to actin The myosin cross bridge pulls the thin filament inward toward the centre of sarcomere The binding of ATP to the myosin head disconnecting from actin The hydrolysis of ATP leads to repositioning of the myosin head The transport of Ca ion to the back into the sarcoplasmic reticulum