Actin and myosin YB

Muscular contraction mechanism

1. Cell biology Actin and Myosin By Yogesh Bhandari Roll No. 4903

2. M yofibril   (muscle fibril) Definition- A   myofibril   is a basic rod-like unit of a muscle cell. Muscles are composed of tubular cells called  myocytes, known as muscle fibers in striated muscle and these cells in turn contain many chains of myofibrils .

3. Myofibrils are composed of long proteins : Actin Myosin Titin Myofilaments These proteins are organized into filaments Sarcomeres is the basic unit of striated muscle tissue. which repeat along the length of the myofibril in sections

4. A ctin Globular  multi-functional proteins  that form microfilaments  F ound in essentially all  eukaryotic cells M ass is roughly 42-kDa D iameter of 4 to 7 nm.

5. An actin is monomeric  subunit of two types of filaments in cells:  1. Microfilaments - one of the three major components of the  cytoskeleton 2. Thin filaments - part of the contractile  apparatus in muscle cells. It can be present as free  monomer called  G actin (globular ) or as part of a linear  polymer microfilament  called  F-actin  (filamentous ) both are essential for important cellular functions as mobility  and contraction of cells during cell division

6. Myosin are a   super family of  motor proteins  h aving roles in muscle  contraction other   motility processes in  eukaryotes myosin molecules are composed of : head neck tail domain

7. head domain binds the filamentous  actin and uses ATP  hydrolysis neck domain acts as a linker tail domain  mediates interaction with cargo molecules and/or other myosin  subunits Myosin classes 18 classes of myosin 1 to18

8. Muscle contraction It is activation of tension generating sites within muscle fiber occurs when these filaments slide over one another in a series of repetitive events. Muscles contract by sliding the thick (myosin) and thin (actin) filaments along each other

9. When an impulse reaches the muscle fibres of a motor unit it stimulates a reaction in each sarcomere between the actin and myosin filaments t his reaction results in start of a  contraction   thin actin filaments slide over the thick myosin filament known as sliding filament theory of muscle contraction . Mechanism

10. When Calcium is present, the blocked active site of the actin clears. 

11. Step  B: Power stroke: myosin head pivots pulling the actin filament toward the center.  Step  C: The cross bridge detaches when a new ATP binds with the myosin .

12. Thank you…