5. Molecular aspects of muscle contraction, electromechanical coupling

Question 1

what are skeletal muscle

Answer 1

The skeletal muscle cells are giant cells formed by fusion of embryonic muscle cells (myoblasts). A muscle cell is also called a muscle fiber.

Question 2

what it the inner part of the muscle fibre

Answer 2

The inner part of the muscle fibers is densely packed with myofibrils.

Question 3

what is myofibrils made of

Answer 3

Consist of thin, thread-like myofilaments:
▪ Actin
▪ Myosin
o Myosin is twice as thick as actin.

Question 4

smallest unit of the myofibrils called

Answer 4

sarcomere

Question 5

increase in diameter of the muscle fibre

Answer 5

When the diameter of a muscle fiber expands, the
number of parallel myofibrils increas

Question 6

increase muscle fibre length

Answer 6

When a striated muscle fiber grows longer, the number of sarcomers in the myofibrils increases.

Question 7

what is a sarcomere made of

Answer 7

Each sarcomere contains two sets of actin filaments. One of the filaments in each set is anchored in a protein lattice that separates two adjacent sarcomers. These dividing walls are called Z-discs.

Question 8

how are myosin filaments are interconnected

Answer 8

myosin filaments are interconnected by means of a protein lattice, which is visible as a transverse line (M- line) in the middle of the sarcomere.

Question 9

Proteins in the sarcomere: name all

Answer 9

titin
nebulin
alpha-actinin

Question 10

Proteins in the sarcomere: titin originate

Answer 10

Largest protein of the body.
▪ Originates from the Z-lines and ends in the
myosin bundles.

Question 11

Proteins in the sarcomere: titin function

Answer 11

▪ Ensures a precise return of actin and myosin
bundles to their original position even after
extensive stretch.

Question 12

Proteins in the sarcomere: nebulin what does it look like

Answer 12

• A stiff, rod-shaped protein, which remains as an integral part of the filament.

Question 13

Proteins in the sarcomere: nebulin functions

Answer 13

▪ Determine the direction and placement of actin polymerization during the development of the sarcomere.
▪ Protects the developed actin from rearranging effect of other actin-binding proteins.
▪ Ensures that all actin filaments are of precisely the same length.

Question 14

Proteins in the sarcomere: alpha-actinin

Answer 14

Net-like protein
▪ Provides binding site for the actin complexes orienting toward the inner part of the sarcomere.

Question 15

T-tubules where are they and what are they

Answer 15

they are the cell membranes gave thin tubular invaginations that traverse the fiber.
they surround each myofibril on either side of the Z-discs.

Question 16

actin: what is it made of

Answer 16

Consists of globular actin molecules bound together in a coiled double chain.
- Main component is G-actin, which forms polarized actin- fibers winding up in the double helix.

Question 17

actin: where is tropmyosin

Answer 17

On the surface of the helix there is a tropomyosin
molecule.

Question 18

actin: tropomyosin use

Answer 18

When troponin complex binds to tropomyosin, the
tropomyosin slides into the groove of the two stranded actin helix and previously masked myosin-binding sites become uncovered. Cross-bridge cycle starts.

Question 19

Myosin: what is it

Answer 19

Gggregated myosin molecules, with two heavy and four light polypeptide chains.
- The heavy chains are twisted around each other and form an elongated “tail”, a bent “neck”, and a pair of “heads”.
- The four light chains are included in the heads.

Question 20

Myosin: function

Answer 20

The myosin heads can bind to the actin molecules and exert force on them by altering the angle between the tail and the protruding heads of the myosin molecules. This constitutes the basis for the contraction mechanism.

Question 21

Steps of electromechanical coupling 1

Answer 21

1. Ca release through the function of the TRIAD(T-tubulus + two SR terminal cystern;

Question 22

Steps of electromechanical coupling 2

Answer 22

2. Activation of muscle proteins(Ca initiates the connection between Tropomyosin-Tn complex leading to the formation of Acto-Myosin komplex)

Question 23

Steps of electromechanical coupling 3

Answer 23

3. Muscle Contraction(If Ca is available there will be continuous „power strokes” of cross bridges creating contraction through „walk –along” („ratchet”) mechanism)

Question 24

Steps of electromechanical coupling 4

Answer 24

4. Relaxation follows the process (Ca elimination from IC space as follows) :

Question 25

Steps of electromechanical coupling 4

Answer 25

4. Relaxation follows the process (Ca elimination from IC space as follows) :

Question 26

Steps of electromechanical coupling 4a

Answer 26

a. Na/Ca antiportmechanism (to the EC space !)

Question 27

Steps of electromechanical coupling 4b

Answer 27

b. ATP-dependent Ca-pump to the SR and/or to other compartments (i.e. mitochondria).