5. Molecular aspects of muscle contraction, electromechanical coupling Flashcards
what are skeletal muscle
The skeletal muscle cells are giant cells formed by fusion of embryonic muscle cells (myoblasts). A muscle cell is also called a muscle fiber.
what it the inner part of the muscle fibre
The inner part of the muscle fibers is densely packed with myofibrils.
what is myofibrils made of
Consist of thin, thread-like myofilaments:
▪ Actin
▪ Myosin
o Myosin is twice as thick as actin.
smallest unit of the myofibrils called
sarcomere
increase in diameter of the muscle fibre
When the diameter of a muscle fiber expands, the
number of parallel myofibrils increas
increase muscle fibre length
When a striated muscle fiber grows longer, the number of sarcomers in the myofibrils increases.
what is a sarcomere made of
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.
how are myosin filaments are interconnected
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.
Proteins in the sarcomere: name all
titin
nebulin
alpha-actinin
Proteins in the sarcomere: titin originate
Largest protein of the body.
▪ Originates from the Z-lines and ends in the
myosin bundles.
Proteins in the sarcomere: titin function
▪ Ensures a precise return of actin and myosin
bundles to their original position even after
extensive stretch.
Proteins in the sarcomere: nebulin what does it look like
- A stiff, rod-shaped protein, which remains as an integral part of the filament.
Proteins in the sarcomere: nebulin functions
▪ 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.
Proteins in the sarcomere: alpha-actinin
Net-like protein
▪ Provides binding site for the actin complexes orienting toward the inner part of the sarcomere.
T-tubules where are they and what are they
they are the cell membranes gave thin tubular invaginations that traverse the fiber.
they surround each myofibril on either side of the Z-discs.
actin: what is it made of
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.
actin: where is tropmyosin
On the surface of the helix there is a tropomyosin
molecule.
actin: tropomyosin use
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.
Myosin: what is it
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.
Myosin: function
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.
Steps of electromechanical coupling 1
- Ca release through the function of the TRIAD(T-tubulus + two SR terminal cystern;
Steps of electromechanical coupling 2
- Activation of muscle proteins(Ca initiates the connection between Tropomyosin-Tn complex leading to the formation of Acto-Myosin komplex)
Steps of electromechanical coupling 3
- Muscle Contraction(If Ca is available there will be continuous „power strokes” of cross bridges creating contraction through „walk –along” („ratchet”) mechanism)
Steps of electromechanical coupling 4
- Relaxation follows the process (Ca elimination from IC space as follows) :
