Anatomy:
The membrane of the muscle cell is the sarcolemma (sarco is Greek for flesh); it is electrically excitable via the acetylcholine receptor.
The cytoplasm of the muscle cell is called sarcoplasm; it contains glycogen, glycolytic enzymes, and mitochondria (red muscle is that color due to cytochromes and Mb in the muscle tissue)
The contents of the muscle cells is myofibrils, which are about 1 mm in diameter; these fibrils exhibit light and dark bands:
I bands = isotropic (same over the surface)
A bands = anisotropic = not the same over surface (have double refraction or birefringence of light)
Muscle tissue has two types of filaments, thick and thin
Thick filaments:
Thick filaments are made up of:
1. myosin (has ATPase activity that is activated by actin)
2. C-protein
3. M-line proteins I and II
the two myosin chains form a coiled-coil rod
Thin filaments:
Thin filaments are made up of:
1. F (fibrous)-actin is made of polymerized G (globular)-actin
2. Tropomyosin (32 kdal x 2 chains)
3. Troponins (3)
TnC = 18 kdal ( binds Ca2+)
TnI = 24 kdal (I = Inhibitory, interacts with actin)
TnT = 37 kdal (binds Tropomyosin).
Sliding filament model:
The sliding filament model proposes that muscle bundles shorten by having the myosin portion of the thick filaments inch along the F-actin of the thin filaments. This takes place in a manner similar to the interleafed fingers of two hands moving past each other.
Biochemical reaction sequence:
1. Nerve ending releases acetylcholine; the muscle membrane is depolarized via the acetylcholine receptor.
2. Scarcoplasmic reticulum releases bound Ca ions. In resting muscle the concentration of Ca ions is less than 10-62+ away from the myofibrils.
3. Ca ion binds to TnC (prior to this binding, tropomyosin blocks the ability of myosin head unit (S1) to bind to actin).
4. myosin + ATP ---> H+ myosin-ADP-Pi (this process is activated by Ca ions and actin)
5. TnC (+ Ca) permits tropomyosin to move; the movement permits the interaction: myosin-ADP-Pi + actin --> actomyosin complex + ADP + Pi (free)
6. Myosin undergoes a large conformation change and releases ADP.
7. Another molecule of ATP binds and causes S1 of myosin to release from actin.
Careful study of the process of muscle contraction shows that the power stroke is not directly related to ATP hydrolysis (that only coils the spring!) rather it is related to the release of ADP!
