What is the contraction for who has

The contractile apparatus of the muscle cells form the myofilaments Actin (7 nm diameter) and Myosin (15n nm diameter). By means of muscle-specific intermediate filaments (desmin), they are arranged so regularly that the Horizontal stripes results.

During the contraction in the sarcomere, myosin is the active part as a motor protein. The myosin monomer consists of one Tail, one movable neck and one Headboard, which forms a cross bridge with the actin filament and loosens it again. This causes the actin filaments towards the M line slide along the myosin heads to shrink the sarcomere. This reduces the width of the I- and desH-band. The A-band always stays the same width.

Actin:
from two almost identical α-helices from polypeptides

Myosin:
from two identical, heavy chains for the tail, neck and head and two pairs of light chains.


The Muscle contraction so found in Sarcomere instead of. The muscle has in the relaxed state little [Ca2 +] in the sarcoplasm. As a result, the myosin binding site on the actin filament for myosin Not is accessible. Are responsible for it Companion proteins, especially the allosteric Protein troponin C. As soon as the intracellular [Ca2 +] is increased, it binds Troponin C . Troponin C changes shape and pushes that Tropomyosin (another accompanying protein), which previously covered the docking point for myosin, away so that myosin can dock on the actin filament.

After binding to the actin filament, the myosin molecule also undergoes a series of changes (in simplified form):

  1. Rotate the head in the direction of the M-band (= center of the sarcomer) first Shape change. This movement pulls that Actin filament by about 10 nm with itself.
  2. Myosin binds ATP moleculewhat another Shape change leads. This reduces the affinity for actin and Myosin leaves the actin binding site Come on.
  3. As a result, the myosin undergoes a further change in shape, which leads to Splitting of ATP into ADP and Pi leads. There will also be something warmth free.
  4. The Splitting of ATP leads to another Shape changeso that the head is back in his Original position returns and is ready for a new bond on the actin filament.

The muscle contraction comes in rise of intracellular[Ca2 +] ahead. This increase is initiated by a Action potentialthat is generated at the neuromuscular synapse and transmitted through the entire sarcolemma. So that the whole muscle fiber is captured by the action potential at the same time, there is Sarcolemma invaginations, the T tubules,in the depth the muscle fiber. As a result, the action potential leads to a shortening of all sarcomeres within milliseconds. Furthermore, each myofibril is part of a network of sarcoplasmic reticulum (SR) surround which [Ca2 +] contains. A distinction is made between the longitudinal and the circular SR (= terminal cysts). If the sarcolemma is now depolarized, this leads to a very rapid increase in the [Ca2 +] i around the myofibrils, which leads to the aforementioned changes in shape of the actin-myosin complex.





Accompanying proteins
at regular intervals on the actin filament:
- troponin complex
  • Troponin I.
  • Troponin C
  • Troponin T
- tropomyosin
- tropomodulin







Rigor mortis: After death, the myosin heads can no longer detach from the actin filament due to a lack of ATP:
—> Rigor mortis


Sarcoplasmic reticulum:
  • longitudinal tubes
  • Circular hoses = terminal cisterns

Using theT tubules (= Invaginations of the sarcolemma) that will sarcoplasmic reticulum notified about the expiring action potential at the Sarkolemm and to Release of [Ca2 +] caused. The T-tubules run across to the longitudinal axis of the myofibrils in the skeletal muscle always at the transition from the A to the I band between two also circular ones terminal cysts. Thus they form one with the T-tubule Triad. Get on each sarcomere of a myofibril two triadsbecause there are two transitions from the A to the I band per sarcomere.

Note:
The triad of a T-tubule and two terminal cysts is mostly found in the skeletal muscle because the T-tubule system and the SR system are very well developed there.