Monday 13 May 2013

Contraction of Skeletal muscles

Sarcolema:

It consists of true cell membrane (plasma membrane). Outer coat of thin layer of polysaccharides that contain numerous thin collagen fibrils. At the end of muscle fiber this layer fuses with the tendon fiber. Tendon fibers bundle up to form muscle tendon, which then inserts into bones.

Myofibrils:

Actin + Myosin = Myofibrils.
Actin and Myosin are responsible for actual muscle contraction. Myosin fibrils have light and dark bands. Light bands contain only Actin and are called I bands. Dark bands contain both Actin and myosin thus called A bands.

Cross Bridges: 

projections from the sides of myosin filament.

Interaction between cross bridges and actin causes contraction. Ends of actin filament are also attached to
Z-discs. From this disc these filaments extend in both directions to interdigitate. Z-disc passes crosswise across myofibril and also from myofibril to myofibril, thus attaching the myofibrils across the muscle fiber. This is why all the muscle fiber has light and dark bands. These bands give the muscle their sriated appearance. Myofibril between two Z-discs is called Sarcomere. 

Titin:

Side by side relation between actin and myosin is brought about by a large number of filamentous molecules of proteins called titin. It is very springy.

Sarcoplasma:

The spaces between myofibrils is filled with intracellular fluid called sarcoplasm. Pottasium + magnesium + phosphate + protein enzymes. The mitochondria produce ATP.

Sarcoplasmic Reticulum:

Present in the sacroplasm.

General principles of contraction:

  1. Action potential travels along a motor nerve to its ending on the muscle fibers.
  2. The nerve on its endings secrete Acetylcholine (Ach).
  3. Ach causes opening of multiple Ach gated cation channels.
  4. This allows Na influx to the muscle fiber membrane. thus depolarization leading to opening of voltage gated sodium channels.
  5. The action potential travels along the membrane of muscle fibers.
  6. Thus depolarization of membrane and also the sarcoplasmic reticulum releases large quantities of calcium ions that have been stored with it.
  7. Calcium ions Initiate attractive forces between actin and myosin causing them to slide along side each other.
  8. After a fraction of a second calcium moves back to the reticulum thus its removal ceases the contraction.

Molecular mechanism:

The forces generated by the cross bridges bring about contraction. 

Sliding filament mechanism:

In contracted state the ends of actin filaments overlap. Z-discs are pulled upto the ends of myosin filaments. Myosin filament is made up of 200 or more individual myosin molecules. The arms and heads together form the cross bridges. Myosin filaments are twisted so that each successive pair of cross bridges is axially displaced from the previous one by 120 degrees. Myosin head functions as ATPase enzyme. F-actin protein molecule is the backbone of actin filament. Each strand contains polymerized G-actin molecules. Attached to these are ADP to which myosin cross bridges are attached to cause contraction. In resting state, tropomyosin lies on top pf the active sites of actin strands so that attraction between actin and myosin filament doesnot take place. Troponin-topomyosin complex is the antoagonist for contraction. There are three types of Troponin:
  1. Troponin I = strong affinity for actin.
  2. Troponin T = Strong affinity for tropomyosn.
  3. Troponin C = strong affinity for Calcium ions.

ATP as a source of contraction:

Head of cross bridges bind with ATP, this causes the ATP to cleave into ADP which stays bound. When active sites on actin filament uncover myosin heads bind there. This causes conformational changes in head, the head tilts towards the arm thus power stroke for pulling Actin filament is generated. After this the ADP and phosphate is released and the cycle repeats again.

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