Contraction of Skeletal Muscle

Physiologic anatomy of Skeletal Muscle



Skeletal Muscle Fibre

Myofibrils are composed of Actin and Myosin filaments: 
Each muscle fiber consists of millions of myofibrils, and each myofibril is composed of 1500 myosin filaments and 3000 actin filaments lying side by side. These are large polymerised protein molecules that are responsible for muscle contraction. The tick filaments myosin and the the thin filaments actin: 


Light and dark bands: The myosin and the actin filaments partially interdigitate and thus cause light and dark bands. The light bands contain only actin filaments and are called I Bands. The dark bands contain myosin filaments as well as the ends of actin filaments, where they overlap the myosin, and are called A bands.


Cross-Bridges: The small projections from the sides of myosin filaments are cross-bridges. They protrude from the surfaces of the myosin filament along its entire length except the centre. Interaction between these cross bridges and the actin filaments causes contraction. 


Z Disks: The ends of the actin filaments are attached to Z disks. The Z disc passes through the myofibrils across the muscle fiber. The entire muscle fiber therefore has light and dark bands, giving muscle the striated appearance. 

Sarcomere: The portion of the myofibril that lies between two successive Z disks is called the sarcomere. During rest the actin filaments overlap the myosin filaments and just bearly overlap one another.


Genetic Mechanisms of Muscle contraction


An action potential travels along a motor nerve to its end on muscle fiber, and the nerve secretes a small amount of neurotransmitter acetylcholine.

The acetylcholine acts on a local area of the muscle membrane to open the acetyl-choline channels, which allows the sodium ions to flow into the muscle fiber.

The action potential travels along the muscle fiber membrane , causing the sarcoplasmic reticulum to release calcium ions into the myofibrils that have been stored in the retinaculum.

The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide together; this is the contractile process.

After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum, where they remain stored until a muscle action potential arrives; this removal of the calcium ions from the myofibrils causes the muscle contraction to cease.

Molecular Mechanism of Muscle contraction

Muscle contraction occurs by a sliding filament mechanism. Mechanical forces generated by the interaction of myosin cross-bridges with the actin filaments cause the actin filaments to slide inward among the myosin filaments. Under resting conditions these forces are inhibited, but when an action potential travels over the muscle fiber membrane, the sarcoplasmic reticulum releases large quantities of calcium ions which activate the the forces between the actin and the myosin and contraction begins.

The myosin filament is composed of multiple myosin molecules. 

The important feature of the myosin head is that it functions as an ATPase enzyme, which allows the head to cleave ATP and thus to energise the contraction process.

The actin filament is composed of actin, tropomyosin, and troponin. Each actin filament is about 1 micrometer long. The basis of the actin filaments are inserted strongly into the Z discs, whereas the other ends protrude in both directions into the adjacent sarcomeres to lie in the spaces between the myosin molecules. 

Interaction of Myosin, Actin Filaments, and Calcium Channel Ions to cause contraction:

The actin filament is inhibited by the troponin-tropomysin complex; activation is stimulated by calcium channel ions. 

Inhibition by the troponin-tropomyosin complex. The active sites of the normal actin filament or relexed muscle are inhibited or physically covered by the troponin-tropomysin complex. Consequently the sites cannot attach the heads of the myosin filaments to cause contraction until the inhibitory effect of the troponin-tropomyosin complex inhibited.

Activation by calcium channel ions.  The inhibitory effect of the troponin-tropomyosin complex is inhibited by the presence of calcium ions. Calcium ions combine with troponin and, causing the troponin complex to tug on the tropomyosin molecule. This uncovers the active sites, thus allowing contraction to proceed.

A walk along theory can explain how activated actin filament and the myosin cross-bridges interact to cause contraction. When myosin head attaches to an active site, the head tilts automatically toward the arm that is dragged along the actin filament. This tilt of the head is called the power stroke, Immediately after tilting, the head automatically breaks away from the active site. The head then returns to its normal perpendicular direction. In this position, it combines with a new active site farther along the actin filament. Thus, the heads of the cross bridges bend back and forth and step by step walk along the actin filament, pulling the ends of the actin filaments towards the centre of the myosin filament.

Degree of Actin and Myosin Filament Overlap - Effect on Tension Developed by the contracting muscle. 

The strength of contraction is maximal when there is maximal overlap between actin filaments and the cross-bridges of the myosin filaments. As the sarcomere shortens there is increasing overlap and increasing muscle tension. Full tension is maintained at a sarcomere length of 2.0 micrometers because the actin filament has overlapped all the cross-bridges of the myosin filament. Any more contraction causes the actin filaments to overlap - causing muscle tension to decrease. 

Energetics of Muscle Contraction: 

Muscle contraction requires ATP to perform three main functions. 

Most of the ATP is used to activate the walk along mechanism of muscle contraction
Calcium is pumped back into the sarcoplasmic reticulum after contraction ends.
Sodium and potassium ions are pumped through the muscle fiber membrane to maintain an appropriate ionic environment for the propagation of action potentials.

There are 3 main sources of energy for muscle contraction.

The concentration of ATP in the muscle fiber is sufficient to maintain full contraction for only 1 to 2 seconds. 

Phosphocreatine: carries high-energy bond tht is similar to that of ATP but has more free energy. Maximal contraction for 5 to 8 seconds 
The breakdown of Glucogen to pyruvic acid and lactic acid - releasing energy for converting ADP to ATP. The rate of ATP formation is 2,5 times as fast as when ATP formation when foodstuff reacts with oxygen. Muscle contraction for 1 minute.
Oxidative metabolism occurs when oxygen is combined with various cellular foodstuffs to liberate ATP. 95% of sustained contraction. 
Characteristics of Whole Muscle Contraction
Isometric contractions do not shorten the muscle, isotonic contractions do shorten the muscle. 
Isometric contractions - no muscle shortening
Isotonic contraction - Muscle shortens and the tension remains consistant.

Fast fibers are adapted for powerful muscle contraction, whereas slow fibers are adapted for prolonged muscle activity. 
Fast fibers (white muscle) are:
 larger for greater strength contraction, 
have extensive sarcoplasmic reticulum for rapid release of calcium ions, and 
have lare amount of glycolytic enzymes for rapid release of energy, and 
have lower capillary and fewer mitochondria because oxidative metabolism is of secondary importance.
Slow fibers:
smaller muscle fibers
are innervated by smaller nerve fibers
have high capillarity and large number of mitochondria to support the high levels of oxidative metabolism
contain large amounts of myoglobin, which give slow muscle their reddish appearance.
Muscle hypertrophy is the increase in the total mass of the cell - actin and myosin filaments 
Muscle atrophy - disuse atrophy, decay in the contractile proteins - and atrophy occurs immediately after nerve supply is lost.
 
  

Summary: Mlecular basis for contraction (Animation)