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12. 5. 2012.

Muscle fiber contraction

An alpha-motor neuron is a neuron that connects with and innervates many muscle fibers. A single alpha-motor neuron and all the muscle fibers it supplies are collectively termed a motor unit. The synapse or gap between the alpha-motor neuron and a muscle fiber is reffered to as a neuromuscular junction. This is where communication between the nervous and muscular system occurs.

Action potential

The events that trigger a muscle fiber to contract are complex. The process, that is described on the second picture, is inititated by an electric signal, or action potential, from the brain or spinal cord to an alpha-motor neuron. The action potential arrives at the alpha-motor neuron’s dendrites, specialized receptors on the neuron’s cell body. From here, the action potential travels down the axon to the axon terminals, which are located very close to the plasmalemma. When the action potential arrives, these nerve endings secrete a neurotransmitter substance called acetylcholine(Ach), which binds to receptors on the plasmalemma(second picture, part a). If enough Ach binds to the receptors, the action potential will be transmitted the full length of the muscle fiber as ion gates open in the muscle cell membrane and allow sodium to enter. This process is reffered to as depolarization. An action potential mus be generated in the muscle cell before the muscle cell can act. 

Role of calcium in the muscle fiber

In addition to depolarizing the fiber membrane, the action potential travels over the fiber’s network of tubules(T-tubules) to the interior of the cell. The arrival of an electrical charge causes the adjacent SR to release large quantities of stored calcium ions(Ca2+) into the sarcoplasm.
In the resting state, tropomyosin molecules cover the myosin-binding sites on the actin molecules, preventing the binding of the myosin heads. Once calcium ions are released from the SR, they bind to the troponin on the actin molecules. Troponin, with its strong affinity for calcium ions, is believed to then initiate the contraction process by moving the tropomyosin molecules off the myosin-binding sites on the actin molecules. This is shown in the picture 2,part c. Because tropomyosin normally cover the myosin-binding sites, it blocks the attraction between the myosin cross-bridges and actin molecules. However, once the tropomyosin has been lifted off the binding sites by troponin and calcium, the myosin heads can attach to the binding sites on the actin molecules.

Sliding filament theory: How muscle creates movement

When muscle contracts, muscle fibers shorten. How do they shorten? The explanation for this phenomenon termed the sliding filament theory. When the myosin cross-bridges are activated, they bind with actin, resulting in a conformational change in the cross-bridge, which causes the myosin head to tilt and drag the thin filament toward the center of the sarcomere. This tilting of the head is reffered to as the power stroke. The pulling of the thin filament past the thick filament shortens the sarcomere and generates force. When the fibers are not contracting, the myosin head remains in contact with the actin molecule, but the molecular bonding at the site is weakened or blocked by tropomyosin.
Immediately after the myosin head tilts, it breaks away from the active site, rotates back to its original position, and attaches to a new active site farther along the actin filament. Repeated attachments and power strokes cause the filaments to slide past one another, giving rise to the term sliding filament theory. This process continues until the ends of the myosin filaments reach the Z-disks, or until the Ca2+ is pumped back into the sarcoplasmic reticulum. During this sliding(contraction), the thin filaments move toward the center of the sarcomere and protrude into the H-zone, ultimately overlapping. When this occurs, the H-zone is no longer available.

Energy for muscle contraction

Muscle contraction is an active process requiring energy. In addition to the binding site for actin, a myosin head contains a binding site for adenosine triphosphate(ATP). The myosin molecule must bind with ATP for muscle contraction to occur because ATP supplies the needed energy.
The enzyme adenosine triphosphatase(ATPase), which is located on the myosin head, splits the ATP to yield adenosine diphosphate(ADP), inorganic phosphate(Pi), and energy. The energy released from this breakdown of ATP is used to power the tilting the myosin head. Thus, ATP is the chemical source of energy for muscle contraction.

End of muscle contraction

Muscle contraction continues as long as calcium is available in the sarcoplasm. At the end of a muscle contraction, calcium is pumped back into the SR, where it is stored until a new action potential arrives at the muscle fiber membrane. Calcium is returned to the SR by an active calcium-pumping system. This is another energy-demanding process that also relies on ATP. Thus,energy is required for both the contraction and relaxation phases.
When the calcium is pumped back into the SR, troponin and tropomyosin return to the resting conformation. This blocks the linking of the myosin cross-bridges and actin molecules and stops the use of ATP. As a result, the thick and thin filaments return to the their original relaxed state.

The molecular events of a contractile cycle – changes in the myosin head during various phases of the powerstroke:
1)      Tight binding in the rigor state. The cross-bridge is at a 45 degrees angle relative to the filaments.
2)      ATP binds to its binding site on the myosin. Myosin then dissociates from actin.
3)      The ATPase activity of myosin hydrolyzes the ATP. ADP and Pi remain bound to myosin.
4)      The myosin head swings over and binds weakly to a new actin molecule. The cross-bridge is now at 90 degrees relative to the filaments.
5)      Release of Pi initiates the power stroke. The myosin head rotates on its hinge, pushing the actin filament past it.
6)      At the end of the power stroke, the myosin head releases ADP and resumes the tightly bound rigor state. 

 “Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney

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