#6: Muscle Contraction Flashcards
Twitch
Single contraction in a relaxation cycle observed in a skeletal muscle fiber. Can last 10 to 100 milliseconds. Much longer than an action potential. Height, duration, and slope of twitch curve can vary from fiber to fiber.
Single twitch tension is determined by length of sarcomere. Does not represent the maximum force a muscle fiber can produce.
Recruitment
Describes a multiple motor unit summation (adding).
For example, take two motor units, X and Y. X stimulates 5 muscle fibers, and Y stimulates 7 muscle fibers. If done together through recruitment, they stimulate 12 muscle fibers together, evoking two action potentials simultaneously.
Increase & Decrease in Force of Muscle Contraction
How can you explain if muscle action potential is supposed to be an all-or-none response?
Tension that is developed depends on how stretched the muscle is. When actin and myosin contract past each other, the tension that the muscle fiber generates is directly proportional to the number of cross-bridges formed between the thick and thin filaments.
If muscle fiber is extremely stretched, little overlapping between thick and thin filaments. So very few cross bridges are formed, therefore very little force is generated.
The optimal length happens when you have the maximum thick and thin filaments overlapping. Numerous cross-bridges, therefore generating maximal force.
If you suppress the muscle fibers, the thick and thin filaments have too much overlapping, so the thick filaments can move the thin filaments only to a short distance before the thin filaments start to overlap with the opposite ends of the other thin filament.
Summation
If you apply multiple stimuli within a short duration especially when you factor previous stimulus has not completely disappeared, there will be an increase in the contractile force. This process is known as summation.
Tetanus
As the stimulus frequency footing increases, the average tension of each contraction remains pretty constant. This process is called tetanus.
If a period of relaxation is allowed in between each stimulus, it’s called incomplete tetanus. Otherwise, it’s called complete tetanus, when muscle fibers have achieved a maximal state of contraction.
Why is tetanic tension so much greater than twitch tension?
In twitch, action potential causes release of calcium ions, to saturate the tripolin. All myosin binding sites are initially available. The calcium begins pumping back into the sarcoplasmic reticulum. Tripolin tripo-myosin complex reformed and reblocked the many binding sites available to the actin.
In tetanus, with the continuation of the action potential, there’s enough Ca+2 to saturate the tripolin at all time, and the tripo myosin is ready to be binded by actin, and so the maximal forces can be generated. So the forces are much greater.
Muscle Fatigue
How does it occur?
1) Conduction failure - with action potential repeatedly firing muscles, opening those K+ channels, they’ll cause ions to move out of cells and into the very small volume of T tubules, and this extra outflux of K+ will cause depolarization of cell, contributing to conduction failure. Action potential will not be able to be generated.
2) Lactic Acid buildup - constant conversion of glucose to lactate, and lactic acid will start to build up. Acidification of muscle causes # of protein malfunctions including actin, myosin, and proteins involved in the Ca+2 release.
3) Inhibition of Cross-Bridge Cycling - imagine you hydrolyze alot of ATP, the buildup of ADP and phosphate may directly inhibit cross-bridge cycling, which delays cross bridge detachment from actin and prevents more forces from generating.
