Sliding-filament theory of muscular contraction Flashcards
What does the sliding-filament theory explain?
How muscles contract to produce movement.
Explain sliding-filament theory in its simplest form. In 5 steps.
During contraction, actin filaments at each end of the sarcomere slide inward along the myosin filaments.
The inward movement pulls the Z-lines closer to the center of the sarcomere thereby shortening the muscle fiber.
As the actin filaments move over the myosin filaments, both the H-zone and I-band decrease in size.
The action of the myosin crossbridges pulling on the actin filament is what drives the movement of the actin filament.
Because the flexion of the myosin crossbridge results in only a very small displacement of the actin filament, numerous rapid, repeated flexions across many crossbridges are necessary to produce measurable muscle movement.
What happens during the resting phase, under normal conditions? In 3 steps.
- There is a low concentrations of calcium in the myofibril because most of it is stored in the sarcoplasmic reticulum.
- Very few myosin crossbridges are attached to actin. Although the actin binding sites are covered, myosin and actin still interact weakly.
- This weak bond becomes strong (resulting in muscle tension) when the actin binding site is exposed following the release of stored calcium.
What happens in the exitation-contraction coupling phase? In 6 point.
- Before the myosin crossbridges can flex, they must first attach to the actin filament.
- Sarcoplasmic reticulum is stimulated, releases calcium ions.
- Calcium binds to troponin, triggering a shift in another protein, tropomyosin, which runs along the groove of the actin filament’s double helix.
- this shift exposes the binding sites on actin, allowing the myosin crossbridges to attach more rapidly to the actin filament.
- As the myosin croddbridges pull the actin filaments towards the center of the sarcomere, force is produced.
- The amount of force generated by the muscle at any given moment is directly proportional to the number of myosin crossbridges attached to actin filaments at the time.
During the muscle contraction phase, where does the energy come from?
- the hydrolysis (breakdown) of adenosine triphosphate ATP into adenosine diphosphate ADP and phosphate.
How is ATP hydrolyzed by ADP?
- It is catalyzed by the enzyme myosin adenosine triphosphatase (ATPase).
How is myosin adenosine triphosphatase utilized in the contraction phase?
The enzyme myosin triphosphatase enzyme catalyses the ATP to ADP reaction.
2. For the myosin head to detach from the actin site and return to its original position, a new ATP modecule must replace the ADP on the myosin crossbridge globular head.
3. This allows the contraction process to continue (if calcium is available to bind to troponin) or leads to relaxation if calcium is no longer present.
What other important role does calcium play?
In other skeletal functions, like regulating glycolytic and oxidative energy metabolism and protein synthesis and degradation.
What happens in the recharge phase?
Measurable muscle shortening ovvurs only when the sequence of events - binding of calcium to troponin, coupling of the myosin crossbridge with actin, the power stroke, dissociation of actin and myosin, and resetting of the myosin ATPase is available to catalyze ATP breakdown.
When does the relaxation phase take place?
When the stimulation of the motor nerve ceases.
What happens during the relaxation phase?
Calcium is pumped back into the sarcoplasmic reticulum, which prevents the continued interaction between actin and myosin filaments.
2. This results in the actin and myosin filaments returning to their rebound state, leading to muscle relaxation.
Summarize the 5 steps of muscle contraction.
- Initiation of ATP splitting (by myosin ATPase) causes myosin head to be in an energized state that allows it to move into a position to be able to form a bond with actin.
- The release of phosphate from the ATP splitting process then causes the myosin head to change shape and shift.
- This pulls the actin filament in toward the center of the sarcomere and is referred to as the power stroke; ADP is then released.
- Once the power stroke has occurred, the myosin head detaches from the actin but only after another ATP binds to the myosin head because the binding process facilitates detachment.
- The myosin head is now ready to bind to another actin (as described in step 1), and the cycle continues as long as ATP and ATPase are present and calcium is bound to the troponin.
