Cross Bridge Cycle Flashcards
Myosin cross bridges are the __ ___ sites.
force generating
Contraction does not necessarily cause ___ of fiber.
Shortening
Contraction is defined as
generation of force
In the relaxed state, thin and thick filaments overlap only at
ends of A band
Sliding filament model of contraction 1: During contraction, thin filaments slide past ___ filaments so that actin and myosin filaments overlap to a greater degree.
thick
Sliding filament model of contraction: When does sliding begin?
When the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments.
Sliding filament model of contraction: For sliding to begin, the ___ heads on the thick filaments latch onto __ __ ___ on actin in the thin filaments.
myosin,
myosin-binding sites
Sliding filament model of contraction 2: These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the ___ of the sarcomere.
center
Sliding filament model of contraction: What happens when cross bridge attachments form and break several times during a contraction?
It generate tension and propels the thin filaments toward the center of the sarcomere.
Sliding filament model of contraction 3: What is the end result of the generated tension that propels the thin filaments toward the center of the sarcomere?
Causes the muscles to shorten
Sliding filament model of contraction: Notice that as the thin filaments slide centrally, the Z discs to which they attach are pulled __ the M line.
toward
Overall, as a muscle cell shortens, what are the 4 things that occur?
1- I bands shorten
2-Distance between Z discs shorten
3-H zones disappear
4-A bands move closer together; length does not change
Cross bridges occur when
myosin heads bind to actin
Overall, as a muscle cell shortens, what are the 4 things that occur?
1- I bands shorten
2-Distance between Z discs shorten
3-H zones disappear
4-A bands move closer together; length does not change
During muscle shortening, the ___ is pulled toward the M line.
Z discs
Contraction ends when the ___ ___ become inactive, ___ declines, and the muscle fiber ___.
cross bridges
tension
relaxes
A contraction is triggered by a series of molecular events known as the
Cross bridge cycle
The formation of a cross bridge is initiated when ___ ___ released from the sarcoplasmic reticulum bind to troponin.
Calcium ions
The formation of a cross bridge is initiated when calcium ions released from the sarcoplasmic reticulum bind to ___. This binding causes ___ to change shape.
troponin, troponin
____ moves away from the myosin binding sites on actin allowing the myosin to bind actin and form a cross bridge.
Tropomyosin
The myosin head must be activated before cross bridge cycle can begin. This occurs when ___ binds to the head and is hydrolyzed to ADP and inorganic phosphate.
ATP
The myosin head must be activated before cross bridge cycle can begin. This occurs when ATP binds to the head and is hydrolyzed to _________.
ADP and inorganic phosphate.
The energy liberated from the __ _ __ activates the myosin head, forcing it into the cocked position.
hydrolysis of ATP
Cross bridge cycle maybe divided into four steps.
Step 1: __ __
Crossbridge formation
Step 1: Cross bridge formation
The activated myosin head binds to actin, forming a cross bridge. ___ ___ is released and the bond between myosin and actin become stronger.
Inorganic phosphate
What is released to cause the bond between myosin and actin become stronger?
Inorganic phosphate
What is Step 2?
Step 2: The power stroke
Step 2: The power stroke
__ is released and the activated myosin head pivots, sliding the thin myofilament toward the center of the sarcomere.
ADP
What is Step 3?
Step 3: Cross bridge detachment
Step 3: Cross bridge detachment
When another ___ binds to the Myosin head, the link between the myosin head and actin __ and the myosin head detaches.
ATP, weakens
What is Step 4?
Step 4: Reactivation of myosin head
Step 4: Reactivation of myosin head
ATP is hydrolyzed to __ __ ___ __. The energy released during hydrolysis reactivates the myosin head, returning it to the cocked position.
ADP and inorganic phosphate
As long as the binding sites on __ remain exposed, the cross bridge cycle will repeat.
actin
And as the cycle repeats, the thin myofilaments are pulled ___ each other and the sarcomere ___. This shortening causes the whole muscle to contract.
Toward, shortens
Cross bridge cycling ends when calcium ions are actively transported back into the ___ ___.
sarcoplasmic reticulum
___ returns to its original shape allowing tropomyosin to glide over and cover the myosin binding site on actin.
Troponin
Polypeptide strand which is a rod-shaped protein and spirals about the actin core to help stiffen and stabilize it.
Tropomyosin
Successive tropomyosin molecules are arranges end to end along the actin filaments, and in a relaxed muscle fiber, block __ __ __ __ __ so that myosin heads on the thick filaments cannot bind to thin filaments.
myosin-binding sites on actin
Troponin is a globular 3-polypeptide complex. One of its polypeptides is an inhibitory subunit that binds to
actin
Another type of troponin binds to tropomyosin and helps position it on
actin
The third type of troponin binds
calcium ions
