Lecture 2 - Lucas

Question 1

when the cross bridge cycle occurs, ——- occurs

Answer 1

contraction

Question 2

the cross bridge cycle is the ____

Answer 2

interaction between myosin heads (thick filaments) and actin (thin filaments) during muscle contraction.

Question 3

step 1 of cross bridge cycle

Answer 3

ATP is bound to the myosin head, in its bent neck position, not bound to actin.

Actin binding sites are blocked by tropomyosin due to the absence of calcium (Ca²⁺)

ATP binding causes the release of myosin from actin, resetting it for the next cycle so it doesnt prematurely bind to actin.

Question 4

step 2 of crossbridge cycle

Answer 4

ATP is hydrolyzed into ADP and Pi by the myosin ATPase (on myosin head).
The hydrolysis charges the spring in the myosin neck, putting the head into a high-energy, cocked position.

its energy is stored in the spring and is ready to bind to actin

Question 5

step 3 of crossbridge cycle

Answer 5

Myosin in its charged, cocked position weakly binds to actin (no bent neck).

The initial attachment is weak and reversible because Pi and ADP are still bound to the myosin head. no force generated yet

calcium binds to troponin which moves tropomyosin away from myosin binding site

Question 6

step 4 of cross bridge cycle

Answer 6

**Release of Pi gives myosin head bent neck (STRONG BOND)

Energy release from the spring causes the myosin head to snap back to its bent position.
The pull on the actin filament towards M line results in filament sliding, generating force and a power stroke.

sacromere shortens and generates force

Question 7

step 5 of cross bridge cycle

Answer 7

After the power stroke, ADP is released from the myosin head.

Key Action:
The myosin head remains strongly bound to actin in the absence of a new ATP molecule.

Importance:
This is the rigor state if no ATP is available, where myosin and actin are tightly locked together.

Question 8

step 6 of cross bridge cycle

Answer 8

A new ATP molecule binds to the myosin head and release of the strong bond, which resets the cycle

Question 9

Between Step 4 and Step 6, the actin-myosin bond is _____ and unbreakable unless ATP is present.

Answer 9

strong

Question 10

there is rigor if a _____ bond persists

Answer 10

strong

Question 11

why is calcium required

Answer 11

to reveal actin binding site

Question 12

Sliding Filament Mechanism

Contraction leads to the sliding ______ , which ________ the sarcomere.

Answer 12

actin filaments past myosin, which sortens sacromere

Question 13

Key Changes in Sarcomere during contraction:
Z-lines: ____
A-band: _____
I-band: _____
H-zone: ______

Answer 13

z lines: move closer together to each other, towards myosin (to the right)
A band: remains same length
I band: shortens (to the right)
H zone: narrows or dissapears

Question 14

does contraction always mean shortening

Answer 14

NO! Contraction can happen without shortening

Question 15

what are other types of contraction that dont lead to shortening

Answer 15

lengthening or isometric (stays the same), it really depends b/w the force by the muscle and the load on it

Question 16

contraction refers to the ____ activity

Answer 16

cross bridge activtiy

Question 17

what is calcium’s role with tropomyosin

Answer 17

1) anchor binds troponin to g actin

2) calcium binds to troponin, inducing a conformational change.

3) troponin binds to TM, which moves tropomyosin away from the actin binding sites,

allowing myosin heads to attach.

Question 18

what happens when theres no ATP

Answer 18

myosin heads cannot detach from actin, cannot reset for next cycle whcih causes rigor mortis

Question 19

what happens when theres no calcium

Answer 19

Tropomyosin blocks myosin binding sites and won’t move out the way, preventing cross-bridge formation.

Question 20

sacromere tension length relationship

contraction occurs in all _ in a fiber _________

Answer 20

sacromeres, simultaenously

Question 21

Sarcomere Length-Tension Relationship

tell me the different scenarios of scaromere length

Answer 21

short sacromere length (z disc to z disc): Filaments overlap too much, titin is right against z discs, no space for myosin to pull z discs closer or bind to actin. Zero tension 0/20

slight longer length: still some blockage, not maximal contraction 10/20

optimal length: stretched out titin, maximal force, actin is not bumped into each other at all, no overlap 20/20

plateau: even more stretched out, still maximal force, not increased force 20/20, passive tension increase (not active)

too long: myosin isnt touching actin anymore, zero tension, not enough overlap between myosin and actin 0/20

Question 22

the force of contraction is proprtional to

Answer 22

number of cross bridges formed, if no more crossbridges are formed, then it doesnt matter how stretched the sacromere is, there wont be any more force

Question 23

the ascending limb deals with what type of overlap of sacromeres

Answer 23

too short to slightly longer to optimal

Question 24

the plateau portion of sacromere length is for

Answer 24

optimal to slightly longer

Question 25

the descending limb portion of sacromere deals with what scenario

Answer 25

too long, myosin isnt even touching actin at this point

Question 26

what happens to g actin and myosin vs thin filament and actin if theres no calcium or atp

Answer 26

Pure g-actin & myosin:

Binding: Yes, weak binding occurs.
Since there is no tropomyosin or troponin, the binding site on actin is fully exposed.
Without ATP, the bond remains weak and reversible, as the strong binding requires the release of Pi during ATP hydrolysis.

Thin filaments & myosin:

Binding: No binding occurs.
Tropomyosin blocks the myosin-binding sites on actin.
Without Ca²⁺, tropomyosin cannot shift to expose these sites, preventing any interaction between actin and myosin.

Question 27

in the shortes sacromere length, what type of compression is there bc of all the overlap

Answer 27

A band compression

Question 28

when the sacromere is too long, is there any touching with the myosin and actin

Answer 28

none at all

Question 29

during the optimal state, is there an H zone

Answer 29

no H zone!!! its the most optimal

Question 30

in the worst steps of sacromere length (A band compression and actin overlap), what happens to calcium

Answer 30

• TN-C has low affinity for calcium
• less calcium available

Question 31

optimal sacromere length in numbers

Answer 31

2.1 to 2.2 micrometers