Module 4: Lecture 2

Question 1

what is the A band based on?

Answer 1

the thick filaments and how far they travel
- your thin filaments are within it as well

Question 2

why is the I Band called the light band?

Answer 2

because there is no thick filaments in it so light is allowed through it

Question 3

does muscle contraction always cause a ‘shortening’ of the muscle?

Answer 3

no

Question 4

what does contraction refer to?

Answer 4

an active state in which your actin and myosin are forming cross bridges and generating force

Question 5

what is the sliding filament mechanism caused by?

Answer 5

cross-bridge cycling

Question 6

why can a cross bridge form?

Answer 6

because of the overlap of the filaments and the binding sites

Question 7

what is the sliding filament mechanism a result of?

Answer 7

the action potential stimulating skeletal, smooth and cardiac muscles

Question 8

what is linked to internal changes in calcium release and contraction?

Answer 8

changes in the membrane potential of muscles

Question 9

what is the signal that allows your action and myosin cross bridges to begin to form and actually generate force?

Answer 9

when cytosolic calcium increases in a resting myofibre

Question 10

what is the only purpose of the action potential in the muscle fiber?

Answer 10

to stimulate the release of calcium

Question 11

are thin filaments a single polypeptide chain?

Answer 11

no, it is a quaternary structure, meaning it is made up of multiple different protein chains

Question 12

what is the single polypeptide chain component of actin called?

Answer 12

globular actin (G-actin)

Question 13

what is a single polypeptide chain?

Answer 13

a long chain of amino acids that fold back in on itself

Question 14

how can ‘F-actin’ be formed?

Answer 14

if you polymerize a whole bunch of g-actins into a long chain

Question 15

what is formed from two F-actins intertwining?

Answer 15

an actin helix
- the main structure of thin filaments

Question 16

is there a binding site on every actin molecule?

Answer 16

yes, it is where the myosin head can physically attach to the actin molecule and where the cross bridge is formed

Question 17

how do we make sure that myosin does not attach to the actin to form a cross bridge binding sites at rest?

Answer 17

we physically block the binding sites with tropomyosin that intertwine on the actin helix

Question 18

what is the sensor on the actin for when the muscle is activated or not?

Answer 18

troponin

Question 19

what are the regulatory molecules?

Answer 19

troponin and tropomyosin

Question 20

what does troponin regulate?

Answer 20

whether tropomyosin is blocking or not blocking that active site

Question 21

how many actin molecules can one single tropomyosin molecule block?

Answer 21

seven actin molecules
- all regulated by one single troponin protein

Question 22

each tropomyosin molecule is held in such _________ binding position by the _________ molecule.

Answer 22

1. inhibitory
2. troponin

Question 23

what are the three troponin subunits?

Answer 23

1. T
2. I
3. C

Question 24

which troponin subunit is physically interacting with the tropomyosin molecule itself?

Answer 24

troponin T
- this is what is holding it in close proximity to the actual tropomyosin molecule

Question 25

which troponin subunit keeps it in the inhibitory state and ensures that the binding sites are blocked?

Answer 25

troponin I

Question 26

when we want muscle contraction, which subunit of troponin do we have a conformational change in?

Answer 26

troponin I subunit that removes inhibitory blocking

Question 27

which troponin subunit is responsible for sensing when a muscle contraction should be occuring?

Answer 27

troponin C - calcium binds to troponin C and induces the conformational change to remove the inhibition

Question 28

what is our thick filament made up of?

Answer 28

a whole bunch of myosin molecules with the head regions in both directions away from M line

Question 29

what is a single myosin molecule composed of?

Answer 29

two large polypeptide heavy chains that are intertwined with their tail regions in this helix form and four smaller light chains - they combine to generate a large molecule with two globular heads and one long tail - two large polypeptide heavy chains and four smaller light chains

Question 30

where on a myosin molecule is the site for where a cross-bridge with actin forms?

Answer 30

globular heads

Question 31

what are the two binding sites on a single large polypeptide heavy chain globular head?

Answer 31

1. actin binding site 2. ATP binding site

Question 32

the light chains on the polypeptide heavy chains are for what?

Answer 32

they are specialized proteins that stabilize the head and also act as a site of regulation - critical for stability - they are meant to stabilize the myosin head and the interactions and regulate its activity

Question 33

what are the two light chains on the heavy chains called?

Answer 33

one is called the essential light chain and the other is called the regulatory light chain

Question 34

what energy is used for generating your cross bridges/contraction?

Answer 34

ATPase

Question 35

why are the myosin molecules in the two ends of each thick filament oriented in opposite directions?

Answer 35

so that the power stroke of the cross-bridges move the thin filaments toward the centre of the sarcomere

Question 36

what happens during the power stroke?

Answer 36

the head of the myosin physically bends and pulls the myosin head inward

Question 37

what is the sequence of events that allow for the cross-bridge cycle?

Answer 37

1. binding between the myosin head and the thin filament 2. movement of the cross-bridge through power stroke, cross bridge bends, pulling thin myofilament inward 3. detachment of the cross-bridge: detach myosin from the thin filament 4. re-energize the myosin head so it can re-attach to a thin filament to repeat the cycle

Question 38

every time a myosin undergoes power stroke, it is pulling ____________________________________.

Answer 38

that actin filament closer and closer towards the M line

Question 39

why do we have two heads on a single myosin molecule?

Answer 39

to generate more force than a single head

Question 40

what is the A Band defined as?

Answer 40

the width of your thick filaments

Question 41

from rest to contracted sarcomere, what happens to the A-band, I-band, H-zone, and overall sarcomere length?

Answer 41

A-band: no change I-band: shorten H-zone: shorten Sarcomere: shorten