L10 - Skeletal Muscle 2

Question 1

What is the crossbridge cycle?

Answer 1

the process by which the myosin head attaches to the actin binding site, and pulls itself along the actin filament

Question 2

What does calcium’s interaction with the troponin/tropomyosin complex result in?

Answer 2

exposes the actin binding sites to which myosin attaches, enabling this “ratcheting” action to occur

Question 3

How do muscle contractions occur?

Answer 3

thick filaments (myosin) attach to thin filaments (actin), and pull themselves along using mobile "heads" which bind to the long actin molecule and "walk" along it
this pulls both ends of the sarcomere closer together, hence shortening the fiber

Question 4

What is the structure of the thick filament?

Answer 4

dotted with a great number of myosin heads extending off the filament in all directions

Question 5

What is the structure of the thin filament?

Answer 5

appears as a long string of pearls (two actin polymers twisted together) surrounded by a cable of tropomyosin which blocks the myosin binding sites on the actin molecules

Question 6

What does each myosin protein consist of?

Answer 6

a long chain of intertwined proteins each with two hinged head protruding off the end

Question 7

What is the myosin head made up of?

Answer 7

a heavy chain and a light chain

Question 8

What is the heavy chain of the myosin head?

Answer 8

an enzyme known as myosin ATPase which has the capacity to hydrolyse ATP, and also contains the actin binding site

Question 9

Why is the myosin ATPase important?

Answer 9

the enzymatic activity of myosin ATPase is essential in providing energy for the crossbridge cycle from ATP

Question 10

What is tropomyosin?

Answer 10

long protein polymers which surround the actin polymer, and partially block the myosin binding sites

Question 11

What must happen in order for the myosin head to bind strongly to the actin filament?

Answer 11

tropomyosin must be shifted off the binding sites on the actin polymer, completely exposing them to the myosin head, which can then form a strong bond

Question 12

What causes tropomyosin to be shifted off the binding sites?

Answer 12

Ca2+, when released from the sarcoplasmic reticulum, binds to troponin, which has the capacity to bind calcium and shift its conformation
this conformational change to troponin causes tropomyosin to shift to the side

Question 13

What happens when tropomyosin shifts to the side?

Answer 13

the binding sites are exposed enabling myosin to bind strongly to the actin filament

Question 14

What is the power stroke?

Answer 14

when a myosin head binds to actin, and pulls it forward

Question 15

What happens after the power stroke?

Answer 15

the myosin head resets its position, binding to an actin molecule further along the chain and pulls that forward in the same manner

Question 16

What happens when more Ca2+ is released from the sarcoplasmic reticulum?

Answer 16

more actin binding sites will be available and more crossbridges will be formed between the thick and thin filaments resulting in higher contractile force of that particular fiber

Question 17

What happens when more crossbridges are present?

Answer 17

there will be more contractile force

Question 18

What is the rigor state of the myosin head?

Answer 18

when the myosin head is tightly bound to the actin molecule at at 45˚ angle

Question 19

What allows the myosin head to be released from the actin filament in its rigor state?

Answer 19

ATP binding to the myosin head

Question 20

What does the energy released by myosin ATPase go into?

Answer 20

rotating the myosin head into a 90˚ position, which, in the absence of Ca2+ will bind weakly to the actin filament, and remain there

Question 21

What initiates the power stroke?

Answer 21

binding of Ca2+ to troponin, exposure of the binding site on the actin molecule and the formation of a strong bond between myosin with the actin filament

Question 22

What is a “spark”

Answer 22

the release of a specific volume of Ca2+ from the SR

Question 23

What remains constant, regardless of the volume of cytosolic Ca2+? What does this mean if only one action potential occurs?

Answer 23

re-uptake of Ca2+ into the SR via Ca2+-ATPase
if only one action potential occurs, and hence one spark, a small amount of Ca2+ is released overall, and quickly taken back up into the SR, producing only a very low force, short-duration contraction of the muscle fiber

Question 24

What happens if the frequency of action potentials in the muscle fiber is increased? What happens when the frequency reaches a critical point?

Answer 24

Ca2+ sparks will begin to summate, leading to a build-up of cytosolic Ca2+
when a high enough frequency is achieved, the increased levels of cytosolic Ca2+ expose more crossbridge binding sites, increasing the number of active crossbridge cycles, and hence increasing the force of contraction

Question 25

When is the maximum contractile force for an entire muscle achieved?

Answer 25

when all motor units are recruited and undergoing tetanic contractions

Question 26

What is an isometric contraction?

Answer 26

a contraction which produces a force equal to the load that is opposing it (fibers do not shorten but we are still using ATP because myosin heads are slipping)

Question 27

What is a concentric contraction?

Answer 27

a contraction which produces a force greater than the load that is opposing it (fibers to shorten)

Question 28

Do fast-twitch or slow-twitch muscle fibers have a higher maximum velocity?

Answer 28

fast-twitch

Question 29

What is the relationship between force and velocity?

Answer 29

as force increases, velocity decreases

Question 30

What is an eccentric contraction?

Answer 30

a contraction which produces a force that is less than the load that is opposing it (fibers to shorten)

Question 31

Will short wide muscles or long thin muscles generate higher velocity?

Answer 31

long thin muscles

Question 32

What is the power of a muscle?

Answer 32

the product of force and velocity

P = F x V

Question 33

What is power proportional to in muscles?

Answer 33

POWER ∝ CSA x LENGTH (MUSCLE VOLUME)

Question 34

Will short wide muscles or long thin muscles generate higher maximum force?

Answer 34

short wide muscles

Question 35

Will short wide muscles or long thin muscles generate the greatest power?

Answer 35

both will generate the same power (but will do so at different velocities)

Question 36

How can power of a muscle be increased?

Answer 36

by increasing muscle size (building up to increase cross sectional area) and/or by increasing muscle velocity

Question 37

Consider the full range of a muscle contraction during a biceps curl. Given what you know about the sliding filament theory, at which stage of the curl would the biceps be able to exert the most force?

Answer 37

the middle range of muscle contraction

this is because muscle fibers are at optimal length