Section 2

Question 1

Explain what is meant by excitation-contraction coupling

Question 2

Describe the sliding filament theory.

Question 3

Describe what happens during the power stroke.

Question 4

Explain the role of calcium in skeletal muscle contraction.

Question 5

Cross bridges form between _______ and _________

Answer 5

Cross bridges form between the myosin HEAD and actin

Question 6

Describe what occurs during contraction.

Answer 6

During contraction, the thin filaments move inwards over the thick filaments.

When this happens, the z-lines move closer together. This occurs simultaneously along the entire fibre and all sarcomeres shorten to the same degree.

Neither the length of the thin filaments or thick filaments themselves changes, just the degree of overlap. As a result, the whole muscle shortens in what is called a
concentric contraction.

Question 7

What is the power stroke?

Answer 7

the interaction between myosin and actin that leads to a shortening of the sarcomere.

Question 8

What are the steps of the cross-bridge cycle?

Answer 8

1. Binding: Myosin cross-bridge binds to actin molecule
2. Power stroke: The myosin head bends, pulling thin myofilament inward.
3. Chevron
4. Detachment: Cross-bridge detaches at end of power stroke and returns to original
   conformation.
5. Binding: Cross-bridge binds to more distal actin molecule; cycle repeats.

Question 9

What is the result of a power stroke in muscle contraction?

Answer 9

The result of a power stroke is the actin molecules being pulled closer to the center of myosin molecules.

Question 10

How does the actin movement change in each successive cross-bridge cycle?

Answer 10

On each successive cross-bridge cycle, the actin is pulled even closer to the center of the myosin molecules.

Question 11

How many actin molecules surround each myosin molecule on each end during a power stroke?

Answer 11

Each myosin molecule is surrounded by six actin molecules on each end, all of which are pulled inward simultaneously in muscular contraction.

Question 12

Are all cross-bridges actively pulling actin at the same time during muscle contraction?

Answer 12

No, at any given time, not all cross-bridges are actively pulling actin. Some are holding the actin in position while others prepare for the next power stroke.

Question 13

How many heads does each myosin molecule have, and how do they act in muscle contraction?

Answer 13

Each myosin molecule has two heads, which act independently. Only one of them may be attached to actin at any given time.

Question 14

What is the energy source for the power stroke in muscle contraction?

Answer 14

The energy for the power stroke comes from excitation-contraction coupling, which converts an electrical signal into a muscle contraction.

Question 15

What is excitation-contraction coupling in muscle contraction?

Answer 15

Excitation-contraction coupling is the process of converting an electrical signal (initiated by acetylcholine) into an actual muscle contraction.

Question 16

What role does acetylcholine (ACh) play in muscle contraction?

Answer 16

Acetylcholine is released into the neuromuscular junction, causing permeability changes and initiating an action potential that propagates across the muscle membrane, ultimately leading to muscle contraction.

Question 17

What are the two membrane structures in skeletal muscle cells that transmit the signal for muscle contraction?

Answer 17

The two membrane structures are the Sarcoplasmic Reticulum (SR) and the T-tubules.

Question 18

What is the function of the Sarcoplasmic Reticulum (SR) in muscle cells?

Answer 18

The SR is a membranous structure that serves as a storage site for calcium ions (Ca2+). It is involved in regulating calcium levels for muscle contraction.

Question 19

What are Transverse tubules (T-tubules), and where are they located in muscle fibers?

Answer 19

T-tubules are invaginations of the plasma membrane that run perpendicular to the muscle fibers. They are located at the junction of A and I bands and play a role in transmitting the electrical signal for muscle contraction.

Question 20

What happens when the plasma membrane depolarizes in muscle cells?

Answer 20

The wave of depolarization spreads across the plasma membrane and goes deeper into the cells through the T-tubules, ultimately transmitting the electrical signal for muscle contraction

Question 21

What is the role of T-tubules in muscle cells?

Answer 21

T-tubules are invaginations of the plasma membrane that transmit the electrical signal for muscle contraction from the surface of the muscle fiber to deeper regions of the muscle cell.

Question 22

What are dihydropyridine receptors, and where are they located in muscle cells?

Answer 22

Dihydropyridine receptors are voltage-sensors located on the surface of T-tubules. They sense the wave of depolarization as it travels down the T-tubules.

Question 23

What are ryanodine receptors, and where are they located in muscle cells?

Answer 23

Ryanodine receptors are located on the sarcoplasmic reticulum (SR), opposite to the dihydropyridine receptors on the T-tubules. They are a type of calcium (Ca2+) channel.

Question 24

How does the wave of excitation influence the ryanodine receptors of the SR?

Answer 24

he dihydropyridine receptors on the T-tubules sense the wave of depolarization and influence the ryanodine receptors of the SR to undergo a conformational change, leading to the opening of the ryanodine receptors and the release of calcium (Ca2+) into the cytoplasm.

Question 25

Why is the release of calcium (Ca2+) important in muscle contraction?

Answer 25

The release of calcium (Ca2+) is crucial because it is the primary trigger that allows skeletal muscles to contract by enabling cross-bridge formation between myosin and actin.

Question 26

Why can’t muscle contraction occur in a relaxed muscle?

Answer 26

In a relaxed muscle, contraction cannot take place because tropomyosin and troponin are positioned to prevent cross-bridge formation by blocking the myosin binding site on the actin molecules. Calcium release is necessary to change the conformation of these regulatory proteins and initiate muscle contraction.

Question 27

What is the cause of muscle relaxation?

Answer 27

Muscle relaxation is caused by decreased nerve activity at the neuromuscular junction, leading to the cessation of acetylcholine (Ach) release.

Question 28

How does acetylcholinesterase contribute to muscle relaxation?

Answer 28

Acetylcholinesterase is an enzyme that rapidly hydrolyzes acetylcholine (Ach), removing any remaining Ach from the neuromuscular junction. This action stops the generation of action potentials in the skeletal muscle fiber.

Question 29

What happens when action potentials are no longer generated in the skeletal muscle fiber during muscle relaxation?

Answer 29

When action potentials cease, the sarcoplasmic reticulum (SR) stops releasing stored calcium (Ca2+), and Ca2+-ATPase pumps on the SR actively pump calcium back into the SR for future use.

Question 30

What is the consequence of calcium removal from the cytosol during muscle relaxation?

Answer 30

Without calcium (Ca2+), the troponin-tropomyosin complex can once again cover the actin molecules, preventing cross-bridge formation. This results in muscle lengthening and relaxation.

Question 31

Can you define acetylcholinesterase?

Answer 31

Acetylcholinesterase is an enzyme responsible for the rapid hydrolysis (breakdown) of acetylcholine, a neurotransmitter. It plays a crucial role in terminating the action of acetylcholine at the neuromuscular junction and other synapses.

Question 32

Action potential generated in response to binding of acetylcholine and subsequent end-plate potential is propagated across surface membrane and down ___ of muscle cell

Answer 32

T-tubules

Question 33

Action potential in T-tubule triggers ___ release from sarcoplasmic reticulum.

Answer 33

calcium

Question 34

Calcium ions released from lateral sacs bind to ___ on actin filaments; leads to ___ being physically
moved aside to uncover cross-bridge binding sites on actin.

Answer 34

Calcium ions released from lateral sacs bind to troponin on actin filaments; leads to tropomyosin
being physically moved aside to uncover cross-bridge binding sites on actin.

Question 35

________ cross-bridges attach to actin and bend, pulling actin filaments toward the centre of the
sarcomere; powered by energy provided by ________.

Answer 35

Myosin cross-bridges attach to actin and bend, pulling actin filaments toward the centre of the sarcomere; powered by energy provided by ATP

Question 36

Calcium is actively taken up by the ___ when there is no longer an action potential

Answer 36

sarcoplasmic reticulum

Question 37

With ___no longer bound to ___, ___ slips back to its blocking position over binding sites on
actin; contraction ends; actin slides back into its original resting position

Answer 37

With CALCIUM no longer bound to TROPONIN, TROPOMYOSIN slips back to its blocking position over binding sites on actin; contraction ends; actin slides back into its original resting position

Question 38

What allows for ATP-powered cross-bridge cycling during muscle contraction?

Answer 38

The exposure of actin binding sites on the actin filaments allows for ATP-powered cross-bridge cycling.

Question 39

What happens when ATP binds to the ATPase site on myosin?

Answer 39

When ATP binds to the ATPase site on myosin, it splits into ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing stored energy that is transferred to the myosin cross-bridge.

Question 40

How is the myosin cross-bridge prepared for the power stroke?

Answer 40

The myosin cross-bridge is “cocked” and ready for the power stroke after ATP splits into ADP and Pi, releasing energy.

Question 41

What happens in the presence of calcium ions (Ca2+) during muscle contraction?

Answer 41

In the presence of Ca2+, the troponin-tropomyosin complex exposes the actin molecules, allowing the myosin cross-bridge to bind with the actin and initiate the power stroke.

Question 42

What occurs when there is an absence of calcium ions (Ca2+) during muscle contraction?

Answer 42

In the absence of calcium ions, the myosin cross-bridge remains “cocked” and contraction will not occur.

Question 43

What happens during the power stroke of muscle contraction?

Answer 43

During the power stroke, inorganic phosphate (Pi) is released, and when the power stroke is complete, ADP is also released, but the cross-bridge remains bound to the actin.

Question 44

What causes the cross-bridge to detach from actin during muscle contraction?

Answer 44

Binding of a new ATP molecule to the myosin cross-bridge causes it to detach from actin and return to its “un-cocked” state.

Question 45

What is rigor mortis, and why does it occur after death?

Answer 45

Rigor mortis is the stiffening of muscles after death. It occurs because the concentration of calcium ions (Ca2+) increases in cells, causing cross-bridges to remain “cocked,” leading to muscle contraction. This continues until ATP is depleted, after which the muscles remain contracted. Rigor mortis eventually resolves when muscle proteins decay.

Question 46

What is the latent period in muscle contraction, and what happens during this period?

Answer 46

The latent period in muscle contraction is a brief delay before contraction starts. During this period, cross-bridge cycling begins.

Question 47

How long does a single skeletal muscle action potential typically last?

Answer 47

A single skeletal muscle action potential lasts only 1-2 milliseconds (msec).

Question 48

When does peak tension occur in muscle contraction, and what is it?

Answer 48

Peak tension, the greatest tension generated while still exerting force against an external load, usually occurs between 40-120 msec after cross-bridge cycling begins. The exact time varies depending on factors like muscle fiber type and location in the body.

Question 49

What is the relaxation time in muscle contraction, and what happens during this period?

Answer 49

The relaxation time is the duration of time it takes for all calcium (Ca2+) to be removed from the muscle. It generally takes 50-200 msec from peak tension. During this time, the muscle relaxes.

Question 50

What is a muscle twitch, and what causes it?

Answer 50

A muscle twitch is the minimum contraction of a whole muscle caused by a single action potential in the nerve exciting the muscle.

Question 51

Why does the temporal relationship between the action potential and the mechanical response become important in muscle mechanics?

Answer 51

Understanding the temporal relationship is crucial for studying muscle mechanics and how muscles contract efficiently. The timing of events during muscle contraction influences the overall function of the muscle.