4) Muscle Fiber Contraction Flashcards by Echo Quinton

In order for skeletal muscle cells to contract, each cell must be stimulated by: ?

In order for skeletal muscle cells to contract, each cell must be stimulated by a process of a motor neuron

Recall:
Every muscle fiber (muscle cell) is Innervated by ONE motor neuron only
A motor neuron may innervate many muscle fibers (cell)

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What is a Motor Unit?

Motor Unit: Single alpha motor neuron, its axon and all of the muscle fibers it activates
- Functional unit of the motor system: Represents smallest increment in force that can be generated

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What is Motor unit recruitment?

Motor unit recruitment: Activation of muscle fibers by activation of their motor neuron

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What is excitation-contraction coupling

Excitation-contraction coupling → sequence of events beginning with excitation of a motor neuron, resulting in contraction of muscle fibers

Action potential -> Motor neuron releases Acetylcholine -> Ca++ released from SR -> Muscle contracts

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What is the Size Principle?

Size principle:
- When stimulus travels down, it activates the smallest motor neuron first
- Smallest alpha-motor neurons recruited first
- Smaller cell volume means that the same stimulus has a greater effect on the cell’s resting membrane potential

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Type I
- Fibers per motor neuron:
- Motor neuron size:
- Motor neuron conduction velocity:

Type IIa
- Fibers per motor neuron:
- Motor neuron size:
- Motor neuron conduction velocity:

Type IIx
- Fibers per motor neuron:
- Motor neuron size:
- Motor neuron conduction velocity:

Complete the table:
Type I
- Fibers per motor neuron: </= 300
- Motor neuron size: Smallest
- Motor neuron conduction velocity: Slowest
- Small Axon has slower transmission

Type IIa
- Fibers per motor neuron: >/= 300
- Motor neuron size: Larger
- Motor neuron conduction velocity: Faster
- Fatigue resistant

Type IIx
- Fibers per motor neuron: >/=300
- Motor neuron size: Largest
- Motor neuron conduction velocity: Fastest

Type I: Slow ; Smallest ; Recruited first
Type IIa: Fast, fatigue resistant, intermediate size; Recruited second
Type IIx: Fastest, Fatigue (b/c anaerobic only), Largest; Recruited Third

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How would one develop more force?

Recruit more motor units

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Motor unit recruitment

What is the Principle of orderly recruitment?

Motor units are activated on the basis of a fixed order of fiber recruitment
- as the intensity of activity increases, the number of fibers recruited increases in the following order in an additive manner:
- type I → type IIa → type IIx

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Size principle → order of ? of motor units is directly related to the size of their ?
- Type I motor units are recruited ? in graded movement as they have smallest ?
- ? in a muscle always recruited in same order

Size principle → order of recruitment of motor units is directly related to the size of their motor neuron (ie. motor units with smaller motor neurons will be recruited first)
- Type I motor units are recruited first in graded movement as they have smallest motor neuron
- Motor units in a muscle always recruited in same order

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Role of calcium in muscle fiber

Action potential causes the release of large quantities of ? from the ? to the sarcoplasm

At rest: ? molecules block the myosin-binding sites on the actin molecules, preventing ?

Following release, calcium binds to ?
Result?

Action potential causes the release of large quantities of calcium from the sarcoplasmic reticulum (SR) to the sarcoplasm

At rest: tropomyosin molecules block the myosin-binding sites on the actin molecules, preventing binding of the myosin heads
Following release calcium binds to troponin C

Troponin C bound to calcium moves tropomyosin off the myosin-binding sites
Myosin heads can attach to the binding sites on the actin molecules

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What is the sliding filament theory?

Muscle contraction and force regulation in skeletal muscle occurs through the relative sliding of and the interaction between the contractile filaments Actin and Myosin

Actin pulled inward toward Z-line -> slide inward => sarcomere shortens

Two filament sarcomere model works well for explaining properties of isometrically and concentrically contracting muscle
- Does not explain Eccentric contractions (Titin theory)

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What types of contractions does the Sliding Filament Theory work to explain? Where does it fall short?

Two filament sarcomere model works well for explaining properties of isometrically and concentrically contracting muscle
- Does not explain Eccentric contractions (Titin theory)

Muscle contraction and force regulation in skeletal muscle occurs through the relative sliding of and the interaction between the contractile filaments Actin and Myosin

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Sliding Filament Theory:

Upon contraction, how does the sarcomere change?
A band
I band
Z lines
H zone

A-band remains constant
I band shortens
Z lines move closer together
H zone gets smaller or disappears

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In sliding filament theory:
? filaments must slide between the ?
? changes length

In sliding filament theory:
Actin filaments must slide between the myosin
Sarcomere changes length -> shortens as cross-bridge cycling occurs

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Sliding Filament Theory:
- The actin filaments are pulled towards the ? by the ?
- A small force or movement is generated at each ? (≈ 5pN or 11nM).
- Many thousands of active cross-bridges → ?

Sliding Filament Theory:
- The actin filaments are pulled towards the H zone by the cross-bridges or myosin heads.
- A small force or movement is generated at each cross-bridge (≈ 5pN or 11nM).
- Many thousands of active cross-bridges → Force of contraction

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Cross bridge Cycle:
At Rest: ? blocks myosin-head binding site on actin
Removed by?

Cross bridge Cycle:
At Rest: tropomyosin blocks myosin-head binding site on actin
Removed by: Calcium released from SR binds to Troponin C (TnC) to pull tropomyosin from actin

Cross-bridge cycle:
Step 1. ? binds to myosin head -> ? of actin-myosin complex

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released
6. Return to attached state

Cross-bridge cycle:
WHAT HAPPENS NEXT
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed (Myosin ATPase activity), causing myosin heads to return to their resting conformation (“Cocked”) Faster Myosin ATPase = Faster crossbridge cycling
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released
6. Return to attached state

Cross-bridge cycle:
WHAT HAPPENS NEXT?
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin (Rebinds Actin at new place to continue pulling Actin toward Z-line)
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released
6. Return to attached state

What happens next?

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other (myosin bends pulling actin inward)
5. ADP is released
6. Return to attached state

What happens next?

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released
6. Return to attached state

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released

WHAT happens next?

Cross-bridge cycle:
1. ATP binds to myosin head -> dissociation of actin-myosin complex (Actin and myosin separate)
2. ATP is hydrolyzed, causing myosin heads to return to their resting conformation
3. A cross-bridge forms and the myosin head binds to a new position on actin
4. P is released // Myosin heads change conformation, resulting in the power stroke // Filaments slide past each other
5. ADP is released
6. Return to attached state

Cross-bridge cycling

During the cross-bridge cycle, contractile proteins convert the energy of ? into mechanical energy (Force and/or movement)

During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into mechanical energy (Force and/or movement)

Cross-bridge cycling

During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into ?

During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into mechanical energy (Force and/or movement)

# Cross-bridge cycling What are the five steps of cross-bridge cycling? i) **?** - Reduces affinity of myosin for actin - Myosin head is released from the actin - Muscle is relaxed; no force production ii) **?** - ATP → ADP + Pi + myosin (Products of hydrolysis remain on myosin head) - Myosin head pivots into "cocked" position - Still no force produced III) **?** - Increased affinity of the myosin-ADP + Pi complex for actin - Isometric force can be produced - Still no **concentric** contraction iv) **?** - There is a conformational change in the myosin head about the hinge - Actin is pulled about 11nM along the myosin filament - **Concentric** contraction occurs v) **?** - The myosin head remains bound to the actin until another ATP binds and initiates another cycle

During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into mechanical energy (Force and/or movement) i) **ATP binding to myosin head** - Reduces affinity of myosin for actin - Myosin head is released from the actin - Muscle is relaxed; no force production ii) **ATP hydrolysis:** - ATP → ADP + Pi + myosin (Products of hydrolysis remain on myosin head) - Myosin head pivots into "cocked" position - Still no force produced III) **Myosin head binds to a new position on the actin filament** - Increased affinity of the myosin-ADP + Pi complex for actin - Isometric force can be produced - Still no **concentric** contraction iv) **Release of Pi from myosin triggers the power stroke** - There is a conformational change in the myosin head about the hinge - Actin is pulled about 11nM along the myosin filament - **Concentric** contraction occurs v) **ADP is released from the myosin head** - The myosin head remains bound to the actin until another ATP binds and initiates another cycle

# Cross-bridge cycling Describe the 5 steps of Cross-bridge cycling i) ATP binding to myosin head - Reduces affinity of **?** for **?** - **?** is released from the **?** - Muscle is **?** - Is force produced**?** ii) ATP hydrolysis: - ATP → **?** (Products of hydrolysis remain on **?**) - What happens to the Myosin head **?** - Is force produced**?** III) Myosin head binds to a new position on the actin filament - Increased affinity of the **?** for actin - **Type of?** force can be produced - Still no **?** contraction iv) Release of Pi from myosin triggers the *power stroke* - There is a conformational change in the **?** about the hinge - Actin is pulled about **?** along the **?** - **Type?** contraction occurs v) ADP is released from the myosin head - The myosin head remains bound to the actin until **?**

During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into mechanical energy (Force and/or movement) i) ATP binding to myosin head - Reduces affinity of **myosin** for **actin** - **Myosin head** is released from the **actin** - Muscle is **relaxed** - **No Force Production** ii) ATP hydrolysis: - ATP → **ADP + Pi + myosin** (Products of hydrolysis remain on **myosin head**) - Myosin head **pivots into "cocked" position** - **Still no force produced** III) Myosin head binds to a new position on the actin filament - Increased affinity of the **myosin-ADP + Pi complex** for actin - **Isometric** force can be produced - Still no **concentric** contraction iv) Release of Pi from myosin triggers the *power stroke* - There is a conformational change in the **myosin head** about the hinge - Actin is pulled about **11nM** along the **myosin filament** - **Concentric** contraction occurs v) ADP is released from the myosin head - The myosin head remains bound to the actin until **another ATP binds and initiates another cycle**

The actin-myosin complex is known as the **?**

The actin-myosin complex is known as the **rigor complex (rigor mortis)** - ATP is required to release myosin & actin (not a major regulator) - As long as **ATP** is present muscle will contract until they fatigue (ATP essential for contraction) - After death ATP stops being produced, actin-myosin remain bound

Myosin head contains binding site for **?** and **?**

Myosin head contains binding site for **ATP** and **Actin**

What enzyme splits ATP into ADP (adenosine diphosphate) and Pi (Inorganic phosphate)? Where is this enzyme located?

Enzyme **Adenosine triphosphatase (ATPase)** located on the **Myosin head** splits ATP into ADP and Pi

How do you stop muscle contraction?

1) Stop Action Potential 2) Remove Calcium from sarcoplasm - Remove TnC-Calcium interaction = Tropomyosin moves back to block myosin binding site on actin

Two transport proteins that pump calcium across the plasma membrane out of the cell? ie pump Ca++ from the muscle cytosol into extracellular space

Na+/Ca++ exchanger (NCX) in sarcolemma (PM) - Requires ATP Surface Ca++ pump (PMCA) - Requires ATP ## Footnote What pumps Ca++ into the Sarcoplasmic Reticulum (SR)? SERCA - Sarcoplasmic reticulum Ca++-ATPase (SERCA) - Returns majority of Ca++ to the SR

What pumps Ca++ into the Sarcoplasmic Reticulum (SR)?

SERCA - Sarcoplasmic reticulum Ca++-ATPase (SERCA) - Returns majority of Ca++ to the SR ## Footnote Works with **Calsequestrin** - binds Ca++ in the SR - Reduce apparent [Ca++] to allow Ca++ to be concentrated within the SR

What protein buffers increased Calcium levels in the SR?

Calcium binding protein: **Calsequestrin** - binds Ca++ in the SR - Reduce apparent [Ca++] to allow Ca++ to be concentrated within the SR

What is the Role of: **Calsequestrin**

Calcium binding protein: **Calsequestrin** - binds Ca++ in the SR - Reduce apparent [Ca++] to allow Ca++ to be concentrated within the SR - As **SERCA** pumps Ca++ into SR, Calsequestrin binds so that more calcium can be added