4) Muscle Fiber Contraction Flashcards
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)
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
What is Motor unit recruitment?
Motor unit recruitment: Activation of muscle fibers by activation of their motor neuron
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
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
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
How would one develop more force?
Recruit more motor units
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
