Skeletal Muscle Contraction Mechanism

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Questions and Answers

What initiates the muscle contraction according to the sliding filament mechanism?

Crossbridge formation between myosin heads and G-actins (correct)

Depolarization of the muscle fiber

ATP hydrolysis within the muscle fiber

Release of calcium ions from the sarcoplasmic reticulum

Which statement correctly describes the two states of crossbridges?

High-force state is when the muscle is contracting and requires more energy (correct)

Low-force state means the muscle is actively shortening

High-force state occurs when muscle is relaxed

Low-force state is associated with muscle contraction

What is the primary role of myosin crossbridges in muscle contraction?

To facilitate ATP storage

To connect thick and thin filaments (correct)

To regulate calcium ion concentration

To inhibit muscle relaxation

Why is titin significant in the context of skeletal muscle?

It contributes to muscle elasticity and stability Signup and view all the answers

In skeletal muscle metabolism, what is the main function of ATP?

To provide energy for muscle contraction and relaxation Signup and view all the answers

What occurs after Myosin binds to actin during muscle contraction?

Myosin releases ADP and Pi Signup and view all the answers

In the process of muscle contraction, what role does ATP play?

ATP binds to myosin and causes it to release actin Signup and view all the answers

What is the initial state of myosin before the power stroke occurs?

Myosin is in a cocked position with ATP bound Signup and view all the answers

What specifically enables the myosin head to swivel during contraction?

Release of Pi Signup and view all the answers

What happens to actin during the power stroke?

Actin moves toward the M line Signup and view all the answers

How is energy generated for muscle contraction?

From ATP hydrolysis to ADP and Pi Signup and view all the answers

What results from the hydrolysis of ATP during muscle contraction?

Myosin is energized and cocked Signup and view all the answers

Which type of filament moves during muscle contraction?

Actin filaments Signup and view all the answers

What causes the myosin head to perform a power stroke during muscle contraction?

Dissociation of inorganic phosphate (Pi) from the myosin head Signup and view all the answers

What occurs immediately after the power stroke is completed?

ADP leaves the myosin head Signup and view all the answers

What happens when a new ATP molecule binds to the myosin head?

Myosin head detaches from the actin filament Signup and view all the answers

What characterizes the rigor mortis state in muscles?

Muscle in a contracted state due to lack of ATP Signup and view all the answers

Which part of the muscle contraction process helps pull the actin filaments toward the M line?

Rotation of the myosin head by 45 degrees Signup and view all the answers

In the context of the sliding filament theory, what is the role of tropomyosin?

Inhibits the exposure of myosin binding sites on actin Signup and view all the answers

What triggers the shortening of the sarcomere during muscle contraction?

Rotation of the myosin head Signup and view all the answers

What condition occurs if there is a lack of ATP available to the muscle?

The muscle becomes stiff and contracted Signup and view all the answers

What is the role of Ca2+ in the contraction of skeletal muscle fibers?

It binds to low-affinity sites on troponin C, causing a conformational change. Signup and view all the answers

What happens to the myosin head when ATP binds to it?

It detaches from the thin filament (actin). Signup and view all the answers

Which statement about the high-affinity sites on troponin C (TnC) is correct?

They are always occupied by Ca2+ under physiological conditions. Signup and view all the answers

During muscle contraction, which changes occur in the sarcomere structure?

The H zone disappears while the A band remains constant. Signup and view all the answers

What does the sliding filament theory describe?

How thick and thin filaments slide past each other in muscle contraction. Signup and view all the answers

What is the result of Ca2+ binding to the low-affinity sites on TnC?

It weakens TnI's attachment to actin, allowing tropomyosin to move. Signup and view all the answers

Which part of the skeletal muscle fiber is primarily involved in crossbridge formation?

Sarcomere. Signup and view all the answers

What occurs immediately after ATP is hydrolyzed on the myosin head?

The myosin head rotates to a cocked position. Signup and view all the answers

Which of the following conditions would prevent muscle contraction?

Absence of calcium ions. Signup and view all the answers

What is required for the contraction cycle of skeletal muscle fibers to repeat?

Electrical signals from the somatic motor neuron Signup and view all the answers

During relaxation of muscle, what happens to the calcium levels?

Calcium is removed from the low-affinity sites in TnC. Signup and view all the answers

Which molecule is responsible for initiating muscle contraction by binding to troponin?

Calcium (Ca2+) Signup and view all the answers

What happens to muscle fibers when the levels of Ca2+ decrease?

Tropomyosin blocks the myosin binding site on actin Signup and view all the answers

What effect does tropomyosin have in a relaxed muscle state?

It covers the myosin-binding sites on actin. Signup and view all the answers

During excitation-contraction coupling, what is the role of acetylcholine (ACh)?

To activate Na+ channels and initiate an action potential Signup and view all the answers

What happens to the I band during muscle contraction?

It shortens along with the H zone. Signup and view all the answers

What initiates the conformational change in the troponin complex?

Calcium binding to the low-affinity sites on TnC. Signup and view all the answers

What is the primary function of ATP in muscle contraction?

To power the muscle contraction cycle Signup and view all the answers

How does the transition between contracted and relaxed states occur in muscle fibers?

Through the binding and releasing of ATP and Ca2+. Signup and view all the answers

When does the 'latent period' occur in muscle contraction?

Between action potential initiation and tension development Signup and view all the answers

What substance is primarily synthesized during resting state in muscle fibers to store energy?

Creatine phosphate Signup and view all the answers

How does the concentration of sarcoplasmic Ca2+ affect muscle tension?

Higher Ca2+ levels result in more crossbridge formation Signup and view all the answers

What is the primary mechanism by which muscle fibers regenerate ATP during contraction?

Creatine phosphate and aerobic respiration Signup and view all the answers

What structural change occurs in the DHP receptor during excitation-contraction coupling?

It opens Ca2+ release channels Signup and view all the answers

What occurs after myosin heads release ADP during contraction?

The power stroke is completed Signup and view all the answers

How does relaxation of muscle fibers occur?

Ca2+ is pumped back into the sarcoplasmic reticulum Signup and view all the answers

What must happen to Ca2+ for muscle contraction to cease?

Ca2+ must be removed from the cytoplasm Signup and view all the answers

Which part of the muscle fiber is primarily involved in the propagation of action potentials?

T-tubules Signup and view all the answers

What triggers skeletal muscle contraction?

Acetylcholine binding to receptors in sarcolemma Signup and view all the answers

Which sequence of structures correctly represents the pathway of action potentials necessary for skeletal muscle contraction?

Somatic neuron, sarcolemma, T-tubules Signup and view all the answers

Which of the following correctly identifies the source of calcium release during muscle contraction?

Terminal cisterns of the sarcoplasmic reticulum Signup and view all the answers

During muscle contraction, which structure do thin filaments move toward?

Z disc Signup and view all the answers

What process energizes the myosin head for muscle contraction?

Hydrolysis of ATP Signup and view all the answers

Which of the following correctly describes the process at the neuromuscular junction?

Sodium ions enter muscle cell after acetylcholine binds Signup and view all the answers

What happens to creatine phosphate levels during periods of high ATP demand?

They decrease Signup and view all the answers

Which enzyme is responsible for transferring a phosphate group from creatine phosphate to ADP?

Creatine kinase Signup and view all the answers

Study Notes

Skeletal Muscle

Topic: Sliding filament mechanism, contraction-relaxation cycle, and muscle metabolism
Objective 1: Detail of the sliding filament mechanism for muscle contraction and the contraction-relaxation cycle
Objective 2: Detail of excitation-contraction coupling
Objective 3: Muscle metabolism with respect to ATP production

Sliding Filament Mechanism for Muscle Contraction and Contraction-Relaxation Cycle

Crossbridge formation: Myosin heads and actin filaments interlock, allowing myosin heads to bind to specific sites on actin. This is the driving force behind muscle contraction.
Two states of crossbridges: Low-force (muscle relaxing but maintaining tone) and high-force (muscle contracting). Some crossbridge formation exists during relaxation (low-force).
Troponin C (TnC): Within skeletal muscle, TnC has two high-affinity sites always occupied by Ca2+ (under physiological conditions). Two low-affinity sites bind and release Ca2+ as cytoplasmic [Ca2+] changes.
High-affinity sites (structural sites) are sites with sustained activation from constant Ca2+ binding.
Low-affinity sites located at TnC affect Tnl binding to actin. Myosin binding sites are covered by Tnl, attached to tropomyosin.
Binding of Ca2+ to low-affinity sites leads to troponin complex conformational change. This weakens Tnl attachment to actin allowing tropomyosin to move. Also, myosin-binding sites on actin are exposed, aiding cross-bridge formation.

Excitation-Contraction (EC) Coupling

4 main events in excitation-contraction coupling:
Acetylcholine (ACh) release from somatic motor neuron
ACh initiates an action potential in the muscle fiber
Action potential travels along the sarcolemma into T-tubules, triggering Ca2+ release from the sarcoplasmic reticulum (SR)
Ca2+ binds to TnC, initiating muscle contraction
Sequence of events for excitation-contraction coupling:
Somatic motor neuron releases ACh at the neuromuscular junction.
ACh binds to receptors on the motor end plate (muscle fiber).
Receptor activation of Na+ channels increases Na+ permeability.
Action potentials travel along sarcolemma.
Muscle action potential travels along sarcolemma, into t-tubules. DHP receptors change shape.
DHP receptors open RyRs (Ca2+ release channels) in SR. Ca2+ is released into cytoplasm.
Ca2+ binds to TnC, allowing actin-myosin binding.
Myosin heads execute power strokes.
Actin filaments slide toward center of sarcomeres (contraction).
To end a contraction, Ca2+ must be removed from sarcoplasm.
SR pumps Ca2+ back into lumen.
Ca2+ release from TnC: tropomyosin moves back to blocking position, blocking actin's myosin binding site (relaxation).
Myosin heads are released, elastic elements pull filaments to their relaxed position.

Muscle Metabolism

ATP: ATP is required for muscle contraction cycles. The available ATP reserve in skeletal muscle tissues sustains contraction for very few seconds
Additional ATP generation: Creatine phosphate, anaerobic respiration, aerobic respiration are the three ways for muscle fibers to generate more ATP.
Creatine Phosphate (PCr): A high energy molecule that donates a phosphate group to ADP to generate ATP. This is the initial/first source of ATP. This process is very rapid and occurs at the beginning of muscle contraction. The initial amount of ATP plus the amount released from creatine phosphate provides sufficient energy for about 15 seconds of maximum muscle contraction.
Anaerobic Cellular Respiration: Generates 2 ATP molecules per glucose. It's used during heavy exercise, where a lack of oxygen causes pyruvic acid to be converted to lactic acid. Lactic acid diffuses into the blood and can be converted to glucose in the liver. Produces enough ATP to sustain muscle contraction for 30 to 40 seconds.
Aerobic Cellular Respiration: Utilizes oxygen to produce 36 ATP molecules per glucose. This process utilizes pyruvic acid, fatty acids, and amino acids for aerobic ATP production. This sustained method produces energy for minutes to hours. This is critical during rest and moderate exercise.

Oxygen Debt

Excess oxygen required after exercise is called oxygen debt.
Oxygen debt function:
Convert lactic acid back into glycogen.
Resynthesize PCr (phosphocreatine) and ATP in muscle fibers.
Replace oxygen removed from myoglobin.
Sustain the increased chemical reactions resulting in elevated body temperature.
Sustain the extra workload of organs (e.g., heart and lungs) after stopping exercise.
Allow for extra energy consumption for tissue repair.

Sample Questions

Question 1 & Answer: Contraction of myofibrils within a muscle fiber begins when calcium is released from the sarcoplasmic reticulum.
Question 2 & Answer: The sequence of structures for action potential propagation is: somatic neuron, sarcolemma, T tubules
Question 3 & Answer: Calcium is released from the terminal cisterns of sarcoplasmic reticulum.
Question 4 & Answer: During muscle contractions, thin filaments are pulled toward the Z disc.
Question 5 & Answer: ATP hydrolysis reaction energizes the myosin head.
Question 6 & Answer: ACh is released into the neuromuscular junction, binding to receptors, stimulating the entry of sodium ions, and initiating the generation of an action potential, deep into the t-tubule.
Question 7 & Answer: Concentration of creatine phosphate is highest during exercise.
Question 8 & Answer: Creatine kinase catalyzes the transfer of a phosphate group.
Question 9 & Answer: Majority of lactic acid is converted back into glucose in the liver
Question 10 & Answer: Myoglobin has the ability to bind to oxygen.

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Description

Explore the complex processes involved in skeletal muscle contraction, focusing on the sliding filament mechanism and the contraction-relaxation cycle. Understand the role of troponin C and the excitation-contraction coupling in muscle metabolism and ATP production. This quiz will deepen your comprehension of how muscles operate and generate force.