Molecular Basis of Muscle Contraction
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Questions and Answers

What triggers the cross-bridge cycle to start again after the myosin head is detached?

  • Binding of phosphates to myosin
  • Decrease in muscle temperature
  • Increase in calcium concentration (correct)
  • Release of tropomyosin from actin

What happens during muscle relaxation when intracellular Ca2+ concentration decreases?

  • Tropomyosin blocks the myosin-binding site (correct)
  • Ca2+ binds to troponin C
  • Myosin binds tightly to actin
  • ATP is hydrolyzed and released

What is the effect of repeated stimulation on the muscle regarding calcium concentration?

  • It triggers a decrease in the power stroke of myosin.
  • It causes a decrease in intracellular calcium levels.
  • It results in a cumulative increase in intracellular [Ca2+]. (correct)
  • It leads to a decrease in muscle metabolism.

Which of the following statements describes the state of muscle during tetanus?

<p>Muscle remains contracted due to high calcium levels. (C)</p> Signup and view all the answers

What causes Rigor Mortis in susceptible individuals?

<p>Persistent high levels of intracellular calcium. (C)</p> Signup and view all the answers

Which process is primarily responsible for the reaccumulation of Ca2+ during muscle relaxation?

<p>Active transport by Ca2+-ATPase (SERCA) (D)</p> Signup and view all the answers

What is the low-energy position of the myosin head in the cross-bridge cycle?

<p>45° angle (B)</p> Signup and view all the answers

What happens to the myosin binding site when calcium concentration decreases?

<p>The site is blocked by tropomyosin. (A)</p> Signup and view all the answers

How does malignant hyperthermia affect muscle contraction?

<p>It causes muscles to remain in a state of hypercontracture. (C)</p> Signup and view all the answers

What role does troponin I play in muscle contraction?

<p>Inhibits interaction between actin and myosin. (B)</p> Signup and view all the answers

What initiates the sequence of events leading to muscle contraction?

<p>Action potential generation at the muscle end plate (A)</p> Signup and view all the answers

What role do voltage-dependent sodium channels play in muscle contraction?

<p>They allow sodium to enter the muscle cell, causing depolarization (A)</p> Signup and view all the answers

What change occurs in the dihydropyridine receptor (DHPR) during excitation-contraction coupling?

<p>It undergoes a conformational change (A)</p> Signup and view all the answers

During the cocked state of the myosin head, what occurs?

<p>ATP is hydrolyzed to ADP and phosphate (A)</p> Signup and view all the answers

What effect does calcium binding to troponin have during muscle contraction?

<p>It leads to the displacement of tropomyosin (D)</p> Signup and view all the answers

What is the primary result of the sliding filament mechanism in muscle contraction?

<p>The distance between the Z lines decreases (B)</p> Signup and view all the answers

Which state of the myosin head involves a blocking of the actin binding site by tropomyosin?

<p>Released state (C)</p> Signup and view all the answers

What happens during the crossbridge state of muscle contraction?

<p>Calcium binds to troponin, allowing myosin to bind to actin (D)</p> Signup and view all the answers

The action potential in muscle cells travels along which structure?

<p>T-tubules (A)</p> Signup and view all the answers

What primarily happens during the relaxation phase of muscle contraction?

<p>Calcium ions are actively transported out of the muscle cell (B)</p> Signup and view all the answers

Flashcards

Excitation-Contraction Coupling

The process of a muscle contracting after being stimulated by a nerve.

Neuromuscular Junction

A specialized junction between a motor neuron and a muscle fiber where neurotransmitters are released.

Action Potential

The potential difference across the muscle cell membrane that triggers muscle contraction.

Troponin

The protein that binds to calcium and causes a conformational change in tropomyosin, exposing myosin binding sites on actin.

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Tropomyosin

A protein that blocks myosin binding sites on actin, preventing muscle contraction in the absence of calcium.

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Sliding Filament Theory

The process where thick (myosin) and thin (actin) filaments slide past each other, shortening the sarcomere.

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Sarcomere

A repeating unit of a muscle fiber that contains all the elements required for muscle contraction.

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Crossbridge Cycle

The process where myosin heads bind to actin, pull the filaments, and detach in a cyclical process.

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Calcium Release

The influx of calcium into the cytoplasm, which initiates the crossbridge cycle.

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Calcium Reuptake

The return of calcium to the sarcoplasmic reticulum, which stops the crossbridge cycle and allows muscle relaxation.

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Tetanus

The state where a muscle is unable to relax due to sustained calcium levels and continuous cross-bridge cycling.

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Calcium Release from SR

The release of calcium from the sarcoplasmic reticulum (SR) triggers muscle contraction. This is a key step in the excitation-contraction coupling process.

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ATP and Muscle Relaxation

The binding of ATP to the myosin head causes it to detach from actin, allowing the muscle to relax. ATP is essential for muscle relaxation and contraction.

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Rigor Mortis

This refers to the state after death when muscles become stiff. It occurs due to the depletion of ATP, which prevents myosin detachment from actin. Eventually, it dissipates as muscle proteins break down.

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Power Stroke

The myosin head attaches to actin and pulls it, causing the muscle to contract. This is a key event during the cross-bridge cycle.

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Tropomyosin and Muscle Relaxation

Tropomyosin blocks the myosin binding site on actin, preventing muscle contraction. Calcium binding to troponin moves tropomyosin away, allowing contraction.

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Malignant Hyperthermia

This is a rare condition where the body's metabolism increases dramatically because of uncontrolled calcium release in skeletal muscles, leading to high temperatures.

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Ryanodine Receptor Mutation

A mutation in the ryanodine receptor (RyR) leads to increased calcium release from the SR, causing muscle over-contraction and increased metabolism.

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Active Muscle Relaxation

Muscle relaxation is an active process that requires energy (ATP) to detach the myosin head from actin and pump calcium back into the SR.

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Study Notes

Molecular Basis of Muscle Contraction

  • Muscle contraction is preceded by nerve fiber excitation.
  • Excitation-contraction coupling describes events from muscle excitation to contraction.
  • Action potential stimulates intracellular calcium ion ([Ca2+]) increase.
  • Depolarization of T-tubules opens Ca2+ release channels in the sarcoplasmic reticulum (SR).
  • Increased intracellular [Ca2+] causes Ca2+ to bind to troponin C.
  • Cross-bridge cycling occurs.
  • Calcium is reaccumulated by the SR.
  • Relaxation occurs when intracellular [Ca2+] decreases.

Learning Outcomes

  • Students will explain the events between action potential generation at the endplate and muscle contraction/relaxation.
  • Students will explain the cross-bridge mechanism.

Excitation-Contraction Coupling Steps

  • Excitatory postsynaptic potential (EPSP) formation by sodium influx via the nicotinic acetylcholine receptor.
  • Depolarization of the muscle cell membrane with subsequent opening of voltage-gated sodium channels.
  • Action potential formation spreading to T-tubules.
  • Conformational change in the dihydropyridine receptor (DHPR).
  • Direct protein-protein interaction leading to ryanodine receptor (RyR1) opening.
  • Calcium influx from the sarcoplasmic reticulum.

Cross-Bridge Cycle/Walk-Along Theory

  • Actin and myosin filaments slide past each other causing shortening of the sarcomere.
  • The interaction between myosin and actin is required for contraction.
  • The cross-bridge cycle begins with ATP binding to myosin, detaching it from actin.
  • ATP hydrolysis causes the myosin head to "cock" into a high-energy position.
  • Myosin binds to actin.
  • Phosphate release triggers the power stroke, pulling actin inwards.
  • ADP release causes myosin to return to a low-energy position.

The Cross-Bridge Cycle

  • Released state: Myosin binding site blocked by tropomyosin, myosin in low-energy 45° position with bound ATP.
  • Cocked state: ATP hydrolyzed to ADP + Pi, myosin head moves to a 90° high-energy position.
  • Crossbridge state: Calcium binds to troponin, tropomyosin moves, uncovering myosin binding sites on actin; myosin head binds to actin.
  • Power stroke: Phosphate release triggers myosin head movement along actin, muscle contraction occurs, myosin head returns to low-energy 45° position.
  • Detached state: ATP binds to myosin, causing it to detach from actin; if calcium concentration remains high, cycle repeats.

Muscle Relaxation

  • Calcium is reaccumulated in the sarcoplasmic reticulum through the Ca2+-ATPase (SERCA).
  • Intracellular calcium concentration decreases.
  • Tropomyosin blocks myosin heads.
  • Muscle relaxes.

Mechanisms of Tetanus

  • Repeated action potentials lead to continuous calcium ion release resulting in sustained contraction.

Malignant Hyperthermia

  • Triggered by volatile inhalational anesthetics, or succinylcholine.
  • Ryanodine receptor mutations lead to persistent high intracellular calcium.
  • Hyper-contracture, increased muscle metabolism, and heat production occur.

Questions

  • What is rigor mortis?
  • What are three functions of ATP in muscle contraction?
  • Is muscle relaxation an active or passive process?

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Molecular Basis of Muscle Contraction PDF

Description

This quiz focuses on the molecular mechanisms involved in muscle contraction, particularly the process of excitation-contraction coupling. Students will learn to explain the sequence of events from action potential generation to muscle relaxation, including the roles of calcium ions and troponin in the contraction cycle.

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