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?

Muscle remains contracted due to high calcium levels. Signup and view all the answers

What causes Rigor Mortis in susceptible individuals?

Persistent high levels of intracellular calcium. Signup and view all the answers

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

Active transport by Ca2+-ATPase (SERCA) Signup and view all the answers

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

45° angle Signup and view all the answers

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

The site is blocked by tropomyosin. Signup and view all the answers

How does malignant hyperthermia affect muscle contraction?

It causes muscles to remain in a state of hypercontracture. Signup and view all the answers

What role does troponin I play in muscle contraction?

Inhibits interaction between actin and myosin. Signup and view all the answers

What initiates the sequence of events leading to muscle contraction?

Action potential generation at the muscle end plate Signup and view all the answers

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

They allow sodium to enter the muscle cell, causing depolarization Signup and view all the answers

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

It undergoes a conformational change Signup and view all the answers

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

ATP is hydrolyzed to ADP and phosphate Signup and view all the answers

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

It leads to the displacement of tropomyosin Signup and view all the answers

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

The distance between the Z lines decreases Signup and view all the answers

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

Released state Signup and view all the answers

What happens during the crossbridge state of muscle contraction?

Calcium binds to troponin, allowing myosin to bind to actin Signup and view all the answers

The action potential in muscle cells travels along which structure?

T-tubules Signup and view all the answers

What primarily happens during the relaxation phase of muscle contraction?

Calcium ions are actively transported out of the muscle cell Signup and view all the answers

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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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.