Muscle Contraction: Actin-Myosin Complex Formation

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Questions and Answers

What initiates the activation of MLCK in smooth muscle contraction?

Dephosphorylation of myosin heads

Hyperpolarization of the membrane

Increased intracellular calcium binding with Calmodulin (correct)

Opening of potassium channels

What happens to myosin heads when MLCP is activated?

They interact with Calmodulin

They are inactivated by dephosphorylation (correct)

They bind with actin to promote contraction

They become phosphorylated

Which event directly follows the opening of Na+ channels if the threshold is reached?

Membrane begins to repolarize

Membrane depolarizes rapidly (correct)

Myosin heads become activated

K+ channels open and K+ starts to leave

In the sequence of smooth muscle contraction, which statement accurately describes the signaling pathway?

Increased intracellular calcium levels trigger smooth muscle contraction. Signup and view all the answers

What is the result of potassium channels remaining open during the action potential phase?

Hyperpolarization of the membrane occurs. Signup and view all the answers

What is the initial biochemical action taken by the head of myosin during the formation of the actin-myosin complex?

The head of myosin hydrolyzes ATP. Signup and view all the answers

Which scenario describes what happens when a small load is placed on the muscle during contraction?

There is a sliding of actin over myosin, resulting in muscle shortening. Signup and view all the answers

What triggers the detachment of the myosin head from the actin filament?

Release of ADP and Pi from the myosin head. Signup and view all the answers

What effect does a large load have on the interaction between actin and myosin?

Flexion of the myosin head occurs without muscle shortening. Signup and view all the answers

What is the role of ATP in the reactivation of the myosin head?

It hydrolyzes into ADP and Pi, energizing the myosin head. Signup and view all the answers

What happens to the membrane potential immediately after K+ channels open at the peak of action potential?

The membrane hyperpolarizes as K+ leaves the cell. Signup and view all the answers

What is the role of the Na+/K+ transporter after the action potential occurs?

It restores the resting potential of the membrane. Signup and view all the answers

Which channel type is primarily responsible for the regeneration of the action potential along the axon?

Voltage-gated Na+ channels Signup and view all the answers

What state does the membrane enter when it becomes hyperpolarized after an action potential?

It enters a refractory period. Signup and view all the answers

Which of the following molecules activates myosin heads in smooth muscle contraction?

MLCK Signup and view all the answers

What results from the opening of K+ channels during the action potential?

Decreased membrane potential Signup and view all the answers

What occurs if the threshold of excitation is not reached?

No action potential will fire. Signup and view all the answers

Which component in smooth muscle contraction inhibits the myosin head activity?

MLCP (myosin-light chain phosphatase) Signup and view all the answers

What initiates the propagation of an action potential along an excitable membrane?

Rapid depolarization past the threshold of excitation Signup and view all the answers

During the refractory period, what is the primary ion that exits the cell?

Potassium ions Signup and view all the answers

What is the normal resting membrane potential (RMP) of a nerve cell?

-70mV Signup and view all the answers

What happens to sodium ion channels during the repolarization phase of an action potential?

They close Signup and view all the answers

What leads to the depolarization of the axon membrane during an action potential?

Sodium influx Signup and view all the answers

Which of the following best describes the function of action potentials?

They convey information rapidly along the axon Signup and view all the answers

What occurs immediately after a stimulus causes the membrane to depolarize?

Opening of sodium ion channels Signup and view all the answers

What ensures that a cell cannot fire again during the refractory period?

Hyperpolarization caused by potassium leaving Signup and view all the answers

What initiates the depolarization process in a neuron?

Opening of Na+ channels Signup and view all the answers

Which condition must be met for an action potential to propagate along the membrane?

Threshold potential must be reached Signup and view all the answers

During which stage of the action potential do K+ channels open and Na+ channels close?

Repolarization phase Signup and view all the answers

What primarily influences the resting potential of a neuron?

Greater leakage of K+ than Na+ through leak channels Signup and view all the answers

What happens immediately after the peak of the action potential?

K+ ions leave the cell and Na+ channels close Signup and view all the answers

In what scenario might the depolarization process not travel down a neuron's membrane?

If threshold potential is not reached Signup and view all the answers

What is the refractory period in a neuron?

State where no action potential can occur Signup and view all the answers

What role do leak channels play in maintaining resting potential?

They provide access for K+ ions to exit more readily. Signup and view all the answers

Study Notes

Formation of Actin-Myosin Complex

Myosin head binds with ATP, initiating the contraction cycle.
ATPase activity hydrolyzes ATP into ADP and inorganic phosphate (Pi), releasing energy.
The energized myosin head moves perpendicularly towards actin, leading to attachment.

Effects of Power Stroke

With a small load, actin slides over myosin, resulting in muscle shortening.
With a large load, myosin head flexes and stretches the elastic neck without actin sliding.

Detachment of Myosin Head

Release of ADP and Pi allows new ATP to bind to the myosin head.
The new ATP has a low affinity for actin, causing the myosin head to detach from actin.

Reactivation of Myosin Head

The bound ATP splits into ADP and Pi again, energizing the myosin head.
The reactivated head moves towards actin filaments, ready for another cycle of contraction.

Skeletal and Smooth Muscle Comparison

Skeletal muscle has a resting membrane potential of approximately -70mV, influenced by ion concentration differences.
Action potentials involve rapid depolarization followed by repolarization.
Depolarization reaches a threshold, opening Na+ channels and allowing Na+ influx.

Action Potential Mechanism

Potassium ion conductance is significantly higher than sodium during the resting state.
A stimulus leads to depolarization of target cells toward the threshold potential.
If the threshold is reached, Na+ channels open and rapid depolarization occurs.
K+ channels open at peak action potential, leading to hyperpolarization.
The cell resets its membrane potential as K+ channels close and Na+/K+ transporter restores resting potential.

Key Players in Smooth Muscle Contraction

Calmodulin binds with intracellular calcium (Ca²+) to activate myosin light chain kinase (MLCK).
MLCK phosphorylates and activates myosin heads, promoting contraction.
Myosin light chain phosphatase (MLCP) dephosphorylates and inactivates myosin heads, leading to relaxation.

Sequence of Events in Action Potential

A sensory stimulus depolarizes the membrane toward the threshold.
After repolarization, hyperpolarization occurs, placing the membrane in a refractory state.
Restoration to resting potential involves closure of K+ channels and transporters.

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Description

This quiz explores the intricate process of actin-myosin complex formation, detailing how the head of myosin binds with ATP and the subsequent power stroke mechanism. Understanding these processes is crucial for comprehending muscle contraction and its physiological implications.