Muscle Physiology: EC Coupling Overview
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Questions and Answers

What is the primary function of calmodulin in smooth muscle contraction?

  • It acts as a second messenger to increase ATP production.
  • It directly initiates the action potential in muscle fibers.
  • It facilitates the release of calcium from the extracellular fluid.
  • It binds to calcium and activates myosin-light-chain-kinase. (correct)

What characterizes malignant hyperthermia?

  • Decreased heart rate and muscle contractions.
  • Absence of gap junctions in cardiac muscles.
  • Mutation in RyR leading to increased body temperature and muscle rigidity. (correct)
  • Improved performance of muscle fibers during exercise.

Which type of channels are involved in calcium entry for smooth muscle contraction?

  • Voltage-gated, ligand-gated, second messenger-gated, and mechanically-gated channels. (correct)
  • Only ligand-gated channels.
  • Only mechanically-gated channels.
  • Calcium channels that only respond to electrical stimuli.

How do cardiac muscle cells effectively communicate with one another?

<p>Using gap junctions that form an electrical syncytium. (A)</p> Signup and view all the answers

What distinguishes the T tubules in cardiac muscles from those in skeletal muscles?

<p>Cardiac T tubules have a larger diameter and are well developed. (D)</p> Signup and view all the answers

What initiates the contraction of a muscle cell during excitation-contraction coupling?

<p>Release of calcium ions from the sarcoplasmic reticulum (A)</p> Signup and view all the answers

What structure is responsible for transmitting the action potential deep into the muscle fiber?

<p>T Tubules (B)</p> Signup and view all the answers

Which process follows the depolarization of the T-tubule membrane?

<p>Opening of calcium channels in the sarcoplasmic reticulum (D)</p> Signup and view all the answers

What is the role of sodium ions during the excitation phase of muscle contraction?

<p>They are responsible for the formation of the action potential. (D)</p> Signup and view all the answers

How does excitation-contraction coupling primarily link excitation to contraction?

<p>By linking depolarization of the membrane to calcium release. (B)</p> Signup and view all the answers

What anatomical feature facilitates the connection between the T-tubule and the sarcoplasmic reticulum?

<p>Triad (A)</p> Signup and view all the answers

What happens during muscle fiber relaxation?

<p>Calcium ions are reabsorbed into the sarcoplasmic reticulum. (D)</p> Signup and view all the answers

Which type of muscle is characterized by a different mechanism of excitation-contraction coupling compared to skeletal muscle?

<p>Cardiac muscle (D)</p> Signup and view all the answers

What does the DHPR (dihydropyridine receptor) primarily function as in muscle contraction?

<p>A voltage sensor (D)</p> Signup and view all the answers

What triggers the release of calcium ions during muscle contraction?

<p>Conformational change of DHPR (A)</p> Signup and view all the answers

What role does troponin C play in muscle contraction?

<p>It binds to calcium and initiates contraction (B)</p> Signup and view all the answers

What effect does the calcium-troponin complex have on the tropomyosin protein?

<p>It induces a change that exposes myosin binding sites (D)</p> Signup and view all the answers

What process occurs to shorten the sarcomere during muscle contraction?

<p>Release of ADP and inorganic phosphate (A)</p> Signup and view all the answers

Which pump is responsible for muscle relaxation by moving calcium back into the sarcoplasmic reticulum?

<p>SERCA pump (D)</p> Signup and view all the answers

In an excited muscle, how does the presence of calcium ions in the cytosol affect muscle contraction?

<p>It promotes muscle contraction by allowing myosin to bind to actin (C)</p> Signup and view all the answers

What is the condition of calcium levels in the cytosol during a resting muscle state?

<p>Low calcium concentration (C)</p> Signup and view all the answers

Flashcards

Excitation-Contraction Coupling (EC Coupling)

The process linking muscle excitation (depolarization) to calcium release, triggering contraction.

T-tubules

Invaginations of the sarcolemma (muscle cell membrane) that carry the action potential deep into the muscle fiber.

Calcium Release Mechanism

Depolarization of the T-tubule membrane triggers the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction.

Action Potential

A rapid change in the membrane potential of a nerve or muscle cell, which leads to the activation of the processes needed for contraction and other functions.

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Sarcoplasmic Reticulum (SR)

A specialized endoplasmic reticulum in muscle cells that stores calcium ions and releases them in response to an action potential.

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Neuromuscular Junction

The synapse between a motor neuron and a muscle fiber, where the nerve impulse initiates muscle contraction.

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Depolarization

A change in a cell's membrane potential that makes the inside of the cell less negative.

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Motor End Plate

Region of the muscle fiber's membrane where the motor neuron releases neurotransmitters.

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Smooth muscle calcium release

Smooth muscle cells release calcium from extracellular sources and/or the sarcoplasmic reticulum (SR) via different gated channels, not just RyR(ryanodine receptor)

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Cardiac muscle cell coupling

Cardiac muscle cells are electrically connected via many gap junctions which allow action potentials to spread rapidly throughout the tissue.

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Malignant hyperthermia cause

A mutation in the ryanodine receptor (RyR) causes uncontrolled calcium release in muscle cells, leading to uncontrolled muscle contractions.

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Cardiac muscle T-tubules

Cardiac muscle cells have highly developed T-tubules that are larger than in other muscle types.

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Cardiac muscle DIAD

Cardiac muscle cells have a unique structure where one t-tubule is present for every one SR cistern.

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DHPR (dihydropyridine receptor)

A voltage sensor in the T-tubule membrane, that responds to action potentials and triggers calcium release.

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RyR (Ryanodine receptor)

A receptor in the sarcoplasmic reticulum that opens in response to signals from DHPR, releasing calcium.

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Calcium release during muscle contraction

An action potential causes the release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction.

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Troponin C

A protein component of thin actin filaments that binds to calcium, triggering a conformational change in tropomyosin.

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Tropomyosin's role in muscle contraction

A protein that, when calcium binds to troponin, moves exposing the myosin-binding sites on actin, allowing muscle contraction.

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Myosin-actin interaction

Myosin heads bind to exposed actin sites, pulling actin filaments, leading to muscle shortening.

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SERCA pump

The enzyme responsible for muscle relaxation, pumping calcium back into the sarcoplasmic reticulum.

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Muscle relaxation process

Calcium is pumped back into the sarcoplasmic reticulum, allowing tropomyosin to block myosin-binding sites, ending muscle contraction.

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Study Notes

Excitation-Contraction Coupling (EC Coupling)

  • EC coupling is the link between muscle excitation and muscle contraction.
  • Action potentials trigger muscle cell contraction.
  • Calcium ions control whether or not contraction occurs.
  • The link between muscle excitation and calcium release is excitation-contraction coupling.

Objectives

  • Understand EC coupling.
  • Understand the passage of impulses.
  • Understand the function of T-tubules.
  • Understand how calcium is released.
  • Know the muscle contraction sequence.
  • Understand muscle relaxation.
  • Compare smooth and cardiac muscle differences.
  • Understand applied aspects of EC coupling.

What is EC Coupling?

  • Action potentials initiate muscle contraction.
  • Calcium regulates contraction.
  • Excitation-contraction coupling links muscle excitation to calcium release from the sarcoplasmic reticulum.

The Passage of the Impulse

  • Nerve impulses travel to the post-synaptic membrane via neuromuscular junctions.
  • Conformational changes result in sodium ion influx into muscle fibers.
  • Sodium accumulation depolarizes the membrane, creating an end-plate potential.
  • The potential increases towards the action potential threshold.

The T-tubule

  • T-tubules are invaginations of the sarcolemma that extend into the muscle fiber.
  • T-tubule lumen is continuous with the extracellular fluid (ECF).
  • Depolarization during action potentials occurs across the T-tubule membrane.
  • Terminal cisterns, swellings of sarcoplasmic reticulum (SR), are on each side of T-tubules.

How Depolarization Opens a Calcium Channel

  • Dihydropyridine receptor (DHPR) is a voltage sensor in the T-tubule membrane.
  • DHPR is closely associated with the foot of the calcium channel.
  • The action potential passes to the L-tubules, containing the ryanodine receptor (RyR).
  • The voltage sensor changes shape when depolarized, causing the RyR to open, releasing calcium.

The Contraction Sequence

  • Accumulated calcium initiates and maintains sarcomere contraction.
  • Free calcium binds to troponin C on thin actin filaments.
  • This forms the active calcium-troponin complex.

Binding of Calcium to Troponin C

  • Calcium binds to troponin C.
  • This causes a conformational change in tropomyosin.
  • This exposes myosin binding sites on actin filaments.

Myosin Head Binding

  • Myosin heads bind to actin.
  • ADP and IP release trigger power strokes, shortening sarcomeres.

Relaxation

  • SERCA pump (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase pump) actively pumps calcium back into the sarcoplasmic reticulum.

Differences in Smooth Muscles

  • Different calcium channels (voltage-gated, ligand-gated, second messenger-gated, or mechanically-gated) exist depending on smooth muscle type.
  • Calcium can enter from the extracellular fluid.
  • IP3 (inositol triphosphate) is a second messenger that opens channels and releases calcium from the SR.
  • Calcium binds to calmodulin.
  • Myosin light chain kinase (MLCK) activates the myosin head through phosphorylation.

Differences in Cardiac Muscles

  • Cardiac muscle cells form a syncytium (electrically connected).
  • Gap junctions, made of connexins, link cells.
  • T-tubules are well-developed.
  • Only one terminal cistern per T-tubule (a diad).

Applied Aspects (e.g., Malignant Hyperthermia)

  • Malignant hyperthermia is caused by RyR mutations in L-tubules.
  • Symptoms include increased body temperature, muscle contractions, rigidity, and heart rate with high fever.

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Excitation-Contraction Coupling (EC Coupling) PDF

Description

Explore the intricate process of excitation-contraction coupling, where electrical impulses lead to muscle contraction. This quiz covers essential concepts such as the function of T-tubules, calcium release, and differences between muscle types. Enhance your understanding of how muscle contraction and relaxation occur through this vital physiological mechanism.

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