Muscle Biology and Contraction

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Questions and Answers

What is the primary function of skeletal muscles?

  • Protection of internal organs
  • Storing nutrients
  • Production of blood cells
  • Facilitating movement (correct)

Which characteristic allows muscle cells to respond to stimuli?

  • Elasticity
  • Contractility
  • Extensibility
  • Excitability (correct)

Which connective tissue layer surrounds an entire skeletal muscle?

  • Endomysium
  • Epimysium (correct)
  • Perimysium
  • Fascia

What is the smallest contractile unit of muscle called?

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

Which type of muscle is under voluntary control?

<p>Skeletal muscle (B)</p> Signup and view all the answers

What is found between muscle fibers and serves as an electrical insulator?

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

Which of the following structures consists of actin and myosin?

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

What is the role of mitochondria in muscle cells?

<p>Energy production (C)</p> Signup and view all the answers

What initiates the power stroke during muscle contraction?

<p>Phosphate separation from the myosin head (D)</p> Signup and view all the answers

Which process is NOT directly involved in the muscle contraction cycle?

<p>Degradation of acetylcholine by acetylcholinesterase (A)</p> Signup and view all the answers

What is the role of tropomyosin in thin myofilaments?

<p>It covers the active points of actin (A)</p> Signup and view all the answers

What role does ATP play in muscle contraction?

<p>It provides energy for myosin head movement (C)</p> Signup and view all the answers

When muscle relaxation begins, what happens to Ca+2 ions?

<p>They are pumped back into the sarcoplasmic reticulum (B)</p> Signup and view all the answers

What comprises a triad in skeletal muscle?

<p>1 T tubule and 2 terminal cisternae (D)</p> Signup and view all the answers

During the resting state of muscle, what covers the active sites of actin?

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

Which protein is primarily responsible for connecting actin to the Z line?

<p>α-ACTININ (B)</p> Signup and view all the answers

What happens to the troponin-tropomyosin complex when Ca+2 binds to troponin?

<p>It changes confirmation to expose actin sites (A)</p> Signup and view all the answers

What is the primary component of thick myofilaments?

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

Which component is responsible for reestablishing the sodium-potassium gradient in a muscle cell after contraction?

<p>Sodium-potassium ATPase (B)</p> Signup and view all the answers

What region of the sarcomere consists only of thick filaments?

<p>H zone (C)</p> Signup and view all the answers

What is the main function of dystrophin in skeletal muscle fibers?

<p>It links actin to the dystroglycan in the outer cell membrane (A)</p> Signup and view all the answers

What is primarily stored in muscle cells to provide quick energy for contraction?

<p>Creatine phosphate (C)</p> Signup and view all the answers

What happens when an action potential reaches the triad?

<p>Calcium channels in the terminal cisternae open (D)</p> Signup and view all the answers

Which of the following proteins prevents excessive stretching of the sarcomere?

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

What is the main energy source used by muscles for contraction during the first 1-2 seconds?

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

What process occurs after the depletion of creatine phosphate stores during muscle contraction?

<p>Aerobic glycolysis (D)</p> Signup and view all the answers

Which term describes the stimulation of additional motor units to increase muscle contraction strength?

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

What factor does NOT influence the strength of muscle contraction?

<p>Duration of muscle activity (D)</p> Signup and view all the answers

What primarily results in muscle fatigue during repeated stimulation?

<p>Decreased tension in the muscle fiber (C)</p> Signup and view all the answers

What is the relationship between motor unit size and precision of movement?

<p>Larger motor units provide less precise control (D)</p> Signup and view all the answers

Which of the following is true about motor neurons?

<p>A motor neuron can innervate multiple muscle fibers. (D)</p> Signup and view all the answers

What occurs in the muscle fibers when they are stimulated repeatedly?

<p>Diminished tension over time (C)</p> Signup and view all the answers

Flashcards

Muscle Excitability

The ability of a muscle cell to respond to a stimulus, causing an electrical signal (action potential) to travel along the cell membrane.

Muscle Contractility

The ability of a muscle cell to shorten and thicken, generating force to produce movement.

Muscle Extensibility

The ability of a muscle cell to stretch or extend without being damaged.

Muscle Elasticity

The ability of a muscle cell to return to its original shape after being stretched or contracted.

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Sarcomere

The smallest contractile unit of a muscle fiber, consisting of overlapping thin (actin) and thick (myosin) filaments.

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Sarcoplasm

The cytoplasm of a muscle cell, containing organelles and other molecules necessary for muscle function.

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Sarcoplasmic Reticulum (SR)

The specialized endoplasmic reticulum of a muscle cell, responsible for storing and releasing calcium ions needed for muscle contraction.

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Endomysium

A layer of connective tissue that surrounds each muscle fiber, providing electrical insulation and support.

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Motor end plate

The part of a muscle fiber that is innervated by a nerve ending.

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Myofibril

A structural unit of muscle tissue that runs along the length of the fiber. It is composed of thin and thick filaments.

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Actin

A protein found in the thin filaments of muscle fibers. It has a binding site for myosin, which allows for muscle contraction.

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Tropomyosin

A protein that covers the active sites of actin. It prevents myosin from binding to actin, which keeps the muscle relaxed.

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Troponin

A protein that is attached to tropomyosin. It is responsible for moving tropomyosin away from the active sites of actin when calcium is present, allowing muscle contraction.

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Titin

A regulatory protein that prevents excessive stretching of the sarcomere.

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Nebulin

A protein that produces F-actin from G-actins.

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Alpha-actinin

A protein that connects actin filaments to the Z line, which is the boundary between sarcomeres.

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Muscle Contraction

The process by which muscle fibers shorten and generate force, involving the interaction between myosin and actin filaments.

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Acetylcholinesterase (AChE)

The enzyme that breaks down acetylcholine (Ach) into choline and acetate in the synaptic cleft.

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SERCA (Sarcoplasmic Reticulum Calcium ATPase)

A calcium-dependent ATPase located in the SR membrane that pumps calcium ions back into the SR during muscle relaxation.

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Myosin

A protein that forms the thick filaments in muscle fibers, responsible for binding to actin and creating the power stroke.

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What is the order of energy systems used during muscle contraction?

ATP is used first, then creatine phosphate, followed by glucose-glycogen breakdown, finally ending with aerobic glycolysis in the mitochondria.

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What is a motor neuron?

A single nerve cell that extends from the brain or spinal cord to a muscle, stimulating muscle fiber contraction.

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What is a motor unit?

The combination of a single motor neuron and all the muscle fibers it innervates.

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What is recruitment?

The process of activating more motor units to increase muscle force and power.

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What is muscle fatigue?

A type of muscle fatigue that occurs when a muscle fiber is repeatedly stimulated, resulting in a decrease in tension even with stimulation.

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How does muscle fatigue affect muscle tension?

The decrease in muscle tension due to repetitive stimulation, even when the stimulation continues. It's also characterized by a reduced rate of shortening and a slower rate of relaxation.

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What is motor unit size?

The size of a motor unit, determined by the number of muscle fibers it innervates. Small motor units control precise movements, while large motor units create powerful movements.

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What is muscle contractility?

The ability of a muscle to shorten, thicken, and generate force to produce movement.

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Study Notes

Skeletal Muscle Contraction

  • Skeletal muscles are responsible for movement, posture maintenance, heat production, and protection of internal organs.
  • Muscle cells exhibit excitability (respond to stimuli), contractility (shorten to generate force), extensibility (extend without damage), and elasticity (return to original shape).

Muscle Types

  • Skeletal muscle: Striated, voluntary, multi-nucleated, and attached to bones.
  • Smooth muscle: Non-striated, involuntary, uni-nucleated, and found in internal organs.
  • Cardiac muscle: Striated, involuntary, uni-nucleated, and found only in the heart.

Skeletal Muscle Structure

  • Organized into: myofilaments (actin and myosin), myofibrils, muscle fibers, muscle fascicles, and skeletal muscle.
  • Connective tissue layers: endomysium (surrounds individual muscle fibers), perimysium (surrounds muscle fascicles), and epimysium (surrounds the entire muscle).
  • Tendons connect muscles to bones.

Sarcomere Structure

  • The functional unit of the muscle.
  • Contains repeating units of actin and myosin filaments.
  • I band: thin actin filaments
  • H zone: thick myosin filaments.
  • A band: both thick and thin filaments
  • M line: connects the thick filaments, and Z lines: connect thin filaments.

Muscle Proteins

  • Important intracellular proteins: Dystrophin, titin, actinin, desmin, and nebulin.
  • Dystrophin-glycoprotein complex: maintains muscle integrity.
  • Titin: prevents overstretching of sarcomeres.

Sarcotubular System

  • T tubules: invaginations of the sarcolemma to transmit action potentials.
  • Sarcoplasmic reticulum (SR): stores calcium ions that trigger muscle contraction.
  • Terminal cisternae: portions of SR that surround T tubules (triad).

Neuromuscular Junction

  • The junction between a motor neuron and muscle fibers.
  • Acetylcholine (ACh): neurotransmitter released to initiate muscle contraction.
  • Acetylcholinesterase (AChE): enzyme that degrades ACh, stopping the signal.

Molecular Mechanisms of Muscle Contraction

  • Steps involved in muscle contraction, including the roles of ATP, calcium, actin, troponin, and tropomyosin.
  • Sliding filament theory: Myosin filaments pull on actin filaments, causing muscle fibers to shorten.
  • Phosphate separation initiates the power stroke and also moves the filaments in opposite directions.
  • ATP hydrolysis and its role in the cross-bridge cycle are essential for muscle contraction.

Muscle Fatigue

  • Reduced muscle tension resulting from repeated stimulation.
  • Decreased rate of shortening and slower rate of relaxation.
  • Factors contributing to muscle fatigue include decreased ATP, metabolites, and ion imbalances.

Skeletal Muscle Fiber Types

  • Type 1 (slow-twitch): slow contraction speed, high endurance, and oxidative metabolism.
  • Type IIa (fast-oxidative-glycolytic): fast contraction speed, moderate endurance, and both oxidative and glycolytic metabolism.
  • Type IIx (fast-glycolytic): fastest contraction speed, low endurance, and substantial glycolytic metabolism.

Muscle Energy Metabolism

  • ATP stores: immediate energy source.
  • Creatine phosphate: short-term energy reserve.
  • Glycolysis: anaerobic ATP production from glucose.
  • Oxidative phosphorylation; aerobic ATP production in the mitochondria..

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