Skeletal Muscle Contraction: Physiology

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Questions and Answers

Which event immediately precedes the release of acetylcholine (ACh) at the neuromuscular junction?

  • Depolarization of the T-tubules
  • Influx of sodium ions into the muscle fiber
  • Activation of troponin by calcium
  • Arrival of an action potential at the axon terminal (correct)

What is the primary role of the sarcoplasmic reticulum in skeletal muscle contraction?

  • To generate ATP for muscle contraction
  • To provide structural support to the sarcomere
  • To store and release calcium ions (correct)
  • To transmit action potentials to the interior of the muscle fiber

During muscle contraction, what prevents myosin from binding to actin when the muscle is at rest?

  • The presence of ATP bound to the myosin head
  • The absence of calcium ions in the sarcoplasm
  • The position of tropomyosin blocking the binding sites on actin (correct)
  • The physical separation of actin and myosin filaments

What is the role of ATP in muscle relaxation?

<p>ATP is required to pump calcium back into the sarcoplasmic reticulum. (B)</p> Signup and view all the answers

What characterizes the latent period of a muscle twitch?

<p>The period of isoelectric activity, during which the muscle shows no mechanical activity. (A)</p> Signup and view all the answers

What is the immediate consequence of calcium ions binding to troponin?

<p>Exposure of myosin-binding sites on actin (B)</p> Signup and view all the answers

How does the duration of the relaxation period compare to the contraction period in a typical muscle twitch?

<p>The relaxation period is longer than the contraction period. (A)</p> Signup and view all the answers

Which of following occurs during the contraction period?

<p>Calcium binds to troponin, allowing actin and myosin filaments to slide past each other. (A)</p> Signup and view all the answers

What is the primary reason why fast-twitch muscle fibers contract more quickly than slow-twitch fibers?

<p>Fast-twitch fibers express a faster isoform of myosin ATPase. (B)</p> Signup and view all the answers

How does increasing the temperature of a muscle typically affect the characteristics of a muscle twitch?

<p>Decreases the duration, decreases the latent period, and increases the force of contraction (B)</p> Signup and view all the answers

What is the primary mechanism by which endurance training enhances the ability of slow-twitch muscle fibers to sustain twitches over time?

<p>Enhancing oxygen utilization within the muscle fiber (A)</p> Signup and view all the answers

What is the primary cause of muscle fatigue during high-intensity exercise?

<p>Accumulation of metabolites, such as lactic acid (C)</p> Signup and view all the answers

In the context of muscle physiology, what does 'recruitment' refer to?

<p>The increase in the number of motor units activated as stimulus intensity increases (B)</p> Signup and view all the answers

What is the effect of curare on skeletal muscle contraction?

<p>Blocks the action of acetylcholine at the neuromuscular junction, causing muscle paralysis (C)</p> Signup and view all the answers

Which of the following best describes the mechanism by which anticholinesterases affect skeletal muscle contraction?

<p>Inhibiting the breakdown of acetylcholine in the synaptic cleft. (A)</p> Signup and view all the answers

Which of the following is a characteristic of a muscle experiencing fatigue?

<p>A fatigue curve can be obtained when the nerve muscle preparation is stimulated repeatedly. (D)</p> Signup and view all the answers

What best describes the contractile property of the gastrocnemius muscle?

<p>It is studied using kymograph. (D)</p> Signup and view all the answers

The contractile property of frog's gastrocnemius-sciatic nerve preparation can be studied by applying:

<p>Electric stimulator and kymograph together. (A)</p> Signup and view all the answers

What is one of the primary factors that influences temperature's effect on nerve conduction velocity?

<p>The activation energy of ion channels (D)</p> Signup and view all the answers

What is the physiological basis for the 'warm-up' effect in skeletal muscles, where performance improves with increasing temperature?

<p>Temperature-sensitive muscle proteins improving muscle performance (C)</p> Signup and view all the answers

In both skeletal and cardiac muscles under a relaxed state, a low intracellular calcium ion concentration is maintained and the tropomyosin-troponin complex suppresses interaction between molecular motor myosin and actin filament (F-actin); this is termed the OFF state. What is the main characteristic of the ON state?

<p>The myosin is allowed to interact with actin, producing active force (D)</p> Signup and view all the answers

When a nerve muscle preparation is stimulated repeatedly, a fatigue curve is often obtained. Which of the following is least likely to contribute to this phenomenon?

<p>Increased efficiency in calcium reuptake by the sarcoplasmic reticulum (C)</p> Signup and view all the answers

Which form of fatigue is characterized by a quicker recovery time (within 30 minutes) and is typically associated with activities like circuit training?

<p>High frequency fatigue (D)</p> Signup and view all the answers

As individuals age, what is a key change observed in muscle twitch responses?

<p>Decreased capacity for calcium handling. (A)</p> Signup and view all the answers

What mechanisms explain how the muscle fiber length is related to the force of muscle contraction?

<p>At optimal lengths, most actin and myosin are available for cross-bridge formation, which can produce the greatest amount of force. (C)</p> Signup and view all the answers

For therapeutic interventions, electrical stimulation is sometimes used. What is the main goal when applying electrical stimulation for the purpose of helping someone rehabilitate?

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

What is muscle tetanus?

<p>It refers to a sustained contraction of a muscle that occurs when it is stimulated at a high frequency without allowing it to relax between stimuli. (A)</p> Signup and view all the answers

In incomplete (unfused) tetanus, what happens as the muscle is stimulated repeatedly at a moderate frequency?

<p>Partial relaxation occurs between stimuli, demonstrating a wave-like pattern. (C)</p> Signup and view all the answers

What effect does a higher frequency of stimulation have on muscle contraction, distinguishing tetanus from a single muscle twitch?

<p>Sustained and smooth contraction (C)</p> Signup and view all the answers

Which of the following scenarios most closely resembles the physiological state of complete tetanus in a muscle?

<p>Holding a heavy object steady (D)</p> Signup and view all the answers

Which of the following is observed with a shorter duration of time, when studying simple muscle twitch?

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

If the intensity of the stimulus increases for the muscle twitch response, what happens?

<p>There is recruitment in the twitch response. (D)</p> Signup and view all the answers

Which factor would cause the recorded amplitude of the isoelectric line in a muscle twitch to be altered?

<p>Application of the stimulus (B)</p> Signup and view all the answers

If a muscle fiber is stretched beyond its optimal length, what would be the initial effect on its ability to generate force during a twitch?

<p>Decreased force generation due to reduced overlap of actin and myosin (A)</p> Signup and view all the answers

In a muscle stimulated to exhibit unfused tetanus, what would be the effect on muscle tension if the stimulation frequency were significantly increased?

<p>Muscle tension would increase until it reaches a plateau, transitioning to fused tetanus. (D)</p> Signup and view all the answers

A researcher is studying muscle twitches in vitro and notices that the twitches become weaker and less frequent over time, despite consistent stimulation. If they add a compound that significantly increases the concentration of calcium ions within the sarcoplasm, what would they likely observe?

<p>A transient restoration of twitch amplitude followed by rapid fatigue. (C)</p> Signup and view all the answers

What describes the mechanism of how cold influences muscle contraction?

<p>Cold decreases the nerve condition velocity, decreasing muscle force (A)</p> Signup and view all the answers

What best describes why fast-twitch fibers are more dominant in explosive activities like sprinting or weightlifting?

<p>They dominate in activities that require strength and power training. (B)</p> Signup and view all the answers

Flashcards

Muscle twitch

A single contraction-relaxation cycle in a muscle fiber.

Latent period

Time between stimulus application and the start of muscle contraction.

Contraction period

The phase where muscle shortening occurs.

Relaxation period

Muscle returns to its original length.

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Fast-twitch fibers

Type of muscle fiber for shorter, quicker twitches.

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Slow-twitch fibers

Muscle fiber results in longer twitches with slower relaxation.

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Muscle fatigue

A decrease in muscular activity due to repeated stimulus.

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Fatigue cause: Depletion of Ach

This is caused by depletion of Ach at neuromuscular junction.

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Fatigue cause: Metabolite buildup

This is caused by accumulation of metabolites like lactic acid.

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Fatigue cause: Energy depletion

This is caused by depletion of muscle ATP/glycogen.

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Muscle tetanus

Sustained muscle contraction due to high-frequency stimulation.

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Incomplete tetanus

Muscle stimulated repeatedly at a moderate frequency with partial relaxation.

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Complete tetanus

Muscle stimulated at a very high frequency with no relaxation.

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Stimulus frequency

High or low applied to stimuli to the muscle will produce these responses.

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Isolated Contraction

Is muscle contraction brief or isolate in a twitch?

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High frequency fatigue

Muscle fatigue after short burst activities

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Low frequency fatigue

Muscle fatigue recovery taking 24-72 hours

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Study Notes

  • Physiology of skeletal muscle involves understanding the simple muscle twitch (SMT).

Skeletal Muscle Contraction

  • Somatic motor neurons release acetylcholine (ACh) at the neuromuscular junction.
  • ACh initiates a muscle action potential upon binding.
  • Action potential in the T-tubules alters the conformation of DHP receptors.
  • DHP receptors open calcium release channels allowing calcium to enter the cytoplasm.
  • Calcium binds to troponin exposing myosin binding sites on actin.
  • Myosin heads then execute a power stroke allowing filaments to slide.
  • Actin filaments slide to the center of the sarcomere.

Sarcomere Structure

  • The sarcomere is the contractile unit of muscle.
  • Sarcomeres contain actin (thin filaments) and myosin (thick filaments.)
  • Key structures include the Z disc, tropomyosin, troponin, tropomodulin, and M line.

Nerve & Muscle Preparation using Frog

  • The contractile properties of muscle tissue can be studied using the frog's gastrocnemius-sciatic nerve preparation.
  • The gastrocnemius muscle and sciatic nerve are dissected from the frog for experimentation.

Apparatus

  • A kymograph is used to record muscle contractions.
  • An electric stimulator adjusts the stimulus intensity and frequency.

Simple Muscle Twitch (SMT)

  • SMT involves a single contraction followed by relaxation from a single maximum stimulus.
  • SMT is the smallest unit of muscle contraction, triggered by a single action potential.
  • Three phases: Latent period, contraction period and relaxation period.

Phases of a Simple Muscle Twitch

  • Latent period is the time between stimulus application and muscle contraction: it lasts about 0.01 seconds.
    • Conduction occurs along the motor axon to the neuromuscular junction (NMJ)
    • NMJ transmission occurs where the muscle depolarizes.
    • Calcium ions are released into the cytoplasm.
    • There is no visible contraction; the muscle prepares to contract.
  • Contraction period: muscle shortening occurs between 10-100 milliseconds varied by muscle type upward deflection.
    • Calcium binds to troponin, allowing actin and myosin filaments to slide.
    • Force is generated, causing the muscle fiber to contract and tension rises.
  • Relaxation period: the muscle returns to its original length after contraction occurring about 0.05 seconds and is a downward deflection back the baseline.
    • It is longer than the contraction phase.
    • Calcium flows back to the sarcoplasmic reticulum
    • Actin and myosin filaments return to their resting positions, decreasing tension and returning to baseline.

Factors Affecting SMT

  • Type of muscle fibers.
  • Initial muscle fiber length.
  • Temperature.
  • Fatigue.

Muscle Fiber Types

  • Fast-twitch fibers (e.g., eye muscles) generate shorter, quicker twitches.
  • Slow-twitch fibers (e.g., postural muscles) generate longer twitches with slower relaxation.

Clinical Relevance: Exercise and Muscle Training

  • Fast-twitch fibers (Type II) help with explosive activities i.e. sprinting.
  • Slow-twitch fibers (Type I) help with endurance activities.
  • High-intensity training improves fast-twitch fiber recruitment efficiency.
  • Endurance training improves oxygen use in slow-twitch fibers.

Effect of Temperature on Muscle Contraction

  • Warm muscles exhibit shorter duration and higher amplitude twitches.
  • Cold muscles exhibit longer duration and lower amplitude twitches.
  • Increased temperature increases nerve conduction velocity, frequency of stimulation, and muscle force.
  • Enzyme activity increases with temperature, improving muscle contraction efficiency.
  • Higher temperatures increase elasticity and extensibility, increasing muscle force.
  • "The higher temperature dependence of skeletal muscle may allow it to contract relatively quickly upon warming up, even from slight warming due to light movement or exercise."
  • Muscle performance is improved by warming the muscle by acting as a temperature sensor.
  • Warming up before is important to reduce injury and help with indepence.

Clinical Relevance: Muscle Relaxation States

  • A low intracellular calcium ion concentration is maintained and the tropomyosin-troponin complex suppresses interaction between myosin and actin, termed the OFF state.
  • Calcium binding to troponin from an increase concentration allows myosin and actin to bind termed the ON state.
  • Heat functions as "thermal signaling," inducing the ON state regardless of calcium ions.

Effect of Fatigue

  • Repeated stimulation of a nerve muscle preparation results in a fatigue curve.
  • Fatigue is the decrease in muscular activity from continuous stimuli.
    • Ach depletion at the neuromuscular junction.
    • Accumulation of metabolites like lactic acid.
    • Reduction of ATP or glycogen levels.
    • Psychogenic causes.

Types of Frequency Fatigue

  • Low-frequency fatigue occurs during endurance activities and takes 24-72 hours to recover.
  • High-frequency fatigue occurs during circuit training and takes 30 minutes or less to recover.

Comparison Table of Muscle Properties with Various Conditions

  • Room temperature: normal latent period, amplitude, and duration.
  • Warmth: decreased latent period and duration, increased amplitude.
  • Cold/Fatigue: increased latent period and duration, decreased amplitude.
  • Older adults have slower twitch responses due to changes in calcium handling.
  • Rehabilitation programs for aging populations should focus on preserving muscle function.

Therapeutic Interventions

  • Therapists use electrical stimulation to induce muscle twitches for rehabilitative purposes.
  • Neuromodulation devices like TENS and FES maintain muscle tone or restore function.

Muscle Tetanus

  • Muscle tetanus refers to a prolonged contraction caused by a high stimulation frequency.
  • Muscle does not allow time to relax between stimuli and represents maximal contraction and plays an important role in coordinated smooth movements.

Types of Tetanus

  • Incomplete (Unfused) Tetanus involves repeated stimulation at a moderate frequency where partial relaxation occurs.
  • Tension increases with stimulus but does not plateau.
  • Complete (Fused) Tetanus involves stimulation at a very high frequency without relaxation.
    • Twitches fuse into a single, smooth contraction.
    • Represents the maximum force the muscle can produce.

Key Differences: Twitch vs. Tetanus

  • Twitch by single stimulus, briefs, and has a temporary rise with low amounts.
  • Tetanus by repeated high-frequency stimulus, strong and continuous, and has Continuously high with maximal amount.

Examples of Tetanus

  • Holding a heavy object steady.
  • Clenching jaw tightly.
  • Maximal effort during powerlifting.

Sample Questions

  • During the relaxation phase there is active movement of calcium back into the sarcoplasmic reticulum.
  • Type II muscle fibers also known as fast-twitch have the shortest duration.
  • Troponin binds to calcium ions resulting in contraction.
  • The speed of a muscle twitch id dependent on the type of muscle fiber involved.
  • Pumping of the calcium ions back into the sarcoplasmic reticulum NOT a response to contraction.

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