Human Physiology Chapter on Muscle Function
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Questions and Answers

What primarily determines the strength of contraction in mammals?

  • Number of motor units activated (correct)
  • Frequency of action potentials
  • Level of calcium ions in the muscle
  • Type of muscle fiber utilized

    • Which statement about fast-twitch fibers is true?

  • They are always glycolytic.
  • They are less fatigue resistant than slow-twitch fibers. (correct)
  • They can either be glycolytic or oxidative. (correct)
  • They are primarily oxidative.

    • What characterizes complete tetanus in muscle fibers?

  • There is no fusion of action potentials.
  • It occurs at lower stimulation frequencies compared to incomplete tetanus.
  • Calcium ion transients do not play a role.
  • The twitch force is completely fused and maximized. (correct)

    • Which process is NOT associated with muscle fatigue?

    <p>Excessive calcium ion accumulation</p> Signup and view all the answers

    What is a major characteristic of slow-twitch fibers?

    <p>They are fatigue resistant.</p> Signup and view all the answers

    Which factor is NOT a focus when examining muscle fatigue?

    <p>Motor unit recruitment efficiency</p> Signup and view all the answers

    Which type of fiber is primarily involved in rapid movement?

    <p>Glycolytic fast-twitch fibers</p> Signup and view all the answers

    What happens to calcium ion transients at high rates of stimulation in twitch fibers?

    <p>They demonstrate a complete fusion.</p> Signup and view all the answers

    What is a key characteristic of slow-twitch fibers (Type I) compared to fast-twitch fibers (Type II)?

    <p>Fatigue resistant</p> Signup and view all the answers

    Which type of muscle fatigue is characterized by a fast onset and fast recovery?

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

    Which of the following best describes the role of motor units in muscle contraction?

    <p>Motor units can vary in size, affecting force output.</p> Signup and view all the answers

    What primarily influences muscle fatigue during prolonged activity?

    <p>The intensity and duration of stimulation.</p> Signup and view all the answers

    What defines a motor unit in skeletal muscle?

    <p>One motor neuron and all the muscle fibers it innervates</p> Signup and view all the answers

    Which factor most significantly influences the type of muscle fiber utilized during high-intensity exercise?

    <p>Type of motor units being recruited</p> Signup and view all the answers

    In terms of muscle contraction, what distinguishes twitch from tetanus?

    <p>Twitch is a single contraction; tetanus is a prolonged contraction from multiple stimuli.</p> Signup and view all the answers

    How does muscle fatigue primarily occur during prolonged exertion?

    <p>Accumulation of metabolic byproducts and depletion of calcium</p> Signup and view all the answers

    Why are skeletal muscle fibers referred to as multinucleated cells?

    <p>They form from the fusion of many individual muscle cells.</p> Signup and view all the answers

    What characteristic of cardiac muscle distinguishes it from skeletal muscle?

    <p>It has a longer refractory period.</p> Signup and view all the answers

    What is the primary mechanism through which skeletal muscle contraction initiates?

    <p>Release of acetylcholine at the neuromuscular junction</p> Signup and view all the answers

    Which classification accurately represents the types of muscle fibers based on their contraction attributes?

    <p>Type I fibers are slow-twitch and fatigue-resistant.</p> Signup and view all the answers

    Match the types of muscle fibers with their characteristics:

    <p>Slow-twitch fibers (Type I) = Fatigue 'resistant', repetitive role Fast-twitch fibers (Type II) = White appearance, fewer mitochondria</p> Signup and view all the answers

    Match the types of muscle fatigue with their descriptions:

    <p>High frequency fatigue = Associated with low stimulation frequency Metabolic fatigue = Prolonged energy depletion Long duration fatigue = Slower onset, prolonged recovery</p> Signup and view all the answers

    Match the characteristics with their corresponding muscle fiber types:

    <p>Slow-twitch fibers (Type I) = Primarily used in endurance activities Fast-twitch fibers (Type II) = Lower endurance capacity</p> Signup and view all the answers

    Match the classification of muscle fiber types with their specific features:

    <p>Type I fibers = Fatigue-resistant, postural function Type II fibers = Fatigues quickly, used in sprinting</p> Signup and view all the answers

    Match the types of muscle fatigue with their onset and recovery characteristics:

    <p>High frequency fatigue = Occurs with low-frequency stimulation Metabolic fatigue = Gradual onset, prolonged effects Long duration fatigue = Progressive fatigue over time</p> Signup and view all the answers

    Match the muscle types with their characteristics:

    <p>Striated = Includes cardiac and skeletal muscle Smooth = Involuntary muscle type Cardiac = Constant contraction importance Skeletal = Voluntary muscle with strength and agility</p> Signup and view all the answers

    Match the definitions with the terms related to muscle cells:

    <p>Sarcoplasm = The inside of the muscle cell ATPase = An active transport mechanism for ions Motor unit = A motor neuron and the muscle fibers it innervates Cytoplasm = The cellular material within the muscle fiber</p> Signup and view all the answers

    Match the functions of muscle in mammals:

    <p>Locomotion = Movement and travel Respiration = Breathing assistance Thermogenesis = Heat production Vision = Light perception and processing</p> Signup and view all the answers

    Match the factors influencing muscle fiber characteristics:

    <p>Diameter of fibers = 5 to 80 micrometers Length of fibers = From millimeters to tens of centimeters Nuclei = Multinucleated structure Innervation = One motor neuron per fiber</p> Signup and view all the answers

    Match the muscle fiber types with their properties:

    <p>Fast-twitch = Higher force output, fatigues quickly Slow-twitch = Lower force output, more endurance Skeletal = Striated muscle controlling voluntary movements Cardiac = Striated muscle responsible for heart contractions</p> Signup and view all the answers

    Match the ion-related terms with their descriptions:

    <p>Calcium ions = Required for muscle contraction initiation PK = Potassium ion concentration PNa = Sodium ion concentration PCl = Chloride ion concentration</p> Signup and view all the answers

    Match the terms related to muscle contraction with their meanings:

    <p>Depolarization = Change in membrane potential to a less negative value Repolarization = Return of the membrane potential to resting state Tetanus = Sustained muscle contraction Fatigue = Decreased ability to generate force after prolonged activity</p> Signup and view all the answers

    Match the effects of ion concentration changes with their potential outcomes:

    <p>Increase in Cl permeability = Potentially increased rate of repolarization Decrease in Cl permeability = Potential delay in repolarization Increase in PNa = Faster depolarization Decrease in PK = Slower action potential generation</p> Signup and view all the answers

    Match the term with its description related to muscle contraction:

    <p>Twitch = An all or none contractile event Complete tetanus = Continuous contraction at high stimulation rates Calcium transient = Short-term increase in calcium ion concentration during contractions Incomplete tetanus = unfused twitches resulting in a sustained but fluctuating force</p> Signup and view all the answers

    Match the muscle fatigue causes with their factors:

    <p>CNS fatigue = Involves central nervous system effects PNS fatigue = Relates to peripheral nervous system involvement Neuro-muscular junction fatigue = May involve neurotransmitter depletion Muscle-related fatigue = Includes energy depletion and ionic imbalances</p> Signup and view all the answers

    Match the type of tetanus with its characteristics:

    <p>Fused tetanus = Characterized by sustained muscle contraction Unfused tetanus = Characterized by partial fusion of twitches Complete tetanus = Results in maximal muscle force output Incomplete tetanus = Results in variability of muscle force output</p> Signup and view all the answers

    Match the following concepts related to muscle fatigue with their corresponding descriptions:

    <p>K+ buildup = Depolarizes t-tubule leading to action potential failure Na+ channel inactivation = Prevents further contraction Ca2+ release = Essential for muscle contraction initiation Rest periods = Allow repolarization and resetting of Na+ channels</p> Signup and view all the answers

    Match the elements of muscle structure with their functions:

    <p>T-tubules = Conduct action potentials into the muscle fiber Myoplasm = Site for metabolic processes within the muscle Sarcoplasmic reticulum (SR) = Stores and releases calcium ions DHPR = Voltage sensor involved in excitation-contraction coupling</p> Signup and view all the answers

    Match the following frequencies with their effects on muscle force and fatigue:

    <p>100 Hz = High frequency stimulation leading to rapid fatigue Low frequency = Allows for recovery and potential force increase Fast onset (~5sec) = Rapid muscle fatigue initiation 1 min rest = Time for K+ pumping and Na+ channel resetting</p> Signup and view all the answers

    Match the muscle contraction events with their mechanisms:

    <p>Calcium release = Triggers muscle contraction through actin and myosin interaction Action potential propagation = Initiates contraction by altering membrane potential Cross-bridge cycling = The process that generates muscle tension Energy consumption = ATP is required for contraction and relaxation</p> Signup and view all the answers

    Match the stimulus frequency with its resulting contraction type:

    <p>7 Hz = Three distinct twitches 15 Hz = Begins to show incomplete tetanus 50 Hz = Results in a fused tetanus Low frequency = Permits individual twitch responses</p> Signup and view all the answers

    Match the following components of the muscle contraction mechanism with their roles:

    <p>Sarcolemma = Cell membrane crucial for action potential propagation Sarcoplasmic reticulum (SR) = Storage site for Ca2+ T-tubule = Facilitates action potential transmission into the muscle fiber DHPR molecule = Key in Ca2+ release during contraction</p> Signup and view all the answers

    Match the following phases of muscle response with their respective mechanisms:

    <p>Fatigue (decline) = Reduction in force output Force production = Active muscle contraction phase K+ repolarization = Restoration of the membrane potential Action Potential failure = Inability to trigger further contractions</p> Signup and view all the answers

    Match the following physiological events with the type of muscle fatigue they represent:

    <p>High Frequency Fatigue = Characterized by rapid onset and reduced force Low Frequency Fatigue = Gradual onset with extended activity Electrolyte imbalance = Involves K+ and Na+ changes affecting muscle function Mechanical fatigue = Result of prolonged muscle contraction leading to metabolic changes</p> Signup and view all the answers

    Match the following ion movements with their effects during muscle contraction:

    <p>K+ movement = Repolarizes the fiber Ca2+ influx = Triggers muscle contraction Cl- presence = Reduces K+ build-up K+ build-up = Depolarizes t-tubule</p> Signup and view all the answers

    Match the following fatigue recovery durations with their characteristics:

    <p>Fast recovery (~98%) = 1 minute High frequency fatigue onset = 30 seconds Cytoplasmic Ca2+ levels = Associated with fatigue decline Complete recovery = Longer than 1 minute</p> Signup and view all the answers

    Match the following muscle environments with their effects on ion movement:

    <p>Extracellular space with K+ = Promotes depolarization Intracellular space with Cl- = Aids repolarization Sarcoplasmic reticulum release = Increases cytoplasmic Ca2+ T-tubule depolarization = Influences fiber excitability</p> Signup and view all the answers

    Match the following types of muscle fatigue with their mechanisms:

    <p>High frequency fatigue = Decline in force due to Ca2+ inhibition Fatigue from K+ build-up = Associated with prolonged activity Recovery with Cl- = Aids in K+ homeostasis Initial level retention = Fast recovery mechanism</p> Signup and view all the answers

    Match the following descriptions with their respective muscular phenomena:

    <p>Recovery from high frequency fatigue = Requires 1 minute rest Force decline under fatigue = Correlates with Ca2+ availability K+ build-up reduction = Facilitated by extracellular Cl- Depolarization of T-tubules = Leads to calcium release</p> Signup and view all the answers

    What is a primary factor contributing to metabolic fatigue in muscle cells?

    <p>Accumulation of lactate</p> Signup and view all the answers

    Which metabolic process is primarily active during long-duration exercise in muscle fibers?

    <p>Aerobic respiration</p> Signup and view all the answers

    How does calcium ion concentration affect muscle contraction during fatigue?

    <p>It decreases the force by reducing the release of calcium from the sarcoplasmic reticulum.</p> Signup and view all the answers

    Which condition is most likely to lead to increased lactate production in muscle tissue?

    <p>High-intensity interval training</p> Signup and view all the answers

    Which of the following best describes the body's metabolic response to increasing exercise intensity?

    <p>Increased lactate production alongside a rise in aerobic pathways</p> Signup and view all the answers

    Which factor contributes to decreased contractile function during high-intensity exercise?

    <p>Increased Pi concentration</p> Signup and view all the answers

    What role does lactate play in metabolism during exercise?

    <p>It stimulates glucose production in the liver.</p> Signup and view all the answers

    How does low ATP concentration affect calcium release in muscle fibers?

    <p>It decreases RyR opening.</p> Signup and view all the answers

    What is a common result of increased H+ concentration in muscle during intense exercise?

    <p>Reduced contractile function significantly.</p> Signup and view all the answers

    Which condition would likely contribute to muscle fatigue in McArdle's disease patients?

    <p>Inability to produce lactate.</p> Signup and view all the answers

    Which effect does increased Mg2+ have on calcium dynamics during muscle contraction?

    <p>It makes it harder for DHPR to open RyR.</p> Signup and view all the answers

    In the context of anaerobic metabolism, what is the byproduct of glucose and its implications?

    <p>Lactic acid, which contributes to fatigue.</p> Signup and view all the answers

    What metabolic response occurs as exercise intensity increases?

    <p>Increased anaerobic energy production.</p> Signup and view all the answers

    Which metabolic pathway primarily contributes to ATP production during high-intensity anaerobic exercise?

    <p>Conversion of glucose to lactate</p> Signup and view all the answers

    What is the primary result of ATP hydrolysis in muscle fibers?

    <p>Contractile proteins resetting</p> Signup and view all the answers

    How does lactate production affect muscle fatigue during intense exercise?

    <p>It decreases intracellular pH, leading to fatigue</p> Signup and view all the answers

    Which of the following best describes the calcium's role during muscle contraction?

    <p>Calcium binds to troponin, facilitating actin-myosin interaction</p> Signup and view all the answers

    What metabolic changes occur in the muscle during light exercise?

    <p>Increased phosphate and stable ATP levels</p> Signup and view all the answers

    Which condition correctly reflects ATP regeneration during aerobic metabolism?

    <p>Requires oxygen and produces carbon dioxide and water</p> Signup and view all the answers

    What effect does an increase in ADP and AMP have during intense exercise?

    <p>Increases glycolytic enzyme activity</p> Signup and view all the answers

    How does the body primarily generate ATP in a resting state?

    <p>Beta-oxidation of fatty acids</p> Signup and view all the answers

    What immediate effect does intense exercise have on muscle metabolism?

    <p>Increased AMP and ADP concentration</p> Signup and view all the answers

    Which of the following is a consequence of prolonged anaerobic exercise on muscle fibers?

    <p>Decrease in muscle pH due to acidification</p> Signup and view all the answers

    Study Notes

    Resting Em

    • The resting Em is influenced by the ratio of permeability of potassium (PK), sodium (PNa), and chloride (PCl) ions across the cell membrane.

    Effects of Changing PCl on Depolarization and Repolarization

    • Increasing PCl would make depolarization faster and repolarization slower.
    • Decreasing PCl would make depolarization slower and repolarization faster.

    Muscle Types

    • Striated muscle
      • Cardiac muscle - responsible for heart contractions.
      • Skeletal muscle - responsible for voluntary movement.
    • Smooth muscle (visceral) - found in internal organs and responsible for involuntary movements.

    Muscle Contraction

    • In all muscle types, contraction is initiated by an increase in intracellular calcium ion concentration ([Ca2+]).
    • This rise in [Ca2+] is triggered by electrical and/or chemical stimulation.

    Skeletal Muscle: Model for Muscle Fatigue

    • Skeletal muscle is used as a model to understand muscle fatigue.

    Muscle Terminology

    • Sarcoplasm, myoplasm, cytoplasm, and intracellular all refer to the interior of the muscle cell or fiber.
    • ATPase or pump describes active processes that use ATP hydrolysis to move ions across membranes against their concentration gradient.

    Functions of Muscle in Mammals

    • Locomotion - movement.
    • Respiration - breathing.
    • Blood circulation - pumping blood throughout the body.
    • Digestion - breaking down food.
    • Excretion - removing waste products.
    • Reproduction - involved in the reproductive process.
    • Vocalization - producing sounds.
    • Vision - movement of the eye.
    • Thermogenesis - heat production.

    General Characteristics of Skeletal Muscle Fibers

    • Length: Millimeters to centimeters.
    • Diameter: 5 to 80 micrometers.
    • Multinucleated: Contain multiple nuclei located at the periphery of the cell.

    Structural Overview of Skeletal Muscle

    • Each skeletal muscle fiber is innervated by one motor neuron.

    Motor Unit

    • A motor unit comprises a single motor neuron and the muscle fibers it innervates.
    • Motor unit size varies from 3-6 muscle fibers in fine control muscles (eye, hand) to 2000 muscle fibers in powerful muscles (gastrocnemius, trapezius).

    Strength of Contraction

    • The strength of contraction is generally determined by the number of activated motor units.

    Classes of Muscle Fibers

    • Twitch fibers:
      • Have a single excitatory synapse.
      • Generate all-or-none action potentials and contractile events (twitches).
      • Involved in rapid movements.
    • Slow-twitch fibers (Type I):
      • Fatigue resistant.
      • Oxidative (use oxygen for energy).
      • Responsible for postural and repetitive movements.
    • Fast-twitch fibers (Type II):
      • Can be glycolytic or oxidative and glycolytic.
      • Produce rapid movements.
      • Fatigue faster.

    Tetani

    • When twitch fibers are stimulated at high enough rates, the action potentials do not fuse, but the [Ca2+] transients do.
    • This leads to a fusion of the twitch force responses, resulting in complete or incomplete tetani.
    • Complete or incomplete tetani are also known as fused or unfused tetani.
    • Fused tetanus can be several times greater than the twitch force.

    Muscle Fatigue

    • Muscle fatigue is a decline in peak force production and muscle power output.
    • It can be caused by a combination of factors in the central nervous system (CNS), peripheral nervous system (PNS), neuromuscular junction, and the muscle itself.
    • This discussion focuses on factors related to muscle fatigue, particularly the t-tubules.

    Muscle Fatigue: T-Tubules and the SR

    • The T-tubules (transverse tubules) are invaginations of the sarcolemma (muscle cell membrane) that extend into the muscle fiber.
    • The sarcoplasmic reticulum (SR) is a network of intracellular membranes that stores and releases calcium.
    • The T-tubules and SR play a crucial role in muscle contraction and relaxation.

    What is Muscle Fatigue?

    • Muscle fatigue is typically defined as a decrease in peak force production and muscle power output during intense or prolonged activity.
    • The degree of fatigue depends on the intensity and duration of activity, as well as the type of muscle fiber involved.

    Muscle Fiber Types

    • Slow-twitch fibers (Type I):
      • Red appearance due to high levels of myoglobin.
      • Many mitochondria for aerobic energy production.
      • Lower force output and slower contraction speed.
      • Fatigue resistant, suited for sustained activity.
      • Responsible for posture and repetitive movements.
    • Fast-twitch fibers (Type II):
      • White appearance due to lower myoglobin content.
      • Fewer mitochondria, relying more on anaerobic metabolism.
      • Higher force output and faster contraction speed.
      • Fatigue faster, suitable for explosive activities.

    Types of Muscle Fatigue

    • High-frequency fatigue:
      • Fast onset and recovery.
      • Occurs due to high-frequency stimulation of muscle fibers.
    • Metabolic fatigue:
      • Caused by the accumulation of metabolic byproducts, such as lactate and hydrogen ions.
    • Long duration fatigue:
      • Develops over prolonged periods of activity.
      • Caused by depletion of energy stores, damage to muscle fibers, and hormonal changes.

    High-frequency Fatigue in a Toad Muscle

    - High-frequency stimulation (100Hz) of a toad gastrocnemius muscle leads to this type of fatigue.

    Resting Membrane Potential (Em)

    • The normal ratio of membrane permeability to potassium (PK), sodium (PNa), and chloride (PCl) is 1 : 0.01 : 2.
    • Changing this ratio effects resting membrane potential (Em).
    • If the ratio changes to 1 : 0.01 : 1, the resting Em will be less negative than normal.
    • If the ratio changes to 1 : 0.01 : 0, the resting Em will be more negative than normal.
    • Keeping the ratio at 1 : 0.01 : 2, the resting Em remains normal.
    • Increasing PNa to 10, the resting Em will be less negative than normal.
    • Chloride permeability (PCl) has a significant effect on membrane potential.

    Action Potential

    • Increasing chloride permeability (PCl) would speed up repolarization of the action potential.
    • Increasing chloride permeability (PCl) would make it easier to repolarize because it will allow chloride ions to move into the cell, countering the positive charge of sodium ions that enter during depolarization.

    Muscle Types

    • There are two main classifications of muscle:
      • Striated (striped)
        • Cardiac: Responsible for pumping blood.
        • Skeletal: Responsible for movement.
      • Smooth (visceral) Responsible for involuntary movement in internal organs.

    Muscle Contraction

    • Muscle contraction is initiated by a rise in intracellular ionized calcium concentration ([Ca2+]).

    Muscle Terminology

    • Sarcoplasm, myoplasm, cytoplasm, and intracellular are all terms used to describe the inside of a muscle cell or fiber.

    Muscle Functions

    • Locomotion: Movement.
    • Respiration: Breathing.
    • Blood circulation: Pumping blood.
    • Digestion: Breakdown of food.
    • Excretion: Removal of waste.
    • Reproduction: Reproduction.
    • Vocalisation: Sound production.
    • Vision: Sight.
    • Thermogenesis: Heat production.

    Skeletal Muscle

    • Skeletal muscle fibers: Extend from tendon to tendon.
    • Skeletal muscle fiber length: Millimeters to tens of centimeters.
    • Skeletal muscle fiber diameter: 5 to 80 μm (micrometers).
    • Skeletal muscle fibers are multinucleated cells with peripheral nuclei.
    • Each skeletal muscle fiber is innervated by one motor neuron.
    • Motor unit: One motor neuron together with all muscle fibers it innervates.
    • Size of motor unit: Varies.
      • Fine movement control: 3-6 muscle fibers (some ocular muscles, muscles in the hand and arm)
      • Power: 2000 fibers (gastrocnemius, trapezius).

    Muscle Contraction Strength

    • The strength of muscle contraction is determined by the number of motor units activated.

    Muscle Fiber Classes

    • Twitch Fibers:
      • One excitatory synapse in the middle region of the fiber.
      • Generate "all or none" action potentials.
      • Respond with "all or none" contractile events (twitches).
      • Involved in rapid movement of body parts.
    • Slow-twitch Fibers:
      • Fatigue resistant (oxidative).
      • Red appearance.
      • Many mitochondria.
    • Fast-twitch Fibers:
      • Can be either glycolytic or oxidative and glycolytic.
      • Rapid movements.
      • White appearance
      • Fewer mitochondria.
    • Superfast

    Tetanus

    • Tetanus: A state of sustained muscle contraction.
    • Complete tetanus: Force responses are completely fused (no relaxation) when twitch fibers are stimulated at a high enough rate.
    • Incomplete tetanus: Force responses are incompletely fused (some relaxation) when twitch fibers are stimulated at a moderate rate.
    • The fused tetanus force can be several fold greater than the twitch force.

    Muscle Fatigue

    • Muscle fatigue: A decline in peak force production and muscle power output during intense or prolonged muscle activity.
    • Muscle fatigue depends on:
      • Intensity and duration of stimulation.
      • Muscle fiber type.
      • Central nervous system (CNS) and peripheral nervous system (PNS)
      • Neuro-muscular junction.
      • Muscle itself (including t-tubules).

    Broad Classification of Muscle Fiber Types

    • Slow-twitch fibers (Type I):
      • Fatigue resistant
      • Postural or repetitive role
    • Fast-twitch fibers (Type II):
      • Fatigue faster
      • "Explosive" contraction

    Muscle Fiber Types: Detailed

    • Slow-twitch fibers (Type I):
      • Fatigue resistant.
      • Many mitochondria.
      • Lower force output.
      • Postural or repetitive role.
      • Red appearance.
      • Oxidative metabolism.
    • Fast-twitch fibers (Type II):
      • High force output.
      • Fatigues faster
      • "Explosive" contraction.
      • White appearance.
      • Glycolytic metabolism.

    Types of Muscle Fatigue

    • High frequency fatigue:
      • Fast onset.
      • Fast recovery.
    • "Metabolic" fatigue:
      • Accumulation of metabolic byproducts such as lactic acid.
      • Can occur during prolonged exercise.
    • Long duration fatigue:
      • Caused by depletion of energy stores
      • Can occur during endurance exercise.

    High Frequency Fatigue

    • High frequency fatigue:
      • Fast onset: Occurs within seconds when muscle fibers are stimulated at high frequencies.
      • Fast recovery: Muscle function returns quickly after a brief rest.
    • Mechanism:
      • K+ build-up in the t-tubules (transverse tubules) leads to depolarization, inactivation of sodium channels, and failure of action potentials.
      • DHPR (dihydropyridine receptor) on the t-tubules is not activated.
      • No activation of DHPR means no calcium release from the sarcoplasmic reticulum (SR), leading to a decrease in muscle force.
    • With short rest:
      • Potassium is pumped back into the cell and diffuses out of the restricted space.
      • The membrane repolarizes and sodium channels reset, allowing muscle function to recover.

    Recovery from High Frequency Fatigue

    • Presence of chloride ions: Helps speed up recovery by repolarizing the membrane and reducing potassium build-up.
    • Absence of chloride ions: Result in slower recovery from high frequency fatigue, as potassium build-up continues to interfere with action potential propagation.

    High Frequency Fatigue Summary

    • High-frequency fatigue results in a rapid decline in force, followed by quick recovery. This mechanism is primarily caused by potassium buildup in the t-tubule, leading to depolarization, the inactivation of sodium channels, and failure of action potentials.

    Types of Muscle Fatigue

    • High frequency fatigue: characterized by fast onset and fast recovery.
    • ‘Metabolic’ fatigue: onset time and recovery time will depend on the intensity and duration of activity.
    • Long duration fatigue: characterized by slow onset and slow recovery, likely due to the depletion of energy stores.

    Stages of Metabolic Fatigue

    • Stage A: Less force due to reduced Ca2+ release and/or decreased force development by contractile proteins.
    • Stage B: Less force due to decreased sensitivity of the Ryanodine receptors (RyR) to Ca2+ due to high ADP and AMP concentrations in the cytosol.
    • Stage C: Less force due to lower levels of ATP decreasing RyR opening and Ca2+ release.
    • Stage D: Less force due to increased Mg2+ concentration in the cytosol, making it harder for DHPR (dihydropyridine receptors) to open RyR.

    What Causes Metabolic Fatigue?

    • Increased Inorganic Phosphate ([Pi]): It decreases the free [Ca2+] in the Sarcoplasmic Reticulum (SR) by forming Calcium Phosphate (CaP).
    • Increased Mg2+: Competes with ATP for the stimulatory site of the RyR reducing Ca2+ release.
    • Increased ADP & AMP: Competes with ATP for the stimulatory site on the RyR, blocking the opening of the RyR and Ca2+ release.
    • Decreased ATP: ATP is needed to open RyR and power the Ca2+ pump in the SR. Reduced ATP levels decrease RyR opening and Ca2+ release.
    • Increased H+ (Decreased pH): While it can have some effect on excitation-contraction coupling (ECC), it does not significantly affect overall ECC.

    Lactate and Muscle Fatigue

    • Lactate myth: The idea that lactate causes fatigue originated in 1929. However, the correlation between lactic acid and fatigue does not mean it's the direct cause.
    • Lactate's role: It stimulates glucose production in the liver (tissue-to-tissue signalling), acts as a readily diffusible fuel for the heart and brain, and is involved in the release of human growth hormone.
    • McArdle's disease: Patients lack glycogen phosphorylase, meaning they cannot produce lactate during glycolysis, yet they still experience rapid fatigue. This evidence contradicts lactate being the key to fatigue.

    ATP Utilization and its Relationship to Fatigue:

    • Short-term ATP replenishment: (~50 mM) Creatine phosphate (CrP) + ADP → ATP + Cr.
    • Long-term ATP replenishment: From glucose and oxygen, or from glucose alone during anaerobic conditions.

    Sites of ATP Usage:

    • Muscle Contraction: ATP is used by the contractile proteins to power the sliding filament mechanism.
    • Ca2+ pump: Required for the Ca2+ pump located in the SR membrane to restore Ca2+ back into the SR after each contraction.
    • DHPR: ATP is needed to enable the DHPR to open RyR channels in the SR.

    Metabolic Changes and Their Impact on Fatigue

    • Light exercise: Inorganic Phosphate ([Pi]) levels increase.
    • Intense exercise: Increased levels of phosphate, lactate, H+, Mg2+, ADP, and AMP, coupled with decreased levels of ATP, pH, glycogen, and Cr. These changes can lead to a reduction in overall muscle function.

    Key sites that Contribute to Fatigue:

    • SR: Increased [Pi] and [Mg2+] can interfere with Ca2+ release from the SR. Reduced ATP levels can also affect Ca2+ pump function, leading to a decrease in Ca2+ availability for contraction
    • DHPR: Increased [Mg2+] can make it harder for the DHPR to open RyR channels.
    • RyR: Increased ADP and AMP compete with ATP for the stimulatory site of the RyR. Increased [Mg2+] also impacts its function.

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    Description

    This quiz explores muscle types, contraction mechanisms, and the role of ion permeability in resting membrane potential. Understand the differences between striated and smooth muscle, as well as the factors influencing depolarization and repolarization. Test your knowledge on how skeletal muscle serves as a model for muscle fatigue.

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