Topic 7

Muscle Terminology

  • Myo/mys: refers to muscle

  • Sarco: refers to flesh

  • Sarcolemma: plasma membrane of muscle cell

  • Sarcoplasm: cytoplasm of muscle cell

  • Sarcoplasmic reticulum: endoplasmic reticulum of muscle cell

  • Muscle fiber: synonym for muscle cell

  • Tendons & Aponeuroses: connective tissues that attach muscle to bone or to other muscles

  • Attachments:

    • Origin: the immovable attachment point of a muscle

    • Insertion: the movable attachment point of a muscle

Types of Muscle

  • Skeletal Muscle: voluntary muscle responsible for movement

  • Smooth Muscle: involuntary muscle found in walls of hollow organs

  • Cardiac Muscle: involuntary muscle that makes up the heart

Functions of Skeletal Muscle

  • Movement: creates movement by contracting and pulling on tendons attached to bones

  • Support: supports visceral organs and shields tissue from injury through the abdominal wall

  • Posture: maintains body position and stability through continuous contraction

  • Temperature Regulation: skeletal muscles, comprising around 40% of body mass, contribute to maintaining body temperature

  • Communication: facilitates various forms of human communication (e.g., speaking, writing, facial expressions and gestures.)

Special Characteristics of Muscle

  • Excitability: ability to receive and respond to stimuli

  • Contractibility: can generate force when stimulated

  • Extensibility: can be stretched beyond resting length

  • Elasticity: recoils to resting length after being stretched

Review of Muscle Features for Contraction

  • Highly cellular structure

  • Well-vascularized

  • Elongated cells that enhance contraction

  • Presence of myofilaments within muscle cells

    • Made up of actin and myosin (proteins)

Muscle Anatomy: Gross and Microscopic Level

Gross Anatomy of Muscle

  • Epimysium: sheath of dense, irregular connective tissue surrounding the entire muscle

  • Perimysium: connective tissue surrounding a bundle of muscle fibers called a fascicle

  • Endomysium: thin connective tissue layer surrounding each muscle fiber, containing capillaries and nerves

  • Purpose of connective tissue: anchors and supports nerves and blood vessels; continuous with tendons and attach to periosteum

Microscopic Anatomy of Muscle

  • Muscle Fibers: elongated, cylindrical cells with a diameter of 10-100 μm and lengths up to 30 cm (similar to hair)

  • Myofibrils: packed in sarcoplasm, composed of thick and thin myofilaments

  • Striations: visible due to the arrangement of repeating A bands (dark) and I bands (light)

The Sarcomere - Functional Unit of Muscle Fiber

  • A Band: area containing thick (myosin) filaments

  • I Band: area containing thin (actin) filaments

  • Z Line: anchors the sequential sarcomeres

  • H Zone: lighter region in the middle of the A band where myofilaments do not overlap

  • M Line: center of the sarcomere

Myofilament Structure

Thick Filaments (Myosin)

  • Composed of myosin with binding sites for both actin and ATP

  • Cross-bridge formation during contraction

  • Hydrolyzes ATP for energy during movement

Thin Filaments (Actin and Regulatory Proteins)

  • Actin: has binding site for myosin

  • Tropomyosin: covers myosin binding site

  • Troponin: interacts with Ca2+ to remove the blocking action of tropomyosin

Muscle Physiology

Regulation of Contraction of Skeletal Muscle Fiber

  1. Generation and transmission of an Action Potential (AP) along the sarcolemma.

  2. Excitation-contraction coupling, which begins at the neuromuscular junction (NMJ).

Neuromuscular Junction (NMJ)

  • Definition: synapse between a motor neuron and a muscle fiber

  • Events at NMJ:

    1. Action potential arrives at the axon terminal.

    2. Voltage-gated Ca2+ channels open, allowing Ca2+ to diffuse into the cell.

    3. Ca2+ triggers acetylcholine (ACh) release by exocytosis.

    4. ACh binds to receptors on the sarcolemma, depolarizing it.

    5. Stimulation ends when acetylcholinesterase breaks down ACh.

Excitation-Contraction Coupling

  • Two important structures involved:

    1. Transverse Tubules (T-tubules): continuous with sarcolemma and conduct AP deep into the fiber

    2. Sarcoplasmic Reticulum (SR): smooth ER network surrounding each myofibril, responsible for storing and releasing Ca2+.

  • Steps:

    1. Action Potential moves along sarcolemma and down T-tubules.

    2. Ca2+ ions are released from SR.

    3. Ca2+ binds to troponin, removing the blocking action of tropomyosin.

    4. Contraction begins via cross-bridge cycle.

Sliding Filament Model of Contraction

  • Relaxed State: Filaments overlap slightly.

  • Contracted State: Increased overlap of actin and myosin filaments is observed.

  • Visual effects:

    • Distance between Z lines decreases.

    • Size of I band reduces.

    • H zone disappears.

    • No change to A band size.

Role of Ca2+ in Muscle Contraction

  • Low Sarcoplasmic Ca2+ Levels: Tropomyosin blocks active sites on actin, preventing myosin attachment (muscle fiber relaxed).

  • High Sarcoplasmic Ca2+ Levels: Ca2+ binds to troponin, causing tropomyosin to move away and initiating the cross-bridge cycle.

Cross-Bridge Cycle

  • Occurs only in the presence of Ca2+:

    1. Cross-bridge forms between myosin and actin.

    2. Power stroke occurs, pulling actin toward the center of the sarcomere.

    3. Cross-bridge detaches, requiring another ATP molecule.

    4. Myosin head is reset for another cycle.

Rigor Mortis

  • Muscle stiffness following death caused by:

    • Excess Ca2+ escaping from SR and depletion of ATP, leading to inability of cross-bridges to detach.

Motor Units and Muscle Contraction

Motor Unit

  • Definition: one motor neuron and all the muscle fibers it innervates.

  • Variability: motor unit size varies, with fewer fibers for precise movements and more fibers for bulk force.

Muscle Twitch

  • Definition: motor unit's response to a single AP of its motor neuron.

  • Phases of a Muscle Twitch:

    1. Latent Period: time before contraction begins.

    2. Contraction Phase: muscle contracts.

    3. Relaxation Phase: muscle relaxes.

Wave Summation and Tetanus

  • Wave Summation: continuous stimulation that doesn't allow the muscle to relax completely, leading to increased tension and potentially reaching peak tension.

  • Tetanus: a sustained muscle contraction caused by a high frequency of APs firing, resulting in sustained contraction.

How does the muscle as a whole control the magnitude of generated force?

  •     By changing the number and and size of motor units recruited.

Muscle Tone

  • Definition: Muscles maintain a continuous partial contraction, integral for posture, balance, and injury prevention.

  • Muscle contraction is defined as the generation of force rather than shortening; shortening occurs when the generated tension exceeds resistance.

    • Your muscles are never “fully relaxed.”

    • Reflexive, coordinated control over motor units also creates muscle tone - the ability of a muscle to maintain continuous and passive partial contraction.

    • Muscle tone maintenance is important for posture, balance, keeping muscles ready to respond, and preventing injury.

Types of Muscle Contractions

  • What is Contraction?

    • Contraction is the generation of force.

      • Shortening occurs only when tension generated by cross-bridging exceeds opposing force.

  • Isotonic Contraction: muscle generates force with a change in muscle length.

    • Concentric: muscle shortens as force exceeds external load.

      • Force generated is greater than the external load.

    • Eccentric: muscle lengthens as force is less than external load.

      • Force generated is less than the external load.

  • Isometric Contraction: muscle generates force without a change in muscle length (a stalemate situation).

Muscle Metabolism

  • ATP: direct energy source for muscle contraction.

  • ATP stores are depleted after about 5 seconds of maximal effort.

  • Replenishing ATP Stores:

    1. Direct phosphorylation of ADP by creatine phosphate (CP).

    2. Anaerobic pathway (glycolysis).

    3. Aerobic pathway (cellular respiration).

Energy Sources for Muscle Contraction

Direct Phosphorylation

  • At peak levels of exertion, aerobic metabolism can only provide about one-third of needed ATP.

Anaerobic Pathway

  • Produces ATP for 30-40 seconds of maximum contraction.

  • Lactic acid is produced, which can be recycled to pyruvic acid and used by mitochondria for ATP or to rebuild glycogen.

Aerobic Pathway

  • Supplies most ATP during rest and light to moderate exercise:

    • Initial fuel from muscle glycogen transitioning to blood glucose, fatty acids, and amino acids with prolonged exercise.

Classification of Muscle Fibers

  • Muscle fibers classified by:

    1. Speed of muscle twitch (slow vs fast).

    2. Preferred metabolic pathway for ATP synthesis (oxidative vs glycolytic).

  • Types of Skeletal Muscle Fibers:

    • Slow Oxidative: high endurance, high myoglobin, low glycogen stores (e.g., suited for endurance activities).

    • Fast Oxidative: intermediate characteristics, suited for moderate exercise.

    • Fast Glycolytic: low endurance, high power output, suited for short, intense efforts.

Comparison of Muscle Tissue Types

  • Skeletal Muscle: Striated, voluntary, ATP from mostly anaerobic sources, and has T-tubules, with regulation by troponin.

  • Cardiac Muscle: Striated, involuntary, ATP derived from mostly aerobic sources, has T-tubules, and regulated by troponin.

  • Smooth Muscle: Non-striated, involuntary, ATP remains from aerobic sources, lacks T-tubules, regulated by calmodulin.

Concept Checks

Concept Check #1

  • Question: What is the function of wave summation in muscle action?

    • A. To prevent muscle injury.

    • B. To prevent muscle fatigue.

    • C. To produce sustained muscle contractions.

    • D. To produce repetitive, coordinated periods of contraction and relaxation.

Concept Check #2

  • Question: Which of the following describes rigor mortis?

    • A. Muscle decay following death caused by bacterial breakdown.

    • B. Muscle softening following death caused by the loss of myofilaments.

    • C. Muscle stiffening following death caused by excess ATP availability and lack of Ca2+.

    • D. Muscle stiffening following death caused by Ca2+ escaping from SR and depletion of ATP.