Ch. 11 - Skeletal Muscle Physiology

Chapter 11: Skeletal Muscle Physiology (BIOL 214)

Universal Characteristics of Muscle

  • Excitability (responsiveness): Ability to respond to chemical signals, stretch, and electrical changes across the plasma membrane.

  • Conductivity: Local electrical excitation sets off a wave of excitation that travels along the muscle fiber.

  • Contractility: Capability of muscle to shorten when stimulated.

  • Extensibility: Ability of muscle to be stretched between contractions.

  • Elasticity: Ability of muscle to return to its original resting length after being stretched.

Characteristics of Skeletal Muscle

  • Definition: Voluntary, striated muscle usually attached to bones.
        - Multinucleate: Allows formation of long cells (up to 30 cm).

  • Striations: Alternating light and dark transverse bands due to the arrangement of internal contractile proteins.

  • Voluntary Control: Typically under conscious control; exceptions exist (e.g., pain reflexes, diaphragm).

Structure of Skeletal Muscle Fibers

  • Tissue Wrappings:
        - Endomysium: Connective tissue around an individual muscle fiber.
        - Perimysium: Connective tissue surrounding muscle fascicles.
        - Epimysium: Connective tissue surrounding the entire muscle.

  • Tendons: Attachments between muscle and bone matrix; continuous with collagen fibers of tendons and connective tissue of the bone matrix.

  • Collagen Properties: Extensible and elastic; stretches slightly under tension and recoils when released, resisting excessive stretching and protecting muscles from injury.

  • Function of Collagen: Returns muscle to resting length, contributes to power output and muscle efficiency.

The Muscle Fiber

  • Sarcoplasmic Structure:
        - Sarcoplasma: Cytoplasm of the muscle fiber containing:
            - Myofibrils: Long protein cords occupying most of the sarcoplasm.
            - Glycogen: Stored carbohydrate for energy during exercise.
            - Myoglobin: Red pigment providing oxygen required for muscle activity.

  • Nuclei:
        - Multiple Nuclei: Flattened nuclei pressed against the sarcolemma.
        - Myoblasts: Stem cells that fused to form each muscle fiber early in development.
        - Satellite Cells: Unspecialized myoblasts remaining between muscle fibers and endomysium; play a role in regeneration of damaged skeletal muscle.

  • Mitochondria: Packaged into spaces between myofibrils for energy production.

Sarcoplasmic Reticulum and T Tubules

  • Sarcoplasmic Reticulum (SR): Smooth endoplasmic reticulum forming a network around each myofibril.
        - Terminal Cisterns: Dilated end-sacs of SR that cross the muscle fiber from one side to the other; serve as calcium reservoirs, releasing calcium to activate contraction.

  • T Tubules: Tubular infoldings of the sarcolemma that penetrate the cell and emerge on the other side.

  • Triad: A complex made up of one T tubule and two terminal cisterns associated with it.

Myofilaments

  • Thick Filaments:
        - Composed of several hundred myosin molecules; each molecule has a tail and two globular heads arranged in a helical array around the bundle.
        - Bare Zone: Area in the middle of the thick filament devoid of heads.

  • Thin Filaments:
        - Composed of fibrous (F) actin with a string of globular (G) actin subunits.
        - Tropomyosin: Blocks six or seven active sites on G actin subunits.
        - Troponin: A small, calcium-binding protein located on each tropomyosin molecule.

  • Elastic Filaments:
        - Made of titin, a huge protein that runs through the core of thick filaments, anchoring them to Z discs and M lines.
        - Functions to stabilize, position thick filaments, prevent overstretching and provide recoil.

  • Contractile and Regulatory Proteins:
        - Contractile proteins consist of myosin and actin for contraction duty.
        - Regulatory proteins (tropomyosin and troponin) act as switches for contraction activation by calcium release.

  • Dystrophin: A clinically important protein linking actin to membrane proteins that connect to the endomysium; genetic defects result in muscular dystrophy.

Striations

  • Definition: Result from the precise organization of myosin and actin in cardiac and skeletal muscle cells.
        - A Bands (Dark): Correspond to thick filament overlap with thin filaments.
        - I Bands (Light): Correspond to regions containing only thin filaments.
        - Z Disc: Anchorage for thin and elastic filaments, bisecting I bands, providing a zig-zag appearance.

  • Sarcomere: Defined as a segment from Z disc to Z disc; the functional contractile unit of muscle fibers where shortening occurs through filament sliding without length change.

Structural Hierarchy of Skeletal Muscle

  • Muscle: A contractile organ, usually attached to bones by tendons, composed of bundles of muscle fibers (fascicles).

  • Fascicle: A bundle of muscle fibers within a muscle, supplied by nerves and blood vessels.

  • Muscle Fiber: A single muscle cell, elongated and enclosed in a specialized plasma membrane.

  • Myofibril: A bundle of protein myofilaments within a muscle fiber, surrounded by SR and mitochondria, exhibiting striations; it's made up of several sarcomeres.

  • Myofilaments: Fibrous protein strands, including thick (myosin) and thin (actin) filaments that carry out the contraction process.

The Nerve—Muscle Relationship

  • Stimulus Requirement: Skeletal muscle does not contract unless stimulated by a nerve; severed or poisoned nerve connections lead to paralysis.
        - Denervation Atrophy: Shrinkage of paralyzed muscle when the nerve is disconnected.

Motor Neurons and Motor Units

  • Somatic Motor Neurons: Nerve cells serving skeletal muscles from the brainstem and spinal cord.
        - Each muscle fiber is only supplied by one motor neuron, but each neuron can branch to multiple fibers; this creates an “all-or-nothing” contraction response.

  • Motor Unit: Consists of one nerve fiber and all muscle fibers it innervates; contract in unison providing wide-area weak contractions.
        - Small Motor Units: Comprising 3-6 fibers per neuron, allowing fine control (e.g., eye muscles).
        - Large Motor Units: Comprising hundreds of fibers for powerful contractions (e.g., gastrocnemius).

Neuromuscular Junction

  • Definition: The synapse where the nerve fiber meets the muscle fiber, forming distinct terminals with synaptic vesicles filled with acetylcholine (ACh).

  • Components: Includes axon terminal, synaptic cleft, and receptors on the sarcolemma; lack of receptors can lead to weakness (e.g., in myasthenia gravis).

Electrically Excitable Cells

  • Definition: Muscle fibers exhibit voltage changes in response to stimulation; a resting membrane potential of approximately -90 mV is maintained by sodium-potassium pumps.

  • Depolarization: Occurs when sodium channels open, causing the inside of the membrane to become positive.

  • Action Potential: The quick up-and-down voltage shifts, propagating down the muscle fiber's membrane and causing contraction.

  • Impulse: A wave of excitation that travels along the cell membrane; a chain reaction of action potentials.

Neuromuscular Toxins and Paralysis

  1. Spastic Paralysis: Continuous contraction due to cholinesterase inhibitors that prevent ACh degradation, potentially causing suffocation.

  2. Tetanus: Caused by bacteria blocking glycine release, leading to overstimulation and spastic paralysis.

  3. Flaccid Paralysis: Muscles become limp through ACh receptor blockage (e.g., curare) or blockade of ACh release (e.g., botulism).

Phases of Muscle Contraction and Relaxation

  • Excitation: Nerve action potentials lead to muscle action potentials.

  • Excitation–Contraction Coupling: Links action potentials to myofilament activation.

  • Contraction: Muscle fiber develops tension and can shorten.

  • Relaxation: Muscle fiber relaxes and returns to original length when stimulation ends.

Length–Tension Relationship and Muscle Tone

  • Definition: The amount of tension generated by a muscle depends on its length before stimulation. Optimal length generation occurs with minimal overlap between filaments.

  • Muscle Tone: The nervous system maintains partial contraction to ensure muscles are near the optimal length.

Rigor Mortis

  • Definition: Stiffness occurring post-mortem due to calcium release and inability to relax muscle fibers; peaks around 12 hours after death and lasts for 48-60 hours.

Twitch and Contraction Phases

  • Twitch: A quick cycle of contraction and relaxation after stimulus reaches threshold.

  • Latent Period: Delay between stimulus and contraction due to excitation and internal tension generation.

  • Contraction Phase: Period during which the muscle generates tension.

  • Relaxation Phase: Time when tension decreases as SR reabsorbs Ca²⁺.

Contraction Strength of Twitches

  • Factors Affecting Strength:
        - Starting muscle length, fatigue levels, muscle temperature, and hydration.
        - Higher voltages recruit larger motor units, enhancing contraction strength.
        - Higher stimulus frequencies can lead to temporal summation and ultimately tetanus.

Types of Contraction

Isometric Contraction

  • Definition: Internal tension is produced without changing muscle length.

  • Application: Important for postural stability.

Isotonic Contraction

  • Definition: Muscle changes in length but maintains tension.
        - Concentric: Muscle shortens while contracting (e.g., lifting weights).
        - Eccentric: Muscle lengthens while contracting (e.g., lowering weights).

ATP Sources for Muscle Contraction

  • Importance of ATP: All muscle contraction relies on the availability of ATP, oxygen, and organic energy sources (e.g., glucose).

  • ATP Synthesis Pathways:
        - Anaerobic Fermentation: Quick ATP production without oxygen but yielding lactate.
        - Aerobic Respiration: More efficient, producing significantly more ATP but requires a constant supply of oxygen.

Immediate, Short-Term, and Long-Term Energy Sources

  • Immediate Energy: Sourced from phosphagen system using creatine phosphate and adenylate kinases.

  • Short-Term Energy: Utilizes anaerobic fermentation, generating ATP through glycolysis, effective for 30-40 seconds of high-intensity activity.

  • Long-Term Energy: Aerobic respiration begins after 40 seconds, meeting long-term ATP demands.

Muscle Fatigue and Endurance

  • Fatigue: Characterized by muscle weakness from prolonged use, associated with electrolyte loss, fuel depletion, and central fatigue.

  • VO₂ max: Measures maximal oxygen uptake, crucial for sustaining high-intensity efforts.

Excess Postexercise Oxygen Consumption (EPOC)

  • Definition: Metabolic demand post-exercise to restore ATP, myoglobin oxygen reserves, lactate disposal, and elevated metabolic rates post-exercise.

Physiological Classes of Muscle Fibers

  • Slow-twitch Fibers (Type I): Endurance-oriented, resistant to fatigue, rich in myoglobin and mitochondria.

  • Fast-twitch Fibers (Type II): Quick, powerful responses, utilize glycolysis, less resistant to fatigue.

  • Intermediate Fibers: Possess characteristics of both types, found in varied proportions based on muscle function.

Muscular Strength and Conditioning

  • Factors Affecting Strength: Muscle size, fascicle arrangement, motor unit size, and temporal summation.

  • Resistance Training: Increases muscle strength through hypertrophy and increased myofilament synthesis.

  • Endurance Training: Improves fatigue resistance and cardiovascular function.

Response to Stretch in Muscle

  • Stretch Response: Initiates contraction via calcium channel opening.

  • Plasticity: Allows adjustments in muscle tension relative to stretch without losing functionality.

Muscular Dystrophy and Aging

  • Muscular Dystrophy: Genetic conditions causing skeletal muscle degeneration (e.g., Duchenne muscular dystrophy).

  • Aging Effects: Muscle mass and strength decline significantly with age, with fast-twitch fibers showing the earliest atrophy.