Principles of Human Physiology - Muscle Physiology

Principles of Human Physiology

Chapter 12a: Muscle Physiology

General Information
  • Author: Cindy L. Stanfield
  • Edition: Sixth edition
  • Publisher: Pearson Education, Inc. © 2017

Chapter Outline

  • 12.1 Skeletal Muscle Structure
  • 12.2 The Mechanism of Force Generation in Muscle

Learning Outcomes

  • Structural Features: Name the major structural features of a skeletal muscle cell and briefly describe the relationship between structure and function.
  • Crossbridge Cycle: Describe the sequence of events that occurs in the crossbridge cycle and relate this sequence to the sliding-filament model of muscle contraction.

12.1 Skeletal Muscle Structure

  • Skeletal Muscles: Connective tissues bind skeletal muscles to two or more bones via tendons.

    • Exceptions:
    • Connection to skin (example: facial muscles)
    • Connection to cartilage (example: larynx)
    • Other muscles (example: sphincters)

  • Connective Tissues:

    • Epimysium: Outermost layer, continuous with tendon connective tissue.
    • Perimysium: Divides muscle into fascicles, which consist of 100s to 1000s of myofibers.
    • Endomysium: Surrounds individual muscle fibers.
Structure at the Cellular Level
  • Components of a Muscle Fiber (Myofiber):
    • Sarcolemma: Plasma membrane of a muscle fiber.
    • Transverse Tubules: Also known as T-tubules.
    • Multinucleated: Muscle fibers extend the length of the muscle.
    • Sarcoplasm: Cytoplasm of the muscle fiber, contains various organelles.
    • Mitochondria: High presence to meet energy demands.
    • Myofibrils: Protein structures involved in muscle contraction.
Structure at the Molecular Level

  • Myofibrils: Composed of organized thick and thin filaments which are essential for muscle contraction.

  • Contractile Proteins:

    • Myosin: Thick filaments contribute to the striated appearance in skeletal and cardiac muscle.
    • Contains a tail oriented towards the M line and heads that extend towards the I band.
    • Myosin heads have binding sites for actin and ATP (ATPase).
    • Arrangement: Two myosin molecules bound at their tails create thick filaments and crossbridges.
    • Actin: Thin filaments, consist of:
    • G-actin (globular actin) molecules form F-actin (filamentous actin) as a double helical strand.
    • F-actin is anchored at Z lines.

  • Regulatory Proteins:

    • Tropomyosin: Covers myosin-binding sites on actin when the muscle is relaxed.
    • Troponin: A complex of three proteins that binds to actin and tropomyosin, regulating contraction by binding Ca^2+ reversibly.

  • Structural Protein:

    • Titin: Anchors thick filaments between the M line and Z line, provides elastic structural support.

12.2 The Mechanism of Force Generation in Muscle

  • Key Concepts:
    • The sliding filament model describes how muscles contract via the overlapping of actin and myosin filaments.
    • Crossbridge Cycle: Mechanism through which muscles generate force.
    • Excitation-Contraction Coupling: Process of how muscle contractions are initiated and regulated.
    • Muscle cell metabolism provides ATP necessary for the crossbridge cycle.
Sliding-Filament Model

  • Muscle Contraction: Characterized by the shortening of muscle fibers (sarcomeres) when actin and myosin filaments overlap and slide past each other.

  • Changes during Contraction:

    • Overall sarcomere shortens with no change in the A band length.
    • The I band and H zone both shorten.

  • Mechanism: Sliding is due to the cyclical formation and breaking of crossbridges, termed the crossbridge cycle.

The Crossbridge Cycle: Generation of Force

  • Process:

    • Cyclical formation of connections between actin and myosin.
    • Myosin heads undergo conformational changes, pivoting back and forth, powered by ATP hydrolysis.

  • Analogy: The mechanism can be compared to rowing a boat:

    • Crossbridge or Myosin Head: Represents the oar.
    • Linking of Myosin to Actin: Equivalent to the oar contacting the water.
    • Phases Include:
    • Formation of the myosin-actin link (oar into the water).
    • Power stroke (oar pushing through the water).
    • Release of the crossbridge (oar out of the water).
Excitation of the Myofiber

  • Neuromuscular Junction: Area where the motor neuron connects with the muscle fiber.

    • Motor Unit: Each somatic motor neuron innervates several myofibers; each myofiber connects to only one motor neuron.

  • Structure of Neuromuscular Junction:

    • Presynaptic Cell: Peak of the axon terminal of somatic motor neuron.
    • Acetylcholine (ACh): Neurotransmitter released, binding to receptors on the postsynaptic myofiber in the motor end plate.
    • Contains cation channels and AChase (acetylcholinesterase).

  • End-Plate Potential (EPP): Initiates a graded depolarization within the myofiber, leading to the muscle action potential.

Excitation-Contraction Coupling

  • Description: Transition from the excitation of muscle fibers to the activation of crossbridge cycles.

    • Requires release of Ca^2+ from the sarcoplasmic reticulum.
    • Functional roles:
    • Ca^2+ binds to troponin, allowing for tropomyosin to shift and expose binding sites on actin for myosin.

  • Relaxation:

    • Following muscle contraction, Ca^2+ is actively transported back into the sarcoplasmic reticulum, ceasing contraction signals,
    • Tropomyosin re-covers binding sites, leading to muscle relaxation.