Muscular Tissue Anatomy & Physiology Notes

Muscle Functions

  • Movement of bones or fluids (e.g., blood)
  • Maintaining posture and body position
  • Stabilizing joints
  • Heat generation (especially skeletal muscle)

Types and Characteristics of Muscle Tissues

  • Universal Characteristics of Muscles:

    • Excitability (Responsiveness): Responds to chemical signals, stretches, and electrical changes.
    • Conductivity: Local electrical excitation triggers a wave that travels along the muscle fiber.
    • Contractility: Ability to shorten when stimulated.
    • Extensibility: Capability of being stretched between contractions.
    • Elasticity: Returns to original length after being stretched.

  • Three Types of Muscle Tissues:

    • Skeletal Muscle:
    • Long, cylindrical shape
    • Multinucleate
    • Striated
    • Voluntary control
    • Function: Overall body mobility
    • Location: Attached to bones or skin
    • Smooth Muscle:
    • Spindle-shaped cells
    • Uninucleate
    • No striations
    • Involuntary control
    • Function: Propels substances (food, urine, baby)
    • Location: Walls of hollow organs
    • Cardiac Muscle:
    • Branched cells
    • Uni or bi-nucleate
    • Striated
    • Involuntary control
    • Function: Propels blood into circulation
    • Location: Walls of the heart

Skeletal Muscle Fibers

  • Connective Tissue Wrappings:

    • Epimysium: Dense irregular connective tissue surrounding entire muscle.
    • Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).
    • Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

  • Muscle cells are termed muscle fibers:

    • Plasma membrane: Sarcolemma
    • Cytoplasm: Sarcoplasm
    • Smooth ER: Sarcoplasmic reticulum (stores calcium)
    • Provides oxygen: Myoglobin
    • Energy store: Glycogen
    • Contains many rod-like myofibrils aligned with muscle fiber length.

Myofibrils and Sarcomeres

  • Striations:
    • Dark bands: A bands
    • Light bands: I bands
    • Sarcomere: Functional unit extending from one Z disc to another, consisting of A band and parts of I band.
  • Filament Structure:
    • Thick Filaments: Composed mainly of myosin.
    • Thin Filaments: Composed mainly of actin.

Muscle Contraction Mechanism

  • Sliding Filament Theory: Thick and thin filaments slide past each other during contraction, with sarcomeres shortening without changing filament lengths.
  • Role of Calcium:
    • High Ca2+ levels promote binding of myosin to actin, initiating contraction.
    • Low Ca2+ levels block myosin binding sites on actin, leading to relaxation.

Neuromuscular Junction and Muscle Activation

  • Neuromuscular Junction Events:
    • Motor neuron axon terminal releases acetylcholine (ACh).
    • ACh binds to receptors on muscle fiber, initiating an action potential.
    • Action potential travels along sarcolemma and into T-tubules, triggering Ca2+ release from sarcoplasmic reticulum.

Rigor Mortis

  • Occurs post-mortem due to calcium release and lack of ATP, leading to sustained muscle contraction. Initial stiffness appears about 3-4 hours after death, peaking at 12 hours, then diminishing as tissues degrade.

Length–Tension Relationship

  • The amount of tension generated depends on muscle length prior to stimulation. Optimal length yields the greatest force upon contraction.
  • Muscle tone is maintained to ensure muscles are near optimal resting lengths for effective contractions.