Histo lec

Chapter 4: Muscle Tissue

Overview of Muscle Tissue

  • Contractility: Fundamental ability of cells to shorten. High contractility seen in:

    • Pericytes (small blood vessels)

    • Myoepithelial Cells (exocrine glands)

    • Muscle Cells (Myocytes): Exhibit greatest contractility

  • Muscle Tissue Function:

    • Responsible for locomotion and movement of body parts.

    • Highly cellular, composed of muscle fibers supported by connective tissue.

  • Cellular Origin:

    • Muscle cells primarily derived from mesoderm, except for certain eye muscle cells (ectoderm).

  • Cell Structure:

    • Elongated cells known as muscle fibers.

    • Components:

      • Basal Lamina: Envelops muscle fibers.

      • Sarcolemma: Cell membrane of muscle cells.

      • Sarcoplasm: Cytoplasm of muscle cells.

      • Sarcoplasmic Reticulum: Smooth endoplasmic reticulum of muscle cells.

      • Sarcosomes: Mitochondria of muscle cells.

Types of Muscle Cells (Fibers)

  • Three Types:

    • Skeletal Muscle: Striated, voluntary, associated with locomotion of the body.

    • Cardiac Muscle: Striated, involuntary, found only in the heart.

    • Smooth Muscle: Non-striated, involuntary, found in walls of visceral organs and blood vessels.

Skeletal Muscle

  • Organization:

    • Form mouse-shaped organs, referred to as skeletal muscles.

    • Attached to skeleton by tendons.

    • Origin and insertion points defined at muscle attachments.

  • Voluntary Control: Controls movement of limbs, body wall, and face.

  • Contraction Properties:

    • Quick and strong contractions.

Structure of Skeletal Muscle

  • Muscle Fiber Arrangement:

    • Muscle fibers grouped into bundles (fascicles), each enveloped by:

      • Epimysium: Envelopes the entire muscle.

      • Perimysium: Envelopes fascicles.

      • Endomysium: Envelopes each muscle fiber.

  • Muscle Fiber Characteristics:

    • Long, tapering, cylindrical, multinucleated cells.

    • Myoblasts fuse to form fibers, some remain as stem cells (myosatellite cells).

    • Central, numerous oval nuclei in fibers.

Myofibrils and Striations

  • Myofibrils:

    • Numerous per muscle fiber (5,000 to 10,000).

    • Composed of sarcomeres, displaying striations due to:

      • I-band: Light bands (isotropic).

      • A-band: Dark bands (anisotropic).

    • Striations lead to characteristic appearance.

Mechanism of Muscle Contraction

  • Sliding Filament Theory:

    • During contraction, thick (myosin) and thin (actin) filaments slide past each other, shortening the sarcomere.

    • Calcium ions bind to troponin, exposing actin binding sites for myosin.

    • ATP hydrolysis provides energy for myosin head flexing, pulling thin filament towards the sarcomere center.

Cardiac Muscle

  • Location: Found in the heart and large blood vessels.

  • Cylindrical Cells: Shorter and branching, with central nuclei.

  • Structure:

    • Form bundles with surrounding connective tissue (perimysium and endomysium).

    • Intercalated discs unique to cardiac muscle facilitate cell connections.

Smooth Muscle

  • Distribution: Found in walls of organs and blood vessels, termed visceral muscle.

  • Cell Characteristics:

    • Fusiform shape, single oval nucleus, and less organized than striated muscle.

  • Contraction Mechanisms:

    • Filaments slide past each other; calcium from extracellular space triggers contraction via calmodulin and myosin light chain kinase (MLCK).

    • Regulated by autonomic nervous system.

Muscle Repair and Regeneration

  • Skeletal Muscle: Limited regenerative capacity, reliant on myosatellite cells.

  • Smooth Muscle: Variable regeneration; can generate new cells in certain organs (e.g., uterus).

  • Cardiac Muscle: Negligible regenerative capacity; repaired with connective tissue.