Muscle Tissue

Introduction to Muscle Tissue

  • Muscle Tissue

    • A primary tissue type

    • Divided into:

    • Skeletal muscle tissue

    • Cardiac muscle tissue

    • Smooth muscle tissue

Special Characteristics of Muscle Tissue

  • Excitability (responsiveness or irritability): The ability to receive and respond to stimuli.

  • Contractility: The ability to shorten when stimulated.

  • Extensibility: The ability to be stretched.

  • Elasticity: The ability to recoil to resting length.

Muscle Functions

  • Movement of bones or fluids (e.g., blood).

  • Maintaining posture and body position.

  • Stabilizing joints.

  • Heat generation (especially by skeletal muscle).

Organization of Muscle

  • Skeletal Muscle

    • Composition includes:

    • Muscle tissue (muscle cells or fibers)

    • Connective tissues

    • Nerves

    • Blood vessels

  • Connective Tissues

    • Three layers:

    • Epimysium (outer layer)

    • Perimysium (middle layer)

    • Endomysium (inner layer)

Components of Muscle Connective Tissue

  • Epimysium

    • Exterior collagen layer.

    • Connected to deep fascia.

    • Separates muscle from surrounding tissues.

  • Perimysium

    • Surrounds muscle fiber bundles (fascicles).

    • Contains blood vessels and nerve supply to fascicles.

  • Endomysium

    • Surrounds individual muscle cells (muscle fibers).

    • Contains capillaries and nerve fibers contacting muscle cells.

    • Contains myosatellite cells (stem cells) that repair damage.

Muscle Attachments

  • Endomysium, perimysium, and epimysium combine at the ends of muscles to form connective tissue attachment to bone matrix:

    • Tendon (bundle)

    • Aponeurosis (sheet)

Blood Vessels and Nerves in Muscle Organization

  • Muscles have extensive vascular systems that:

    • Supply large amounts of oxygen.

    • Supply nutrients.

    • Carry away wastes.

  • Skeletal muscles are voluntary muscles controlled by:

    • Nerves of the central nervous system (brain and spinal cord).

Characteristics of Skeletal Muscle Fibers

  • Skeletal Muscle Cells

    • Very long, developing through fusion of mesodermal cells (myoblasts).

    • Multi-nucleated (contain hundreds of nuclei).

Microscopic Anatomy of a Skeletal Muscle Fiber

  • Cylindrical cell with a diameter of 10 to 100 µm and length up to 30 cm.

  • Features include:

    • Multiple peripheral nuclei.

    • Many mitochondria.

    • Glycosomes for glycogen storage and myoglobin for oxygen storage.

    • Contains myofibrils, sarcoplasmic reticulum, and T-tubules.

Sarcomere

  • Definition: Smallest contractile unit (functional unit) of a muscle fiber.

    • A myofibril comprises many sarcomeres.

    • Composed of thick (myosin) and thin (actin) myofilaments.

    • Defined by the region between two successive Z discs.

Features of a Sarcomere

  • Thick filaments: Extend the entire length of an A band.

  • Thin filaments: Run the length of the I band and partially into the A band.

  • Z disc: Coin-shaped sheet of proteins anchoring thin filaments and connecting myofibrils.

  • H zone: Lighter midregion where filaments do not overlap.

  • M line: Line of protein myomesin holding adjacent thick filaments together.

Ultrastructure of Thick Filament (Myosin)

  • Composed of the protein myosin with two components:

    • Myosin Tails: Contain 2 interwoven heavy polypeptide chains.

    • Myosin Heads: Contain 2 smaller light polypeptide chains.

    • Act as cross bridges during contraction.

    • Have binding sites for actin (thin filaments) and ATP.

    • Contain ATPase enzymes for energy release.

Ultrastructure of Thin Filament (Actin)

  • Twisted double strand of fibrous protein F-actin.

  • F-actin consists of:

    • G (globular) actin subunits.

    • G actin bears active sites for myosin head attachment during contraction.

  • Regulatory proteins:

    • Tropomyosin: Covers active sites on actin.

    • Troponin: Binds calcium ions.

Sarcolemma and Transverse Tubules

  • Sarcoplasm: Cytoplasm of muscle fiber.

  • Sarcolemma: Cell membrane of a muscle fiber.

  • Transverse Tubules (T-tubules):

    • Continuous with the sarcolemma.

    • Transmit action potential through cell, allowing contraction of the entire muscle fiber simultaneously.

    • Share same properties as the sarcolemma.

Sarcoplasmic Reticulum (SR)

  • A network of smooth endoplasmic reticulum surrounding each myofibril.

  • Pairs of terminal cisternae form perpendicular cross channels.

  • Functions in regulating intracellular Ca$^{2+}$ levels.

  • Concentrates Ca$^{2+}$ via ion pumps and releases it into sarcomeres to initiate muscle contraction.

Structural Components of a Sarcomere

  • Involvement in muscle contraction:

    • Initiating contraction: Ca$^{2+}$ binds to troponin, leading to the exposure of active sites on F-actin.

  • Thick filaments: About 300 twisted myosin subunits;

    • Contain titin strands that recoil after stretching.

  • Myosin Action During Contraction:

    • Myosin heads interact with actin, forming cross-bridges and pivoting, producing muscle motion.

Sliding Filament Theory of Muscle Contraction

  • Thin filaments slide towards M line alongside thick filaments.

  • The width of the A zone stays constant while Z lines move closer together.

Steps in Skeletal Muscle Contraction and Relaxation

Initiation of Muscle Contraction

  1. ACh released, binding to receptors at the synaptic terminal.

  2. Action potential reaches T-tubule; causes SR to release Ca$^{2+}$.

  3. Active site exposure; myosin binds to actin, initiating contraction.

  4. Steps in Muscle Relaxation:

    • ACh broken down by AChE.

    • Sarcoplasmic reticulum recaptures Ca$^{2+}$.

    • Active sites covered, preventing cross-bridge interaction, leading to relaxation.