Skeletal Muscle Structure

  • Overview: Each skeletal muscle is classified as an organ composed of various integrated tissues.

    • Constituents: These tissues include:

    • Skeletal muscle fibers

    • Blood vessels

    • Nerve fibers

    • Connective tissue

  • Connective Tissue Layers (Mysia): Skeletal muscles are surrounded by three layers of connective tissue that provide both structure and compartmentalization:

    • Epimysium:

    • Description: A sheath of dense, irregular connective tissue.

    • Functions:

      • Allows powerful muscle contraction and movement while maintaining structural integrity.

      • Separates the muscle from surrounding tissues and organs, permitting independent movement.

    • Perimysium:

    • Description: A connective tissue layer that encases bundles of muscle fibers called fascicles.

    • Endomysium:

    • Description: A thin connective tissue layer surrounding each muscle fiber.

    • Contains extracellular fluid and nutrients for muscle fiber support, supplied by the blood.

Fascicular Organization

  • Fascicles: Inside the skeletal muscle, muscle fibers are organized into bundles known as fascicles.

    • Function:

    • Allows the nervous system to control specific movements by activating selected muscle fibers within a fascicle.

Muscle Fiber Structure

  • Skeletal Muscle Fibers: Also known as muscle fibers due to their long and cylindrical shape.

    • Size: Fibers can have:

    • Diameters up to 100 μm.

    • Lengths up to 30 cm (11.8 in) in the Sartorius muscle of the upper leg.

    • Development:

    • Formed from the fusion of multiple embryonic myoblasts, each contributing a nucleus, resulting in multinucleated cells to facilitate the production of proteins and enzymes necessary for contraction.

Patterns of Fascicle Organization

  • Connective Tissue Scaffolding: Skeletal muscle is enveloped in connective tissue at three levels.

    • Endomysium: Covers individual muscle fibers.

    • Perimysium: Encases groups of muscle fibers into fascicles.

    • Epimysium: Encloses the entire muscle.

  • Fascicle Arrangement & Classification: The arrangement of fascicles influences force generation and range of motion, leading to several types of classification:

    • Parallel Muscles: Fascicles run parallel to the muscle's long axis. Types include:

    • Flat sheets that expand at the ends (e.g., broad attachments).

    • Rotund muscles with tendons at each end.

    • Muscles with a central belly; e.g., biceps brachii.

    • Fusiform: A type of parallel muscle with a spindle shape (tapering ends).

    • Circular Muscles (Sphincters): When relaxed, they increase the size of openings; when contracted, they decrease them.

    • Examples: Orbicularis oris (around the mouth) and orbicularis oculi (around the eyes).

    • Convergent Muscles: Widespread fibers converge to a single attachment point.

    • Example: Pectoralis major, attaching to the greater tubercle of the humerus.

    • Pennate Muscles: Fascicles align with a central tendon at an angle, allowing for more fibers but less range of motion. Types include:

    • Unipennate: Fascicles are on one side of the tendon (e.g., extensor digitorum).

    • Bipennate: Fascicles are on both sides of the tendon.

    • Multipennate: Muscle fibers wrap around the tendon (e.g., deltoid muscle).

Muscle Tendon/Conection Details

  • Ligaments: Connect bones together, providing stability to joints.

    • Structure: Appear as crisscross bands; somewhat elastic, allowing joint movement without exceeding capacity.

    • Examples: Knee joint ligaments stabilize it, preventing excessive movement.

  • Tendons: Connect muscles to bones.

    • Structure: Tough cords with some elasticity, allowing movement upon muscle contraction.

    • Function: Absorb impact during muscle actions.

Injuries

  • Sprains: Ligament injuries resulting from overstretching or tearing.

    • Common Locations: Ankle, knee, wrist.

    • Symptoms: Pain, swelling, bruising; instability of the joint.

    • Grades:

    • Grade 1: Mild sprain (slight stretch).

    • Grade 3: Complete tear (serious instability).

  • Strains: Tendon injuries due to overstretching or tearing.

    • Common Areas: Leg, foot, back.

    • Symptoms: Similar to sprains; muscle cramping and weakness may occur.

  • Tendonitis: Inflammation of a tendon, often related to aging or overuse.

    • Common in: Athletes, e.g., golfers and baseball pitchers.

    • Symptoms: Pain on movement, swelling, warmth in the affected area.