Skeletal Muscle Notes

Focus on skeletal muscle after covering cardiac and smooth muscle.
Understanding the arrangement of muscles from macroscopic to microscopic levels.

Muscle belly with tendon.
Epimysium: A membrane wrapped around the entire muscle belly for protection.
- Epi means upon or wrapped around.
- Acts as a protective sheath.
Muscle belly consists of muscle fascicles, resembling strands of spaghetti.

Muscle fascicles are bundles of muscle fibers.
Perimysium: Protective membranous sheets around muscle fascicles, separating them.

Epimysium: Around the entire muscle belly.
Perimysium: Around muscle fascicles.
Endomysium: Around individual muscle fibers; endo meaning within.
All membranes blend to form part of the tendon.
These membranous sheaths shorten when the muscle belly shortens, pulling on tendons and bones, creating movement.
Relaxation of muscle leads to relaxation of membranous sheaths and tendons, returning bones to a resting state.

Tendons anchor muscle to bone.
Without tendons, muscles cannot connect and are rendered useless.
Example: Gastrocnemius (calf muscle) with medial and lateral components blending into the Achilles tendon (calcaneal tendon).
Achilles tendon is formed by epimysium, endomysium, and perimysium blending together
Tendons increase the length of a muscle, potentially increasing its force production (leverage momentum).
Ongoing research on tendinopathy and the role of tendons in muscle strain.
Taut or weak tendons may impact muscle flexibility, strength, and injury.
Gastrocnemius is a postural muscle that stabilizes lower limb mechanics.

Muscle belly → Fascicle → Muscle Fiber → Myofibrils
Myofibrils contain myofilaments (actin and myosin), which are responsible for contractile properties.
Myofilaments give skeletal muscle its striated appearance.

Muscle fibers are long and cylindrical, running the entire length of the muscle belly (e.g., 35 cm).
Muscle fibers are multinucleated with nuclei on the peripheries to relay information effectively.
A group of fibers forms a fascicle, and fascicles make up the muscle belly.
Muscle fiber size varies (up to 30 cm long, or as small as 1 mm for fine muscles like those moving the eye).

Myosatellite cells on the periphery of muscle fibers aid in regeneration and development.
Muscle damage (tearing) during strength training leads to regeneration of new muscle tissue.
Myoblasts and myosatellite cells repair damaged tissue, developing immature muscle cells into mature ones.
Increasing muscle size (hypertrophy) involves increasing the number of muscle fibers by repairing damaged muscle tissue.

Muscle fiber = Muscle cell.
Sarcolemma: Plasma membrane of a skeletal muscle cell, isolating it from other fibers and monitoring substance movement.
Sarcoplasm: Gel-like substance within the cell, similar to cytoplasm but specialized for skeletal muscle.
Transverse T Tubules: Tunnel networks within the sarcolemma, relaying neural stimulus throughout the muscle fiber.
- These tubules wrap around each myofibril.
Mitochondria: Densely packed within muscle fibers to provide energy (ATP) for contraction and relaxation.
- The number of mitochondria can vary based on training type (aerobic vs. anaerobic).
Myofibrils: Smaller components within muscle fibers.
- Myofibrils run the entire length of the muscle cell and contain actin and myosin filaments.
Sarcoplasmic Reticulum: Wraps around myofibrils and stores calcium, which is essential for muscle contraction.
- T tubules are dispersed between them.
- Calcium is released from the sarcoplasmic reticulum into the myofibrils upon neural stimulation.

Thick myofilaments: Myosin.
Thin myofilaments: Actin, troponin, and tropomyosin.
Arrangement of thick and thin filaments creates the striated appearance of muscle.
Myofilaments don't run the entire length of the muscle fiber but are compartmented into sarcomeres.

Sarcomeres: Little compartments or carriages that house myosin, actin, troponin, and tropomyosin.
Sarcomeres link together to run the entire length of the muscle fiber.
Contractile proteins (actin and myosin) within each sarcomere.
Regulatory proteins (troponin and tropomyosin).

Myosin (thick filaments).
Actin, troponin, and tropomyosin (thin filaments).
Overlap of filaments creates the striated appearance.
Structural proteins, such as dystrophin, maintain the integrity of the sarcomere.
- Muscular dystrophy is caused by a lack of dystrophin, leading to the collapse of sarcomeres and loss of muscle function.