Skeletal Muscle Basics

Muscle Structure and Function

• A/I bands and H zones are fused together in skeletal muscle.
• DHPR (Dihydropyridine receptor) is a voltage-gated sensor in the T-tubule membrane.
• RYR (Ryanodine Receptor) is a Ca2+ release channel in the SR membrane.

Muscle Fiber Components

• Myofibrils: site of contraction, consisting of thick and thin filaments.
• Thick filaments: • Made up of myosin, composed of 2 heavy chains and 4 light chains. • Heavy chain is the motor domain for binding ATP and releasing to create movement.
• Thin filaments: • Actin: a globular protein that polymerizes to form F-actin filaments. • Tropomyosin: a strand protein wrapped spirally around F-actin filaments. • Troponin: a complex of 3 subunits (T, I, and C) that regulate muscle contraction.

Troponin and Muscle Contraction

• Troponin-C binds to 4 Ca2+ ions, initiating muscle contraction.
• In the absence of Ca2+, tropomyosin inhibits myosin head attachment to actin, preventing contraction.
• When Ca2+ binds to troponin, the complex shifts, allowing myosin heads to bind to actin and initiate contraction.

Skeletal Muscle Characteristics

• Contract in response to signals from somatic motor neurons.
• Cannot initiate their own contraction.
• Functions: movement, maintaining posture, supporting soft tissue, and storing nutrient reserves.

Skeletal Muscle Structure

• Consists of muscle fibers developed from myoblasts.
• Surrounded by connective tissue (epimysium, perimysium, and endomysium).
• Nerves: controlled by somatic nervous system, with each fiber supplied with nerve endings that control contraction.
• Blood vessels: extensive vascular system supplies oxygen and nutrients and removes waste.

Muscle Fiber Components

• Sarcolemma: maintains transmembrane potential.
• Sarcoplasm: rich in glycogen and myoglobin, providing ATP for muscle contraction.
• Myofibrils and sarcoplasmic reticulum (SR): site of muscle contraction and Ca2+ storage.

Transverse Tubules (T-Tubules) and Sarcoplasmic Reticulum (SR)

• T-tubules: passageways from extracellular space to terminal cisternae of SR, allowing nerve message and nutrient passage.
• SR: stores Ca2+ at rest, releasing it upon stimulation, and pumping it back in the absence of stimulation.

Test your knowledge on the basics of skeletal muscles, including their characteristics, functions, and mechanisms of contraction.