Skeletal muscle is a striated, voluntary muscle that makes up ~40-50% of total body mass. It is organized hierarchically from whole muscle → muscle fascicles → muscle fibers (cells) → myofibrils → sarcomeres → filaments (actin and myosin).
Hierarchical Organization
Muscle → Fascicle → Fiber
Muscle: Whole organ (e.g., biceps brachii) wrapped in epimysium (dense connective tissue)
Fascicle: Bundle of muscle fibers wrapped in perimysium
Muscle fiber (cell): Long, cylindrical, multinucleated cell wrapped in endomysium
Fiber → Myofibril → Sarcomere → Filaments
Myofibrils: Long cylindrical organelles filling the fiber; aligned in parallel; give the striated appearance
Sarcomere: Basic contractile unit, bounded by Z discs
Sarcomere Components
Z disc: Anchors thin filaments (actin); defines sarcomere boundaries
I band: Region of thin filaments only (light)
A band: Region of thick filaments (myosin); entire thick filament length
H zone: Center of A band with thick filaments only
M line: Center of sarcomere; links thick filaments together
Memory aid: "Zee Is AHeavy Man" (Z disc, I band, A band, H zone, M line)
Molecular Components
Thick Filaments (Myosin)
Myosin II structure: Two heavy chains (tail + heads) + four light chains
Myosin heads: Contain ATPase activity and actin-binding sites
Skeletal muscle's striated appearance comes from the precise arrangement of actin and myosin in sarcomeres. Memorize: I band = thin only, A band = thick (and thin where overlapping), H zone = thick only, M line = center, Z disc = boundary. DMD vs MG vs LEMS is a classic exam trio.