Structure of Skeletal Muscle

Each skeletal muscle is composed of a series of individual muscle fibers tightly bound together; each fiber represents a single muscle cell. Fiber length and thickness vary with the muscle's specific function. The actual size of a muscle is determined by two factors:

1. The total number of fibers in that muscle; and
2. The size of each individual fiber.

The structural hierarchy (Wilmore & Costill, 1994), from outside in, includes:

• Epimysium — connective sheath around the whole muscle
• Fascicle (fasciculus) — a bundle of fibers, wrapped by the perimysium
• Fibre — the muscle cell, wrapped by the endomysium
• Myofibril — within the fiber
• Myofilament — actin and myosin
• Structural units of the myofibril: A bands, H-zone, Z-line, and the sarcomere (the repeating contractile unit), made of actin and myosin

Muscle ends are made of dense, tough tissue known as tendons, which attach the muscles to the bones.

• Each fiber = one muscle cell; fibers bound together form the muscle.
• Muscle size set by (1) total number of fibers and (2) size of each fiber.
• Connective sheaths: epimysium (muscle), perimysium (fascicle), endomysium (fiber).
• Fiber → myofibrils → myofilaments (actin and myosin).
• Sarcomere = repeating contractile unit; bounded by Z-lines, contains A bands and H-zone.
• Tendons (dense tough tissue) attach muscle to bone.

• muscle-contraction-nature — how these fibers shorten (all-or-none, summation).
• active-tension — tension from actin/myosin crossbridges.
• length-tension-relationship — actin/myosin overlap and crossbridge count.
• muscle-fiber-types — Type I/II fiber differences.
• three-muscle-types — skeletal vs. smooth vs. cardiac muscle.