Chapter 9
Muscles and Muscle Tissue
283
9
Epimysium
Fascicle
Muscle
Tendon
Part of fascicle
Perimysium
Muscle fiber
Part of muscle fiber
Endomysium
Nucleus
Sarcolemma
Myofibril
Sarcomere
Sarcomere
Thin (actin) filament
Thick (myosin) filament
Thin filament
Thick filament
Head of myosin molecule
Actin molecules
Table 9.1
Structure and Organizational Levels of Skeletal Muscle
STRUCTURE AND ORGANIZATIONAL LEVEL
DESCRIPTION
CONNECTIVE
TISSUE WRAPPINGS
Muscle
(organ)
A muscle consists of hundreds to thousands of
muscle cells, plus connective tissue wrappings,
blood vessels, and nerve fibers.
Covered externally by
the epimysium
Fascicle
(a portion of the muscle)
A fascicle is a discrete bundle of muscle cells,
segregated from the rest of the muscle by a
connective tissue sheath.
Surrounded by
perimysium
Muscle Fiber
(cell)
A muscle fiber is an elongated multinucleate cell;
it has a banded (striated) appearance.
Surrounded by
endomysium
Myofibril or Fibril
(complex organelle composed of bundles of
myofilaments)
Myofibrils are rodlike contractile elements that
occupy most of the muscle cell volume. Composed
of sarcomeres arranged end to end, they appear
banded, and bands of adjacent myofibrils are
aligned.
Sarcomere
(a segment of a myofibril)
A sarcomere is the contractile unit, composed of
myofilaments made up of contractile proteins.
Myofilament or Filament
(extended macromolecular
structure)
Contractile myofilaments are of two types—
thick and thin. Thick filaments contain bundled
myosin molecules; thin filaments contain actin
molecules (plus other proteins). The sliding of the
thin filaments past the thick filaments produces
muscle shortening. Elastic filaments (not shown
here) maintain the organization of the A band
and provide elastic recoil when muscle
contraction ends.
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