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(A)

Muscle fasciculus
(B)

(C)
Muscle fiber
H Z A I
band disc band band

(D) Z Sarcomere Z Myofibril
G-actin molecules
(J)

Myofilaments
F-actin filament
(K)

(E) (L)
Myosin filament

Myosin molecule
(M)

(N)
Light Heavy
(F) (G) (H) (I) meromysin meromysin
Figure 9-3. Organization of skeletal muscle from the gross to the molecular level. Skeletal muscle (A) is
organized in to bundles (fascicles) of muscle fibers, where the functional unit of contraction is the sarcomere
(E). Cross-sectional views (F-I) of the sarcomere in (E) are represented. The organization of the myosin fila-
ment (L) and actin filament (K) and their respective components are shown. Actin monomers (J) polymerize
to form filaments (K), whereas the myosin filament is formed by a series myosin molecules (M) comprised of
heavy and light meromysin subunits (N). Source: adapted from Guyton and Hall, 2006. Reproduced with
permission from Elsevier.

The sarcoplasmic reticulum is agranular glycogen inclusions also are found in mus-
(contains no ribosomes) and functions cle fibers.
in excitation–contraction coupling (dis- The transverse tubules (or T systems) are
cussed later in this chapter). A Golgi appa- continuous with the plasma membrane and
ratus, large numbers of mitochondria, and extend into the interior of the muscle fiber