Automated Methods for the Analysis of Skeletal Muscle Fiber Size and Metabolic Type - International Review of Cell and Molecular Biology - page 272

Figure 7.1 The structure of skeletal muscle. (A) An overview of skeletal muscle structure
is shown; skeletal muscle features a “cable-within-a-cable” structure, in which individual
muscle fibers are aligned in parallel with the length of the tissue and are ensheathed by
extracellular matrix proteins. (B) A section of skeletal muscle (vastus lateralis muscle
from rhesus monkey), cut at 90 to the longitudinal axis of the muscle, is shown labeled
for laminin (green) and for slow-type myosin (red). Each enclosed ring in the green
image represents a separate muscle fiber. The red muscle fibers are Type I fibers, which
are specialized for endurance performance and feature metabolism based primarily
upon oxidative phosphorylation.
run parallel to one another, along the longitudinal axis of the muscle
( Fig. 7.1 A). The contractile muscle fibers are ensheathed by variety of
extracellular matrix proteins, which help guide development and support
maintenance of the muscle (e.g., laminin, elastin, fibronectin, and type IV
collagen) or provide structural support and protection (e.g., type
I collagen). When a muscle is sectioned at a 90 angle to the longitudinal
axis of the muscle and labeled for extracellular matrix proteins or proteins
that are found at the cellular membrane of the fibers (sarcolemma),
a “cobble-stone” type pattern is observed, in which the open portions
of the pattern represent cross sections through the muscle fibers
( Fig. 7.1 B). The structure of skeletal muscle is analogous to the “cables-
within-a-cable” design of support cables for suspension bridges, each of
which contains hundreds of small cables, running in a parallel direction.
Contraction occurs when the muscle is stimulated by neuronal input at
neuromuscular junctions, which leads to increases in cytoplasmic calcium
and activation of myosin, the main component of the “thick filaments”;
the activated myosin binds to and slides along actin, the main component
of “thin filaments,” resulting in an overall shortening of the fiber length
( Clark et al., 2002 ).
Muscle fibers can be distinguished into two basic types, which were
initially distinguished based upon the speed of contraction, which are