102
UNIT 2
Basic Tissues
Skeletal Muscle
Figure 6-2.
Organization of skeletal muscle.
A single skeletal muscle (e.g., the biceps) is composed of numerous
fascicles
(“small bundles”). The muscle as a whole is enveloped
in a strong layer of dense connective tissue, the
epimysium
. Each fascicle consists of a large number of
muscle ±
bers
(cells) and is
surrounded by a sheet of less dense connective tissue, the
perimysium
(Fig. 6-2A). Muscle ±
bers are unusual among the cells of
the body in that each contains a large number of nuclei (see Fig. 6-3A), and the nuclei are located around the periphery of the cell
(Figs. 6-2B and 6-3B). Each muscle ±
ber is enveloped by a thin layer of delicate connective tissue, the
endomysium
. An individual
muscle ± ber contains many
myo± brils
, which, in turn, consist of an array of regularly organized thick and thin
myo±
laments
, the
contractile elements of the muscle (Fig. 6-2D). The myo±
laments are visible only with the electron microscope (Figs. 6-2C and 6-4B).
Thick myo± laments are composed of clusters of
myosin
molecules, and the thin myo±
laments are predominantly
actin
molecules
but contain some additional auxiliary molecules that are important for the contraction process. In cross section, the myo±
laments
are arranged in a repeating pattern so that each threadlike cluster of myosin molecules is surrounded by six actin molecules in a
hexagonal array, and the myosin molecule clusters themselves are arranged in a hexagonal array (Fig. 6-4B). Longitudinally, the
actin and myosin molecules form repeating units called
sarcomeres
(Fig. 6-2C). Actin ±
laments are anchored at one end in the
Z line
,
a transverse membrane-like structure. Myosin molecules lie parallel to the actin molecules and partially overlap the actin molecules
that are attached to two adjacent Z lines (Fig. 6-4B). The region in which the myosin and actin overlap is designated the
A band
,
and the region in which only actin molecules are present is designated the
I band
(Figs. 6-2C and 6-4A,B). Muscle contraction is the
result of chemical interactions between the myosin and actin molecules.
Myofibril
Tendon
Muscle
Epimysium
(surrounds entire muscle)
Fascicle
(bundle of
muscle fibers)
Perimysium
(separates fascicles)
Endomysium
(envelops single muscle cell)
Muscle fiber
(muscle cell)
Myofilaments
Myosin
Actin
Z line
Sarcomere
I band
A band
Muscle fiber
(muscle cell)
B
C
A
D
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