256
Chapter 9
A single skeletal muscle cell is known as a
muscle fi
ber.
Each muscle fi ber is formed during development by the fusion
of a number of undifferentiated, mononucleated cells, known
as
myoblasts,
into a single cylindrical, multinucleated cell.
Skeletal muscle differentiation is completed around the time of
birth, and these differentiated fi
bers continue to increase in size
from infancy to adulthood, but no new fi bers are formed from
myoblasts. Adult skeletal muscle fi bers have diameters between
10 and 100 μm and lengths that may extend up to 20 cm.
If skeletal muscle fi bers are destroyed after birth as a
result of injury, they cannot be replaced by the division of
other existing muscle fi bers. New fi bers can be formed, how-
ever, from undifferentiated cells known as
satellite cells,
which
are located adjacent to the muscle fi bers and undergo differen-
tiation similar to that followed by embryonic myoblasts. This
capacity for forming new skeletal muscle fi bers is considerable
but will generally not restore a severely damaged muscle to full
strength. Much of the compensation for a loss of muscle tissue
occurs through an increase in the size
(hypertrophy)
of the
remaining muscle fi bers.
The term
muscle
refers to a number of muscle fi
bers
bound together by connective tissue (
Figure 9–2
). The
relationship between a single muscle fi ber and a muscle is
analogous to that between a single neuron and a nerve, which
is composed of the axons of many neurons. Skeletal muscles
are usually attached to bones by bundles of collagen fi bers
known as
tendons.
In some muscles, the individual fi bers extend the entire
length of the muscle, but in most, the fi bers are shorter, often
oriented at an angle to the longitudinal axis of the muscle.
The transmission of force from muscle to bone is like a num-
ber of people pulling on a rope, each person corresponding to
a single muscle fi ber and the rope corresponding to the con-
nective tissue and tendons.
Some tendons are very long, with the site where the tendon
attaches to the bone far removed from the end of the muscle.
Sarcomere
M line
Z line
Z line
Z line
Z line
H zone
Actin
Tropomyosin
Troponin
Cross-bridge
Thick (myosin) filament
Thin (actin) filament
Figure 9–2
Structure of skeletal muscle.
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