A single skeletal muscle cell is known as a
Each muscle ﬁ ber is formed during development by the fusion
of a number of undifferentiated, mononucleated cells, known
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
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
remaining muscle ﬁ bers.
refers to a number of muscle ﬁ
bound together by connective tissue (
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
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.
Thick (myosin) filament
Thin (actin) filament
Structure of skeletal muscle.