Muscular Biopolymers - Engineered Biomimicry - page 139

A motor neuron plus all the muscle fibers it
stimulates is called a motor unit .
A typical skeletal muscle consists of hundreds
or thousands of very long cylindrical cells called
muscle ibers . The muscle fibers lie parallel to one
another and range from 10 to 100 µm in diam-
eter. While a typical length is 100 µm, some
muscle fibers are up to 30 cm long.
The sarcolemma is a muscle fiber's plasma
membrane, and it surrounds the muscle fiber's
cytoplasm or sarcoplasm. Because skeletal mus-
cle fibers arise from the fusion of many smaller
cells during embryonic development, each fiber
has many nuclei to direct synthesis of new pro-
teins. The nuclei are at the periphery of the cell
next to the sarcolemma, conveniently out of the
way of the contractile elements. The mitochondria
(energy packs) lie in rows throughout the mus-
cle fiber, strategically close to muscle proteins
that use ATP to carry on the contraction process.
Within the muscle fibers are myoibrils , which are
extended lengthwise in the sarcoplasm. Their
prominent light and dark band colors, called
cross-striations , make the whole muscle fiber
appear striped or striated.
Myofibrils form the contractile element of the
skeletal muscle. They are 1-2 µm in diameter and
contain three types of smaller filaments called
myofilaments. . These are thin, thick, and elastic
filaments, respectively. The thin filament is about
8 nm; the thick filaments are about 16 nm.

may even pull the thin filaments of each sarcomere
so far inward that their ends overlap. As the thin
filaments slide inward, structures called Z-discs
come toward each other and the sarcomere short-
ens but the lengths of thick and thin filaments
do not change. The sliding of the filaments and
shortening of the sarcomere cause shortening of
the whole muscle fiber and ultimately the entire
muscle.
An increase in Ca 2 + concentration in the sar-
coplasm starts filaments sliding, whereas a
decrease turns off the sliding process. When a
muscle fiber is relaxed, the concentration of Ca 2 +
in the sarcoplasm is low. This is because the
sarcoplasmic reticulum (SR) membrane contains
Ca 2 + active transport pumps that remove Ca 2 +
from the sarcoplasm. Ca 2 + is stored or seques-
tered inside the SR. As a muscle action potential
travels along the sarcolemma and into the trans-
verse tubule system, Ca 2 + release channels open
in the SR membrane. The result is a flood of Ca 2 +
from within the SR into the sarcoplasm around
the thick and thin filaments. The calcium ions
released from the SR combine with troponin,
causing it to change shape. This shape change
slides the troponin-tropomyosin complex away
from the myosin-binding sites on actin.
Muscle contraction requires Ca 2 + . It also
requires energy in the form of ATP. ATP attaches
to ATP-binding sites on the myosin cross-bridges
(heads). A portion of each myosin head acts as
an ATPase, an enzyme that splits the ATP into
ADP + P , where P symbolizes the terminal phos-
phate group PO 3 4 and ADP is adenosine diphos-
phate, through hydrolysis reaction. This reaction
transfers energy from ATP to the myosin head
even before contraction begins. The myosin
cross-bridges thus are in an activated (energized)
state. Such activated myosin heads spontane-
ously bind to the myosin-binding sites on actin
when the Ca 2 + level rises and tropomyosin slides
away from its blocking position. The shape
change that occurs when myosin binds to actin
produces the power stroke of contraction. Dur-
ing the power stroke, the myosin cross-bridge

6.1.2 Muscle Contraction
In the mid-1950s, Jean Hanson and Hugh Huxley
proposed the sliding filament mechanism of mus-
cle contraction. They stated that skeletal muscle
contraction was due to thick and thin filaments
sliding past one another. During muscle contrac-
tion, myosin cross-bridges pull on the thin fila-
ments, causing them to slide inward toward what
is called the H-zone . As the cross-bridges pull on
the thin filaments, the thin filaments meet at the
center of the sarcomere. The myosin cross-bridges