| Skeletal muscles generally are
connected to the bones of the skeleton by tendons. The part
of the muscle generating the force is the body. The body
contains bundles (fascicles) of muscle cells. Muscle cells are
muscle fibers and are multinucleate. The plasma
membrane is called the sarcolemma.
| The contractile component of muscle
cells is contained within rod-like elements called myofibrils. Myofibrils
have overlapping thick and thin filaments myosin and actin,
The smooth endoplasmic reticulum surrounding the myofibrils is
called sarcoplasmic reticulum which is closely associated
with inward extensions of the sarcolemma called transverse
| Skeletal muscle is striated muscle
due to the orderly arrangement of thick and thin filaments that run
parallel to the long axis of the fiber. Myofibrils are also composed
of repeating units called sarcomeres. Each sarcomere is
bordered by Z lines that anchor the thin filaments. M
lines are in the center of the sarcomeres.
sarcomere is banded with the
|A band -
||appears dArk and is the length of the
|H zone -
||light region in the center of the A band
|I band -
||lIght band where only thin filaments are
| Actin (thin) and myosin (thick)
filaments are attached by cross bridges. Thin filaments are
composed of globular actin linked to form helical strands.
Two regulatory proteins are associated with actin. Tropomyosin
extends over and covers binding sites on actin subunits. Troponin,
a complex of three proteins, uncovers binding sites when bound
Mechanism of Filament Sliding
| Each myosin molecule is composed
of two parts (dimer) each part consisting of a tail twisted around
the other and a head. A thick filament consists of pairs
of myosin molecules with each pair attached by the ends of their tails.
These pairs of myosin molecules are bundled together so that their
heads protrude in a helical pattern at either end with a bare
zone in the center. The head of the myosin
molecule has a site that binds to actin to form crossbridges,
and an ATPase site that
| Extending along the length of each thick
filament from the M line to each Z line is an elastic protein called
titin. Titin gives the sarcomere elasticity so that when it
is stretched it returns to its original position when it is relaxed.
Titin also anchors the thick filaments in the proper position.