Skeletal Muscle Organ Anatomy
Skeletal muscles, which are the organs of the skeletal muscle system,
include three layers of connective tissue that enclose and provide
structure to the muscle as a whole. The outer layer of each muscle fiber
is
wrapped
with
a
layer
of
connective
tissue
called
the epimysium ("epi-" means "outside" or "over," and "-mysium"
refers to "muscle"). The epimysium allows muscle to contract and move
powerfully while maintaining its structural integrity; it also separates
muscle from other tissues and organs. It is composed of a thick layer of
collagen fibers. The epimysium is surrounded by fascia, which is a type
of connective tissue that is found around body organs. Deep fascia
surrounds groups of muscles, sometimes joining with tendons to
strengthen the bone attachment, whereas superficial fascia lies between
muscle and skin. Some fascia contains adipose tissue that insulates and
protects muscle.
Inside each skeletal muscle, muscle fibers (a single muscle cell is called a
muscle fiber; not to be confused with fibers found in connective tissue)
are organized into groups called fascicles, or bundles of muscle fibers
(cells). Some of these fascicles can be seen without a microscope when a
muscle is cut open. Each fascicle contains many long muscle fibers
bound by a middle layer of connective tissue called the perimysium
("peri-" means "around"). Similar to the epimysium, the perimysium
contains collagen fibers, but the perimysium also contains elastic fibers.
Inside each fascicle, each individual muscle fiber is encased in a
connective tissue layer called the endomysium ("endo-" means
"inside"). The endomysium forms a thinner layer than the dense
epimysium and perimysium, and contains areolar and reticular tissues
which form loose, delicate networks. The endomysium of muscle fibers
connect to each other to form a loose complex within a fascicle. Small
blood vessels and motor neurons pass through the endomysium to
support and activate each muscle fiber.
The plasma membrane, or sarcolemma, of a skeletal muscle fiber is
located just under the endomysium. The sarcolemma is the site of action
potential conduction, which triggers muscle contraction. Within the
sarcolemma is the sarcoplasm, the cytoplasm of the muscle cell.
EXAMPLE
Muscular Dystrophy (MD) is a progressive weakening of skeletal
muscles. Duchenne’s Muscular Dystrophy (DMD), the most common
type of MD, is caused by a mutation in the dystrophin gene. Dystrophin
helps the thin filaments of myofibrils bind to the sarcolemma and
maintains equal force transmission through the muscle tissue. Without
sufficient dystrophin, muscle contractions cause the sarcolemma to tear,
causing an influx of Ca2+, leading to cellular damage and muscle fiber
degradation. Over time, muscles are damaged and functional
impairments develop.
DMD is an inherited disorder caused by an abnormal gene found on the
X chromosome. It is one of many genetic diseases which are referred to
as X-linked; X-linked disorders affect males since they only carry one
copy of the X chromosome, which they inherited from their mother. Girls
inherit one X chromosome from their mother and one from their father,
so they are usually not affected. DMD is usually diagnosed in early
childhood. It usually first appears as difficulty with balance and motion,
and then progresses to an inability to walk. It continues progressing
upwards in the body from the lower extremities to the upper body,
where it affects the muscles responsible for breathing. It ultimately
causes death due to respiratory failure, and those afflicted do not
usually live past their twenties.