Microscopic Anatomy of Skeletal Muscle
This is a hand drawn cross section of a skeletal muscle. The diagram depicts the outer covering of a muscle as epimysium, a dense connective tissue The inner bundles of muscle fibers are also called fascicles. The each fascicle covering is made up of perimysium. Within the fascicles are individual muscle fibers. Each muscle fiber is covered with endomysium.
This skeletal muscle cross section is preserved with a collagen stain to bring out the perimysium between the fascicles. Each muscle is composed of thousands of fibers (E), each with a covering of loose connective tissue—endomysium (B). The fibers are organized in parallel bundles called fascicles (D). Each fascicle is surrounded by a perimysium sheath (A); the presence of collagen bundles suggests dense connective tissue. Multiple fascicles are encased in a dense connective tissue—epimysium, which covers the entire muscle organ. Blood vessels, like the one shown at (C), and nerve fibers penetrate to the endomysium to supply the muscle fibers.
This is a closer look of individual muscle fibers that were part of a fascicle. Around each of these fibers is a sheath of endomysium. Skeletal muscle cells are multi-nucleated. Only in a longitudinal cut will you be able to see the multiple nucleus. Within each individual muscle fiber are myofibrils. Therefore, a bundle of myofibrils is one muscle fiber.
The striation in skeletal muscle is due to the presence of a dark and light colored band, respectively they are known as A band and I band. The sacrolemma is the plasma membrane around the muscle fiber.
section of skeletal muscle: The individual muscle fiber
(C) originates from a fused series (syncytium) of myoblasts surrounded by a
common sarcolemma. Because of this, multiple nuclei (B) are found
within each fiber (cell).
The volume of
the cell mostly comprises of contractile filaments (myofilaments); the
nuclei are displaced to the periphery. Myofilaments have subunits called sarcomeres— responsible for the banded
appearance of the cell (striations). The
alternating bands of light and dark (see below) are due to the microfilaments actin
(thin) and myosin (thick). The
light bands are where only actin is present, and the dark bands contain both
actin and myosin. To remember,
the I band is “illuminated,” and the A band is “anilluminated.” The individual cell is wrapped in an endomysium (D), which is
penetrated by capillaries (A) to nourish the cell.
The Anatomy of a sacromere: The thick filament produce the dark A band. The thin filament extends in each direction from the Z line. Where they do not over lap the thick filaments, this creates the light I band. The A band is bisected by the H zone. The I bands are bisected by the A line. The H zone is that portion of the A band where the thick and thin filaments do not overlap. The entire arrangement of thick and thin filaments between the Z lies is what makes up one sacromere. Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fiber of which it is a part.