Anatomy of Skeletal Muscle

in order for us to fully understand our topic it's better we learn the basic anatomy of the skeletal muscle.

Anatomy of Skeletal Muscle
A single skeletal muscle, such as the triceps muscle, is attached at its

    * origin to a large area of bone; in this case, the humerus.
    * At its other end, the insertion, it tapers into a glistening white tendon which, in this case, is attached to the ulna, one of the bones of the lower arm.

As the triceps contracts, the insertion is pulled toward the origin and the arm is straightened or extended at the elbow. Thus the triceps is an extensor. Because skeletal muscle exerts force only when it contracts, a second muscle — a flexor — is needed to flex or bend the joint. The biceps muscle is the flexor of the lower arm. Together, the biceps and triceps make up an antagonistic pair of muscles. Similar pairs, working antagonistically across other joints, provide for almost all the movement of the skeleton.

The Muscle Fiber (Figure 1)

Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. The fibers are bound together by connective tissue through which run blood vessels and nerves.
Each muscle fibers contains:

    * an array of myofibrils that are stacked lengthwise and run the entire length of the fiber;
    * mitochondria;
    * an extensive smooth endoplasmic reticulum (SER);
    * many nuclei.

The multiple nuclei arise from the fact that each muscle fiber develops from the fusion of many cells (called myoblasts).

The number of fibers is probably fixed early in life. This is regulated by myostatin, a cytokine that is synthesized in muscle cells (and circulates as a hormone later in life). Myostatin suppresses skeletal muscle development. Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene. These discoveries have already led to the growth of an illicit market in drugs supposedly able to suppress myostatin.

In adults, increased strength and muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue. In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.
Because a muscle fiber is not a single cell, its parts are often given special names such as

    * sarcolemma for plasma membrane
    * sarcoplasmic reticulum for endoplasmic reticulum
    * sarcosomes for mitochondria
    * sarcoplasm for cytoplasm

although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells.

    * The nuclei and mitochondria are located just beneath the plasma membrane.
    * The endoplasmic reticulum extends between the myofibrils.

Seen from the side under the microscope, skeletal muscle fibers show a pattern of cross banding, which gives rise to the other name: striated muscle.

The striated appearance of the muscle fiber is created by a pattern of alternating

    * dark A bands and
    * light I bands.

    * The A bands are bisected by the H zone running through the center of which is the M line.
    * The I bands are bisected by the Z disk.

Each myofibril is made up of arrays of parallel filaments.

    * The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin.
    * The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins:
          o troponin and
          o tropomyosin.

The anatomy of a sarcomere
The entire array of thick and thin filaments between the Z disks is called a sarcomere.

    * The thick filaments produce the dark A band.
    * The thin filaments extend in each direction from the Z disk. Where they do not overlap the thick filaments, they create the light I band.
    * The H zone is that portion of the A band where the thick and thin filaments do not overlap.
    * The M line runs through the exact center of the sarcomere. Molecules of the giant protein, titin, extend from the M line to the Z disk. One of its functions is to provide a scaffold for the assembly of a precise number of myosin molecules in the thick filament (294 in one case). It may also dictate the number of actin molecules in the thin filaments.