Skeletal Muscle

Skeletal muscles are in association with bones forming the skeletal system. These muscles form from 40 to 50% of body mass. In human beings, about 600 muscles are identified. Skeletal muscles are voluntary and striated. These muscles are supplied by somatic nerves.

Striations, control and nerve supply of muscles

Muscle	Striations	Control	Nerve supply
Skeletal muscle	Present	Voluntary	Somatic nerves
Cardiac muscle	Present	Involuntary	Autonomic nerves
Smooth muscle	Absent	Involuntary	Autonomic nerves

Myofibrils or myofibrillae are fine parallel filaments present in sarcoplasm of the muscle cell. Myofibrils run through the entire length of the muscle fiber.

In cross section of a muscle fiber, the myofibrils are separated from one another by sarcoplasm. In some muscle fibers, some of the myofibrils are arranged in groups. These groups of myofibrils are called Cohnheim's areas or fields. The diameter of the myofibril is 0,2 to 2,0 microns. And, the length of a myofibril varies between 1,0 to 4,0 cm depending on length of the muscle fiber.

MICROSCOPIC STRUCTURE OF MYOFIBRIL

Light microscopic studies show that, each myofibril consists of a number of alternating light and dark bands. These bands are otherwise called the sections, segments or discs.

Dark band is called “A” band. “A” band is anisotropic. If polarized light is passed through the muscle fiber at this area, the light rays are refracted at different directions (An = not; iso = it; trops = turning). Light band is isotropic. Rays of polarized light, passed through the muscle fiber at this area, are refracted at the sail angle. So, this band is called “I” band.

The light band is otherwise called J band and the dark bands called Q disc (Querscheibe = cross disc).

In an intact muscle fiber, “I” band and “A” band of adjacent myofibrils are placed side by side. This gives the appearance of characteristic cross striations in muscular fiber.

A narrow lighter area called “H” zone (H = hell = light-in German, “H” = Henson - discoverer) is seen at the middle of “A” band. “I” band is divided into two by a narrow line called “Z” line (in German Zwischenscheibe = between discs). The portion of myofibril in between two “Z” lines is called sarcomere.

Fig.8. Structure of skeletal muscle.

Sarcomere

Sarcomere is structural and functional unit of the skeletal muscle. Each sarcomere extends between two “Z” lines of myofibril. Thus, each myofibril contains many sarcomeres arranged in series throughout the length of myofibril.

Each myofibril consists of alternated dark “A” band and light “I” band (Fig. 8). In the middle of “A” band, there is a light area called “H” zone. In the middle of “H” zone middle part of myosin filament lies. It is called “M” line. Similarly, “'I” band is divided into two equal portions by means of a narrow line called “Z” line. Part of myofibril between two “Z” lines is sarcomere. When muscle is in relaxed state, the average length of each sarcomere is 2 to 3 microns.