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Structure of Skeletal Muscle

Skeletal muscles remain attached at the firm and non-movable part and the freely movable part of the bones. The tendons by which they are attached to the bones, are made up of connective tissue comprised of collagen. Skeletal muscle consists of numerous physiological units, called fibre or muscle cells. Bundles of muscle fibres are grouped as fascicule, which are held together and enclosed by collagen fibres and by connective tissue. The bundle of fasciculi is surrounded by a tough external layer called ascia, lying below the skin. Each muscle fibre is an elongated slender cell. It is syncytial, i.e. multinucleate. Its membrane is called sarcolemma and its cytoplasm is called sarcoplasm. The mitochondria of the cell are called sarcosomes and the endoplasmic reticulum is designated as sarcoplasmic reticulum. Each fibre contains many of thin and rod-like myofibrils, which bear characteristic cross-striations.

The myofibrils are arranged in a number of sections, called sarcomeres, joined end to end all along the length of the muscle fibre. Electron microscope reveals that the sarcomeres are delineated by a very thin and comparatively dense Z-line. A dark anisotropic band (A-band) is present in the center of the sarcomere. Adjacent to this lies a light isotropic band (I-band). Alternate arrangement of dark and light bands gives the striated appearance to a skeletal muscle. At the center of the A-band, a comparatively less dark zone called H-zone is present. The M-line is present in the center H-zone, is formed by threads that connect the myofilaments. The Z-line is located at the center of the I-band.

The sarcoplasm contains many thick and thin filaments. In each sarcomere, the thin filaments are present at the two ends, whereas the thick filaments are located centrally. The two adjacent sarcomeres have almost continuous thin filament having the limiting boundary of Z-line. The thick and thin filaments are alternatively arranged and they form overlaps. The H-zone contains only thick filaments, while the I-band contains only thin filaments. The remainder of A-band has both thick and thin filaments.

The thin filaments consist of actins and the thick filaments of myosin. Both actin and myosin are contractle proteins. Actin has low molecular weight filamentous protein. It occurs in two forms, the monomeric G-actin and the polymeric -actin.  The thin filaments also contain the contractile protein, called tropomyosin. It is a rod-shaped fibrous protein; the rods link end to end to form two helical strands, which are wrapped around the F-actin.

Another small globular protein molecule, called troponin, masks the active sites of F-actin. Each myosin molecule has two components, a tail and a head. The tail is formed of heavy meromyosin (H-MM), while the head is formed of light meromyosin (L-MM). Myosin head has contractile property as well as ATPase like actin; it can form a cross-bridge with the active site present on the actin.

The Mechanism of muscle contraction

Red and White muscles
The skeletal muscles are of two types, red and white muscles. The basis of such a classification is the presence of a red pigment, called myoglobin in it. The red muscles contain very high amount of myoglobin whereas the white muscles contain very low amount. Myoglobin can store oxygen, which is utilized by the mitochondria for the synthesis of ATP as and when required. The differences between the red and white muscles are given in table given below:

Table - Comparison between Red and White Muscles

Red muscle

White muscle





More in number

Less in number

Blood capillaries






Sarcoplasmic reticulum



Skeletal Muscles
Muscles functioning in movement are attached to two bones across a joint, contraction then pulls one bone toward (or away from) the other. The muscle end attached to the immovable bone is called the origin; the muscle end attached to the movable bone is called the insertion. Muscles that move apart and away from the median line are called abductor muscles. The terms flexor and extensor indicate the type of movements muscles effect. The biceps of the forearm is a flexor (flexes or bends the forearm), and the triceps of the forearm is an extensor (extends or straightens the forearm). These muscles act as opposing pairs.

Naming Skeletal Muscles
The names of most of the muscles of the body are based on several types of characteristics. Learning the terms used to indicate specific characteristics helps in remembering the names of muscles.
  1. Muscle name may indicate the direction of the muscle fibres. Rectus fibres usually run parallel to the midline of the body. Transverse fibres run perpendicular to the midline. Oblique fibres are diagonal to the midline. Muscles named according to directions of fibres include the rectus abdominis, transverse abdominis and external oblique
  2. A muscle may be named according to location. The temporalis is near the temporal bone, the tibialis anterior is near the tibia.
  3. Size is another characteristic useful in naming muscles. The term maximus means largest, minimus means smallest, longus long, brevis short. Examples include the gluteus maximus, gluteus minimus, abductor longus, peroneus brevis.
  4. Some muscles are named for their number of origins. The biceps brachii has two origins, the triceps brachii three and the quadriceps femoris four.
  5. Other muscles are named on the basis of shape. Common examples include the deltoid (meaning triangular) and trapezius (meaning trapeziod)
  6. Muscles may be named after their origin and insertion. The sternocleidomastoid originates on the sternum and clavicle and inserts at the mastoid process of the temporal bone.
  7. Another characteristic used for naming muscles is action. We have already seen abductor, flexor and extensor muscles. Other such names are levator (produces an upward movement), depressor (produces a downward movement), Supinator (turns the palm upward or anteriorly), pronator (turn the palm downward or posteriorly), invertor (turns the sole of the foot outward), tensor (makes a body part more rigid), and rotator (moves a bone around its longitudinal axis).

Some Important Muscles of Human Body

Important skeletal muscles: Man has nearly 700 skeletal muscles in the body. Some of the important muscles, their location and action are listed in table below.

Table - Some important muscles of the body






Orbicularis oris

Surrounds opening of mouth

Closes lips, compresses lips, protudes lips and shapes lips during speech


Zygomaticus major

Zygomatic or check bone

Draws angle of mouth upward and outward as in smiling or laughing


Orbicularis oculi


Closes eye



Maxilla and zygomatic arch

Elevates mandible as in closing mouth


Superior rectus


Rolls eye ball upward


Rectus abdominis

Abdominal region

Flexes vertebral column


Transverse abdominis


Compresses abdomen



Below the ribs

Forms floor of thoracic cavity, important for respiratory movements


External intercostals


Elevates ribs during inspiration


Internal intercostals


Draws adjacent ribs together during forced expiration


Levator ani

Anal region

It forms the funnel shaped floor of the pelvic cavity. It contains openings for the anal canal in both sexes and the vagina in the female


Pectoralis major

Arms and pectoral girdle

Flexes, adducts and rotates arms medially



Pectoral girdle

Rotates arms medially


Biceps brachii


Flexes, and supinates forearm



Behind the thighs

Extends and rotates thigh lately



Back of the knee

Movements of legs

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