Useful Notes on ‘Ultrastructure of Striated Muscle’

The muscle fibres are usually found parallel to each other and are grouped in bundles or fasciculi.

Each muscle is composed of many such bundles. The fasciculi are arranged in a par­ticular pattern to accommodate the movements of a limb or other body region.

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The muscle fibres, fasciculi and the muscle as a whole are invested by connective tissue that forms a continuous frame work.

The outer part of this frame work, the epimysium, covers the whole muscle.

Thin collagenous partitions extending inward to surround the individual fasciculi comprise the perimysium and the delicate network of connective tissue that invests the individual muscle fibres is the endomysium.

The connective tissue serves to bind these individual contractile fibres together and to integrate their action.

The striated or skeletal muscles are attached to bones by means of tendons which are found at the ends of muscles.

A tendon is com­posed of dense, white, fibrous (inelastic) connective tissue. An expan­ded tendon consisting of a fibrous or membranous sheet is called an aponeurosis.

Each striated or skeletal muscle has blood vessels and afferent (sensory) and efferent (motor) nerves. Blood passes along the blood vessels, delivers nutrients to the muscles and carries away their waste products. The nerves link the muscles and the central nervous system.

The striated or skeletal muscle fibres are elongated cylindrical cells of variable size. The length of these fibres or cells may vary from a few millimeters to few centimeters (upto 10 cm), while their thickness may vary from 10µ to 1OO µ in diameter and depending on location the length of a muscle fibre may extend from few microns to centimeters.

Each musele fibre is bounded externally by a tough, ex­ceedingly thin elastic membrane called the sarcolemma. Inside the sarcolemma there is a semi fluid substance called the sarcoplasm.

In muscles and muscle contraction the sarcoplasm a large number of delicate bundles of Idngitudinally running myofibrils of 2n in diameter are embedded. They are a sort of intracellular organoids of the muscle fibres.

Each muscle fibre is multinucleate and the nuclei are usually found just beneath the sarco- lemma in the sarcoplasm.

Microscopic studies reveal that striated or skeletal muscles show, both longitudinal and transverse striations.

The longitudinal striations are due to myofibrils or myofilaments composed of the coniractile proteins, actin and myosin.

The transverse striations occur periodically along the longitudinal axis of the muscle fibre, these are due to alternating light and dark bands.

The light bands are widest transverse striations and are generally termed isotropic or I bands, while dark bands are termed anisotropic or A bands.

Microscopic examination in polarized light shows that anisotropic or A bands are birefringent or double refractive, while on the other hand the light bands or I bands give simple refraction.

Electron microscopic studies reveal that the alternating light and dark bands are produced by the arrangement of two types of protein filaments.

The thick filaments (about 110A in diameter and 1.60µ in length in fixed vertebrate muscle) lie parallel to one another about 450A apart and form the anisotrophic band, they are probably made of myosin protein.

The thin filaments (about 50A in diameter and in length in vertebrate muscle) are disposed in an orderly array between the thick filaments, they are probably made of actin protein.

Both of these types of filaments appear within the A band on each side of a centrally bisecting H band which contains only thick myosin filaments, the centres of which are slightly thickened, giving the image of another band4ermed the M band.

Finally in the centre of each I band there is a band of dense amorphous material termed the Z-disc.

Dense Z-discs cross the centre of each I band, thereby, dividing the myofibril into smaller units as sarcomeres.

A sarcomere, thus, is a repeating unit of the myofibril and represents the distance from one Z-disc to the next.

It is regarded as the unit of contractile activity in the striated muscle fibres of vertebrates and arthropods.

The length of a sarcomere is related to contractility. In general the length of a sarcomere is inversely proportional to the speed at which a fibre con­tracts and relaxes.

The two different kinds of filaments are jointed together by cross-bridges which are structurally a part of the thick filaments and which project outward.

These cross-bridges are thought to be signi­ficant in the sliding or ratchet-like action of the filaments which according to a popular theory of contraction, is the significant event in the shortening of the muscle (H. E. Huxley, 1958, 1960, A. F. Huxley and J. Hanson, 1960).

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