Muscles

Muscles

In human, muscular movements are involved.

Types:

There are three types of muscle tissue; striated or striped, nonstriated or unstriped or smooth, and cardiac, according to their location, structure and function.

Structure of Skeletal Muscle

The striated muscle forms 80% or more of the mass of soft tissues in a vertebrate body and found in the body wall and the limbs. It also occurs in the tongue, pharynx and beginning of oesophagus.

Skeletal muscle are having a connective tissue sheath on the outer side and is called epimysium.

A transverse section of it shows a number of bundles or fasciculi.

Each fasciculus is surrounded by connective tissue cover called perimysium.

Within a fasciculus, are present a large number of muscle fibres, each surrounded by connective tissue cover endomysium.

There is a broad band of fibrous connective tissue beneath the skin or around muscles called fascia.

Each muscle fibre is cylindrical, uniform in diameter. Sarcolemma is present on outer side and at places is invaginated to form T or transverse tubules. A skeletal or striated muscle fibre is multinucleated or syncytial.

Ultra Structure of Skeletal Muscle Fibre

Structure:

A striated muscle consists of long, narrow, cylindrical, unbranched fibres with blunt ends.

Each fibre is bounded by an elastic sarcolemma and contains many elongated, flattened nuclei characteristically located near the sarcolemma.

Multinucleate condition results from cell fusion. Hence, a striated muscle fibre is a syncytium.

The striated muscle fibres contain numerous mitochondria and glycogen granules for the supply of adequate energy.

The myofibrils of a striated muscle fibre show alternating dark and light cross bands, the striations, or stripes, hence the name of the muscle.

The dark bands are called anisotropic or A bands.

Each A band has at its middle a light zone termed Henson's line, or H zone.

The light bands are isotropic and are known as the isotropic or I bands.

Each I band is crossed through its centre by a dark membrane, the membrane of Krause, or Z line.

This membrane continues right across the whole fibre and joins the sarcolemma surrounding the fibre.

It seems to hold the myofibrils together and to carry the signals for the contraction of the fibrils inward from the T-tubules (transverse tubules).

The latter are invaginations of sarcolemma into the fibre adjacent to the Z lines.

The part of the myofibril between two successive Z lines functions as a contractile unit termed the sarcomere.

The sarcoplasm also contains a protein pigment myoglobin, which can take up, store, or give up oxygen like haemoglobin.

Electron microscope reveals that each sarcomere is a bundle of fine longitudinal myofilaments of two types : primary and secondary.

(i) Primary Myofilaments:

The primary myofilaments are thicker and confined to the A band only.

They are composed of the protein myosin; bear minute projections called cross-bridges of the protein meromyosin; and are free at both the ends.

(ii) Secondary Myofilaments:

The secondary myofilaments are thinner and occur in I bands, but extend for some distance into the A band between the primary myofilaments.

This partial overlapping of the primary myofilaments by the secondary myofilaments imparts dark appearance to the A bands.

The secondary myofilaments are composed of the proteins actin, tropomyosin and troponin; have a smooth surface; and are attached to the Z lines by one end, being free at the other end.

The secondary (actin) myofilaments are more numerous than the primary (myosin) myofilaments.

Six actin myofilaments surround each myosin myofilament and each actin myofilament is surrounded by three myosin myofilaments.