Complete information on the structure and functions of muscle fibres

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Muscular tissue is a contractile tissue, which is speciallised for contraction when excited. It is made up of elongated contractile cells called muscles fibres. The adjacent muscle cells are held together by aerolar tissue.

Origin:

Muscular tissue developes from mesoderm except ciliary muscles of eye, which is ectodermal in origin.

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Types:

There are three types of muscles found in the body i.e. (i) Skeletal or striated or voluntary, (ii) Visceral or non-striated or involuntary, (iii) Cardiac or striated or involuntary.

Skeletal muscle: The muscles, which are attached to bones, are called skeletal muscle. It is also called voluntary muscle as it is controlled by our wills. Skeletal muscles are liable to fatigue. It is striated for appearance under the microscope. The skeletal muscle is supported by connective tissues. Each whole muscle bulk is covered by connective tissue coat called epimysium. The whole muscle bulk is divided into maller bundles called fascicule. Each fasciculus is covered by connective tissue covering called perimysium. Each fasciculus consists of a large number of muscle cells and each muscle cell is covered by a connective tissue coat called endomysium.

Histological structure of muscle fibre- Each muscle fibre is a cylindrical and multi-nucleated cell. It is the structural unit of muscular tissue .Each fibre is surrounded by an electrically excitable specialized cell membrane known as sarcolemma. Electrical potential of inner surface is -0.1volt (negative) with respect to outer surface. The cytoplasm of muscle cell is called sarcoplasm. The sarcoplasm of each muscle cell contains multiple muscle fibrils called myofibrils. The sarcoplasm also contains sarcosome (mitochondria), Golgi body, myoglobin, sarcoplasmic reticulum. ATP and phosphocreatin molecules.

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Structure of a Myofibril:

Myofibrils appear striated because it bears dark and light bands. This is characteristic feature of skeletal muscles. In cross section myofibrils act as fine dots distributed in a group of polygonal area and are separated from adjacent bundles by clear sarcoplasm. The separated fibrils of sarcoplasmic area are called field of cohnheim. The dark band is doubling refractive (anisotropic) when studied on polarized light. Hence the name of a A-band. The light band is monorefractive (isotrpic) under polarized microscope from which the name is derived as I-band. The middle region of the A-band has a lighted area called Henson’s disc or H-disc. The I-band is bisected by a protinaceous line called Z-line or Krauses membrane. A segment of fibre between two successive Z-lines is called sarcomere. A sarcomere is the contractile unit of muscle fibre. At the mid point of the H-disc there is narrow dark line called M-line. The darker band of A-band is called O-band. On both ides of Z-lines or in I-band, there is a darker transverse line called N-line.

Fine Structure:

The striation of myofibrils are due to periodic variation in density i.e. concentration of myofilaments. There are two types of contractile myofilaments- thick filament and thin filment. Thick myofilaments are made up of protein myosin. Thin myofilaments are composed of more proteins- actin, tropomyosn and troponin.

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Myosin:

Myosin is a contractile protein. It is double headed globular protein with a long tail. The head binds to the actin of thin filament and the tail undergoes folding during muscles contraction.

Actin:

Actin is the major constituent of the thin filament. It is composed of G-actin monomores which are polymerized to form F-actin. For-actin is a double-stranded helix of actin monomers.

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Tropomyosin:

Tropomyosin is a double-stranded α-helical rod. It lies in grooves between two helical strands of actin.

Troponin- Troponin is a complex of three globular polypeptide chains. They are designated as TPC(calcium binding sub-unit) TP-T(tropomyosin binding sub-unit) and TPI(inhibitory sub-unit).

Sarcotubular system-In the sarcoplasm of the muscle cells, the myofibrils are surrounded by membrane bound tubules and channels called sarcoplasmic reticulum.It sores Ca++ ions. The paralell terminal cisternae are separated by a transverse tubule which is known as T-tubule.Ts tubules are generally called T-system. T-system functions for quick transmission of impulse from the cell surface to the interior.

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Muscle Contraction:

Contraction- When the muscle is stimulated, membrane depoalrisations occurs and Ca++ ions released from sacroplasmic reticulum. Then the myosin filament extends from the thick filament and attaches to the actin unit of thin filament. The tail of myosin filaments bends using energy from hydrolysis of ATP. Thus, the thin filament moves towards center of the sacromere and contraction of muscle occurs.

Relaxation:

When ATP binds to the myosin head, the myosin detaches from the actin and relaxation occurs.

Involuntary or smooth muscle- These are without cross striations, therefore they are called unstriated. They work involuntary under the control of autonomous nervous system and hence called involuntary muscles. These are present on the walls of internal hollow organs like alimentary canal, gall bladder, uterus, urinary bladder, blood vessels, various ducts etc.

These are spindle shaped and uninucleate with the nucleus at the center. Sacrolemma is absent and each fibre is covered by its plasmalemma. Myosin and actin fibrils are scattered longitudinally in its cytoplasm. Some spherical dense bodies are associated to action filaments.

Types of Smooth Muscles:

Two types of smooth muscles have been identified. They are:

(i) Multiunit smooth muscles and

(ii) Single unit smooth muscles

In case of multiunit smooth muscle each muscle fibre is separated from neighbouring ones by a layer of glycoprotein, separate nerves innervate each muscle fibre.Hence each muscle fibre functions independently. Example-Erector pili muscles attached to hair roots.

In single unit smooth muscles sheets or bundle of muscle fibres work as a single unit. The nerve supply is also for the entire unit unlike that in multiunit smooth muscle. Such smooth muscles are found in the alimentary canal, uterus, bile duct, uterus, gall bladder etc.Peristalsis, vasoconstriction, vascodilation, etc are the functions carried on by single unit smooth muscles.

Cardiac Muscle or Striated:

It is located in the myocardium of heart. This muscle is made up of uninucleated, short cylindrical cells which interconnect forming a network. The sacroplam contains longitudinal myofibrils which have alternate dark and light bands. There are transverse partitions between two muscle cells called intercalated disc. The cardiac muscles form a functional syncytium. The sinuauricular mode, auriculoventricular node and purkinje fibres consist of self excitatory fibres. Cardiac muscles work nonstop till death. It is immune to tetanus and fatigue.

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