Zoology Assignment Help >> Muscular Tissue, Kinds of Muscles, Structure, Muscle Fibrils

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MUSCULAR TISSUE

Muscles are so named because many of them resemble a mouse with their tendon representing the tail.

Muscle is a contractile tissue which brings about movements. It can be regarded as motors of the body. About 40% to 50% of our body mass is of muscles. Muscle cells are always elongated slender like cells and called muscle fibres.

Kinds of muscles: The muscles are of three types-

(1) Skeletal,

(2) Smooth,

(3) Cardiac

Skeletal muscle: It is also called striped, striated, somatic or voluntary muscle. Its main features are-

1. They are most abundant and found attached to skeleton.

2. They show cross striations under microscope and are best differentiated muscles.

3. They are supplied by somatic nerve and hence are under voluntary control.

4. Response to stimuli is quick and hence get fatigued early.

5. They are under highest control of cerebral cortex.

6. They help in adjustment with external environment.

7. Each muscle fibre is a multi¬nucleated cylindrical cell.

8. Each muscle fibre contains a group of myofilbrits. They are separated by Cohnheim's area.

9. They are composed of myofilaments.

10. Myofilaments are made up of three types of proteins viz.-actin, myosin and tropomyosin.

Structure:

1. Each muscle is made up of several small and parallel bundles of muscle fibres. They are called fasciculi.

2. Fibres of each fasciculi are bound together by a fibrillar connective tissue called endomycium.

3. All the fasciculi are bound together by another connective tissue sheath called perimycium.

4. Whole muscle is having a outer sheath of connective tissue called epimycium.

5. At the end of muscle, by which it joins with bone is the extension of muscle called tendon.

Structure of muscle fibre:

Muscle fibres are unbranched, cylindrical and elongated. Its covering membrane is called sarcolemma. It is again composed of an outer layer of reticular fibres and an Inner layer of membrane. Inner layer is attached to muscle fibre at the Z line and possibly at the M line also. It is polarized and has about 100 mV +ve potential at its outer surface.

Cytoplasm of muscle fibre is called sarcoplasm. On the basis of contents of sarcoplasm, muscle fibres are of two types-

(1) Red fibres- They are less responsive to stimuli and get lately fatigued. They contain more myoglobin, cytochrome and more' and large sized mitochondria. They derive their energy mainly by oxidation reactions. They are smaller and also called slow fibres.

(2) White fibres-They are comparatively long and quick responsive to stimuli. They are also called fast fibres. They contain less ATP and easily fatiguable. They acquire energy mostly by glycolysis.

Because each muscle fibre is having numerous nuclei, they are considered to be multicellular syncytial body. They are developed by the fusion of many embryonic stem cells.

Each muscle fibre is studded with numerous parallel myofibrils or sarcostyle. They provide striations to the whole muscle.

Each myofibril is composed of 1500 myosin filaments and 3000 actin filaments. Myofibrils have A and I bands. A bands are anisotropic to polarized light and are made up of myosin filaments and ends of actin filaments while I bands are isotropic to polarized light and are made up of only actin filaments. A band is about 100 Å thick and 1.5  long while I band is 50 Å thick and 1.0  long.

Each I band is again divided into two equal halves by thin fibrous zig-zag partition called Z band or Z disc or Krause's membrane. Actin filaments are attached to Z disc and extend on either side and join adjacent actin filaments and maintain continuity. The distance between two Z discs is called sarcomere which is functional unit of muscle. A sarcomere comprises a complete A band and two halves of I band on either side. Z line is again divided by N line.

The middle region of A band is comparatively lighter and its terminal parts are darker due to overlapping of actin and myosin filaments. The middle lighter region is called H zone. In the middle of H zone is M line.

Molecular layout of muscle fibril:

The muscle fibrils are composed of-

1. Myosin,

2. Actin,

3. Tropomyosin.

(1) Myosin-Each myosin filament is composed of about 200 myosin molecules. Their molecular weight is about 4,90,000. It is composed of light meromyosin and heavy meromyosin. Junction between these two is called hinges.

Heavy meromyosin portions of myosin molecules protrude from all sides of myosin filaments. They are called cross bridges. Heads of these bridges lie in apposition to actin filament. Each pair of cross bridge is displaced by 60°.

(2) Actin-It is composed of

(i) Actin

(ii) Tropomyosin

(iii) Troponin.

Its backbone is made up of double stranded F-actin molecule. Each F-actin is composed of polymerized G-actin molecules. Its molecular weight is 47,000.

(3) Tropomyosin: Its molecular weight is about 70,000. It is loosely attached to F-actin strand. In resting stage, it covers all active sites on actin strand so that no contraction will occur unless tropomyosin is replaced.

Mechanism of action: Theory of sliding filament for muscle contraction was put forth by H. E. Huxley and Hanson. Main features of this theory are-

1. A sarcomere's 2/3 length is occupied by A band and 1/3 by I band.

2. At the time of contraction A band maintains its length while I band shortens and eventually disappears. It causes reduction of about 35% in the length of sarcomere. At this stage H zone also disappears.

3. Shortening in the length of fibre is brought about by a sliding movement of actin filament over myosin filament towards M line.

4. This is done by rapid formation and breaking of cross bridges at the spurs of myosin filament.

Chemistry of mechanism:

1. Because strong affinity of actin with myosin, it forms a contractile complex actomyosin.

2. Tropomyosin and troponin exerts inhibitory effect over formation of this complex. They are in the form of troponin-tropomyosin complex. Until this complex is broken, the muscle will not contract.

3. Because troponin is having strong affinity to Ca++, so when the muscle is excited by motor impulse, there is a release of Ca++ from the sarcoplasmic reticulum. Now the troponin combines with Ca++.

4. Actin is now free to form actomyosin complex with myosin at the spurs of myosin filament, so the cross bridges are formed at the spurs.

5. Energy required for this process comes from ATP.

6. Dragging of actin filament over myosin filament towards M line is called power stroke.

7. Immediately after this Ca++: are forcefully pumped into E.R. and troponin¬tropomyosin complex is formed again which exerts inhibitory effect.

Smooth muscle: They are also called plain, unstriped and involuntary muscles. Main features of these muscles are-

1. They often encircle or surround viscera.

2. They do not exhibit cross striations.

3. They are supplied by autonomic nervous system.

4. They respond slowly to stimuli, being capable of sustained contraction¬ and therefore do not get fatigued easily.

5. They provide motive power for the regulation of internal environment of the body.

6. Fibre is elongated spindle shaped cell, having a Single centrally placed nucleus and myofibril shows longitudinal striation.

They are generally of two types¬

(1) Multiunit smooth muscle: Each fibre is a separate unit and innervated separately e.g. muscle of iris etc.

(2) Single unit smooth muscle:

Fibres are in group or aggregation or bundles. They are bound by gap junctions. When the contraction begins in any part of the muscle, automatically the whole muscle contracts.

The multiunit smooth muscles are principally neurogenic while single unit smooth muscles are chiefly myogenic. Thus the autonomic nerve to them is to modify contraction rather than to initiate contraction.

Mechanism of action: At chemical and mechanical level, the principle is basically same as that of striated muscle fibre.

Cardiac muscle: Walls of heart are made up of cardiac muscles and hence it is called myocardium. It is intermediate in structure, being striated and at the same time involuntary. It is meant for automatic and rhythmic contractions. Muscle cells are uninucleate.

Each normal cardiac muscle or fibre ends in a Z line at each end. Gap junctions also occur between the folds of the joining fibres. These junctions are called "intercalated disc". Cross striations are less prominent.

Parts of a muscle

Two ends:

(a) Origin: It is an end at which muscle is fixed during contraction.

(b) Insertion: It is an end at which muscle moves during contraction.

Two Parts:

(i) Fleshy part: It is also called belly. It is contractile.

(ii) Fibrous part: It is non¬contractile and inelastic. It is also called tendon or aponeurosis.

Myone: It is also called motor unit.

It is a single alfa motor neuron together with muscle fibre supplied by it.