Smooth muscle

3. Smooth involuntary muscle, present chiefly in hollow organs. Contraction of smooth muscle is not under our control; and smooth muscle is, therefore, also called involuntary muscle. It is supplied by autonomic nerves.

SKELETAL MUSCLE

Skeletal muscle is made up essentially of long, cylindrical “fibres”. The length of the fibres is highly variable, the longest being as much as thirty centimetres in length. The diameter of the fibres also varies considerably (ten to sixty micrometers). Each “fibre” is really a symplast with hundreds of nuclei along its length. The nuclei are elongated and lie along the periphery of the fibre, just under the cell membrane (which is called sarcolemma). The cytoplasm (or sarcoplasm) is filled with numerous longitudinal fibrils that are called myofibrils.

The most striking feature of skeletal muscle fibres is the presence of transverse striations in them. The striations represent alternate dark and light bands that strech across the muscle fibre. The dark bands are called A-bands, while the light bands are called I-bands. (“A” and “I” correspond to the second in the words dark and light.

Running across the middle of each I-band there is a thin dark line called the Z-band. The centre of the A-band is traversed by a lighter band called H-zone. Running through the centre of the H-band a thin dark line can be made out. This is M band.

Connective tissue framework of muscles. Muscles are pervaded by a network of connective tissue fibres that support muscle fibres and unite them to each other. Individual muscle fibres are surrounded by delicate connective tissue that is called the endomysium. Individual fasciculi are surrounded by stronger sheath of connective tissue called the perimysium. Connective tissue that surrounds the entire muscle is called the epimysium. At the junction of a muscle with a tendon the fibres of the endomysium, the perimysium and the epimysium become continuous with the fibres of the tendon.

CARDIAC MUSCLE

The structure of cardiac muscle has many similarities to that of skeletal muscle; but there are important differences as well.

Similarities between cardiac and skeletal muscle. These are the follows. Like skeletal muscle, cardiac muscle is made up of elongated “fibres” within which there are numerous myofibrils. The myofibrils (and, therefore, the fibres) show transverse striations similar to those of skeletal muscle. A, I, Z and H bands can be made out in the striations. The connective tissue framework, and the capillary network around cardiac muscle fibres are similar to those in skeletal muscle.

Differences between cardiac and skeletal muscle. These are as follows.

1. The fibres of cardiac muscle do not run in strict parallel formation, but branch and anastomose with other fibres to form a network.

2. Each fibre of cardiac muscle is not a multinucleated symplast as in skeletal muscle, but is a chain of cardiac muscle cells (or cardiac myocytes) each having its own nucleus. Each myocyte is about eighty micrometers long and about fifteen micrometers broad.

3. The nucleus of each myocyte is located centrally (and not peripherally as in skeletal muscle).

4. At places, the myofibrils merge with each other. As a result of these factors, the myofibrils and striations of cardiac muscle are not as distinct as those of skeletal muscle.

5. With the light microscope the junctions between adjoining cardiac myocytes are seen as dark staining transverse lines running across the muscle fibre. These lines are called intercalated discs.

SMOOTH MUSCLE

Smooth muscle is made up of long spindle shaped cells (myocytes) having a broad central part and tapering ends. The nucleus, which is oval or elongated, lies in the central part of the cell. The length of smooth muscle cells is highly variable (fifteen micrometers to five hundred micrometers). The cells are so arranged that the thick central part of one cell is opposite the thin tapering ends of adjoining cells. Each myocyte is surrounded by a network of delicate fibres, that holds the myocytes together. The fibres between individual myocytes become continuous with the more abundant connective tissue that separates fasciculi or layers of smooth muscle.

Each smooth muscle cell is bounded by a plasma membrane. Adjacent smooth muscle cells communicate through nexuses (gap junctions).

Questions:

1. Classification of muscle tissue.

2. Classification of striated muscle.

3. Structural features of skeletal muscle.

4. Structural features of cardiac muscle .

5. Structuiral features of smooth muscle.

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