Muscular Tissue

Last Updated on Thursday, 16 December 2010 01:13 Written by Sandesh Sunday, 15 August 2010 04:51

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MUSCULAR TISSUE:

The property of excitability, contractibility, extensibility and elasticity is very well developed in the muscular tissue. For this reason, it is also known as contractile tissue.

Excitability- capacity of cell to respond various types of stimuli

Contractibility-capacity to contract

Extensibility-ability to stretch beyond normal length

Elasticity-ability to return to its original shape after contraction

Muscles make up about 40% of the body weight in mammals. They are supplied with blood vessels which provide nutrition and carries away metabolic wastes.

Origin: Muscular tissue is derived from Mesoderm.

General Structure:

• The muscular tissue is composed of muscles cells or muscles fibers
• They do not divide but increase in size due to growth
• The cytoplasm of muscles fibers is known as sarcoplasm and the cell membrane is known as sarcolemma.
• The sarcoplasm contain sarcoplasmic reticulum, Golgi, mitochondria etc.
• Each muscles fiber contains one or more nuclei.
• Each muscle fiber contains numerous thin myofibrils arranged along the long axis of fibers
• The myofibrils lie parallel to one another and each myofibril in turn is made up of thick and thin myofilaments. Thick filaments are composed of protein known as myosinand thin filaments are composed of actin.

General Function:

1. Motion: Muscles fibers are responsible for the movement of body parts and locomotion, beating of heart, churning of food in stomach, pushing food through intestine etc.
2. Maintenance of body posture: Contraction of skeletal muscles holds the body in stationary positions such as sitting and standing.
3. Heat production: Contraction of muscles results in the production of heat

Classification of Muscular tissue:

1. Striated Muscles
2. Unstriated Muscles
3. Cardiac Muscles

1. Striated Muscle Tissue (Skeletal/ Voluntary muscles tissue):

Location: Generally found attached to bones, in head, trunk and limb region, thus known as skeletal tissue. Also found in tongue, pharynx and in the beginning of oesophagus.

Structure: Striated tissue is long, slender, cylindrical and unbranched. The muscles fibers of striated tissue are arranged parallel to each other and are bound by connective tissue. They are longer than other types of tissues and vary in length in between 1000 to 45,000 microns. They are multinucleated and the nuclei are placed not in center but along the peripheral region. Each muscle fiber is bounded by an elastic sheath called sarcolemma. The cytoplasm of muscle fiber is called sarcoplasm. Each muscle fiber contains a large number of myofibrils. The myofibril is made up of two types of myofilaments – actin and myosin. These are proteins which are alternately arranged to give light and dark bands respectively. So, this muscle is called as the striated muscle.

Actin and myosin in some places overlap which produce darker appearance called as A-bands. The light bands between A-bands are called I-bands where only actin filaments are present. At the middle of each dark band (A-bands) where filaments do not overlap and only myosin filaments are present is called H-band.

Line running through the middle of each light band is called the Z-line. The distance between two Z-lines is called as Z-band which represents the functional unit of muscle. This unit is capable of contraction and is called a sarcomere. The myofibril, and therefore the muscle fibre, is made up of thousands of sarcomeres.

Striated muscles fibers suffer fatigue (tiredness) and they are voluntary i.e can be moved with our own will.

2. Unstriated Muscles (Smooth/Involuntary/ Visceral):

Location: Wall of visceral organs such as stomach, intestine, genital, urinary and respiratory organs and wall of blood vessels.

Structure: The unstriated or smooth muscles vary in length in between 15 to 5000 microns. They are long, narrow, spindle shaped cells with wide central region and tapering ends. The muscle fibers or cells are uninucleated with a centrally place nucleus. They fibers may occur in bundle or individually. The sarcoplasm and sarcolemma are quite thin and indistinct in smooth muscles. Sarcoplasm contains indistinct myofibrils.

Smooth muscles fiber undergoes slow and rhythmic contractions and relaxation controlled by autonomic nervous system. Thus they are also known as involuntary muscles. They do not suffer fatigue.

3. Cardiac Muscles:

Location: confined to the muscular wall of heart and to the roots of large blood vessels joining the heart.

Structure: The cardiac muscles are long, cylindrical and freely branched so as to form a network. They have very thin and indistinct sarcoplasm. These muscle fibers are uninucleated with centrally place nucleus. Sarcoplasm contains myofibrils which run along the long axis of muscle fibers. Myofibrils show dark and light band or striations which are not quite prominent (noticeable). Cardiac muscle fiber shows the presence of intercalated discs at the junction of cell membrane. The intercalated discs represent relay station for the transfer of wave of contraction from one fiber to another.

Cardiac muscles are involuntary in function under the control of autonomic nervous system. They undergo rapid but rhythmic contractions to bring about heart cycle. The do not suffer fatigue.