Short Essay on Tissue (396 Words)!
The tissue constituting the muscles and responsible for different movements of the body muscle tissue is made up of long, excitable cells that are capable of considerable contraction, these are arranged in a parallel pattern within the cytoplasm of the muscle cells.
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There are large number of microfilaments that are made of contractile proteins – acting and myosin. Since this is needed for movement it is one of the most abundant tissues.
(a) Striated skeletal muscles:
These muscles are mostly attached to the skeleton and controls motor movements and posture. They are cylindrical and have rounded ends. Each muscle fibre shows dark and light striations and is covered by a sheath, the sarcolemma.
Its cytoplasm is called sarcoplasm in which are present large numbers of contractile myofibrils. Each dark band has a light zone, the H-zone and each light band has a dark line or Z-line in it. The portion between two adjacent Z-line (Zwischenscheibe line) is called sarcomere.
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Striated muscle fibres are multinucleate. Myofibrils are formed of acting and myosin filaments. Skeletal muscles are also called voluntary muscles because the neurons which innervate it come from the somatic or voluntary branch of the nervous system.
[For contraction mechanism refer chapter movement and locomotion]
(b) Unstriped (or smooth muscles or visceral):
These involuntary muscles are found in the walls of hollow organs such as alimentary canal, uterus, and blood vessels. Cells of this muscle are spindle shaped and pointed at their ends. Striations are absent (due to different arrangement of actin and myosin filaments) and uninucleate.
Smooth muscles may be multiunit e.g., ciliary muscles, and iridial muscles of iris and walls of large arteries and airways or single unit e.g., alimentary canal, urinary bladder, uterus, ureters and oviducts.
(c) Cardiac muscles:
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They are involuntary, striated (because it also has acting and myosin filaments arranged into sarcomeres) and none fatigued fibres which are found in the wall of heart where they form myocardium.
Each cardiac muscle fibre is formed of several cells joined end to end. They are striated and uninucleated fibres. These muscles relay signals from cell to cell during a heartbeat.
Characteristic features are the presence of intercalated discs, and cross connections between the muscle fibres. Intercalated discs are irregular transverse thickening of sarcolemma that contain desmosomes and hold cardiac muscle fibres together and gap junction that aid in conduction of muscle action potentials.