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Chapter-23 Biochemistry of Muscle Structure and Function

BOOK TITLE: Essentials of Biochemistry (For Medical Students)

Author
1. B Shivananda Nayak
ISBN
9789350905241
DOI
10.5005/jp/books/11965_23
Edition
2/e
Publishing Year
2013
Pages
14
Author Affiliations
1. University of the West Indies, Trinidad and Tobago, West Indies; Subbaiah Institute of Medical Sciences, Shivamogga, Karnataka, India
Chapter keywords

Abstract

Muscles in the human body have biochemical specialization and which makes them to perform many physiological functions. Three different kinds of muscles are heart muscle, smooth muscle and skeletal muscle. Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. Each muscle fiber contains an array of myofibrils that are stacked lengthwise and run the entire length of the fiber. Each fiber is composed of myofibrils and each myofibril is made up of arrays of parallel filaments which is a series of repeating structural units called sarcomeres· Sarcomeres are composed of thick (myosin) and thin (actin) filaments. The thick filaments have a diameter of about 15 nm and composed of numerous protein strands which are called myosin filaments· Each myosin filament is composed of a two twisted strands called heavy chains and two other, but each different, pairs of twisted strands, called light chains. These light chains are found in the myosin heads. The thin filaments have a diameter of about 5 nm· Actin exists as a polymer of repeating globular proteins called G-actin· Two actin filaments are twisted into a single-stranded filament. These strands are anchored at one end to the Z-disk, lying in the groove formed by the actin filaments are a series of rod-shaped protein molecules called tropomyosin. Each tropomyosin molecule is 6-7 G-actin molecules in length· Bound to the end of each tropomyosin is a third protein called troponin. Troponin consists as three small bound protein molecules. The troponin-I (TnI), troponin-T (TnT) and troponin-C (TnC). One molecule is bound to the actin filament, one to the tropomyosin, and the third is available to bind with Ca2+. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction (NMJ). Together myosin, actin, tropomyosin, and troponin make up over three-quarters of the protein in muscle fibers. Mutations in the genes encoding these proteins may produce defective proteins and resulting defects in the muscles. Among the most common of the muscular dystrophies are those caused by mutations in the gene for dystrophin. Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction.

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