Proteins

Proteins Contraction of the muscle depends on the presence of contractile proteins (Figure . ) such as actin and myosin in the myofilaments. The thick filaments are composed of the protein myosin which are bundled together whose heads produce at opposite ends of the filament. Each myosin molecule is made up of a monomer called meromyosin. The meromyosin has two regions, a globular head with a short arm and a tail. The short arm constitutes the heavy meromyosin (HMM). The tail portion forms the light meromyosin (LMM). The head bears an actin-binding site and an ATP- binding site. It also contains ATPase enzyme that split ATP to generate energy for the contraction of muscle. The thin filaments are composed of two interwined actin molecules. Actin has polypeptide subunits called globular actin or G–actin and filamentous form or F-actin. Each thin filament is made of two F-actins helically wound to each other. Each F-actin is a polymer of monomeric G-actins. It also contains a binding site for myosin. The thin filaments also )DVFLFOH (SLP\VLXP %ORRGYHVVHO )DVFLFOH ZUDSSHGE\ SHULP\VLXP (QGRP\VLXP EHWZHHQLQGLYLGX PXVFOHILEUHV 0XVFOHILEUH 3HULP\VLXP 7HQGRQ 6DUFROHPPD 0LWRFKRQGULRQ \RILEULO 1XFOHXV /LJKW,EDQG 'DUN$EDQG 7KLQ DFWLQ ILODPHQW =GLVF +]RQH 7KLFN P\RVLQ ILODPHQW ,EDQG $EDQG OLQH OLQH =GLVF ,EDQG =GLVF =GLVF 6DUFRPHUH Figure . Organizational Level of a Skeletal Muscle XI Std Biology-Zoology Chapter- sliding-filament theory to explain muscle contraction. According to this theory, overlapping actin and myosin filaments of fixed length slide past one another in an energy requiring process, resulting in muscle contraction. The contraction of muscle fibre is a remarkable process that helps in creating a force to move or to resist a load. The force which is created by the contracting muscle is called muscle tension. The load is a weight or force that opposes contraction of a muscle. Contraction is the creation of tension in the muscle which is an active process and relaxation is the release of tension created by contraction. Muscle contraction is initiated by a nerve impulse sent by the central nervous system (CNS) through a motor neuron. The junction between the motor neuron