Muscles

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Introduction

The cardiac contraction and mechanical function are generated by the sarcomere, which is the contractile unit of the muscle. Each fiber of the muscle is striated and made up of many myofibrils which are also striated in the same pattern of alternating dark and light bands. The portion of a myofibril between two Z lines constitutes a single contractile unit denoted as sarcomere. Each sarcomere is composed of thick and thin protein filaments. The thick filaments are called myosin , the thin filaments are called actin
Figure1: The diagram shows the fiber arrangement in a striated muscle.
Figure1: The diagram shows the fiber arrangement in a striated muscle.
The picture is taken from: http://en.wikipedia.org/wiki/Muscle


When the muscle is activated by the membrane action potential, which changes the intracellular free calcium concentration, the myosin heads interact with the actin binding sites. This interaction is creating cross – bridges (XB`s), the motor units of the sarcomere.
The XB`s slide one over the other and generate force by the convergent of chemical energy, derived from ATP hydrolysis, to mechanical work.
Each myosin head is 19 [nm] long and 5 [nm] width. The density per cubic millimeter of muscle contain 40*1012  myosin heads, and generate stress of 2 [Kg] per square centimeter.
The regulation of the XB attachment is been done on the thin filament by the complex of tropomyosin (Tm), and troponin (Tn) with the three Tn subunits: troponin I (TnI), troponin T (TnT) and troponin C (TnC).

Figure 2: the structure of troponin and interactions with other proteins
Figure 2: the structure of troponin and interactions with other proteins
The picture is taken from http://www.med4you.at/


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