The cardiac contraction and mechanical function are
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
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).
- Tropomyosin (Tm) forms an α helical coiled-coil
dimmer. Each tropomyosin molecule extends across seven actin monomers
in the thin filament and overlaps with neighboring tropomyosin to form
a continuous structure.
- The troponin complex:
TnC: is the myofibril protein that by binding calcium transmits the
signal. It interacts both with TnI and TnT
TnT: is a tropomyosin-binding component and integrates the entire
complex into the thin
TnI: inhibits ATP-ase activity of acto-myosin.
Figure 2: the structure of troponin and interactions with
The picture is taken from http://www.med4you.at/
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