Sarcomere

The sarcomere (from Greek sarx, Sarko's "meat" and meros "part" ) is the smallest functional unit of the Muskelfibrille ( myofibril ) and thus the muscles.

Each sarcomere is composed mainly of three microscopic contractile proteins to filaments ( protein fibers ) composed, which are composed of actin, myosin, and titin. The sarcomere has a strong, regular structure and is in the longitudinal direction of the myofibril, which in turn is directed along the muscle fiber, arranged one behind the other and ranked. In the idle state, it has a length of about 2 to 2.5 microns.

Sections of the sarcomere

Z- discs ( "intermediate" discs and Z or Z- strip lines ), and in which are connected to the relatively thin and therefore lighter actin filaments that limit the sarcomere at its ends.

At the Z-disks "hang" on both sides of actin filaments (English: thin filament ), are the thick and therefore in the microscopic image darker myosin filaments between the ends thereof. An actin filament protrudes so in two adjacent sarcomeres.

The myosin filaments are connected by the elastic titin protein with two adjacent Z - discs and thereby centered in the middle of the sarcomere.

The protein family of myosin, the motor protein of actin. There are known, each with several subtypes more than 15 different classes. Typically, a myosin molecule of a tail, the other structures ( to be transported vesicles ) or another Myosinschwanz is connected, and a little head. The myosin heads represents an ATPase, so it can cleave ATP and change with the energy gained the angle of the bowl. Since the head can bind to actin filaments, the molecule moves as along the filament. In the center of the sarcomere and thus the myosin filaments (English: thick filament ) are the M- discs ( medium slices, also M- strips or M- lines called ). It is assumed that M- and Z-disks of the mechanical transverse stabilization of actin and myosin filaments are used.

The area around the M- discs is referred to ( Hensen zone named after Victor Hensen ) as H- zone, there are only myosin filaments. The area around the Z-disk is called the I-band (I = isotropic), there are only actin filaments. The region between two I- bands is referred to as band A (A = anisotropic).

The myosin filament is linked to M and Z through the elastic disc Titinfilament which runs on myosin filament attached to the M disc. Current evidence indicates that the Titinfilament to center the myosin filament during contraction between the Z- discs and play a spring function, so it is wound down feathery to the Z- discs. The ends of actin and Myosin-/Titinfilamente overlap. In summary, the sequence of Sarkomerabschnitte follows: Z - I - A - H - M - H - A - I - Z. A sarcomere thus consists entirely of an A - band and two half I- bands.

Muscle contraction

When a muscle contraction the actin filaments move along with the Z-disks in the direction of M disc and their associated myosin filaments. Although the filaments do not change their proper length, with the exception of titin in the contraction, the length of the sarcomere modified upon contraction the sarcomere is shorter, longer in elongation.