Are thin filaments composed of actin


Synonyms: Sarcomer, S-line, S-stripe, S-gang
English: sarcomere

1 definition

Under Sarcomere one understands the smallest contractile unit of the muscle. Sarcomeres are made up of myofilaments and are located in what are known as myofibrils.

2 structure

The sarcomere is composed of a large number of individual structural proteins, which can be roughly divided into three classes according to their function:

A part of these proteins forms two types of filaments, the thickness and the thin Filament.

2.1 Thin filament

The thin filaments or actin filaments are connected to the Z-disks via the anchoring protein actinin and interlock with the thick filaments between them. They essentially consist of actin, tropomyosin, troponin and nebulin.

2.2 Thick filament

The thick filaments or myosin filaments consist mainly of myosin, which is grouped into bundles by the C proteins. The anchoring protein titin creates an "elastic" connection to the next Z-disk at the end of the thick filaments.

Electron microscopic examinations show that the Z-disks are the starting point for all other filaments in their formation. A sarcomere extends from Z to Z disk and has a total length of approx. 2-2.5 µm and a diameter of approx. 1 µm. When contracting, the sarcomere shortens by approx. 0.4 μm.

3 classification

Various subdivisions can be identified within a sarcomere. The Z-disks already mentioned are also called Z lines or Z bands designated. Areas that only contain actin filaments are called I-stripes or I-bands because they are simply refractive (isotropic). The myosin-containing regions are called A-stripes or A-bands because they are double refractive (anisotropic). Since the middle zone of the myosin appears lighter, it is called the H-stripe. Due to the median thickening of the myosin filaments, the so-called M-stripe arises. A sarcomere as a whole can accordingly also be called S-stripes are designated.

4 function

The sarcomere is indirectly involved in muscle contraction, as it shortens when actin and myosin slide into one another (see under sliding filament theory).

5 pathophysiology

Damage to the sarcomeres can, for example, be symptoms of so-called hypertrophic cardiomyopathy (HCM) or sore muscles.

5.1 Hypertrophic cardiomyopathy (HCM)

This hypertrophic cardiomyopathy affects the entire heart muscles and is characterized by a thickening of the sarcomeres and thus of the entire muscle. Symptoms of this disease are often shortness of breath and a feeling of pressure in the chest. Hypertrophic cardiomyopathy is one of the most common causes of sudden cardiac death in adolescents and adults.

5.2 sore muscles

The sore muscles can be explained by disorders of the sarcomere structure. Since the late 1990s at the latest, it has been assumed that eccentric loads lead to microtraumas of the actin filaments (AF). The AF play a crucial role in muscle contraction. They are attached to the Z-disk - it is in this area that the injuries to the structure are suspected.