Myofibril organization
In skeletal and cardiac muscle tissues, contraction is provided by long organelles — myofibrils. They are located parallel to the long axis of the fiber or cell and stretch along its entire length.
- In cardiac muscle, myofibrils occupy from 40% of the cytoplasm volume (sarcoplasm), and their number reaches hundreds in a single cardiomyocyte.
- In skeletal muscle, they occupy up to 70% of the volume, and there can be up to 1400 of them in a single muscle fiber (myosymplast).
Transverse striation and sarcomere boundaries
The characteristic optical effect of transverse striation arises due to the strictly ordered structure of myofibrils. Areas of varying density alternate along the organelle:
- Light bands — I-discs.
- Dark bands — A-discs.
Since these zones coincide in adjacent parallel myofibrils, the entire muscle fiber appears striated. Right in the middle of the light I-disc runs a dark boundary — Z-line (telophragma). The segment of the myofibril enclosed between two adjacent Z-lines is the sarcomere.
Ultrastructure and mechanism of contraction
Each sarcomere contains thousands of longitudinally oriented protein filaments — myofilaments. They are divided into two types:
- Thin (actin).
- Thick (myosin).
At rest, myofilaments are distributed unevenly, which creates the visible discs. At the edges of the sarcomere, near the Z-lines, there are only thin filaments. In the center, only thick ones are concentrated. Between these areas, there are overlap regions where both types of filaments are present together.
The mechanism of contraction consists in the mutual sliding (overlapping) of actin and myosin filaments. Their overlap zone increases, as a result of which the sarcomere becomes shorter. The shortening of thousands of sarcomeres along a single myofibril leads to the contraction of the entire muscle.
Briefly: answers to topical questions
What is the name of the line passing through the middle of the dark A-disc of the sarcomere?
The line passing through the middle of the dark A-disc is called the mesophragma, or M-line. In this region, the central part of the thick myosin filaments is fixed by M-protein. The length of the thick filaments determines the width of the dark A-disc.
What is the name of the zone in the center of the sarcomere containing only thick myosin filaments?
The zone in the center of the sarcomere containing only thick myosin filaments is called the H-zone. It is located in the very center of the dark A-disc. The formation of this gap is due to the fact that the ends of the thin actin filaments, directed towards each other from adjacent Z-lines, do not meet in the center of the sarcomere. This ensures the optical inhomogeneity of the fiber.
What regulatory proteins are part of thin myofilaments in addition to actin?
In addition to actin, thin myofilaments contain the regulatory proteins troponin and tropomyosin. They are located on the surface of actin:
- Tropomyosin — forms thin filaments and covers the active sites of actin at rest.
- Troponin — caps the ends of tropomyosin filaments and physically controls its position.
Together, these proteins form a single complex that, in the relaxed state of the muscle fiber, blocks the active centers of actin and prevents interaction with myosin heads.
To remember which filaments are thin and which are thick: AcTin — Thin (Tonic), MyOsin — Bulky (thick).
Class and exam questions
What limits the sarcomere?
The sarcomere is located between two adjacent Z-lines (telophragms).
What structures form the light and dark discs?
Light regions (I-discs) are formed by thin (actin) myofilaments, and dark regions (A-discs) — by thick (myosin) ones.
Do myofilaments shorten themselves during muscle contraction?
No, the protein filaments themselves do not change length. Sarcomere contraction occurs due to their sliding relative to each other.
What else to study on this topic
- Structural organization of the muscle fiber (myosymplast and satellite cells)
- Structural features of myocardial functional fibers
- Embryonic development of skeletal and cardiac muscle tissues
- Differences in the regulation by the somatic and autonomic nervous systems
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