MYOFILAMENTS: TYPES, STRUCTURE AND ORGANIZATION - BIOLOGY - 2025

The myofilaments are contractile proteins of myofibrils, which are structural units of muscle cells, cells elongate termed muscle fibers.

Muscle fibers and their components have particular names. For example, the membrane, cytoplasm, mitochondria, and endoplasmic reticulum are known as sarcolemma, sarcoplasm, sarcosomes, and sarcoplasmic reticulum, respectively.

Structure of myofilaments (Source: Mikael Häggström, used with permission. Via Wikimedia Commons)

In the same way, the contractile elements inside are jointly called myofibrils; and the contractile proteins that make up myofibrils are called myofilaments.

There are two types of myofilaments: thin and thick. The thin filaments are mainly made up of three proteins: F-actin, tropomyosin, and troponin. The thick filaments, for their part, are made up solely of another protein known as myosin II.

In addition to these, there are other proteins associated with both thick and thin filaments, but these do not have contractile functions, but rather structural ones, among which are, to name a few, titin and nebulin.

Types of muscle fibers

The particular arrangement of the myofilaments that make up the myofibrils gives rise to two types of muscle fibers: striated muscle fibers and smooth muscle fibers.

Striated muscle fibers, when examined under an optical microscope, show a pattern of striations or transverse bands that are repeated throughout their entire surface and that give the name, to the muscle that contains them, of striated muscle. There are two types of striated muscle fibers, skeletal and cardiac.

Muscle fibers that do not show this pattern of transverse bands are called smooth fibers. They are the ones that make up the muscles of the vascular walls and the viscera.

Structure

Thin myofilaments

These myofilaments are composed of F actin and two associated proteins: tropomyosin and troponin, which have regulatory functions.

F actin, or filamentous actin, is a polymer of another smaller globular protein called G actin or globular actin, with a molecular weight of around 42 kDa. It has a binding site for myosin, and is arranged in two chains arranged as a double helix composed of approximately 13 monomer per turn.

F-actin filaments are characterized by having two poles: one positive, directed towards the Z disk, and the other negative, disposed towards the center of the sarcomere.

Tropomyosin is also made up of a double-helix polypeptide double chain. It is a 64 kDa protein that forms filaments that are located in the grooves left by the double helix chains of the thin F-actin filaments, as if "filling" the empty spaces in the helix.

At rest, tropomyosin covers or "covers" the binding sites of actin for myosin, preventing the interaction of both proteins, which is what causes muscle contraction. Around each thin filament and about 25-30m from the start of each tropomyosin is another protein called troponin.

Troponin (Tn) is a protein complex made up of three globular polypeptide subunits called troponin T, C and I. Each tropomyosin molecule has an associated troponin complex that regulates it, and together they are responsible for regulating initiation and termination. of muscle contraction.

Thick myofilaments

The thick filaments are polymers of myosin II, weighing 510 kDa and consisting of two heavy chains of 222 kDa each and four light chains. Light chains are of two types: 18 kDa essential light chains and 22 kDa regulatory light chains.

Each myosin II heavy chain is in the shape of a rod with a small globular head at its end that projects almost 90⁰ and has two binding sites, one for actin and one for ATP. This is why these proteins belong to the family of ATPases.

A thick filament is made up of more than 200 myosin II molecules. The globular head of each of these molecules acts like a "paddle" during contraction, pushing the actin to which it is attached so that it slides towards the center of the sarcomere.

Organization

In a skeletal striated muscle fiber, the myofibrils occupy most of the sarcoplasm, and are arranged in orderly, longitudinal clusters throughout the cell.

In a longitudinal section seen with an optical microscope, light bands, called Bands I, and dark bands, called Bands A are observed. These bands correspond to the ordered arrangement of the myofibrils, and therefore of the myofilaments that compose them.

In the center of Band I there is a dark and thin line called Line or Z Disk. The center of each Band A has a lighter area known as Band H, which is divided centrally by a darker line called Line M.

Delimited between two Z lines, a structure called sarcomere is described, which is the functional unit of the skeletal muscle. A sarcomere is composed of contractile myofilaments arranged in an orderly fashion in bands A, H, and a hemi-band I at each end.

The I bands contain only thin filaments, the A band contains thick filaments intertwined at their two ends with fine filaments, and the H band only contains thick filaments.

How are myofilaments organized within sarcomeres?

Both thick and thin myofilaments can be seen by examining a skeletal skeletal muscle sample under an electron microscope. These are said to "interdigitate" or "intertwine" with each other in a sequential, ordered and parallel arrangement.

The thin filaments originate from the Z discs and extend on each side in the opposite direction and toward the center of each adjacent sarcomere. From the Z discs at each end of the sarcomere, in relaxed muscle, actin travels to the start of the H band on each side.

Thus, in the muscle fibers of relaxed skeletal muscle the thick myofilaments occupy the central area that forms the dark bands or A bands; and the thin filaments extend to both sides of the sarcomere without reaching the center of it.

In a cross section in the region where the thick and thin filaments overlap, a hexagonal pattern can be observed that includes the thick filament in the center and six thin filaments that surround it, and that are located on each of the edges of the hexagon.

This organization of myofilaments in the sarcomere is preserved by the function of a series of proteins associated with myofilaments and which have structural functions, among which titin, alpha actin, nebulin, myomesin and protein C can be highlighted..

Mechanism of contraction

When acetyl choline (a neurotransmitter) is released into the neuromuscular plate by stimulation of a motor neuron, the muscle fiber is excited and voltage-gated calcium channels in the sarcoplasmic reticulum open.

Calcium binds to troponin C, causing a conformational change in tropomyosin, exposing the active sites of actin, thus initiating contraction. When calcium levels drop, tropomyosin returns to its initial position and contraction ceases.

Exposing the actin binding sites to myosin allows both proteins to bind and the myosin to push the actin toward the center of the sarcomere, sliding over the myosin.

During muscle contraction, the Z lines of each sarcomere approach towards the center, approaching the M line, increasing the interdigitation between actin and myosin and reducing the size of the I and H bands. The degree of shortening will depend on the summation of the shortening of each of the sarcomeres of the contracted muscle.

References

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