Definition of striated musculature
Transverse striated muscle is the name given to a certain type of muscle tissue because under polarizing light (for example, a simple light microscope) it looks as if the individual muscle fiber cells have regular transverse striation. Normally, the term is used synonymously for skeletal musculature, since this type of tissue is mainly found here. Some muscles whose function is not to move the skeleton, such as the muscles of the diaphragm, tongue or larynx, are also of this tissue type. However, this transverse striation is also found in the heart muscle, which however has some characteristics specific to it as well as some features that do not occur in the rest of the striated muscles, which is why we usually speak of three different types of muscle tissue: transverse striated muscle, smooth muscle and heart muscle.
Types
There are two different types of striated muscles: the red and the white muscles. The muscle fiber cells of the red muscles have a high content of the oxygen supplier myoglobin, which is responsible for the color of this muscle type due to its red color. This means that red muscles are especially designed for long lasting strain and can be found more often in endurance athletes like marathon runners.
The muscle fibers of white muscles, on the other hand, contain less myoglobin and therefore appear lighter. They are mainly responsible for fast, strong movements and therefore predominate in people where muscle strength is the main factor, such as strength athletes. White muscles can be converted into red muscles through training; whether this is also possible the other way round has not yet been conclusively clarified.
Every skeletal muscle is surrounded by connective tissue (epimysium), from which individual fibers, also known as septums (partitions), come off, which on the one hand surround each individual muscle fiber (endomysium) and on the other hand also combine several muscle fibers as groups (perimysium), so that the so-called muscle fiber bundles are formed. The epimysium merges into the muscle fascia and then into the tendons by which the skeletal muscle can be attached to the skeleton. In anatomy, a distinction is made between the attachment and origin of a skeletal muscle.
The transverse striation is caused by the special structure of the individual muscle fiber cells (myocytes). Apart from the usual cell organelles, which can also be found in the muscle fibers (nucleus, mitochondria, ribosomes, endoplasmic reticulum (which here, however, is formed from a complex tubule system and is called sarcoplasmic reticulum)), these cells consist of thousands of so-called myofibrils. These fibrils are filamentous structures that are densely packed next to each other and run lengthwise through the entire muscle.
These in turn are composed of several sarcomeres. Sarcomeres are a unit of the fibril which in turn consists of the smaller components actin and myosin. Actin and myosin are proteins that are sometimes called contractile proteins, because they ultimately cause our muscles to contract.
Actin and myosin are arranged in the sarcomeres in such a regular pattern that a specific pattern is formed: Both actin (directly) and myosin (via another, very stretchy protein) are attached to the so-called Z-disks. From these disks, an area called the “I-band” follows first, which usually contains only actin. This area therefore appears brighter under the light microscope than the “A-bands” that follow.
This is the area where actin and myosin overlap, more or less depending on the contraction state of the muscle. If the muscle is relaxed, there is a place, the “H zone”, where only myosin but no actin is located. However, when the muscle is contracted, the myosin filaments move closer towards the Z-disks, so they overlap more and more with the actin filaments and the “H zone” becomes shorter and shorter until it finally disappears.
This process is known in medicine as the so-called sliding filament mechanism and is the basis for our muscles to shorten. In order for this process to take place, the muscle needs calcium ions, which it receives on the one hand from the sarcoplasmic reticulum and on the other hand from the cell environment, as well as the energy supplier ATP.If ATP is no longer produced, the contraction of the muscle cannot be released, which is why it remains in this tense state. This happens when an organism dies and the body remains in rigor mortis.
