Myocyte

As a muscle fiber, muscle fiber and cell or myocyte, refers to the cellular, spindle-shaped basic unit of ( striated ) muscles of the skeleton. The muscle cells of the heart muscle and of smooth muscle are no muscle fibers. Nevertheless, in particular, the muscle cells of the heart have some similarities with muscle fibers.

Muscle fibers contain as carrier, its function longitudinally aligned in parallel to several hundreds of muscle fibers ( myofibrils ). Depending on the number of muscle fibers in cross-section measure approximately 0.01 mm to 0.1 mm and can be a few millimeters up to several centimeters long, depending on the type and length of each muscle. A skeletal muscle multiple muscle fibers are combined to form fiber bundles ( so-called " meat fibers " ) of 0.1 mm to 1 mm in diameter, whose ends are attached as a rule on tendons to bone. In a contraction of the muscle fiber, these skeletal elements can be brought closer together.

Muscle cells are multinucleate, such as liver cells and osteoclasts and some other body cells.

• 3.1 Development and Growth
• 3.2 Energy Metabolism
• 3.3 contraction

Method of construction

A muscle fiber is an elongated multinucleated cell, the cell nuclei mostly just beneath the cell membrane of the muscle cell, the sarcolemma, lie. Foothills of the sarcolemma double over in many places with tube-like folds and form thus perpendicular to the surface and transverse to the longitudinal axis of the muscle cell, a system called transversal tubules ( T-tubules, transverse tubules; Latin tubulus ' tube '. ), Via the well deep located in the muscle cell areas can quickly be achieved by an excitation when an action potential is conducted via the sarcolemma. These invaginations go down to the immediate vicinity of the cavities of another tube system of foothills of the ( smooth ) endoplasmic reticulum ( sarcoplasmic reticulum SR ). These are now parallel to the longitudinal axis of the muscle cell oriented - in other words between the myofibrils located along or surrounding they - Completed view of the chamber of a system of longitudinal tubules (L tubules longitudinal tubules ) that serves as a reservoir of calcium ions. On both sides of this Ca2 encounter -storing chambers of the L- system to a pull-through folding of the T- system, so that their SR membranes of the folded membrane of the sarcolemma rest and receptors can directly contact each other on the opposing membranes.

If a muscle fiber excited about the myoneural junction by a nerve cell - at the motor endplate by the corresponding (lower ) motor neuron its motor unit ( in my opinion ) - and the action potential (AP ) of the muscle cell is now conducted via the T- system, which are voltage-sensitive ( DHPR ) receptors addressed. Your conformational change in Skelettmuskelfasen directly to activation of the opposite associates ( RYR1 ) receptors in the SR membrane, after which here opens ion channels and Ca2 ions are released from the L- system, which then trigger in the myofibrils as electromechanical coupler contraction: the muscle fiber shortens.

In the cytoplasm of muscle fibers are located next to different numerous mitochondria ( Sarko Omen ). In different concentrations, the sarcoplasmic also oxygen-storing myoglobin as substances, energy-rich substances such as glycogen and enzymes of the aerobic energy included, which can be differentiated biochemically different types of muscle fibers also.

→ Main article: sarcomere

For the main, however, contain muscle fibers tightly packed and arranged in parallel a few hundred myofibrils that extend over the entire length of the muscle fiber. A myofibril is composed of several longitudinally consecutive, sharply defined, uniform compartments, the about 2 microns long sarcomeres, which form the actual kontaktilen units and carry a typical cross-striations ( in polarized light). The reason for this is the high degree of order here in the sarcomere aligned myofilaments as contractile proteins, the 2,000 thin actin and thick myosin filaments around 1,000, which move at a contraction against each other. Since not only the myofilaments of a myofibril, but also the myofibrils in a muscle fiber - in contrast to the smooth muscle cell - all in one direction and strict pull side by side, the striation is also on the entire cell. For clamped in a muscle belly between two Anheftungspunkten, origin and insertion, a skeletal muscle, the muscle fibers of a fiber bundle are now working together in the same direction.

Between the muscle fibers Connective tissue is used as endomysium, which connects it to the radiating tendon, more muscle fibers are taken by a perimysium internum connective tissue and so called the primary beam. Some primary bundle together produce a secondary beam when they are surrounded by a common externum perimysium. These secondary beams eventually spanned the epimysium, which surrounds the muscle as a whole, goes into the muscle fascia, which can combine multiple muscles in a muscle compartment, and separated at shifts from the environment. The task of the connective tissue structures is thus to integrate the muscle force-fit, tear resistant and slidably accrued and to secure its supply. The endomysium consists mostly of reticular fibers, the perimysium of parallel collagen fibers. In this run the supplying blood vessels, efferent lymphatics and impulse-generating nerves.

Fiber types

There are two main types of muscle fibers according to their installations with mitochondria and the enzymes of aerobic metabolism as well as their content of myoglobin, or after the expiration of the Muskelfaserzuckung. Muscle fibers with a relatively higher number of said functional elements and thus better equipment for the longer way of - Available- energy compounds thorough bailing - oxidative ( oxygen-consuming ) metabolism twitch slower and are called (or type 1) Slow - twitch fibers. The fast -twitch muscle fibers rather use the shorter routes of anaerobic energy supply (preferably from glycogen ) - therefore have a lower content, for example to sauerstoffspeicherndem red Muskelfabstoff myoglobin - hot and fast- twitch fibers ( or Type 2). The difference between these types is an adaptation to the prevailing stress pattern; it is so far no definition, as in persistent change in the ( by the AP frequency of the relevant motor neuron specific ) activity pattern, the muscle fibers of a motor unit of a type convertible to another type with time.

S- fibers ( type 1)

S (slow ) fibers are slow twitch and therefore tend to be slow-twitch muscle fibers. But you shorten still so fast that cyclical movements with high refresh rates such as cycling with cadence over 100 rpm are possible - and can be maintained over long periods of time. They are also called dark or red fibers, because they have a dark red color due to the high myoglobin concentration. They are designed for continuous power with limited effort and fatigue very slowly. S fiber is supplied from fine capillaries and gains its energy aerobically, wherein the oxygen required for this is withdrawn from the blood. They are also called type 1 fibers or oxidative fibers.

F- fibers (type 2)

F (fast ) fibers or light-colored or white fibers are fast-twitch muscle fibers. They consume more energy and get tired faster. They are also called type 2 fibers or glycolytic fibers. This F- fibers are usually further divided into two ( sub) types: the more similar the S- fibers in their properties type -2A or FR (almost -resistant ) fibers and the very strong, fast, but quickly tiring type 2B or FF ( fast- fatigue ) fibers.

Other fiber types of skeletal muscle

Other fiber types, some with specific myosin types are found for example in the enormous pressure of chewing developing jaw muscles (eg in the masseter muscle ) or in the finest rapid eye movements exporting muscles of the eyeball (eg in the lateral rectus ). Furthermore, the fetus has a partially special set of myosins, which allow some muscle fibers own specifics beyond the rough classification in S and F fibers.

Physiology

Development and growth

Muscle cells are constructed from merging with each other myoblasts ( syncytium ). Initially, the cell nuclei are present in these myotubes in the center of the cell and provide for the formation and alignment of the myofibrils. When mature myotubes to muscle cells, the nuclei migrate to the edge and forms a basement membrane around each muscle fiber are included in the still mononuclear myoblasts.

Because the nuclei are not capable of division in a muscle fiber, these satellite cells are very important for the growth of muscle cells, as they are the only way to integrate additional nuclei in the muscle cell. They divide into two daughter cells, if necessary, one of which merges with the muscle fiber, while the other grows and eventually divides again. Additional cores are used when the muscle fiber is increased, eg by growth, or shall participate in a healing process.

Energy metabolism

The mobilization, transport and the reduction of high-energy substrates ATP production in the muscle used to perform muscular work. See the

Contraction

→ Main article: muscle contraction

Through interaction of the two proteins of the sarcomere, actin and myosin, a muscle cell can reduce their length (concentric contraction), maintained against resistance ( isometric contraction) or its extension resistance oppose ( eccentric contraction ). In the resting state, the points on actin to which the myosin to bind obscured by another protein tropomyosin. By the arrival of an action potential in the SR for the first time demonstrated by Setsuro Ebashi release of calcium ions is stimulated, which dissolves the blockage by the tropomyosin and thus triggers a contraction of the sarcomere by the so-called Filamentgleiten.

By a single action potential is, however, only triggered when the skeletal muscles, a single short Muskelfaserzuckung, in which the muscle fiber only slightly shortened. To achieve or induce a sustained contraction a higher reduction, must arrive in rapid succession action potentials, thus superimpose a single jerk gradually and are summed ( superposition ). Be distinguished from the first occurring at even higher action potential frequency tetanic contraction, which is the total fusion of single twitches as the maximum possible contraction of the muscle fiber is (smooth or complete tetanus ). Slow twitch fibers are ST- tetanisierbar by action potential series with frequencies from about 20 Hz, the FT fibers need significantly higher fusion frequencies ( about 60 Hz).

In the muscle, the muscle strength is graded not only by different pulse frequencies of the motor neurons, but primarily by the type and number of changing ( and asynchronous) motor units recruited. So no single twitches are even with low muscle tension, such as the reflex -induced muscle tone ( " Reflextonus " ), despite the gravity can maintain, for example, a body involuntarily, mostly visible. Also, it is not a tetanus of the muscle in the conventional stress in vivo.

History

The muscle fibers were first described in 1677 by Antoni van Leeuwenhoek.