Myocyte

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A myocyte (also known as muscle cell ) is a type of cell found in muscle tissue. Myocytes are long tubular cells that develop from myoblasts to form muscles in a process known as myogenesis. There are various special forms of myocytes: cardiac, skeletal, and smooth muscle cells, with various properties. Gastric and skeletal heart muscle cells are referred to as muscle fibers . Cardiomyocytes are muscle fibers that make up the heart chamber, and have a central nucleus. Skeletal muscle fibers help support and move the body and tend to have peripheral nuclei. The smooth muscle cells control unconscious movements such as peristaltic contractions in the esophagus and stomach.

Video Myocyte

Structure

Terminology

Unusual muscle cell microstructure has led cell biologists to make special terminology. However, any special term for muscle cells has a pair used in the terminology applied to other cell types:

Sarcoplasm is the cytoplasm of muscle fibers. Most sarcoplasms are filled with myofibrils, which are long protein straps consisting of myofilaments. Sarcoplasm also consists of glycogen, glucose monomer polysaccharide, which provides energy to cells with high exercise, and myoglobin, a red pigment that stores oxygen until it is necessary for muscle activity.

There are three types of myofilaments:

• Thick filaments, composed of protein molecules called myosin. In muscle band striations, these are the dark filaments that make up the band A.
• Thin filaments consist of a protein molecule called actin. In muscle band striations, these are the light filaments that make up the band I.
• Elastic filaments consist of titin, a large chewy protein; this filaments anchored thick filaments to disk Z.

Together, these miofilaments work to produce muscle contractions.

The sarcoplasmic reticulum, a special type of fine endoplasmic reticulum, forms tissue around each myofibril of muscle fibers. This network consists of the grouping of two ends of dilation cord called cisternae terminal, and a single transverse tubule, or tubule T, which pierces the cell and appears on the other side; together these three components form the triads present in the sarcoplasmic reticulum network, in which each T tubule has two cisternae terminals on each side. The sarcoplasmic reticulum serves as a reservoir for calcium ions, so when the action potential spreads to the T tubule, it signals the sarcoplasmic reticulum to release calcium ions from the membrane channel to awaken the muscle contraction.

Sarcolemma is the cell membrane of striated muscle fibers and receives and performs stimuli. At the end of each muscle fiber, the outer layer of sarcolemma joins the tendon fibers. In the muscle fibers that suppress the sarcolemma are some flattened nuclei; this multinuclear condition is produced from multiple unified myoblasts to produce every muscle fiber, in which each myoblast accounts for one nucleus.

Internal

Cell membranes from myocytes have several special areas, which may include intercalation disks and transverse tubular systems. The cell membrane is covered by a lamina coat that is about 50 μm in width. The laminar layer can be separated into two layers; lamina densa and lamina lucida. Between these two layers there can be several different kinds of ions, including calcium.

Cell membranes anchored to the cell cytoskeleton with anchor fibers about 10 nm wide. These are generally located in the Z lines so they form a groove and the transverse tubules originate. In cardiac myocytes, this forms a toothed surface.

The cytoskeleton is a cell remnant formed and has two main purposes; the first is to stabilize the topography of the intracellular component and the second is to help control the size and shape of the cell. While the first function is important for biochemical processes, the latter is critical in defining the surface to cell volume ratio. This greatly affects the potential electrical properties of the expanding cells. Additionally deviations from standard form and cell size can have a negative prognostic effect.

Myofibrils

Each muscle fiber contains myofibrils, which are very long chains of sarcomeres, cell contractile units. The cells of the biceps brachii muscle may contain 100,000 sarcomomes. Myofibril smooth muscle cells are not arranged into sarkomers. The sarcomas consist of thin and thick filaments. Thin filaments are made of actin and stick to the Z lines that help them line up properly with each other. Troponin is found at intervals along thin filaments. Thick filaments are made of an elongated protein myosin. Sarcomas do not contain any organelles or nuclei. Sarcomeres are marked with a Z line indicating the beginning and end of the sarcometer. Individual myocytes are surrounded by endomyum.

Myocytes are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle tissue, which is enclosed in epimysium sheath. Perimysium contains blood vessels and nerves that provide muscle fibers. Spindle muscles are distributed throughout the muscles and provide sensory feedback information to the central nervous system. Myosin is shaped like a long shaft with a rounded tip pointed to the surface. This structure forms a cross bridge connecting with thin filaments.

Maps Myocyte

Development

A myoblast is a differentiated type of embryonic progenitor cell to give rise to muscle cells. Differentiation is governed by myogenic regulatory factors, including MyoD, Myf5, myogenin, and MRF4. GATA4 and GATA6 also play a role in the differentiation of myocytes.

Skeletal muscle fibers are made when myoblasts coalesce; Therefore, muscle fibers are cells with multiple nuclei, known as myonuclei , with each cell nucleus derived from one myoblast. Combined myoblasts are specific to skeletal muscles (eg, bicep brachii ) and not cardiac muscle or smooth muscle.

Myoblasts in skeletal muscles that do not form muscle fibers differentiate myosatellite cells. These satellite cells remain adjacent to the skeletal muscle fibers, located between the sarcolemma and the basal membrane of the endomisium (the investment of connective tissue that divides the muscular physiula into individual fibers). To reactivate myogenesis, satellite cells must be stimulated to differentiate into new fibers.

Myoblasts and their derivatives, including satellite cells, can now be produced in vitro by directional differentiation of pluripotent stem cells.

Kindlin-2 plays a role in the prolongation of development during myogenesis.

Muscle fiber growth

Muscle fibers grow when done and shrink when not in use. This is due to the fact that exercise stimulates the increase in myofibrils which increases the overall muscle cell size. Well-performed muscles not only increase the size but also can develop more mitochondria, myoglobin, glycogen and higher capillary densities. But muscle cells can not divide to produce new cells, and as a result we have fewer muscle cells as adults than newborns.

src: circres.ahajournals.org

Function

Muscle contractions

When contracting, thin and thick filaments shift to one another by using adenosine triphosphate. This draws the Z disc closer together in a process called the filament shear mechanism. The contraction of all sarcomas results in the contraction of all muscle fibers. This myocyte contraction is triggered by the action potential above the myocyte cell membrane. The potential action uses transverse tubules to obtain from the surface to the inside of myocytes, which are continuous in the cell membrane. The sarcoplasmic reticula is the membrane pouches that touch the transverse tubules but remain separate from. It wraps itself around each sarkomer and filled with Ca ² .

Myocyte excitation causes depolarization of the synapses, the neuromuscular junction, which triggers the action potential. With a single neuromuscular junction, each muscle fiber receives input from one somatic efferent neuron. The potential action in somatic efferent neurons causes the release of acetylcholine neurotransmitters.

When acetylcholine is released, it diffuses across the synapse and binds to the receptor on sarcolemma, a unique term for muscle cells that refers to the cell membrane. It begins an impulse that crosses the sarcolemma.

When the action potential reaches the sarcoplasmic reticulum it triggers the release of Ca ² from Ca ² channel. Ca ² flows from the sarcoplasmic reticulum to the sarcoma with both filaments. This causes the filament to begin to shift and the sarcometer becomes shorter. This requires a large amount of ATP, as it is used both in the attachment and discharge of each myosin head. Very rapid Ca ² is actively transported back to the sarcoplasmic reticulum, which blocks the interaction between thin and thick filaments. This in turn causes muscle cells to relax.

Miscellaneous contractions

There are four main types of muscle contractions: twitch, treppe, tetanus and isometric/isotonic. Twitch contraction is the process described earlier, in which a single stimulus signal for one contraction. In contraction twitch, the length of contraction may vary depending on the size of the muscle cell. During the treppe (or sum) the muscles do not start at maximum efficiency, but they achieve increased contraction strength due to repetitive stimulation. Tetanus involves continuous muscle contraction due to a series of rapid stimuli, which can continue until muscle fatigue. Isometrics is a contraction of skeletal muscle that does not cause muscle movement. But isotonic is the contraction of the skeletal muscle that causes movement.

Special cardiomyocytes located in the sinoatrial node are responsible for generating electrical impulses that control the heart rate.

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Typing fiber

There are many methods used for fiber typing, and the confusion between these methods is common among non-experts. Two often confusing methods are histochemical staining for myosin ATPase activity and immunohistochemical staining for Myosin heavy chain type (MHC). Mycosin ATPase activity is generally - and correctly - referred to simply as "fiber type", and results from direct testing of ATPase activity under various conditions (eg pH). Myosin heavy-chain staining is best described as "MHC fiber type", eg. "Fiber MHC IIa", and the results of different MHC isoform determinations. This method is closely related physiologically, since the type MHC is the major determinant of ATPase activity. Note, however, that these two typing methods are not directly metabolic; they do not directly address the oxidative or glycolytic capacity of the fibers. When "type I" or "type II" fibers are referred to in general, this most accurately refers to the number of types of numerical fibers (I vs II) assessed by the myosin ATPase staining activity (eg "type II" fiber referring to type IIA type IIAX type IIXA... etc.).

Below is a table showing the relationship between these two methods, limited to the types of fibers found in humans. Note the sub-type capitalization used in typing fiber vs. typing MHC, and that some types of ATPases actually contain several types of MHC. Also, subtype B or b is not expressed in humans by either method . Early researchers believed humans to express MHC IIb, which led to the classification of ATPase IIB. However, subsequent research indicates that MHC IIb man is in fact IIx, indicating that IIB is better named IIX. IIb is expressed in other mammals, so it is still seen accurately (together with IIB) in the literature. Non-human fiber types include true IIb fibers, IIc, IId, etc.

Further fiber-typing methods are less formally described, and are present in more spectra. They tend to focus more on metabolic and functional capacity (ie, oxidative vs glycolytic, rapid vs. slow contraction time). As mentioned above, fiber typing by ATPase or MHC does not directly measure or define these parameters. However, many of the various methods are mechanically related, while others are correlated in vivo . For example, the type of ATPase fiber is related to the speed of contraction, because high ATPase activity allows faster cross-bridge cycling. While ATPase activity is only one component of contraction velocity, type I fiber is "slow", in part, because they have a low speed ATPase activity compared to type II fibers. However, measuring contraction velocity is not the same as typing ATPase fiber.

Because of this type of relationship, Type I and Type II fibers have relatively different metabolic, contractile, and motor-unit properties. The table below distinguishes this type of property. This type of property - while partly dependent on the properties of individual fibers - tends to be relevant and measured at the motor unit level, rather than individual fibers.

Fiber color

Traditionally, fiber is categorized depending on its varying color, which is a reflection of myoglobin content. Type I fibers appear red due to high levels of myoglobin. Red muscle fibers tend to have more mitochondria and larger local capillary densities. These fibers are more suitable for durability and slow fatigue because they use oxidative metabolism to produce ATP (adenosine triphosphate). Less oxidative type II white fibers due to relatively low myoglobin and dependence on glycolytic enzymes.

Speed ​​of twitch

Fiber can also be classified on their twitching ability, being fast and slow. These properties are mostly, but not completely, overlapping classifications by color, ATPase, or MHC.

Some authors define fast twitch fibers as one in which myosin can split ATP very quickly. It mainly includes ATPase II type and MHC type II fibers. However, fast twitch fibers also exhibit a higher ability for the electrochemical transmission of action potential and the rapid rate of calcium release and uptake by the sarcoplasmic reticulum. Fast twitch fibers depend on the short-term, glycolytic glycolytic system for energy transfer and can contract and develop tension at 2-3 times the rate of slow twitching. The quick twitch muscles are much better at producing short bursts of strength or speed than slow muscles, and quickly exhaustion.

The slow twitch fibers generate energy for the re-synthesis of ATP through a long-term system of aerobic energy transfer. These include mainly ATPase I and MHC type I fibers. They tend to have low ATPase activity levels, slower contraction rates with poorly developed glycolytic capacities. They contain high mitochondrial volume, and high levels of myoglobin which gives them red pigmentation. They have been shown to have high concentrations of mitochondrial enzymes, so they are tired. Slow-burning muscles are slower than fast twitch fibers, but can contract longer before tiring.

Type distribution

The individual muscles tend to be a mixture of different types of fibers, but the proportions vary depending on the muscle's action and its species. For example, in humans, the quadriceps muscle contains ~ 52% type I fibers, whereas the soleus is ~ 80% type I. Orbicularis oculi eye muscles are only ~ 15% type I. Motor units in muscle, however, have minimal variation between fiber- fiber unit. This fact makes the size of the motorcycle unit recruitment principle feasible.

The total amount of skeletal muscle fibers has traditionally been considered unchanged. It is believed to be no gender or age difference in fiber distribution, however, the type of fiber varies from muscle to muscle and person to person. Inactive men and women (as well as small children) have 45% type II and 55% type I fibers. People at the higher end of any sport tend to exhibit a pattern of fiber distribution, eg. Endurance athletes show a higher level of fiber type I. Sprint athletes, on the other hand, require large quantities of type IIX fibers. Medium-distance event athletes show more or less the same distribution of two types. This is also often the case for athletes such as throwers and jumpers. It has been suggested that different types of exercise can cause changes in skeletal muscle fibers. It is estimated that if you perform endurance type activities for an ongoing period of time, some types of IIX fibers turn into IIA type fibers. However, there is no consensus on this issue. It is possible that type IIX fibers show an increase in oxidative capacity after high intensity resistance training that takes them to a level where they are able to perform oxidative metabolism as effectively as slow twitch fibers from untrained subjects. This will be brought about by an increase in the size and number of mitochondria and associated changes related to no change in the fiber type.

src: circres.ahajournals.org

See also

• The list of human cell types comes from the germ layers

src: jcs.biologists.org

References

src: basictranslational.onlinejacc.org

External links

• Muscle Cell Structure

Source of the article : Wikipedia