Skeletal muscle tissue is striated consisting of elongated, multinucleatemuscle cells calledmuscle fibers, and is responsible for movements of the body. Other tissues in skeletal muscle includetendons andperimysium.[3] Smooth and cardiac muscle contract involuntarily, without conscious intervention. These muscle types may be activated both through the interaction of thecentral nervous system as well as by innervation from peripheralplexus orendocrine (hormonal) activation. Skeletal muscle only contracts voluntarily, under the influence of the central nervous system.Reflexes are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the central nervous system, albeit not engagingcortical structures until after the contraction has occurred.[citation needed]
There are three types of muscle tissue ininvertebrates that are based on their pattern of striation: transversely striated, obliquely striated, and smooth muscle. In arthropods there is no smooth muscle. The transversely striated type is the most similar to the skeletal muscle in vertebrates.[4]
Vertebrate skeletal muscle tissue is an elongated, striated muscle tissue, with the fibres ranging from 3-8 micrometers in width and from 18 to 200 micrometers in breadth. In the uterine wall, during pregnancy, they enlarge in length from 70 to 500 micrometers.[5] Skeletal striated muscle tissue is arranged in regular, parallel bundles ofmyofibrils, which contain many contractile units known assarcomeres, which give the tissue its striated (striped) appearance. Skeletal muscle is voluntary muscle, anchored bytendons or sometimes byaponeuroses tobones, and is used to effectskeletal movement such aslocomotion and tomaintain posture. Postural control is generally maintained as an unconscious reflex, but the responsible muscles can also react to conscious control. The body mass of an average adult man is made up of 42% of skeletal muscle, and an average adult woman is made up of 36%.[6]
Cardiac muscle tissue is found only in the walls of theheart asmyocardium, and it is an involuntary muscle controlled by theautonomic nervous system. Cardiac muscle tissue is striated like skeletal muscle, containing sarcomeres in highly regular arrangements of bundles. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles known asintercalated discs.
Striated skeletal muscle cells in microscopic view
Skeletal muscle is broadly classified into two fiber types:type I (slow-twitch) andtype II (fast-twitch).
Type I, slow-twitch, slow oxidative, orred muscle is dense withcapillaries and is rich inmitochondria andmyoglobin, giving the muscle tissue its characteristic red color. It can carry moreoxygen and sustainaerobic activity.
Type II, fast-twitch muscle, has three major kinds that are, in order of increasing contractile speed:[7][8]
Type IIa, which, like a slow muscle, is aerobic, rich in mitochondria and capillaries and appears red when deoxygenated.
Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short,anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up oflactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.[9]
Type IIb, which is anaerobic,glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents, this is the major fast muscle type, explaining the pale color of their flesh. In laboratory house mice, an intronic single nucleotide polymorphism in theMyosin heavy polypeptide 4 gene[10] causes a great reduction in the amount of Type IIb muscle, yielding the "Mini-Muscle" phenotype, which was discovered based on its greatly reduced (~50%) hind-limb muscle mass.
Thedensity of mammalian skeletal muscle tissue is about 1.06 kg/liter.[11] This can be contrasted with the density ofadipose tissue (fat), which is 0.9196 kg/liter.[12] This makes muscle tissue approximately 15% denser than fat tissue.
Skeletal muscle is a highly oxygen-consuming tissue, andoxidative DNA damage that is induced byreactive oxygen species tends to accumulate withage.[13] The oxidative DNA damage8-OHdG accumulates inheart and skeletal muscle of both mouse and rat with age.[14] Also, DNA double-strand breaks accumulate with age in the skeletal muscle of mice.[15]
Smooth muscle is involuntary and non-striated. It is divided into two subgroups: thesingle-unit (unitary) andmultiunit smooth muscle. Within single-unit cells, the whole bundle or sheet contracts as asyncytium (i.e. amultinucleate mass ofcytoplasm that is not separated into cells). Multiunit smooth muscle tissuesinnervate individual cells; as such, they allow for fine control and gradual responses, much likemotor unit recruitment in skeletal muscle.
Cardiac muscle is involuntary,striated muscle that is found in the walls and thehistological foundation of theheart, specifically the myocardium. Thecardiac muscle cells, (also called cardiomyocytes or myocardiocytes), predominantly contain only one nucleus, although populations with two to four nuclei do exist.[16][17][page needed] Themyocardium is the muscle tissue of the heart and forms a thick middle layer between the outerepicardium layer and the innerendocardium layer.
Coordinatedcontractions of cardiac muscle cells in the heart propelblood out of theatria andventricles to the blood vessels of the left/body/systemic and right/lungs/pulmonarycirculatory systems. This complex mechanism illustratessystole of the heart.
Cardiac muscle cells, unlike most other tissues in the body, rely on an available blood and electrical supply to deliver oxygen and nutrients and to remove waste products such ascarbon dioxide. Thecoronary arteries help fulfill this function.
A chicken embryo, showing theparaxial mesoderm on both sides of the neural fold. The anterior (forward) portion has begun to formsomites (labeled "primitive segments").
All muscles are derived fromparaxial mesoderm. The paraxial mesoderm is divided along theembryo's length intosomites, corresponding to thesegmentation of the body (most obviously seen in thevertebral column.[18] Each somite has three divisions,sclerotome (which formsvertebrae),dermatome (which forms skin), andmyotome (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which formepaxial and hypaxial muscles, respectively. The only epaxial muscles in humans are theerector spinae and small intervertebral muscles, and are innervated by the dorsal rami of thespinal nerves. All other muscles, including those of the limbs are hypaxial, and innervated by theventral rami of the spinal nerves.[18]
During development,myoblasts (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation ofconnective tissue frameworks, usually formed from the somaticlateral plate mesoderm. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongate skeletal muscle cells.[18]
The primary function of muscle tissue iscontraction. The three types of muscle tissue (skeletal, cardiac and smooth) have significant differences. However, all three use the movement ofactin againstmyosin to create contraction.
In skeletal muscle, contraction is stimulated byelectrical impulses transmitted by themotor nerves. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by theneurotransmitteracetylcholine.[19]
Smooth muscle is found in almost allorgan systems such ashollow organs including thestomach, andbladder; in tubular structures such asblood andlymph vessels, andbile ducts; in sphincters such as in the uterus, and the eye. In addition, it plays an important role in the ducts of exocrine glands. It fulfills various tasks such as sealing orifices (e.g. pylorus, uterine os) or the transport of the chyme through wavelike contractions of the intestinal tube. Smooth muscle cells contract more slowly than skeletal muscle cells, but they are stronger, more sustained and require less energy. Smooth muscle is also involuntary, unlike skeletal muscle, which requires a stimulus.
Cardiac muscle is the muscle of the heart. It is self-contracting,autonomically regulated and must continue to contract in a rhythmic fashion for the whole life of the organism. Hence it has special features.[20]
There are three types of muscle tissue ininvertebrates that are based on their pattern ofstriation: transversely striated, obliquely striated, and smooth muscle. In arthropods there is no smooth muscle. The transversely striated type is the most similar to the skeletal muscle in vertebrates.[4]
^Dave, Heeransh D.; Shook, Micah; Varacallo, Matthew (2024),"Anatomy, Skeletal Muscle",StatPearls, Treasure Island (FL): StatPearls Publishing,PMID30725921, retrieved2024-04-22
^abPaniagua, R; Royuela, M; García-Anchuelo, RM; Fraile, B (January 1996). "Ultrastructure of invertebrate muscle cell types".Histology and Histopathology.11 (1):181–201.PMID8720463.
^Larsson, L; Edström, L; Lindegren, B; Gorza, L; Schiaffino, S (July 1991). "MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type".The American Journal of Physiology.261 (1 pt 1): C93–101.doi:10.1152/ajpcell.1991.261.1.C93.PMID1858863.
^Smerdu, V; Karsch-Mizrachi, I; Campione, M; Leinwand, L; Schiaffino, S (December 1994). "Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle".The American Journal of Physiology.267 (6 pt 1): C1723–8.doi:10.1152/ajpcell.1994.267.6.C1723.PMID7545970.Note: Access to full text requires subscription; abstract freely available
^Olivetti G, Cigola E, Maestri R, et al. (July 1996). "Aging, cardiac hypertrophy and ischemic cardiomyopathy do not affect the proportion of mononucleated and multinucleated myocytes in the human heart".Journal of Molecular and Cellular Cardiology.28 (7):1463–77.doi:10.1006/jmcc.1996.0137.PMID8841934.
^Pollard, Thomas D.; Earnshaw, William C.; Lippincott-Schwartz, Jennifer (2008).Cell Biology (2nd ed.). Philadelphia, PA: Saunders/Elsevier.ISBN978-1-4377-0063-3.OCLC489073468.
