A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines.

Skeletal muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

are composed of tubular muscle cells (called muscle fibers or myofibers) which are formed during

embryonic Embryonic may refer to: *Of or relating to an embryo * ''Embryonic'' (album), a 2009 studio album by the Flaming Lips *''Embryonics ''Embryonics'' is a double-CD compilation of tracks by the Australian progressive metal band, Alchemist. It w ...

myogenesis Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development. Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called ''myotubes''. In the early development o ...

. Muscle fibers contain numerous tubular

myofibrils A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. Each myofibr ...

. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as alternating dark and light bands. Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. The

costamere The costamere is a structural-functional component of striated muscle cells which connects the sarcomere of the muscle to the cell membrane (i.e. the sarcolemma).20: 2327-2331 Costameres are sub-sarcolemmal protein assemblies circumferentially a ...

is a different component that connects the sarcomere to the

sarcolemma The sarcolemma (''sarco'' (from ''sarx'') from Greek; flesh, and ''lemma'' from Greek; sheath) also called the myolemma, is the cell membrane surrounding a skeletal muscle fiber or a cardiomyocyte. It consists of a lipid bilayer and a thin oute ...

. Two of the important proteins are

myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin ...

, which forms the thick filament, and

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...

, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The

myosin head The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a he ...

also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Actin molecules are bound to the Z-line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed. The myofibrils of

smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

cells are not arranged into sarcomeres.

Bands

The sarcomeres give

skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

and

cardiac muscle Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...

their striated appearance, which was first described by

Van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...

. * A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs). In electron micrographs of cross-striated muscle, the Z-line (from the

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...

"zwischen" meaning ''between'') appears in between the I-bands as a dark line that anchors the

myofilaments. * Surrounding the Z-line is the region of the I-band (for ''

isotropic Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...

''). I-band is the zone of thin filaments that is not superimposed by thick filaments (myosin). * Following the I-band is the A-band (for ''

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

''). Named for their properties under a

polarized light microscope Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked with ...

. An A-band contains the entire length of a single thick filament. The anisotropic band contains both thick and thin filaments. * Within the A-band is a paler region called the H-zone (from the German "heller", ''brighter''). Named for their lighter appearance under a polarization microscope. H-band is the zone of the thick filaments that has no actin. * Within the H-zone is a thin M-line (from the German "mittel" ''meaning middle''), appears in the middle of the sarcomere formed of cross-connecting elements of the cytoskeleton. Cardiac sarcomere structure

The relationship between the proteins and the regions of the sarcomere are as follows: * ''Actin'' filaments, the thin filaments, are the major component of the I-band and extend into the A-band. * ''Myosin'' filaments, the thick filaments, are bipolar and extend throughout the A-band. They are cross-linked at the centre by the M-band. * The giant protein ''

titin Titin (contraction for Titan protein) (also called connectin) is a protein that in humans is encoded by the ''TTN'' gene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring that is responsible for t ...

'' (connectin) extends from the Z-line of the sarcomere, where it binds to the thick filament (myosin) system, to the M-band, where it is thought to interact with the thick filaments. Titin (and its splice isoforms) is the biggest single highly elasticated protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as sarcomeric ruler and as blueprint for the assembly of the sarcomere. * Another giant protein,

nebulin Nebulin is an actin-binding protein which is localized to the thin filament of the sarcomeres in skeletal muscle. Nebulin in humans is coded for by the gene ''NEB''. It is a very large protein (600–900 kDa) and binds as many as 200 actin monomer ...

, is hypothesised to extend along the thin filaments and the entire I-Band. Similar to titin, it is thought to act as a molecular ruler along for thin filament assembly. * Several proteins important for the stability of the sarcomeric structure are found in the Z-line as well as in the M-band of the sarcomere. * Actin filaments and titin molecules are cross-linked in the Z-disc via the Z-line protein alpha-actinin. * The M-band proteins

myomesin Myomesin is a protein family found in the M-line of the sarcomere structure. Myomesin has various forms throughout the body in striated muscles with specialized functions. This includes both slow and fast muscle fibers. Myomesin are made of 13 do ...

as well as C-protein crosslink the thick filament system (myosins) and the M-band part of titin (the elastic filaments). *The M-line also binds creatine kinase, which facilitates the reaction of ADP and phosphocreatine into ATP and creatine. * The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the

muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...

(based on the

sliding filament model Sliding may refer to: *Sliding (dance), also floating or gliding, a group of footwork-oriented dance techniques *Slide (baseball), an attempt by a baseball runner to avoid getting tagged out *Sliding (motion) See also *Slide (disambiguation) Sl ...

Contraction

The protein

tropomyosin Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in actin-based cytoskeletons. Tropomyosin and the actin skeleton All organisms contain organelles that provide physical integrity to their cells. These type of organelles ar ...

covers the myosin-binding sites of the actin molecules in the muscle cell. For a muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with

troponin C Troponin C is a protein which is part of the troponin complex. It contains four calcium-binding EF hands, although different isoforms may have fewer than four functional calcium-binding subdomains. It is a component of thin filaments, along wit ...

molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin. The concentration of calcium within muscle cells is controlled by the

sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium in bio ...

, a unique form of

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

in the

sarcoplasm Sarcoplasm is the cytoplasm of a muscle cell. It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycogen (a polymer of glucose), myoglobin, a red-colored protein necessary for binding oxygen molecules tha ...

. Muscle cells are stimulated when a

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...

releases the neurotransmitter

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...

, which travels across the

neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

(the synapse between the terminal button of the neuron and the muscle cell).

Acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...

binds to a post-synaptic

nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ne ...

. A change in the receptor conformation allows an influx of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...

ions and initiation of a post-synaptic

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

. The action potential then travels along

T-tubules T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion channels, transporters, and pumps, T-tubules permit ...

(transverse tubules) until it reaches the sarcoplasmic reticulum. Here, the depolarized membrane activates voltage-gated

L-type calcium channel The L-type calcium channel (also known as the dihydropyridine channel, or DHP channel) is part of the high-voltage activated family of voltage-dependent calcium channel. "L" stands for long-lasting referring to the length of activation. This c ...

s, present in the plasma membrane. The L-type calcium channels are in close association with

ryanodine receptor Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons. There are three major isoforms of the ryanodine receptor, which are found in different tissu ...

s present on the sarcoplasmic reticulum. The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum. This mechanism is called

calcium-induced calcium release Calcium-induced calcium release (CICR) describes a biological process whereby calcium is able to activate calcium release from intracellular Ca2+ stores (e.g., endoplasmic reticulum or sarcoplasmic reticulum). Although CICR was first proposed for ...

(CICR). It is not understood whether the physical opening of the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open. The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the contractile apparatus and, thus, muscle cell to relax. Upon muscle contraction, the A-bands do not change their length (1.85 micrometer in mammalian skeletal muscle), whereas the I-bands and the H-zone shorten. This causes the Z lines to come closer together.

Rest

At rest, the

is bound to an ATP molecule in a low-energy configuration and is unable to access the cross-bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into

adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...

(ADP) and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known as ''rigor'', until a new ATP binds the myosin head. This binding of ATP to myosin releases the actin by cross-bridge dissociation. The ATP-associated myosin is ready for another cycle, beginning with hydrolysis of the ATP. The A-band is visible as dark transverse lines across myofibers; the I-band is visible as lightly staining transverse lines, and the Z-line is visible as dark lines separating sarcomeres at the light-microscope level.

Storage

Most muscle cells can only store enough ATP for a small number of muscle contractions. While muscle cells also store

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

, most of the energy required for contraction is derived from

phosphagen Phosphagens, also known as macroergic compounds, are high energy storage compounds, also known as high-energy phosphate compounds, chiefly found in muscular tissue in animals. They allow a high-energy phosphate pool to be maintained in a concentra ...

s. One such phosphagen,

creatine phosphate Phosphocreatine, also known as creatine phosphate (CP) or PCr (Pcr), is a phosphorylated form of creatine that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle, myocardium and the brain to recycle adenosine trip ...

, is used to provide ADP with a phosphate group for ATP synthesis in

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

Comparative structure

The structure of the sarcomere affects its function in several ways. The overlap of actin and myosin gives rise to the length-tension curve, which shows how sarcomere

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...

output decreases if the muscle is stretched so that fewer cross-bridges can form or compressed until actin filaments interfere with each other. Length of the actin and myosin filaments (taken together as sarcomere length) affects force and velocity – longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening. Vertebrates display a very limited range of sarcomere lengths, with roughly the same optimal length (length at peak length-tension) in all muscles of an individual as well as between species.

Arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

, however, show tremendous variation (over seven-fold) in sarcomere length, both between species and between muscles in a single individual. The reasons for the lack of substantial sarcomere variability in vertebrates is not fully known.

References

External links

MBInfo: SarcomereMBInfo: Contractile FiberMuscular Tissues Videos
* - "Ultrastructure of the Cell: sarcoplasm of skeletal muscle" *
Images created by antibody to striationsModel representation of the sarcomere
{{Authority control Cell anatomy Cell movement Muscular system