Sarcomere

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Sarcomere

"A-band" redirects here. For other uses of the term see A band
A band

This is article is about the wireless term. For other uses see A band .The A band is the range of radio frequency up to 250 MHz in the electromagnetic spectrum....
. A sarcomere (Greek sárx = "flesh", méros = "part") is the basic unit of a muscle

MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
's cross-striated myofibril

Myofibril

Myofibrils are cylinder organelles. They are found within muscle cells. They are bundles of actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end....
. Sarcomeres are multi-protein complexes composed of three different filament systems.

A muscle cell from a biceps

Biceps

Biceps may refer to:* The two-headed Biceps brachii muscle on the inside of each upper arm.* The biceps femoris, one of the hamstring muscles of the underside of each thigh....
may contain 100,000 sarcomeres.

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A sarcomere (Greek sárx = "flesh", méros = "part") is the basic unit of a muscle

MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
's cross-striated myofibril

Myofibril

The thick filament system is composed of myosin
Myosin

Myosins are a large family of motor proteins found in eukaryotic Biological tissue. They are responsible for actin-based motility.Following the discovery, by Pollard and Korn, of enzymes with myosin-like function in Acanthamoeba, a large number of divergent myosin genes have been discovered throughout eukaryotes....
protein which is connected from the M-line to the Z-disc by titin
Titin

Titin, also known as connectin, is a protein that is important in the contraction of striated muscle tissues....
. It also contains myosin-binding protein C which binds at one end to the thick filament and the other to Actin.
The thin filaments are assembled by actin
Actin

Actin is a Globular_protein, roughly 42-kDa protein found in all Eukaryote where it may be present at concentrations of over 100 ?M. It is also one of the most highly-Conservation proteins, differing by no more than 20% in species as diverse as algae and humans....
monomers bound to nebulin
Nebulin

Nebulin is an actin-binding protein which is localized to the I-band of the sarcomeres in skeletal muscle. It is a very large protein and binds as many as 200 actin monomers....
, which also involves tropomyosin (a dimer which coils itself around the F-actin core of the thin filament).
Nebulin and titin give stability and structure to the sarcomere.

A muscle cell from a biceps

Biceps

Smooth muscle

Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye....
cells are not arranged into sarcomeres.

Bands

The sarcomeres are what give skeletal and cardiac muscle

Cardiac muscle

Cardiac muscle is a type of involuntary sarcomere muscle found in the walls of the heart, specifically the wikt:myocardium. Cardiac muscle cells are known as cardiac myocytes ....
s their striated appearance.

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

German is a West Germanic languages, thus related to and classified alongside English language and Dutch language. It is one of the world's world language and the most widely spoken mother tongue in the European Union....
"Zwischenscheibe", the band in between the I bands) appears as a series of dark lines.
Surrounding the Z-line is the region of the I-band (for isotropic).
Following the I-band is the A-band (for anisotropic). Named for their properties under a polarizing microscope
Microscope

A microscope is an Laboratory equipment for viewing objects that are too small to be seen by the naked or unaided eye. The science of investigating small objects using such an instrument is called microscopy....
.
Within the A-band is a paler region called the H-band (from the German "Heller", bright). Named for their properties under a polarization microscope
Microscope

A microscope is an Laboratory equipment for viewing objects that are too small to be seen by the naked or unaided eye. The science of investigating small objects using such an instrument is called microscopy....
.
Finally, inside the H-band is a thin M-line (from the German "Mittel", middle of the sarcomere).

The relationship between the proteins and the regions of the sarcomere are as follows:

Actin filaments are the major component of the I-band and extend into the A-band.
Myosin filaments extend throughout the A-band and are thought to overlap in the M-band.
The giant protein titin
Titin

Titin, also known as connectin, is a protein that is important in the contraction of striated muscle tissues....
(connectin) extends from the Z-line of the sarcomere, where it binds to the thin filament 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.
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 as well as M-protein crosslink the thick filament system (myosins) and the M-band part of titin (the elastic filaments).
The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the muscle contraction
Muscle contraction

Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may #Eccentric contraction, #Concentric contraction or #Isometric contraction....
(sliding filament model).

Contraction

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.

The A-band, I-band and Z-line are never visible even at the light-microscope level.

The protein tropomyosin

Tropomyosin

Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin....
covers the myosin binding sites of the actin molecules in the muscle cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin-C 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, a unique form of endoplasmic reticulum. Muscle contraction ends when calcium ions are pumped back out of the sarcomere.

During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine

Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including homo sapiens....
which travels across the neuromuscular junction (the synapse between the terminal bouton of the neuron and the muscle cell). The action potential

Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
then travels along T (transverse) tubules until it reaches the sarcoplasmic reticulum; the action potential from the motor neuron changes the permeability of the sarcoplasmic reticulum, allowing the flow of calcium ions into the sarcomere. The outflow of calcium allows the myosin heads access to the actin cross bridge binding sites, permitting muscle contraction.

Rest

At rest, the myosin head is bound to an ATP

Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
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 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 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.

Storage

Most muscle cells only store enough ATP for a small number of muscle contractions. While muscle cells also store glycogen, most of the energy required for contraction is derived from phosphagens. One such phosphagen

Phosphagen

The phosphagens are energy storage compounds, also known as high-energy phosphate compounds, are chiefly found in muscle tissue in animals. They allow a high-energy phosphate pool to be maintained in a concentration range, which, if it all were Adenosine triphosphate, would create problems due to the Adenosine triphosphate consuming reaction...
is creatine phosphate, which is used to provide ADP with a phosphate group for ATP synthesis in vertebrates.

External links

- "Ultrastructure of the Cell: sarcoplasm of skeletal muscle"