Part of the myosin II structure. Atoms in the heavy chain are colored pink (on the left-hand side); atoms in the light chains are colored faded-orange and faded-yellow (also on the left-hand side).
Myosin unrooted phylogenetic tree
Sliding filament model of muscle contraction.
Crystal structure of myosin V motor with essential light chain – nucleotide-free
State of myosin VI from PDB 2V26 before the power stroke
Phase 1
Phase 2
Phase 3
Phase 4

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.

- Myosin

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Skeletal muscle

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

The myofibrils are composed of actin and myosin filaments called myofilaments, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for muscle contraction.

Muscle contraction

Activation of tension-generating sites within muscle cells.

Types of muscle contractions
In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac
Organization of skeletal muscle
Structure of neuromuscular junction.
Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions
Cross-bridge cycle
Muscle length versus isometric force
Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.
Cardiac muscle
Key proteins involved in cardiac calcium cycling and excitation-contraction coupling
A simplified image showing earthworm movement via peristalsis
Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page.
Electrodes touch a frog, and the legs twitch into the upward position

During a concentric contraction, contractile muscle myofilaments of myosin and actin slide past each other, pulling the Z-lines together.


Smallest functional unit of striated muscle tissue.

Image of sarcomere
Muscle contraction based on sliding filament theory

Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament.

Muscle cell

Also known as a myocyte when referring to either a cardiac muscle cell , or a smooth muscle cell as these are both small cells.

General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril
Diagram of skeletal muscle fiber structure

The thin myofilaments are filaments of mostly actin and the thick filaments are of mostly myosin and they slide over each other to shorten the fiber length in a muscle contraction.

Smooth muscle

Involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations .

Smooth muscle tissue, highlighting the inner circular layer (nuclei then rest of cells in pink), outer longitudinal layer (nuclei then rest of cells), then the serous membrane facing the lumen of the peritoneal cavity
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
A series of axon-like swellings, called varicosities from autonomic neurons, loosely form motor units through the smooth muscle.

There are no myofibrils present but much of the cytoplasm is taken up by the proteins of myosin and actin which together have the capability to contract.

Motor protein

Motor proteins are a class of molecular motors that can move along the cytoplasm of animal cells.

Kinesin walking on a microtubule using protein dynamics on nanoscales

The best prominent example of a motor protein is the muscle protein myosin which "motors" the contraction of muscle fibers in animals.


Family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.

Ribbon diagram of G-actin. ADP bound to actin's active site (multi color sticks near center of figure) as well as a complexed calcium dication (green sphere) are highlighted.
Fluorescence micrograph showing F-actin (in green) in rat fibroblasts
A merged stack of confocal images showing actin filaments within a cell. The image has been colour coded in the z axis to show in a 2D image which heights filaments can be found at within cells.
Structure of the C-terminal subdomain of villin, a protein capable of splitting microfilaments
The structure of a sarcomere, the basic morphological and functional unit of the skeletal muscles that contains actin
Diagram of a zonula occludens or tight junction, a structure that joins the epithelium of two cells. Actin is one of the anchoring elements shown in green.
Ribbon model of actin from rabbitmuscle. The four subdomains can be seen, as well as the N and C termini and the position of the ATP bond. The molecule is oriented using the usual convention of placing the - end (pointed end) up and the + end (barbed end) down.
F-actin; surface representation of a repetition of 13 subunits based on Ken Holmes' actin filament model
Ribbon model obtained using the PyMOL programme on crystallographs of the prefoldin proteins found in the archaean Pyrococcus horikoshii. The six supersecondary structures are present in a coiled helix “hanging” from the central beta barrels. These are often compared in the literature to the tentacles of a jellyfish. As far as is visible using electron microscopy, eukariotic prefoldin has a similar structure.
Ribbon model of the apical γ-domain of the chaperonin CCT
Microfilament formation showing the polymerization mechanism for converting G-actin to F-actin; note the hydrolysis of the ATP.
Atomic structure of Arp2/3. Each colour corresponds to a subunit: Arp3, orange; Arp2, sea blue (subunits 1 and 2 are not shown); p40, green; p34, light blue; p20, dark blue; p21, magenta; p16, yellow.
An actin (green) - profilin (blue) complex. The profilin shown belongs to group II, normally present in the kidneys and the brain.
The protein gelsolin, which is a key regulator in the assembly and disassembly of actin.
Principal interactions of structural proteins are at cadherin-based adherens junction. Actin filaments are linked to α-actinin and to the membrane through vinculin. The head domain of vinculin associates to E-cadherin via α-catenin, β-catenin, and γ-catenin. The tail domain of vinculin binds to membrane lipids and to actin filaments.
Structure of MreB, a bacterial protein whose three-dimensional structure resembles that of G-actin
Giant nemaline rods produced by the transfection of a DNA sequence of ACTA1, which is the carrier of a mutation responsible for nemaline myopathy
Position of seven mutations relevant to the various actinopathies related to ACTA1
Cross section of a rat heart that is showing signs of dilated cardiomyopathy
Image taken using confocal microscopy and employing the use of specific antibodies showing actin's cortical network. In the same way that in juvenile dystonia there is an interruption in the structures of the cytoskeleton, in this case it is produced by cytochalasin D.
Western blot for cytoplasmic actin from rat lung and epididymis
Nobel Prize winning physiologist Albert von Szent-Györgyi Nagyrápolt, co-discoverer of actin with Brunó Ferenc Straub
Chemical structure of phalloidin

The most notable proteins associated with the actin cytoskeleton in plants include: villin, which belongs to the same family as gelsolin/severin and is able to cut microfilaments and bind actin monomers in the presence of calcium cations; fimbrin, which is able to recognize and unite actin monomers and which is involved in the formation of networks (by a different regulation process from that of animals and yeasts); formins, which are able to act as an F-actin polymerization nucleating agent; myosin, a typical molecular motor that is specific to eukaryotes and which in Arabidopsis thaliana is coded for by 17 genes in two distinct classes; CHUP1, which can bind actin and is implicated in the spatial distribution of chloroplasts in the cell; KAM1/MUR3 that define the morphology of the Golgi apparatus as well as the composition of xyloglucans in the cell wall; NtWLIM1, which facilitates the emergence of actin cell structures; and ERD10, which is involved in the association of organelles within membranes and microfilaments and which seems to play a role that is involved in an organism's reaction to stress.


Eukaryotes are organisms whose cells have a nucleus enclosed within a nuclear envelope.

The endomembrane system and its components
Simplified structure of a mitochondrion
Longitudinal section through the flagellum of Chlamydomonas reinhardtii
Structure of a typical animal cell
Structure of a typical plant cell
Fungal Hyphae cells: 1 – hyphal wall, 2 – septum, 3 – mitochondrion, 4 – vacuole, 5 – ergosterol crystal, 6 – ribosome, 7 – nucleus, 8 – endoplasmic reticulum, 9 – lipid body, 10 – plasma membrane, 11 – spitzenkörper, 12 – Golgi apparatus
This diagram illustrates the twofold cost of sex. If each individual were to contribute the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation.
Phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes
One hypothesis of eukaryotic relationships – the Opisthokonta group includes both animals (Metazoa) and fungi, plants (Plantae) are placed in Archaeplastida.
A pie chart of described eukaryote species (except for Excavata), together with a tree showing possible relationships between the groups
The three-domains tree and the Eocyte hypothesis
Phylogenetic tree showing a possible relationship between the eukaryotes and other forms of life; eukaryotes are colored red, archaea green and bacteria blue
Eocyte tree.
Diagram of the origin of life with the Eukaryotes appearing early, not derived from Prokaryotes, as proposed by Richard Egel in 2012. This view implies that the UCA was relatively large and complex.

Motor proteins of microtubules, e.g., dynein or kinesin and actin, e.g., myosins provide dynamic character of the network.


Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues.

A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red).
John Kendrew with model of myoglobin in progress
Chemical structure of the peptide bond (bottom) and the three-dimensional structure of a peptide bond between an alanine and an adjacent amino acid (top/inset). The bond itself is made of the CHON elements.
Resonance structures of the peptide bond that links individual amino acids to form a protein polymer
A ribosome produces a protein using mRNA as template
The DNA sequence of a gene encodes the amino acid sequence of a protein
The crystal structure of the chaperonin, a huge protein complex. A single protein subunit is highlighted. Chaperonins assist protein folding.
Three possible representations of the three-dimensional structure of the protein triose phosphate isomerase. Left: All-atom representation colored by atom type. Middle: Simplified representation illustrating the backbone conformation, colored by secondary structure. Right: Solvent-accessible surface representation colored by residue type (acidic residues red, basic residues blue, polar residues green, nonpolar residues white).
Molecular surface of several proteins showing their comparative sizes. From left to right are: immunoglobulin G (IgG, an antibody), hemoglobin, insulin (a hormone), adenylate kinase (an enzyme), and glutamine synthetase (an enzyme).
The enzyme hexokinase is shown as a conventional ball-and-stick molecular model. To scale in the top right-hand corner are two of its substrates, ATP and glucose.
Ribbon diagram of a mouse antibody against cholera that binds a carbohydrate antigen
Proteins in different cellular compartments and structures tagged with green fluorescent protein (here, white)
Constituent amino-acids can be analyzed to predict secondary, tertiary and quaternary protein structure, in this case hemoglobin containing heme units

Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape.

Myosin light chain

Part of the myosin II structure. Atoms in the heavy chain are colored red on the left-hand side, and atoms in the light chains are colored orange and yellow.

A myosin light chain is a light chain (small polypeptide subunit) of myosin.