The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea.

It is composed of three main components: microfilaments, intermediate filaments, and microtubules, and these are all capable of rapid growth and or disassembly depending on the cell's requirements.

Muscle contraction starts from nerve impulses which then causes increased amounts of calcium to be released from the sarcoplasmic reticulum.

Increases in calcium in the cytosol allows muscle contraction to begin with the help of two proteins, tropomyosin and troponin.

In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton.

This discovery came after the realization that bacteria possess proteins that are homologous to tubulin and actin; the main components of the eukaryotic cytoskeleton.

Intermediate filaments are composed of various proteins, depending on the type of cell in which they are found; they are normally 8-12 nm in diameter.

[2] The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment.

Research has shown that microtubule assembly and stability in the cytoskeleton is compromised causing the neurons to degrade over time.

[20] In Alzheimer's disease, tau proteins which stabilize microtubules malfunction in the progression of the illness causing pathology of the cytoskeleton.

[22] Amyotrophic lateral sclerosis results in a loss of movement caused by the degradation of motor neurons, and also involves defects of the cytoskeleton.

[23] Stuart Hameroff and Roger Penrose suggest a role of microtubule vibrations in neurons in the origin of consciousness.

[27] Myosin motoring along F-actin filaments generates contractile forces in so-called actomyosin fibers, both in muscle as well as most non-muscle cell types.

These filaments, averaging 10 nanometers in diameter, are more stable (strongly bound) than microfilaments, and heterogeneous constituents of the cytoskeleton.

[5] Keratin intermediate filaments in epithelial cells provide protection for different mechanical stresses the skin may endure.

Cortical patches are discrete actin bodies on the membrane and are vital for endocytosis, especially the recycling of glucan synthase which is important for cell wall synthesis.

However, research in the early '90s suggested that bacteria and archaea had homologues of actin and tubulin, and that these were the basis of eukaryotic microtubules and microfilaments.

[43] Three laboratories independently discovered that FtsZ, a protein already known as a key player in bacterial cytokinesis, had the "tubulin signature sequence" present in all α-, β-, and γ-tubulins.

Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis.

Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear.

[48] The cytoskeleton is a highly anisotropic and dynamic network, constantly remodeling itself in response to the changing cellular microenvironment.

Mechanotransduction relies heavily on focal adhesions, which essentially connect the intracellular cytoskeleton with the extracellular matrix (ECM).

Proteins such as focal adhesion kinase (FAK) and Src have been shown to transduce force signals in response to cellular activities such as proliferation and differentiation, and are hypothesized to be key sensors in the mechanotransduction pathway.

[50] As a result of mechanotransduction, the cytoskeleton changes its composition and/or orientation to accommodate the force stimulus and ensure the cell responds accordingly.

[51] A membrane protein that is not closely coupled to the cytoskeleton, for instance, will not produce a significant effect on the cortical actin network if it is subjected to a specifically directed force.

[52] Cells, which are around 10–50 μm in diameter, are several thousand times larger than the molecules found within the cytoplasm that are essential to coordinate cellular activities.

Because cells are so large in comparison to essential biomolecules, it is difficult, in the absence of an organizing network, for different parts of the cytoplasm to communicate.

Examples for intermediate filaments, which have almost exclusively been found in animals (i.e. eukaryotes) are the lamins, keratins, vimentin, neurofilaments, and desmin.

[55] Organelles move along microfilaments in the cytoskeleton driven by myosin motors binding and pushing along actin filament bundles.