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Cytoplasmic nanospaces or cytoplasmic nanojunctions are intracellular regions defined by two closely apposed organellar membranes or by a likewise near approach of an organellar membrane to the plasma membrane, by the signaling complexes that are localized to these membrane domains and by the intervening cytosol and biomolecules therein. The participating membranes are typically separated by a fairly regular gap of less than 50 nm and they are found to extend roughly parallel to each other for a few hundred nanometres.

Current experimental evidence suggests that cytoplasmic nanojunctions provide the cell with regions of restricted diffusion for molecular species that are essential cell signaling messengers, such as, ionic calcium (Ca2+), sodium (Na+) and potassium (K+), but also for macromolecules such as lipids. More concrete examples of processes that appear to be facilitated by these nanojunctions are: excitation-contraction coupling (E-C coupling) in the cardiac muscle and in smooth muscles, the stabilization and control of protein complexes like membrane-bound cellulose synthase, and calcium homeostasis. Cytoplasmic nanospaces are found in eukaryotic cells in plants, animals, fungi and possibly cells belonging to other kingdoms.

These nanojunctions are the key to understanding information transfer within cells. For example, it is within nanospaces that cholesterol can be exchanged, ion concentrations adjusted by orders of magnitude relative to those in the non-junctional cytosol, and intracellular signaling molecules are synthesized, degraded, and their effectors are found. This spatial restriction enables signaling networks to operate in parallel, but use the same basic molecules, thus enabling specific, and possibly wildly different outcomes with little to no cross-talk.

Although the importance of restricting ion, specifically calcium (Ca2+), diffusion originated almost 40 years ago, when the concept of cytoplasmic nanojunctions was proposed (albeit with a different nomenclature), the genesis of cytoplasmic nanospaces and the variety of cellular processes and mechanisms that they can sustain have just started to be understood. More focus attention has been paid to the concept of cytoplasmic nanojunctions by the scientific community in the early '10s of the 21st century, as testified by a number of international meetings on the topic ( Nanospace Biophysics, Peter Wall Institute for Advanced Studies Exploratory Workshop, Vancouver, British Columbia, Canada, October 2010; Vascular & Smooth Muscle Physiology - Nanojunctions and calcium signalling in smooth muscle cells, Edinburgh, UK, December 2011; IUPS 2013 , Birmingham, UK, July 2013) and a surge of scientific articles in peer reviewed journals ( ...) and preprint servers (arxiv, biorxiv, Nature Precedings).

[Must change this paragraph, possibly drawing attention to the similarity between c.n.'s and nmj in terms of providing 'guidance' for otherwise randomly moving ions.] Originally, the concept arose from observation of intracellular nanospaces, but recent (and not) evidence suggest that cytoplasmic nanospaces can be more ubiquitous and include extracellular nanospaces belonging to the same cell (homogeneous---may need to find a better term) and between different cells (heterogeneous---better term?) [homo/hetero-cytotic?].

Historical perspective
CvB's 1977 article hinted at role of Ca2+ influx rate in the generation of contractile force in rabbit aorta smooth muscle tissue.... follow thread of my departmental talk in November 2011. [Mention Nanospace Biophysics workshop]