Connexin 36

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Connexin 36 is a protein found in the gap junctions that play a key role in the Penrose-Hameroff quantum consciousness model. For the purposes of neuroscientific studies, it has proved possible to breed mice that lack connexin 36. These mice, in which the gene for connexin 36 has been deleted, have certain deficits in motor, learning, memory and reproductive capacity. Gamma synchrony in the brain persists, although at reduced amplitude. Otherwise, they appear to observers to by just as conscious as normal mice. This fact has been seized on by critics of the Penrose-Hameroff model claiming that these mice remain conscious either despite having no gap junctions, or despite connexin 36 being the main component of gap junctions. However, both these claims are dubious.

Most animal cells have gap junctions with neighbouring cells. The cells are only 2-4 nm apart at these junctions, with the space bridged by gap junctions that are formed out of proteins called connexins. Six connexin proteins are assembled in each cell to form a connexon, and two such structures, one in each cell, join together to form a gap junction between the two cells.

Humans have at least 14 different types of connexin, each found in a different range of tissues, with at least 10 present in the central nervous system. Most cells have more than one type of connexin present. Connexons may be formed from one type of connexin or from several different types. Moreover, the set of connexins in one cell may be different from the set of connexins in another cell to which the gap junction provides a link. Connexin 36 is present in certain subpopulations of neurons throughout the mammalian brain.

The connexon channels have a width of 1.5nm, which allows ions and smaller molecules to pass from one cell to the other, coupling the cells both electrically and metabolically. The permeability of gap junctions by different sizes of molecules or ions of particular charge can be a function of the connexins that comprise it. Gap junctions flip between open and closed states in response to conditions in the cell. The junctions are viewed as allowing adjacent cells to coordinate their activity, although the specifics of this process remain unknown.

Gap junctions in different tissues have different properties, as a function of being comprised of different connexins. Most cells have examples of more than one type of connexin, while particular types of connexins can be present in a range of tissues. It is common for two types of connexin to be present in the same connexon.

Existing knowledge of connexin distribution and function appears to be a good way from being complete. At the moment, it seems far from clear how important connexin 36 actually is relative to other connexins, and whether the absence of this particular connexin would result in the failure of gap junctions to form or for any that did form to carry out at least some of their normal activities. Some studies suggest the existence of normal neural gap junctions without any connexin 36. The variability of the connexin system seems to argue against the idea that the abscence of just one connexin could knock out the function of any large proportion of gap junctions.

References:-

Molecular Biology of the Cell – Taylor & Francis Group – ISBN 0-8153-4072-9

Psyche-B Archives – January 2005

Visual Neuroscience – Sitaramayya, A. et al – Connexin 36 in bovine retina – 2003, 20, pp.385-95 – Cambridge University Press

Histochemistry and Cell Biology – Carola, M. et al – Immunohistochemical detection of the neuronal connexin 36 – 2002, vol. 117, No. 6, pp. 461-471

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