User:UnderEducatedGeezer/sandbox/Mesaxon

Draft to try to better explain MESAXON for the existing wiki article (and this may not really better explain it)
A Mesaxon is a set of parallel plasma membranes of a Schwann cell. The mesaxon presents somewhat differently in a myelinated axon than in an unmyelinated one. For an unmyelinated axon, the mesaxon is simply the pair of membranes formed by the lips of the invaginated Schwann cell; in a myelinated axon the mesaxon is multiple pairs of parallel membranes, which include an outer mesaxon, an inner mesaxon, and the multiple pairs of membranes formed by the entire spiraling wrapping of the Schwann cell around the axon.

The mesaxon results from an infolding of a Schwann cell when the Schwann cell either partly or completely surrounds a section of an axon or axons. A Schwann cell which surrounds an axon may myelinate it, or not, depending on whether the Schwann cell spirals around the axon multiple times, or simply envelops it once. For an unmyelinated axon, there is only one mesaxon per axon (at each Schwann cell), while a myelinated axon has an outer mesaxon, an inner mesaxon, and a multiple-layered mesaxon within (and comprised of) the myelin sheath.

For an unmyelinated axon, the infolded Schwann cell resembles a capital-letter "U", with the axon held inside the "U". Imagine the "U" being drawn in outline fashion (with each line of the U being hollow so that there is an inner line and an outer line for each 'line' comprising the drawing of the U), such that the outer surface of the Schwann cell is the 'outside' of the "U", and the infolded portion of the Schwann cell's outer surface is the 'inner' or cupped part of the "U", with the axon cradled at the bottom of the "U", and the two top-ends of the "U" folding up together, forming 'lips' almost touching. And since the Schwann cell has a length, the 'lips' of the two top ends of the "U" extend the length of the Schwann cell, and thus form a pair of parallel membranes, which constitutes the mesaxon in the unmyelinated axon. And because a Schwann cell can surround more than one axon that it is not myelinating (each individual axon being at different positions along the initial circumference of the Schwann cell), each axon thus surrounded will have its own mesaxon at each section of the axon invaginated by the Schwann cell.

A myelinated axon can be seen to derive from the above description of an unmyelinated axon, in that one of the top ends of the "U" dives down under the other top-end, and then continues to spiral wrap around and around the axon under previous sections of itself. Those two top ends can be considered 'tongues', with both surfaces of each 'tongue' being made up of the OUTER surface of the Schwann cell, but one surface is outward from the axon, and the other surface in inward toward the axon. As one 'tongue' dives down under the other 'tongue', the diving down 'tongue's' outermost surface is in contact with the other tongue's innermost surface, and THIS set of parallel membranes constitutes the OUTER MESAXON. As the one 'tongue' continues to spiral wrap around and around the axon, up to 150 times, it will eventually stop wrapping, and at the position where that 'tongue' ceases to wrap, it's outermost surface (the surface not nearest to the axon) will be in contact or close to its own innermost surface from an immediately PREVIOUS wrap (due to the spiraling nature of the wrapping), and THIS set of parallel membranes (the outermost surface next to the previous innermost surface) constitutes the INNER MESAXON.

And since these winding/wrapping sheets of membrane have multiple layers of parallel pairs of membranes, they are all the MESAXON of the Schwann cell for that myelinated axon. ---

, or in an infolding folded-inward, which inward-folding holds an axon within those cell surfaces. This mesaxon exists as the invaginated outer surfaces of the Schwann cell run parallel to one another along the length of the portion of the axon it is enveloping. These two parallel sheets of membrane exist in either a myelinated axon, or also in unmyelinated axons.