In the cell plasma of a muscle fiber sit large molecules that carry a negative charge and cannot leave the inner space. That potassium of all things ends up facing them is no whim of nature, but a consequence of what the outer skin of the fiber lets through and what it does not.
Read the Ground PlanPotassium is the cation that occurs far more often inside a muscle fiber than outside it. It is small, singly positive and holds on to its shell of water only loosely — three properties that keep it mobile, while the negatively charged counterparts in the interior remain where they are.
Potassium contributes to normal muscle functionEU-authorized wording · Regulation (EU) No 432/2012
Protein filaments, enzymes, creatine phosphate and the phosphate groups of large carrier molecules are too bulky or too firmly bound to pass the outer skin. Between them they hold by far the largest share of the negative charge in the cell plasma.
The potassium channels of the fiber membrane let more through inward than outward. A bolt of magnesium ions and polyamines sees to that: it slides into the pore from the inner side the moment the way out would come clear.
Three sections that build on one another: what sits fast in the interior, which particle provides the counterpart to it, and why that particle hardly ever leaves.
Every muscle fiber is a closed space with one peculiarity: a large part of what is dissolved inside it never comes out. At the degree of acidity usual in cell plasma, protein filaments, enzymes, creatine phosphate and the phosphate groups of large carrier molecules each carry one or more negative charges. None of these particles fits through the pores of the outer skin.
Electrical neutrality demands that a matching quantity of positive charge stand opposite those charges. The job falls neither to the largest nor to the most common cation, but to the one the resting outer skin lets through at all.
Sodium is plentiful outside the fiber and could, on the arithmetic alone, do the same balancing work. It stays outside nonetheless, because the resting outer skin barely admits it and because a pump in that same skin holds out against it at an energy cost.
That leaves potassium as the only abundant cation mobile enough to arrange itself according to the electrical field inside. It gathers where the fixed negative charges lie and, together with them, makes up the charge balance of the interior.
If the pores simply stayed open, potassium would drain outward at every shift in voltage. The channels in question, however, do not work symmetrically. From the inner side, magnesium ions and polyamines — spermine, spermidine and putrescine, short-chain molecules found in every cell — slide into the pore and block the way out at exactly the moment the electrical field would favor it. The way inward stays clear.
This preferred direction is described at length in the specialist literature. It explains why a fiber keeps most of its potassium stock across many signals, and it is among the reasons the connection between this mineral and the working of the musculature came to be examined by specialists at all.
What was assessed there was the pairing of one mineral with one subject area, not a product and not any of the mechanisms described above. What stands in the register at the end is a single line:
Potassium contributes to normal muscle functionEU-authorized wording · Regulation (EU) No 432/2012
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