Anyone who reads the interior of a muscle fiber as a ground plan finds three groups of occupants there: large molecules that stay; ions that come and go; and water that arranges itself by both. The sections below walk through that ground plan in order and end at the line European law provides for it.
The first look at a ground plan goes to the fixed fittings. In the cell plasma of a fiber those are molecules too bulky to get through the outer skin — and almost without exception they carry a negative charge.
At the degree of acidity usual in cell plasma, protein filaments and enzymes carry a net negative charge. To these come creatine phosphate as a reserve for rapid conversions, the phosphate groups of large carrier molecules and the phosphate residues hanging on stored glycogen. None of these particles leaves the fiber by any ordinary route.
| Charge carrier | Way through the outer skin | Note |
|---|---|---|
| Protein filaments and enzymes | none | largest single group of the charge that sits fast |
| Creatine phosphate | none | present in the fiber in considerable quantity |
| Phosphate groups of large carrier molecules | none | stay bound to their carrier molecule |
| Phosphate residues on glycogen | none | rises and falls with how much reserve has been laid down |
| Chloride | readily passable | follows the electrical field and does not count among the fixed ones |
Roughly ordered after textbook accounts of muscle physiology. The columns serve to place orders of magnitude; none of these terms occurs in the line quoted further down.
That much fixed negative charge calls for a matching quantity of positive charge in the same space. Potassium is the only abundantly available cation mobile enough to take on this part: small, singly charged and wearing a shell of water that it sheds again without difficulty. Sodium does lie outside the fiber in quantity, but it barely gets in through the resting outer skin, and a pump in that same skin holds out against it at an energy cost.
From this division follows the basic furnishing of the interior: plenty of sodium outside, plenty of potassium inside, and between the two an outer skin that at rest is permeable above all to potassium.
Dissolved particles draw water. Because the fixed anions do not leave the interior and the potassium belonging with them stays put as well, the number of dissolved particles inside cannot vary at will. The water content of a fiber hangs directly on this — and with it the form in which the fiber keeps its interior divided at all.
Glycogen is a special case in this respect. It is laid down in the fiber as a reserve, carries phosphate residues and binds water to itself; along with the water, a share of potassium shifts in the same direction. When the reserve is broken down again, the process runs backward. Physiological papers have described this connection for a long time; the line from the register does not take it up.
For understanding this page, one thing about that matters above all: the furnishing of the interior is nothing rigid. It shifts with whatever the fiber happens to be storing, and afterward it returns to its usual division.
An outer skin that lets potassium through ought, in principle, to empty the interior bit by bit. That this does not happen comes down to a peculiarity of the channels involved: they conduct differently well in the two directions.
The pore of these channels is reachable from the inside. As soon as the electrical field favors the way out, dissolved magnesium ions and polyamines slide into the pore and block it. If the field falls the other way, the same particles are pressed back out and the way inward stands open. This behavior has been measured many times at cell membranes.
| Particle | Origin inside the cell | Described behavior at the pore |
|---|---|---|
| Magnesium ion | freely dissolved in the cell plasma | settles into the pore even at a slight outward flow |
| Spermine | Polyamine of the cell metabolism | blocks the way out most markedly of the three |
| Spermidine | Polyamine of the cell metabolism | acts in the same manner, though more weakly |
| Putrescine | Precursor of the other two | the weakest of the three bolts described |
After electrophysiological studies on cell membranes, heavily simplified. None of these particles occurs in the line quoted at the end of this page.
At every electrical event a fiber gives off a little potassium to the outside and fetches it back afterward. Because the way out is bolted at precisely the moment it would open, that loss turns out small. The furnishing of the interior therefore stays the same across long stretches of time — and that is exactly what is meant when an ordinary course of events is spoken of.
At this point the level changes. Everything preceding describes what specialists have measured and published. What a seller may put into words out of all that, by contrast, stands in a register — and there, for potassium and the musculature, exactly one line is entered.
It was assessed by specialists, then taken into the list of permitted statements, and it is held there word for word. Neither a sharpening of it nor a loose retelling would be covered.
Potassium contributes to normal muscle functionEU-authorized wording · Regulation (EU) No 432/2012
Named are a mineral, a process and the word normal. Not named are a quantity, an age, a single muscle group and any form of increase. Sleep, concentration and mood stay outside just as much: each of those fields is reviewed on its own and authorized on its own. The terms from the two tabulations on this page do not occur in the line.
The word normal draws a boundary: what is meant is the process as it takes place without any special occasion, and not its increase, its restoration or its alteration. The wording may moreover be used only for products that count as a source of potassium under European law at all; the minimum content that settles this is laid down elsewhere.
No, none of those terms stands there. They show the reason specialists went after this connection. On a package they may not be used as a statement — that would widen the line as entered and would therefore not be permitted.
Because the mechanical part hangs on other particles, for which separately assessed wordings exist in each case. This edition stays with the one mineral whose line it quotes and puts no second one beside it.
Not for an individual person. The water content of a tissue shifts for many reasons, and a text on the internet has no measured values at hand. Anyone who would like their own intake situation placed in context is in the right place at a medical practice, which has suitable procedures available.
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