How cells pile up potassium on their inside, how sodium-potassium ATPase keeps that gradient in place cycle after cycle, and which statement the European Union has authorized for potassium.
On to the Long ReadDiagram: for every ATP molecule it splits, the pump moves three sodium ions out and brings two potassium ions in.
Slip a fine measuring electrode into a resting skeletal muscle fiber and the meter reads a voltage of roughly minus 85 to minus 90 millivolts. The interior of the cell, in other words, is negative compared with the fluid around it. That voltage is not a side effect. It is the starting position from which the fiber is able to answer a signal in the first place.
It comes from an uneven spread of charged particles. Inside the fiber there are around 150 millimoles of potassium per liter; in the fluid between the cells, only about 4 to 5. With sodium, the picture is reversed. A gradient like this does not form on its own, and it would not last on its own either: ions keep seeping through open channels in the direction that would even things out.
This page tracks potassium as it crosses the membrane. It sets out which pump builds the gradient, which channels turn it into an electrical voltage, and what takes place after a signal so that the fiber can settle back into its starting state.
Viewed from the outside, a muscle fiber just moves. At the level of its outer envelope, though, a tightly timed sequence plays out in which ions change places within a few milliseconds. It can be divided into three stages.
Sodium-potassium ATPase is embedded by the thousands in the membrane of every fiber. It splits one molecule of ATP, puts the energy released into a change of shape, and in the process pumps three sodium ions out and brings two potassium ions in. Since more positive charge leaves the cell than enters it, the pump also adds a few millivolts straight to the membrane voltage.
At rest, certain potassium channels, called inward-rectifier channels, stay open the whole time. Potassium ions follow their concentration gradient outward, and each one takes a positive charge along. What stays behind is a slight surplus of negative charge inside, which holds back further outflow. The resting voltage settles at the point where these two pulls cancel each other out.
Once a signal reaches the fiber, sodium channels open and the voltage swings briefly into positive values. A moment later, voltage-gated potassium channels open. As potassium flows out, the membrane is carried back toward its negative starting value – a step known as repolarization. Only after that can the fiber answer the next signal. For this mineral, the EU has authorized a health claim:
“Potassium contributes to normal muscle function”
EU-authorized wording · Regulation (EU) No 432/2012The potassium that leaves with each signal is a very small amount next to the stock held inside the cell. Across many signals in a row, however, it accumulates, and the pump keeps bringing the potassium back to where it belongs.
An ion, a motor and a set of channels: seen this way, what goes on at the fiber envelope falls into three parts.
Around 98 percent of the potassium in the body is located inside cells, and the biggest portion of that sits in skeletal muscle tissue. Only a small fraction circulates in blood plasma.
“Potassium contributes to normal muscle function”
EU-authorized wording · Regulation (EU) No 432/2012A protein built from a large alpha subunit and a smaller beta subunit. In skeletal muscle fibers, the alpha-2 form is the most common one. Every pump cycle uses up one ATP and takes only a fraction of a second.
When the organism is at rest, a sizable part of the energy turnover in many cells goes to this pump.
Inward rectifiers keep the resting voltage steady, while channels that open with a delay return the membrane to its baseline after a signal. Both kinds of channel are highly selective and let hardly any sodium pass.
Includes a worked example for the resting voltage, a table of the pump cycle’s phases and a section on the narrow T-tubules of the muscle fiber.
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