Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 99

One transporter pump is the sodium-potassium ATPase transporter. It creates a large imbalance between — you guessed it — the concentrations of sodium and potassium inside and outside the cell. It also causes the inside of neurons to be negatively charged inside versus outside, which is necessary for neurons to work.

The sodium-potassium ion pump or transporter is sometimes called an ion exchanger, because it works with two different ions.

The sodium-potassium pump pushes sodium ions outside the cell and potassium ions inside. The exchange ratio is three sodium ions pumped out for every two potassium ions pumped in. These pumps are ubiquitously expressed in neurons and run constantly. By always running, they create a disequilibrium or imbalance in sodium/potassium concentrations across the membrane, as you can see in Figure 3-1.

Figure 3-1: The sodium-potassium pump creates a disequilibrium between sodium and potassium concentrations inside versus outside the cell.

Here are two important facts about the sodium/potassium imbalance:

 Compared to the extracellular fluid, which has high sodium and low potassium concentrations, the cell’s cytoplasm has almost the opposite: very low sodium and high potassium.

 Because the sodium exit to potassium entry ratio is 3:2, a net loss of positive charge occurs inside the cell compared to outside. That is, neurons are negatively charged inside.

Because the negative potential inside neurons is based on the total difference in ionic concentrations between the inside and outside, if the transporter pumps stopped working and all other ion channels closed (in the impermeable state) a neuron’s resting potential (when nothing is passing through) would still exist for a long time. Scientists have done experiments to verify this, using poisons to kill the pumps and channel blockers to close all the ion channels.