Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 101

Ask any neurobiologist, and she’ll tell you that a big challenge in understanding the flow of currents through neural membranes is having to account for the two forces of imbalances: diffusion and voltage. Diffusion causes ions to move from regions of higher to lower concentration. As for voltage, the electrostatic force causes ions to move away from a like charge toward an opposite charge. Neurons have a net negative charge inside with respect to outside, and different concentrations of ions like sodium and potassium inside compared to outside. The following sections explore these ideas in more detail.

The upcoming sections look at how diffusion and electrostatic forces affect ion movement. Let’s look at the forces influencing sodium ions as an example. The sodium concentration is very low inside compared to outside the cell. The inside of the cell is also negatively charged. So, when membrane ion channels that are permeable to sodium open, the concentration gradient pushes sodium from its high concentration outside into the cell. Similarly, the electrostatic force (voltage) will also draw the positively charged sodium from outside to the negatively charged inside. Opening sodium channels will cause a large sodium inrush from both diffusion and the electrostatic force.

What about potassium? Potassium has a higher concentration inside than out, so diffusion tends to move it outside if potassium channels are opened. On the other hand, the negative charge inside the cell attracts positively charged potassium, just like it does sodium.