Cell Membrane Potentials

Cell membranes in general, and membranes of nerve cells in particular, maintain a small voltage or "potential" across the membrane in its normal or resting state. In the rest state, the inside of the nerve cell membrane is negative with respect to the outside (typically about -70 millivolts). The voltage arises from differences in concentration of the electrolyte ions K⁺ and Na⁺.

一般来说，细胞膜，尤其是神经细胞膜，在正常或静息状态下会维持一个小的电压或‘电势’。在静息状态下，神经细胞膜内侧相对于外侧带负电（通常约为-70毫伏）。这种电压来源于K+和Na+电解质离子浓度差异。

If the cell membranes were simply permeable to these ions, they would approach an equilibrium with equal concentrations on each side of the membrane, and hence no voltage difference. This makes it clear that the processes which produce the membrane potential are not simply diffusion and osmosis.

如果细胞膜仅仅允许这些离子通过，它们将达到膜两侧浓度相等的平衡状态，从而没有电压差。这清楚地表明，产生膜电位的过程不仅仅是扩散和渗透。

In the selectively permeable cell membranes are ion channels which allow K⁺ ions to pass to the interior of the cell, but block Na⁺ ions. Negatively charged proteins on the interior of the cell are also denied passage across the membrane. In addition, there are active transport mechanisms at work. There is a process called the sodium-potassium pump which utilizes ATP to pump out three Na⁺ ions and pump in two K⁺ ions. The collective action of these mechanisms leaves the interior of the membrane about -70 mV with respect to the outside.

在选择性通透的细胞膜中，存在离子通道，允许K+离子进入细胞内部，但阻止Na+离子通过。细胞内部的带负电蛋白质也无法穿过膜。此外，还有主动运输机制在起作用。有一种称为钠钾泵的过程，它利用ATP将三个Na+离子泵出细胞，同时将两个K+离子泵入细胞。这些机制的共同作用使膜内电位约为-70 mV，相对于膜外。

For the nerve cell, this equilibrium is disturbed by the arrival of a suitable stimulus. The dynamic changes in the membrane potential in response to the stimulus is called an action potential. After the action potential the mechanisms described above bring the cell membrane back to its resting state.

对于神经细胞来说，这种平衡会被适当刺激的到达所破坏。细胞膜电位对刺激的动态变化称为动作电位。动作电位之后，上述机制会使细胞膜恢复到静息状态。