The membrane potential

• A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential.
• The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.
• In a resting neuron, there are concentration gradients across the membrane for $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript. Ions move down their gradients via channels, leading to a separation of charge that creates the resting potential.
• The membrane is much more permeable to $\text K^+$start text, K, end text, start superscript, plus, end superscript than to $\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript, so the resting potential is close to the equilibrium potential of $\text K^+$start text, K, end text, start superscript, plus, end superscript (the potential that would be generated by $\text K^+$start text, K, end text, start superscript, plus, end superscript if it were the only ion in the system).

Introduction

Suppose you have a dead frog. (Yes, that's kind of gross, but let's just imagine it for a second.) What would happen if you applied an electrical stimulus to the nerve that feeds the frog's leg? Creepily enough, the dead leg would kick!

The Italian scientist Luigi Galvani discovered this fun fact back in the 1700s, somewhat by accident during a frog dissection. Today, we know that the frog's leg kicks because neurons (nerve cells) carry information via electrical signals.

How do neurons in a living organism produce electrical signals? At a basic level, neurons generate electrical signals through brief, controlled changes in the permeability of their cell membrane to particular ions (such as $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text K^+$start text, K, end text, start superscript, plus, end superscript). Before we look in detail at how these signals are generated, we first need to understand how membrane permeability works in a resting neuron (one that is not sending or receiving electrical signals).

In this article, we'll see how a neuron establishes and maintains a stable voltage across its membrane – that is, a resting membrane potential.

The resting membrane potential

Imagine taking two electrodes and placing one on the outside and the other on the inside of the plasma membrane of a living cell. If you did this, you would measure an electrical potential difference, or voltage, between the electrodes. This electrical potential difference is called the membrane potential.

Diagram of a voltmeter measuring the membrane potential. One electrode is outside the cell. The other electrode is in the interior of the cell. The voltmeter shows a -70 mV voltage across the membrane.

_Image modified from "how neurins communicate figure by OpenStax College, Biology (CC BY 4.0)._

Like distance, potential difference is measured relative to a reference point. In the case of distance, the reference point might be a city. For instance, we can say that Boston is $190$190 $\text{miles}$start text, m, i, l, e, s, end text northeast, but only if we know that our reference point is New York City.

For a cell’s membrane potential, the reference point is the outside of the cell. In most resting neurons, the potential difference across the membrane is about $30$30 to $90$90 $\text{mV}$start text, m, V, end text (a $\text{mV}$start text, m, V, end text is $1/1000$1, slash, 1000 of a volt), with the inside of the cell more negative than the outside. That is, neurons have a resting membrane potential (or simply, resting potential) of about $-30$minus, 30 $\text{mV}$start text, m, V, end text to $- 90$minus, 90 $\text{mV}$start text, m, V, end text.

Because there is a potential difference across the cell membrane, the membrane is said to be polarized.

• If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.
• If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.

Diagrams of voltmeters with one electrode inside the cell and one in the fluid outside of the cell. The first voltmeter shows hyperpolarization: it reads -80 mV. The second voltmeter shows the resting potential: it reads -70 mV. The third voltmeter shows depolarization: it reads +40 mV.

_Image modified from "by OpenStax College, Biology (CC BY 4.0)._

All of the electrical signals that neurons use to communicate are either depolarizations or hyperpolarizations from the resting membrane potential.

Where does the resting membrane potential come from?

The resting membrane potential is determined by the uneven distribution of ions (charged particles) between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions.

Types of ions found in neurons

In neurons and their surrounding fluid, the most abundant ions are:

• Positively charged (cations): Sodium ($\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript) and potassium ($\text{K}^+$start text, K, end text, start superscript, plus, end superscript)
• Negatively charged (anions): Chloride ($\text{Cl}^-$start text, C, l, end text, start superscript, minus, end superscript) and organic anions

In most neurons, $\text{K}^+$start text, K, end text, start superscript, plus, end superscript and organic anions (such as those found in proteins and amino acids) are present at higher concentrations inside the cell than outside. In contrast, $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript and $\text{Cl}^-$start text, C, l, end text, start superscript, minus, end superscript are usually present at higher concentrations outside the cell. This means there are stable   concentration gradient    across the membrane for all of the most abundant ion types.

This diagram represents the relative concentrations of various ion types inside and outside of a neuron.

• K+ is more concentrated inside than outside the cell.
• Organic anions are more concentrated inside than outside the cell.
• Cl- is more concentrated outside than inside the cell.
• Na+ is more concentrated outside than inside the cell.

How ions cross the membrane

Because they are charged, ions can't pass directly through the hydrophobic ("water-fearing") lipid regions of the membrane. Instead, they have to use specialized channel proteins that provide a hydrophilic ("water-loving") tunnel across the membrane. Some channels, known as leak channels, are open in resting neurons. Others are closed in resting neurons and only open in response to a signal.

Ion channels. The channels extend from one side of the plasma membrane to the other and have a tunnel through the middle. The tunnel allows ions to cross. One of the channels shown allows Na+ ions to cross and is a sodium channel. The other channel allows K+ ions to cross and is a potassium channel. The channels simply give a path for the ions across the membrane, allowing them to move down any electrochemical gradients that may exist. The channels do not actively move ions from one side to the other of the membrane.

Some ion channels are highly selective for one type of ion, but others let various kinds of ions pass through. Ion channels that mainly allow $\text{K}^+$start text, K, end text, start superscript, plus, end superscript to pass are called potassium channels, and ion channels that mainly allow $\text{Na}^+$start text, N, a, end text, start superscript, plus, end superscript to pass are called sodium channels

$\text {Cl}^-$start text, C, l, end text, start superscript, minus, end superscript

$\text {Na}^+$start text, N, a, end text, start superscript, plus, end superscript$\text {K}^+$start text, K, end text, start superscript, plus, end superscript

In neurons, the resting membrane potential depends mainly on movement of $\text {K}^+$start text, K, end text, start superscript, plus, end superscript through potassium leak channels. Let's see how this works.