Name: how neurons keep a negative resting charge
Uploaded: 2019-03-11T09:21:27.000Z
Duration: 1 min 21 s
Channel: JoVE

The resting membrane potential is the difference in electrical potential between the inside and outside of the cell membrane of neurons at rest when they are not being stimulated. Typically, the value is around negative 70 millivolts, meaning that it is more negative inside.

Cell membranes are selectively permeable because most ions and molecules cannot passively diffuse across them. They often require transmembrane proteins, such as ion channels, to allow them to pass through. When a neuron is at rest, potassium channels are the main type of ion channel that are open.

Another transmembrane protein, the sodium potassium pump, uses energy to continuously move sodium out of the cell, and potassium in. This action creates a concentration gradient, with a higher concentration of potassium inside than outside.

The force of diffusion then causes potassium ions to move down their concentration gradient, through the open potassium channels, and out of the cell. The movement of these positive ions out combined with negatively charged proteins inside the cell creates a negative charge inside the membrane, a negative potential when a neuron is at rest.

the resting membrane potential

<h4>Overview</h4>

The relative difference in electrical charge, or voltage, between the inside and the outside of a cell membrane, is called the membrane potential. It is generated by differences in permeability of the membrane to various ions and the concentrations of these ions across the membrane.

<h4>The Inside of a Neuron Is More Negative</h4>

The membrane potential of a cell can be measured by inserting a microelectrode into a cell and comparing the charge to a reference electrode in the extracellular fluid. The membrane potential of a neuron at rest&mdash;that is, a neuron not currently receiving or sending messages&mdash;is negative, typically around -70 millivolts (mV). This is called the resting membrane potential. The negative value indicates that the inside of the membrane is relatively more negative than the outside&mdash;it is polarized. The resting potential results from two major factors: selective permeability of the membrane, and differences in ion concentration inside the cell compared to outside.

<h4>Membrane Permeability</h4>

Cell membranes are selectively permeable because most ions and molecules cannot cross the lipid bilayer without help, often from ion channel proteins that span the membrane. This is because the charged ions cannot diffuse through the uncharged hydrophobic interior of membranes. The most common intra- and extracellular ions found in the nervous tissue are potassium (K+), sodium (Na+), chloride (Cl-), and calcium (Ca2+). When a neuron is at rest, potassium (K+) channels are the main type of ion channel that is open&mdash;allowing K+ to migrate across the membrane. This permeability, together with the large intracellular concentrations, make the neuron&rsquo;s resting membrane potential determined mainly by the movement of K+.

<h4>Pumps Create Concentration Gradients</h4>

Differences in ion concentration between the inside and outside of neurons are primarily due to the activity of the sodium-potassium (Na+/ K+) pump&mdash;a transmembrane protein that continuously pumps three Na+ ions out of the cell for every two K+ ions it pumps in. This establishes concentration gradients, with a higher concentration of Na+ ions outside of neurons and a higher concentration of K+ ions inside.

Since the membrane is primarily permeable to K+ at rest&mdash;due to the open K+ channels&mdash;K+ can diffuse down its concentration gradient to the region of lower concentration, out of the cell. These positive charges leaving the cell, combined with the fact that there are many negatively charged proteins inside the cell, causes the inside to be relatively more negative.

Eventually, outward diffusion of K+ is balanced by the electrostatic repulsion of positive charges accumulating outside the cell, and electrochemical equilibrium is reached. The net effect is the observed negative resting potential. The resting potential is very important in the nervous system because changes in membrane potential&mdash;such as the action potential&mdash;are the basis for neural signaling.

<h4>Beware the Puffer Fish</h4>

Pufferfish is not often found on many seafood menus outside of Japan, in part because they contain a potent neurotoxin. Tetrodotoxin (TTX) is a very selective voltage-gated sodium channel blocker that is lethal in minimal doses. The median lethal dose (LD50) for mice is 334 &mu;g/kg, compared to 8.5 mg/kg for potassium cyanide. It has also served as an essential tool in neuroscience research. The toxin blocks the flow of Na+ into the cell when the channel opens. It, therefore, disrupts action potentials&mdash;but not the resting membrane potential&mdash;and can be used to silence neuronal activity. Its mechanism of action was demonstrated by Toshio Narahashi and John W. Moore at Duke University, working on the giant lobster axon in 1964.

The Resting Membrane Potential

Overview

The relative difference in electrical charge, or voltage, between the inside and the outside of a cell membrane, is called the membrane ...

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how neurons keep a negative resting charge

Neurons keep a negative resting charge across their membrane. This electrical difference, called the membrane potential, comes from how ions move and how the membrane lets them pass. In a resting neuron, the inside is typically around -70 millivolts and is more negative than the outside, so the cell is polarized.

Two main factors create this resting state. First, the membrane is selectively permeable, which means most ions and molecules cannot cross the lipid bilayer without help from ion channels. Second, ion concentrations are different inside and outside the cell. In nervous tissue, common ions include potassium (K+), sodium (Na+), chloride (Cl-), and calcium (Ca2+).

At rest, potassium channels are the main channels that are open. This makes potassium the ion that most strongly shapes the resting membrane potential. Potassium can move across the membrane more easily than many other ions, and its movement helps set the neuron’s negative charge.

The sodium-potassium pump creates the ion gradients that support this process. This transmembrane protein pumps three Na+ ions out of the cell for every two K+ ions it pumps in. As a result, Na+ builds up outside the neuron, and K+ builds up inside.

Because the membrane is mostly permeable to K+ at rest, potassium diffuses out of the cell down its concentration gradient. As positive charges leave, the inside becomes more negative. Negatively charged proteins inside the cell also contribute to this effect. Eventually, the outward movement of K+ is balanced by the electrical push from the positive charge outside the cell, and electrochemical equilibrium is reached.

This resting potential is important because changes in membrane potential are the basis of neural signaling. One example is the action potential, which depends on these voltage changes. The pufferfish toxin tetrodotoxin (TTX) blocks voltage-gated sodium channels and stops Na+ from entering when the channel opens. It disrupts action potentials, but it does not change the resting membrane potential, so it can be used to silence neuronal activity.

How Neurons Keep a Negative Resting Charge

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key terms and concepts

Explore how selective permeability, the sodium-potassium pump, and potassium movement create the negative resting charge that prepares neurons for signaling.

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How Neurons Keep a Negative Resting Charge

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