What is the charge difference between the inside and outside of the neuron at rest?
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The membrane potential is the difference in electrical charge between the inside and the outside of the neuron. This is measured using two electrodes. A reference electrode is placed in the extracellular solution. The recording electrode is inserted into the cell body of the neuron. Show
TerminologyThere is more than one way to describe a change in membrane potential. If the membrane potential moves toward zero, that is a depolarization because the membrane is becoming less polarized, meaning there is a smaller difference between the charge on the inside of the cell compared to the outside. This is also referred to as a decrease in membrane potential. This means that when a neuron’s membrane potential moves from rest, which is typically around -65 mV, toward 0 mV and becomes more positive, this is a decrease in membrane potential. Since the membrane potential is the difference in electrical charge between the inside and outside of the cell, that difference decreases as the cell’s membrane potential moves toward 0 mV. If the membrane potential moves away from zero, that is a hyperpolarization because the membrane is becoming more polarized. This is also referred to as an increase in membrane potential.  Voltage DistributionAt rest, ions are not equally distributed across the membrane. This distribution of ions and other charged molecules leads to the inside of the cell having a more negative charge compared to the outside of the cell. A closer look shows that sodium, calcium, and chloride are concentrated outside of the cell membrane in the extracellular solution, whereas potassium and negatively-charged molecules like amino acids and proteins are concentrated inside in the intracellular solution. Ion Distribution Creates Electrochemical GradientsThese concentration differences lead to varying degrees of electrochemical gradients in different directions depending on the ion in question. For example, the electrochemical gradients will drive potassium out of the cell but will drive sodium into the cell. Equilibrium PotentialThe neuron’s membrane potential at which the electrical and concentration gradients for a given ion balance out is called the ion’s equilibrium potential. Let’s look at sodium in more detail: When sodium channels open, the neuron’s membrane becomes permeable to sodium, and sodium will begin to flow across the membrane. The direction is dependent upon the electrochemical gradients. The concentration of sodium in the extracellular solution is about 10 times higher than the intracellular solution, so there is a concentration gradient driving sodium into the cell. Additionally, at rest, the inside of the neuron is more negative than the outside, so there is also an electrical gradient driving sodium into the cell. As sodium moves into the cell, though, these gradients change in driving strength. As the neuron’s membrane potential become positive, the electrical gradient no longer works to drive sodium into the cell. Eventually, the concentration gradient driving sodium into the neuron and the electrical gradient driving sodium out of the neuron balance with equal and opposite strengths, and sodium is at equilibrium. The membrane potential of the neuron at which equilibrium occurs is called the equilibrium potential of an ion, which, for sodium, is approximately +60 mV.
Animation 3.1. At rest, both the concentration and electrical gradients for sodium point into the cell. As a result, sodium flows in. As sodium enters, the membrane potential of the cell decreases and becomes more positive. As the membrane potential changes, the electrical gradient decreases in strength, and after the membrane potential passes 0 mV, the electrical gradient will point outward, since the inside of the cell is more positively charged than the outside. The ions will continue to flow into the cell until equilibrium is reached. An ion will be at equilibrium when its concentration and electrical gradients are equal in strength and opposite in direction. The membrane potential of the neuron at which this occurs is the equilibrium potential for that ion. Sodium’s equilibrium potential is approximately +60 mV. The dotted, blue channels represent sodium channels; the striped, green channels represent potassium channels; the solid yellow channels represent chloride channels. ‘Sodium Gradients’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License. View static image of animation. Calculate Equilibrium Potential with Nernst EquationThe gradients acting on the ion will always drive the ion towards equilibrium. The equilibrium potential of an ion is calculated using the Nernst equation: The Nernst Equation [latex]E_{ion}= \displaystyle \frac{61}{z} log \frac{[ion]_{outside}}{[ion]_{inside}}[/latex] The constant 61 is calculated using values such as the universal gas constant and temperature of mammalian cells Z is the charge of the ion [Ion]inside is the intracellular concentration of the ion [Ion]outside is the extracellular concentration of the ion [latex]E_{ion}= \displaystyle \frac{61}{z} log \frac{[ion]_{outside}}{[ion]_{inside}}[/latex] For Sodium: z = 1 [Ion]inside = 15 mM [Ion]outside = 145 mM [latex]E_{ion}= \displaystyle \frac{61}{1} log \frac{145}{15} = 60 mV[/latex] Predict Ion Movement by Comparing Membrane Potential to Equilibrium PotentialIt is possible to predict which way an ion will move by comparing the ion’s equilibrium potential to the neuron’s membrane potential. Let’s assume we have a cell with a resting membrane potential of -70 mV. Sodium’s equilibrium potential is +60 mV. Therefore, to reach equilibrium, sodium will need to enter the cell, bringing in positive charge. On the other hand, chloride’s equilibrium potential is -65 mV. Since chloride is a negative ion, it will need to leave the cell in order to make the cell’s membrane potential more positive to move from -70 mV to -65 mV. Concentration and Equilibrium Potential ValuesWe will use the following ion concentrations and equilibrium potentials:
Table 3.1. Intra- and extracellular concentration and equilibrium potential values for a typical neuron at rest for sodium, potassium, and chloride.
Test Yourself!Video LectureWhat is the charge difference between the inside and outside of an axon?So, when an axon is at rest, the anions give it a negative charge, the sodium pumps keep sodium out and potassium in, and the sodium gates and potassium gates are all closed. Because of the positive-negative difference between the inside and outside, this resting state is called a resting potential.
Is the inside of a neuron positive or negative compared to the outside?Neurons actually have a pretty strong negative charge inside them, in contrast to a positive charge outside. This is due to other molecules called anions.
What ions are inside and outside at rest?4 The Membrane at Rest
Figure 4.1. For a typical neuron at rest, sodium, chloride, and calcium are concentrated outside the cell, whereas potassium and other anions are concentrated inside. This ion distribution leads to a negative resting membrane potential.
What charge is on the inside and outside of membranes?Almost all plasma membranes have an electrical potential across them, with the inside usually negative with respect to the outside. The membrane potential has two basic functions. First, it allows a cell to function as a battery, providing power to operate a variety of "molecular devices" embedded in the membrane.
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