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Chapter 1: Resting Potentials and Action Potentials

John H. Byrne, Ph.D., Department of Neurobiology and Anatomy, McGovern Medical School Revised 01 July 2021

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Video of lecture

Regardless of the enormous intricacy of the brain, it is possible to obtain an expertise of its feature by paying attention to 2 major details:

First, the ways in which individual neurons, the components of the nervous device, are wired together to geneprice actions. 2nd, the biophysical, biochemical, and also electrophysiological properties of the individual neurons.

A excellent location to begin is via the components of the nervous system and also exactly how the electric properties of the neurons endow nerve cells with the capability to procedure and transmit indevelopment.

Video of lecture

1.1 Review to the Action Potential


Figure 1.1 Tap the colored circles (light stimulus) to activate.

You are watching: Define the term threshold as it applies to an action potential.


Theories of the encoding and also transmission of information in the nervous device go ago to the Greek physician Galen (129-210 AD), that argued a hydraulic device whereby muscles contract bereason fluid flowing right into them from hollow nerves. The fundamental concept organized for centuries and was even more elaborated by René Descartes (1596 – 1650) who suggested that animal spirits flowed from the brain via nerves and then to muscles to create motions (See this animation for contemporary interpretation of such a hydraulic theory for nerve function). A major paradigm transition emerged through the pioneering occupational of Luigi Galvani that discovered in 1794 that nerve and muscle might be activated by charged electrodes and said that the nervous device attributes by means of electric signaling (watch this computer animation of Galvani"s experiment). However, tright here was controversy among scholars whether the power was within nerves and also muscle or whether the nerves and muscles were simply responding to the harmful electric shock by means of some intrinsic nonelectric system. The issue was not refixed until the 1930s through the advancement of modern-day digital amplifiers and recording devices that permitted the electrical signals to be taped. One instance is the pioneering occupational of H.K. Hartline 80 years back on electric signaling in the horseshoe crab Limulus . Electrodes were placed on the surconfront of an optic nerve. (By placing electrodes on the surchallenge of a nerve, it is possible to obtain an indication of the alters in membrane potential that are arising in between the external and also inside of the nerve cell.) Then 1-s duration flashes of light of differed intensities were presented to the eye; first dim light, then brighter lights. Very dim lights developed no alters in the task, yet brighter lights created little repetitive spike-like events. These spike-choose events are referred to as action potentials, nerve impulses, or periodically simply spikes. Action potentials are the fundamental events the nerve cells use to transmit indevelopment from one area to an additional.

1.2 Features of Action Potentials

The recordings in the number above illustrate 3 very crucial attributes of nerve activity potentials. First, the nerve activity potential has a brief duration (about 1 msec). Second, nerve activity potentials are elicited in an all-or-nopoint fashion. Third, nerve cells code the intensity of information by the frequency of action potentials. When the intensity of the stimulus is increased, the size of the action potential does not come to be larger. Rather, the frequency or the variety of activity potentials increases. In general, the higher the intensity of a stimulus, (whether it be a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a muscle receptor) the better the number of activity potentials elicited. Similarly, for the motor system, the greater the variety of action potentials in a motor neuron, the higher the intensity of the contraction of a muscle that is innervated by that motor neuron.

Action potentials are of excellent prominence to the functioning of the brain given that they propagate information in the nervous mechanism to the main nervous mechanism and also propagate commands initiated in the main nervous device to the periphery. Consequently, it is vital to understand thoroughly their properties. To answer the questions of exactly how action potentials are initiated and propagated, we should document the potential in between the inside and also external of nerve cells using intracellular recording approaches.

1.3 Intracellular Recordings from Neurons


The potential distinction across a nerve cell membrane can be measured through a microelectrode whose tip is so small (around a micron) that it have the right to pass through the cell without developing any kind of damage. When the electrode is in the bath (the extracellular medium) tright here is no potential tape-recorded bereason the bath is isopotential. If the microelectrode is carefully put into the cell, there is a sharp adjust in potential. The reading of the voltmeter instantaneously alters from 0 mV, to reading a potential distinction of -60 mV inside the cell with respect to the outside. The potential that is recorded as soon as a living cell is impaled through a microelectrode is referred to as the resting potential, and also varies from cell to cell. Here it is displayed to be -60 mV, but can range in between -80 mV and also -40 mV, depending on the certain type of nerve cell. In the absence of any kind of stimulation, the resting potential is mostly constant.

It is also possible to record and examine the action potential. Figure 1.3 illustprices an instance in which a neuron has currently been impaled via one microelectrode (the recording electrode), which is linked to a voltmeter. The electrode records a relaxing potential of -60 mV. The cell has actually also been impaled with a 2nd electrode called the stimulating electrode. This electrode is associated to a battery and a machine that have the right to monitor the amount of existing (I) that flows through the electrode. Changes in membrane potential are produced by closing the switch and also by systematically altering both the dimension and also polarity of the battery. If the negative pole of the battery is associated to the inside of the cell as in Figure 1.3A, an instantaneous adjust in the amount of current will flow with the stimulating electrode, and the membrane potential becomes transiently even more negative. This result should not be surpincreasing. The negative pole of the battery provides the inside of the cell even more negative than it was before. A change in potential that boosts the polarized state of a membrane is referred to as a hyperpolarization. The cell is even more polarized than it was commonly. Use yet a larger battery and the potential becomes also bigger. The resultant hyperpolarizations are graded attributes of the magnitude of the stimuli provided to develop them.


Now take into consideration the instance in which the positive pole of the battery is connected to the electrode (Figure 1.3B). When the positive pole of the battery is associated to the electrode, the potential of the cell becomes even more positive when the switch is closed (Figure 1.3B). Such potentials are dubbed depolarizations. The polarized state of the membrane is lessened. Larger batteries develop also bigger depolarizations. Again, the magnitude of the responses are proportional to the magnitude of the stimuli. However before, an unexplained event occurs when the magnitude of the depolarization reaches a level of membrane potential called the threshold. A totally new form of signal is initiated; the action potential. Keep in mind that if the size of the battery is raised even more, the amplitude of the action potential is the very same as the previous one (Figure 1.3B). The process of eliciting an action potential in a nerve cell is analogous to igniting a fuse with a warm resource. A particular minimum temperature (threshold) is crucial. Temperatures less than the thresorganize fail to ignite the fuse. Temperatures higher than the threshost ignite the fuse just and the threshost temperature and the fusage does not burn any brighter or hotter.

If the suprathreshold present stimulus is lengthy enough, but, a train of activity potentials will be elicited. In general, the action potentials will continue to fire as long as the stimulus continues, via the frequency of firing being proportional to the magnitude of the stimulus (Figure 1.4).


Action potentials are not only initiated in an all-or-nothing fashion, but they are additionally propagated in an all-or-nothing fashion. An activity potential initiated in the cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the method to the synaptic terminals of that motor neuron. Aacquire, the case is analogous to a burning fusage. Once the fuse is ignited, the flame will spread to its finish.

1.4 Contents of the Action Potentials

The activity potential consists of numerous components (Figure 1.3B). The threshold is the worth of the membrane potential which, if reached, leads to the all-or-nopoint initiation of an activity potential. The initial or climbing phase of the activity potential is called the depolarizing phase or the upstroke. The region of the activity potential between the 0 mV level and also the peak amplitude is the overshoot. The rerotate of the membrane potential to the resting potential is referred to as the repolarization phase. Tright here is likewise a phase of the activity potential in the time of which time the membrane potential have the right to be even more negative than the relaxing potential. This phase of the action potential is called the undershoot or the hyperpolarizing afterpotential. In Figure 1.4, the undershoots of the activity potentials do not become even more negative than the resting potential bereason they are "riding" on the continuous depolarizing stimulus.

1.5 Ionic Mechanisms of Resting Potentials

Before researching the ionic mechanisms of action potentials, it is first vital to understand also the ionic mechanisms of the resting potential. The 2 sensations are intimately associated. The story of the resting potential goes ago to the early 1900"s when Julius Bernstein suggested that the resting potential (Vm) was equal to the potassium equilibrium potential (EK). Where

The vital to knowledge the resting potential is the fact that ions are dispersed unequally on the inside and external of cells, and also that cell membranes are selectively permeable to different ions. K+ is especially important for the relaxing potential. The membrane is extremely permeable to K+. In enhancement, the inside of the cell has a high concentration of K+ (i) and the outside of the cell has a low concentration of K+ (o). Hence, K+ will certainly naturally move by diffusion from its area of high concentration to its region of low concentration. Consequently, the positive K+ ions leaving the inner surface of the membrane leave behind some negatively charged ions. That negative charge attracts the positive charge of the K+ ion that is leaving and also has a tendency to "pull it back". Thus, tbelow will be an electrical pressure directed inward that will certainly tend to counterbalance the diffusional pressure directed outward. Ultimately, an equilibrium will be established; the concentration force moving K+ out will certainly balance the electric force holding it in. The potential at which that balance is accomplished is referred to as the Nernst Equilibrium Potential.


An experiment to test Bernstein"s hypothesis that the membrane potential is equal to the Nernst Equilibrium Potential (i.e., Vm = EK) is shown to the left.

The K+ concentration external the cell was systematically varied while the membrane potential was measured. Also shown is the line that is predicted by the Nernst Equation. The experimentally measured points are incredibly cshed to this line. In addition, because of the logarithmic partnership in the Nernst equation, a adjust in concentration of K+ by a aspect of 10 results in a 60 mV readjust in potential.

Keep in mind, yet, that tbelow are some deviations in the figure at left from what is predicted by the Nernst equation. Hence, one cannot conclude that Vm = EK. Such deviations suggest that one more ion is also affiliated in generating the resting potential. That ion is Na+. The high concentration of Na+ exterior the cell and relatively low concentration inside the cell outcomes in a chemical (diffusional) driving pressure for Na+ influx. There is also an electric driving pressure bereason the inside of the cell is negative and this negativity attracts the positive sodium ions. Consequently, if the cell has a small permecapacity to sodium, Na+ will relocate across the membrane and also the membrane potential would certainly be even more depolarized than would certainly be intended from the K+ equilibrium potential.

1.6 Goldman-Hodgkin and also Katz (GHK) Equation

When a membrane is permeable to 2 various ions, the Nernst equation can no much longer be supplied to exactly identify the membrane potential. It is feasible, yet, to apply the GHK equation. This equation explains the potential across a membrane that is permeable to both Na+ and also K+.

Note that α is the proportion of Na+ permecapacity (PNa) to K+ permecapability (PK). Note also that if the permecapability of the membrane to Na+ is 0, then alpha in the GHK is 0, and also the Goldman-Hodgkin-Katz equation reduces to the Nernst equilibrium potential for K+. If the permecapability of the membrane to Na+ is very high and the potassium permeability is extremely low, the terms end up being incredibly large, conquering the equation compared to the terms, and also the GHK equation reduces to the Nernst equilibrium potential for Na+.

If the GHK equation is used to the same information in Figure 1.5, tbelow is a much better fit. The value of alpha needed to acquire this excellent fit was 0.01. This implies that the potassium K+ permeability is 100 times the Na+ permecapability. In summary, the relaxing potential is due not only to the trjiyuushikan.org that tright here is a high permecapacity to K+. There is also a slight permecapability to Na+, which has a tendency to make the membrane potential slightly even more positive than it would certainly have been if the membrane were permeable to K+ alone.


1.7 Membrane Potential Laboratory

Click right here to go to the interactive Membrane Potential Laboratory to experiment via the results of changing outside or internal potassium ion concentration and membrane permecapacity to sodium and potassium ions. Predictions are made making use of the Nernst and the Goldmale, Hodgkin, Katz equations.

Membrane Potential Laboratory

 

Test Your Knowledge

 


If a nerve membrane unexpectedly ended up being equally permeable to both Na+ and K+, the membrane potential would:

A. Not readjust

B. Approach the new K+ equilibrium potential

C. Approach the new Na+ equilibrium potential

D. Approach a worth of about 0 mV

E. Approach a consistent worth of around +55 mV


If a nerve membrane all of a sudden became equally permeable to both Na+ and K+, the membrane potential would:

A. Not change This answer is INCORRECT.

A adjust in permecapability would certainly depolarize the membrane potential considering that alpha in the GHK equation would equal one. At first, alpha was 0.01. Try substituting various values of alpha right into the GHK equation and calculate the resultant membrane potential.

B. Approach the brand-new K+ equilibrium potential

C. Approach the new Na+ equilibrium potential

D. Approach a value of about 0 mV

E. Approach a constant value of about +55 mV


If a nerve membrane unexpectedly ended up being equally permeable to both Na+ and K+, the membrane potential would:

A. Not adjust

B. Approach the new K+ equilibrium potential This answer is INCORRECT. The membrane potential would certainly approach the K+ equilibrium potential only if the Na+ permecapacity was lessened or the K+ permecapacity was boosted. Also tbelow would be no "new" equilibrium potential. Changing the permecapability does not adjust the equilibrium potential.

C. Approach the new Na+ equilibrium potential

D. Approach a value of around 0 mV

E. Approach a continuous worth of about +55 mV


If a nerve membrane suddenly became equally permeable to both Na+ and K+, the membrane potential would:

A. Not change

B. Approach the brand-new K+ equilibrium potential

C. Approach the brand-new Na+ equilibrium potential This answer is INCORRECT. The membrane potential would technique the Na+ equilibrium potential only if alpha in the GHK equation came to be extremely huge (e.g., decrease PK or increase PNa). Also, tbelow would certainly be no "new" Na+ equilibrium potential. Changing the permecapability does not adjust the equilibrium potential; it transforms the membrane potential.

D. Approach a value of about 0 mV

E. Approach a continuous worth of around +55 mV


If a nerve membrane all of a sudden became equally permeable to both Na+ and also K+, the membrane potential would:

A. Not readjust

B. Approach the brand-new K+ equilibrium potential

C. Approach the new Na+ equilibrium potential

D. Approach a worth of about 0 mV This answer is CORRECT! Roughly speaking, the membrane potential would move to a value half method between EK and also ENa. The GHK equation could be supplied to determine the precise worth.

E. Approach a continuous worth of around +55 mV


If a nerve membrane unexpectedly came to be equally permeable to both Na+ and also K+, the membrane potential would:

A. Not adjust

B. Approach the new K+ equilibrium potential

C. Approach the brand-new Na+ equilibrium potential

D. Approach a worth of about 0 mV

E. Approach a constant value of about +55 mV This answer is INCORRECT. The membrane potential would not method a worth of about +55 mV (the approximate worth of ENa) unmuch less tright here was a huge increase in the sodium permeability without a corresponding change in the potassium permecapacity. Alpha in the Goldmale equation would should method a very high worth.


If the concentration of K+ in the cytoplasm of an invertebprice axon is readjusted to a new worth of 200 mM (Note: for this axon normal o = 20 mM and normal i = 400 mM): 

A. The membrane potential would end up being more negative This answer is INCORRECT. The normal worth of extracellular potassium is 20 mM and the normal value of intracellular potassium is 400 mM, yielding a normal equilibrium potential for potassium of about -75 mV. If the intracellular concentration is adjusted from 400 mM to 200 mM, then the potassium equilibrium potential as figured out by the Nernst equation, will equal about -60 mV. Because the membrane potential is commonly -60 mV and also is dependent, to a large degree, on EK, the adjust in the potassium concentration and thus EK would make the membrane potential more positive, not more negative.

B. The K+ equilibrium potential would certainly readjust by 60 mV

C. The K+ equilibrium potential would certainly be about -60 mV

D. The K+ equilibrium potential would certainly be around -18 mV

E. An activity potential would be initiated


If the concentration of K+ in the cytoplasm of an invertebprice axon is changed to a brand-new value of 200 mM (Note: for this axon normal o = 20 mM and also normal i = 400 mM): 

A. The membrane potential would certainly come to be more negative

B. The K+ equilibrium potential would adjust by 60 mV This answer is INCORRECT. The potassium equilibrium potential would certainly not change by 60 mV. The potassium concentration was adjusted just from 400 mM to 200 mM. One can use the Nernst equation to identify the specific value that the equilibrium potential would certainly readjust by. It was initially about -75 mV and as a result of the adjust in concentration, the equilibrium potential becomes -60 mV. Hence, the equilibrium potential does not readjust by 60 mV, it transforms by about 15 mV.

C. The K+ equilibrium potential would be around -60 mV

D. The K+ equilibrium potential would be about -18 mV

E. An action potential would certainly be initiated


If the concentration of K+ in the cytoplasm of an invertebprice axon is readjusted to a brand-new worth of 200 mM (Note: for this axon normal o = 20 mM and also normal i = 400 mM): 

A. The membrane potential would certainly become even more negative

B. The K+ equilibrium potential would certainly change by 60 mV

C. The K+ equilibrium potential would be about -60 mV This answer is CORRECT! This is the correct answer. See the logic explained in responses A and B.

D. The K+ equilibrium potential would be about -18 mV

E. An activity potential would certainly be initiated


If the concentration of K+ in the cytoplasm of an invertebrate axon is adjusted to a brand-new value of 200 mM (Note: for this axon normal o = 20 mM and normal i = 400 mM): 

A. The membrane potential would come to be even more negative

B. The K+ equilibrium potential would adjust by 60 mV

C. The K+ equilibrium potential would certainly be around -60 mV

D. The K+ equilibrium potential would be around -18 mV This answer is INCORRECT.

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Using the Nernst equation, the brand-new potassium equilibrium potential can be calculated to be -60 mV. A worth of -18 mV would certainly be calculated if you substituted o = 200 and i= 400 right into the Nernst equation.

E. An action potential would certainly be initiated


If the concentration of K+ in the cytoplasm of an invertebprice axon is readjusted to a brand-new worth of 200 mM (Note: for this axon normal o = 20 mM and also normal i = 400 mM): 

A. The membrane potential would become even more negative

B. The K+ equilibrium potential would certainly change by 60 mV

C. The K+ equilibrium potential would be about -60 mV

D. The K+ equilibrium potential would certainly be around -18 mV

E. An activity potential would be initiated This answer is INCORRECT. The membrane potential would certainly not depolarize sufficiently to reach threshost (about -45 mV).