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Chapter 1: resting Potentials and activity Potentials

John H. Byrne, Ph.D., department of Neurobiology and Anatomy, McGovern clinical School amendment 01 July 2021

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Video of lecture

Despite the enormous complexity of the brain, that is feasible to attain an knowledge of its duty by paying fist to two major details:

First, the methods in which individual neurons, the components of the concerned system, space wired together to generate behavior. Second, the biophysical, biochemical, and electrophysiological nature of the separation, personal, instance neurons.

A great place to start is through the components of the concerned system and also how the electrical properties of the neurons endow nerve cells with the capacity to process and transmit information.

Video that lecture

1.1 advent to the action Potential


Figure 1.1 madness the colored circles (light stimulus) come activate.

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


Theories that the encoding and transmission of details in the nervous device go back to the Greek doctor Galen (129-210 AD), who suggested a hydraulic mechanism whereby muscles contract due to the fact that fluid flowing right into them from hole nerves. The straightforward theory hosted for centuries and was more elaborated by René Descartes (1596 – 1650) who argued that animal spirits flowed native the brain through nerves and then to muscle to produce movements (See this computer animation for modern interpretation of together a hydraulic theory for nerve function). A major paradigm change occurred with the pioneering job-related of Luigi Galvani who uncovered in 1794 that nerve and also muscle can be set off by fee electrodes and also suggested the the worried system features via electrical signaling (see this computer animation of Galvani"s experiment). However, there to be debate amongst scholars even if it is the power was in ~ nerves and also muscle or whether the nerves and muscles were simply responding to the harmful electrical shock via part intrinsic nonelectric mechanism. The worry was not fixed until the 1930s with the advance of modern electronic amplifiers and also recording devices that allowed the electric signals to it is in recorded. One example is the pioneering job-related of H.K. Hartline 80 years earlier on electrical signaling in the horseshoe crab Limulus . Electrodes were placed on the surface ar of one optic nerve. (By place electrodes ~ above the surface ar of a nerve, it is feasible to obtain an point out of the changes in membrane potential the are emerging between the outside and inside the the nerve cell.) climate 1-s duration flicker of irradiate of varied intensities were presented to the eye; very first dim light, then brighter lights. Very dim lights developed no changes in the activity, but brighter lights produced little repetitive spike-like events. These spike-like events are called action potentials, nerve impulses, or sometimes just spikes. Activity potentials space the basic events the nerve cells use to transmit info from one location to another.

1.2 features of activity Potentials

The recordings in the figure above illustrate three an extremely important features of nerve activity potentials. First, the nerve activity potential has actually a quick duration (about 1 msec). Second, nerve action potentials space elicited in an all-or-nothing fashion. Third, nerve cells password the soot of info by the frequency of action potentials. Once the intensity of the economic stimulation is increased, the dimension of the action potential does not end up being larger. Rather, the frequency or the number of action potentials increases. In general, the better the soot of a stimulus, (whether it be a irradiate stimulus come a photoreceptor, a mechanically stimulus come the skin, or a stretch to a muscle receptor) the greater the number of action potentials elicited. Similarly, because that the engine system, the greater the number of action potentials in a motor neuron, the greater the intensity of the convulsion of a muscle the is innervated by that motor neuron.

Action potentials room of great importance come the work of the mind since castle propagate information in the nervous device to the central nervous system and propagate commands initiated in the central nervous mechanism to the periphery. Consequently, it is vital to recognize thoroughly your properties. To answer the questions of how activity potentials space initiated and propagated, we have to record the potential in between the inside and outside the nerve cells making use of intracellular record techniques.

1.3 Intracellular Recordings indigenous Neurons


The potential difference throughout a nerve cell membrane have the right to be measured with a microelectrode whose tip is so little (about a micron) that it have the right to penetrate the cabinet without producing any damage. As soon as the electrode is in the bathtub (the extracellular medium) over there is no potential recorded due to the fact that the bath is isopotential. If the microelectrode is closely inserted into the cell, over there is a sharp readjust in potential. The analysis of the voltmeter instantaneously changes from 0 mV, to analysis a potential difference of -60 mV within the cell with respect come the outside. The potential the is recorded once a living cell is impaled v a microelectrode is called the resting potential, and also varies from cell to cell. Here it is presented to it is in -60 mV, however can selection between -80 mV and also -40 mV, depending on the particular form of nerve cell. In the lack of any stimulation, the resting potential is normally constant.

It is also possible to record and also study the action potential. Figure 1.3 illustrates an example in i m sorry a neuron has currently been impaled with one microelectrode (the recording electrode), which is connected to a voltmeter. The electrode documents a relaxing potential that -60 mV. The cell has likewise been impaled v a 2nd electrode referred to as the stimulating electrode. This electrode is connected to a battery and also a an equipment that can monitor the quantity of existing (I) the flows through the electrode. Changes in membrane potential are created by closeup of the door the switch and by systematically transforming both the size and polarity of the battery. If the an unfavorable pole that the battery is linked to the within of the cell as in figure 1.3A, an instantaneous change in the lot of existing will circulation through the stimulating electrode, and also the membrane potential becomes transiently more negative. This an outcome should not be surprising. The an adverse pole that the battery provides the within of the cell an ext negative than it to be before. A readjust in potential that rises the polarized state of a membrane is referred to as a hyperpolarization. The cell is more polarized than it was normally. Usage yet a larger battery and also the potential becomes even larger. The resultant hyperpolarizations space graded attributes of the magnitude of the stimuli supplied to create them.


Now take into consideration the instance in which the positive pole of the battery is associated to the electrode (Figure 1.3B). As soon as the positive pole that the battery is linked to the electrode, the potential the the cell becomes more positive as soon as the move is closed (Figure 1.3B). Together potentials are dubbed depolarizations. The polarized state the the membrane is decreased. Bigger batteries develop even bigger depolarizations. Again, the size of the responses space proportional come the size of the stimuli. However, an unusual occasion occurs once the size of the depolarization get a level the membrane potential called the threshold. A completely new form of signal is initiated; the action potential. Keep in mind that if the dimension of the battery is increased even more, the amplitude of the activity potential is the same as the previous one (Figure 1.3B). The procedure of eliciting an activity potential in a nerve cell is analogous to igniting a fuse with a warm source. A particular minimum temperature (threshold) is necessary. Temperatures less than the threshold fail to ignite the fuse. Temperatures better than the threshold ignite the fuse just and the threshold temperature and also the fuse does no burn any brighter or hotter.

If the suprathreshold current stimulus is lengthy enough, however, a train of activity potentials will certainly be elicited. In general, the activity potentials will continue to fire as lengthy as the stimulus continues, with the frequency of firing gift proportional to the size of the economic stimulation (Figure 1.4).


Action potentials room not only initiated in one all-or-nothing fashion, but they are also 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 one undecremented fashion all the means to the synaptic terminals of that motor neuron. Again, the case is analogous to a burning fuse. As soon as the fuse is ignited, the fire will infect its end.

1.4 components of the activity Potentials

The action potential consists of several contents (Figure 1.3B). The threshold is the worth of the membrane potential which, if reached, leader to the all-or-nothing initiation that an action potential. The early or climbing phase the the activity potential is referred to as the depolarizing phase or the upstroke. The an ar of the action potential in between the 0 mV level and also the peak amplitude is the overshoot. The return that the membrane potential come the resting potential is dubbed the repolarization phase. Over there is additionally a phase of the action potential throughout which time the membrane potential can be much more negative than the resting potential. This step of the activity potential is called the undershoot or the hyperpolarizing afterpotential. In number 1.4, the undershoots the the action potentials carry out not become an ext negative than the resting potential since they space "riding" on the continuous depolarizing stimulus.

1.5 Ionic mechanisms of relaxing Potentials

Before evaluating the ionic mechanisms of activity potentials, the is first necessary to recognize the ionic instrument of the resting potential. The two phenomena room intimately related. The story that the relaxing potential goes ago to the beforehand 1900"s once Julius Bernstein argued that the resting potential (Vm) was equal to the potassium equilibrium potential (EK). Where

The key to understanding the relaxing potential is the fact that ion are distributed unequally on the inside and outside that cells, and that cabinet membranes are selectively permeable to various ions. K+ is specifically important because that the resting potential. The membrane is very permeable come K+. In addition, the within of the cell has a high concentration of K+ (i) and the external of the cell has a short concentration the K+ (o). Thus, K+ will certainly naturally relocate by diffusion indigenous its region of high concentration to its an ar of short concentration. Consequently, the hopeful K+ ion leaving the inner surface of the membrane leave behind some negatively charged ions. That negative charge attractive the positive charge the the K+ ion the is leaving and tends come "pull it back". Thus, there will be an electric force command inward that will tend to counterbalance the diffusional force directed outward. Eventually, an equilibrium will be established; the concentration force moving K+ out will certainly balance the electrical force holding it in. The potential in ~ which that balance is accomplished is referred to as the Nernst Equilibrium Potential.


An experiment to check Bernstein"s theory that the membrane potential is same to the Nernst Equilibrium Potential (i.e., Vm = EK) is illustrated to the left.

The K+ concentration outside the cell to be systematically differed while the membrane potential to be measured. Likewise shown is the line the is guess by the Nernst Equation. The experimentally measured point out are an extremely close to this line. Moreover, due to the fact that of the logarithmic connection in the Nernst equation, a readjust in concentration that K+ through a factor of 10 outcomes in a 60 mV readjust in potential.

Note, however, the there room some deviations in the figure at left indigenous what is suspect by the Nernst equation. Thus, one cannot conclude that Vm = EK. Together deviations show that an additional ion is additionally involved in generating the relaxing potential. That ion is Na+. The high concentration the Na+ outside the cabinet and reasonably low concentration inside the cell outcomes in a chemistry (diffusional) driving force for Na+ influx. There is additionally an electric driving force because the inside of the cell is negative and this negativity attractive the hopeful sodium ions. Consequently, if the cell has actually a small permeability come sodium, Na+ will certainly move across the membrane and the membrane potential would certainly be more depolarized than would be supposed from the K+ equilibrium potential.

1.6 Goldman-Hodgkin and also Katz (GHK) Equation

When a membrane is permeable come two various ions, the Nernst equation deserve to no much longer be provided to precisely determine the membrane potential. It is possible, however, to use the GHK equation. This equation defines the potential throughout a membrane that is permeable come both Na+ and also K+.

Note the α is the proportion of Na+ permeability (PNa) come K+ permeability (PK). Note likewise that if the permeability of the membrane come Na+ is 0, then alpha in the GHK is 0, and also the Goldman-Hodgkin-Katz equation reduces come the Nernst equilibrium potential for K+. If the permeability of the membrane to Na+ is an extremely high and the potassium permeability is very low, the terms become really large, dominating the equation contrasted to the terms, and the GHK equation reduces to the Nernst equilibrium potential because that Na+.

If the GHK equation is used to the exact same data in figure 1.5, there is a much much better fit. The value of alpha essential to achieve this great fit to be 0.01. This method that the potassium K+ permeability is 100 times the Na+ permeability. In summary, the relaxing potential is early out not only to the truth that there is a high permeability come K+. There is additionally a slim permeability to Na+, which often tends to make the membrane potential slightly an ext positive 보다 it would have been if the membrane to be permeable come K+ alone.


1.7 Membrane Potential laboratory

Click right here to go to the interactive Membrane Potential laboratory to experiment v the results of transforming external or inner potassium ion concentration and membrane permeability to sodium and also potassium ions. Predictions room made making use of the Nernst and also the Goldman, Hodgkin, Katz equations.

Membrane Potential Laboratory

 

Test her Knowledge

 


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

A. Not readjust

B. Method the brand-new K+ equilibrium potential

C. Approach the brand-new Na+ equilibrium potential

D. Technique a worth of around 0 mV

E. Method a constant value of around +55 mV


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

A. Not readjust This prize is INCORRECT.

A readjust in permeability would depolarize the membrane potential since alpha in the GHK equation would certainly equal one. Initially, alpha was 0.01. Try substituting various values that alpha into the GHK equation and calculate the result membrane potential.

B. Approach the brand-new K+ equilibrium potential

C. Technique the brand-new Na+ equilibrium potential

D. Method a worth of around 0 mV

E. Method a consistent value of about +55 mV


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

A. Not readjust

B. Technique the new K+ equilibrium potential This prize is INCORRECT. The membrane potential would strategy the K+ equilibrium potential only if the Na+ permeability was reduced or the K+ permeability was increased. Also there would certainly be no "new" equilibrium potential. Changing the permeability go not change the equilibrium potential.

C. Technique the new Na+ equilibrium potential

D. Strategy a worth of around 0 mV

E. Technique a constant value of around +55 mV


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

A. Not change

B. Approach the brand-new K+ equilibrium potential

C. Technique 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 ended up being very huge (e.g., decrease PK or boost PNa). Also, there would be no "new" Na+ equilibrium potential. An altering the permeability go not change the equilibrium potential; it transforms the membrane potential.

D. Approach a value of about 0 mV

E. Approach a consistent value of about +55 mV


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

A. Not adjust

B. Strategy the new K+ equilibrium potential

C. Strategy the new Na+ equilibrium potential

D. Strategy a value of about 0 mV This answer is correct! approximately speaking, the membrane potential would move to a value half way between EK and ENa. The GHK equation might be supplied to identify the precise value.

E. Strategy a consistent value of around +55 mV


If a nerve membrane suddenly ended up being equally permeable come both Na+ and also K+, the membrane potential would:

A. Not change

B. Approach the brand-new K+ equilibrium potential

C. Method the new Na+ equilibrium potential

D. Strategy a value of around 0 mV

E. Approach a constant value of about +55 mV This price is INCORRECT. The membrane potential would certainly not strategy a value of around +55 mV (the approximate worth of ENa) uneven there was a huge increase in the salt permeability there is no a corresponding change in the potassium permeability. Alpha in the Goldman equation would need to approach a very high value.


If the concentration of K+ in the cytoplasm of one invertebrate axon is readjusted to a new value the 200 mM (Note: because that this axon common o = 20 mM and also normal i = 400 mM): 

A. The membrane potential would become much more negative This price is INCORRECT. The normal value that extracellular potassium is 20 mM and the typical value of intracellular potassium is 400 mM, yielding a regular 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 certainly equal around -60 mV. Because the membrane potential is usually -60 mV and is dependent, come a big extent, ~ above EK, the readjust in the potassium concentration and also hence EK would make the membrane potential an ext positive, not an ext negative.

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

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

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

E. An action potential would be initiated


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

A. The membrane potential would become more negative

B. The K+ equilibrium potential would change by 60 mV This answer is INCORRECT. The potassium equilibrium potential would certainly not readjust by 60 mV. The potassium concentration was adjusted just indigenous 400 mM to 200 mM. One deserve to use the Nernst equation to determine the precise value that the equilibrium potential would readjust by. It to be initially around -75 mV and as a result of the readjust in concentration, the equilibrium potential i do not care -60 mV. Thus, the equilibrium potential go not adjust by 60 mV, it transforms by about 15 mV.

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

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

E. An action potential would certainly be initiated


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

A. The membrane potential would become much more negative

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

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

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

E. An activity potential would be initiated


If the concentration the K+ in the cytoplasm of one invertebrate axon is readjusted to a brand-new value that 200 mM (Note: because that this axon normal o = 20 mM and also normal i = 400 mM): 

A. The membrane potential would certainly become 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 prize is INCORRECT.

See more: What Is The Formula For Sodium Nitride ? Sodium Nitride

Using the Nernst equation, the brand-new potassium equilibrium potential can be calculated to be -60 mV. A worth of -18 mV would be calculate 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 invertebrate axon is changed to a new value that 200 mM (Note: because that this axon common o = 20 mM and normal i = 400 mM): 

A. The membrane potential would become much more negative

B. The K+ equilibrium potential would change 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 action potential would be initiated This answer is INCORRECT. The membrane potential would not depolarize sufficiently to with threshold (about -45 mV).