After a Nerve Impulse Has Passed Ions Flow Out of the Axon and the Membrane Potential Becomes Again

Chapter i: Resting Potentials and Activity Potentials

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Video of lecture

Despite the enormous complication of the brain, it is possible to obtain an agreement of its function by paying attention to two major details:

• First, the means in which private neurons, the components of the nervous system, are wired together to generate behavior.
• Second, the biophysical, biochemical, and electrophysiological properties of the private neurons.
  

A practiced identify to begin is with the components of the nervous system and how the electrical backdrop of the neurons endow nerve cells with the power to process and transmit information.

1.ane Introduction to the Action Potential

Figure 1.1 Tap the colored circles (calorie-free stimulus) to activate.

Theories of the encoding and transmission of information in the nervous arrangement go back to the Greek physician Galen (129-210 AD), who suggested a hydraulic mechanism by which muscles contract considering fluid flowing into them from hollow fretfulness. The basic theory held for centuries and was further elaborated by René Descartes (1596 – 1650) who suggested that creature spirits flowed from the brain through nerves and and then to muscles to produce movements (See this animation for modernistic interpretation of such a hydraulic theory for nerve function). A major paradigm shift occurred with the pioneering work of Luigi Galvani who plant in 1794 that nervus and muscle could be activated by charged electrodes and suggested that the nervous arrangement functions via electrical signaling (see this blitheness of Galvani's experiment). However, there was contend amongst scholars whether the electricity was within nerves and muscle or whether the nerves and muscles were simply responding to the harmful electric shock via some intrinsic nonelectric mechanism. The issue was not resolved until the 1930s with the development of modern electronic amplifiers and recording devices that immune the electric signals to exist recorded. I case is the pioneering piece of work of H.K. Hartline 80 years ago on electrical signaling in the horseshoe crab Limulus . Electrodes were placed on the surface of an optic nerve. (By placing electrodes on the surface of a nerve, it is possible to obtain an indication of the changes in membrane potential that are occurring between the outside and inside of the nervus jail cell.) So ane-s duration flashes of low-cal of varied intensities were presented to the center; kickoff dim calorie-free, so brighter lights. Very dim lights produced no changes in the activity, but brighter lights produced pocket-size repetitive fasten-like events. These spike-like events are called activeness potentials, nerve impulses, or sometimes only spikes. Activeness potentials are the bones events the nerve cells apply to transmit data from one place to another.

ane.2 Features of Activeness Potentials

The recordings in the figure higher up illustrate iii very important features of nervus action potentials. First, the nerve action potential has a short elapsing (near 1 msec). Second, nerve activity potentials are elicited in an all-or-naught style. Third, nervus cells code the intensity of data by the frequency of activeness potentials. When the intensity of the stimulus is increased, the size of the action potential does not become larger. Rather, the frequency or the number of action potentials increases. In general, the greater the intensity of a stimulus, (whether it be a low-cal stimulus to a photoreceptor, a mechanical stimulus to the pare, or a stretch to a muscle receptor) the greater the number of action potentials elicited. Similarly, for the motor organisation, the greater the number of action potentials in a motor neuron, the greater the intensity of the wrinkle of a muscle that is innervated past that motor neuron.

Activeness potentials are of cracking importance to the functioning of the brain since they propagate information in the nervous system to the central nervous system and propagate commands initiated in the cardinal nervous system to the periphery. Consequently, it is necessary to understand thoroughly their properties. To answer the questions of how action potentials are initiated and propagated, we demand to tape the potential between the within and exterior of nerve cells using intracellular recording techniques.

1.3 Intracellular Recordings from Neurons

The potential deviation across a nervus prison cell membrane can be measured with a microelectrode whose tip is so small (about a micron) that it can penetrate the cell without producing any damage. When the electrode is in the bath (the extracellular medium) there is no potential recorded because the bath is isopotential. If the microelectrode is advisedly inserted into the cell, there is a sharp change in potential. The reading of the voltmeter instantaneously changes from 0 mV, to reading a potential difference of -60 mV inside the cell with respect to the outside. The potential that is recorded when a living cell is impaled with a microelectrode is called the resting potential, and varies from jail cell to jail cell. Here it is shown to be -60 mV, but can range between -80 mV and -40 mV, depending on the particular type of nerve cell. In the absence of any stimulation, the resting potential is generally constant.

Information technology is also possible to record and study the action potential. Effigy 1.three illustrates an case in which a neuron has already been impaled with one microelectrode (the recording electrode), which is continued to a voltmeter. The electrode records a resting potential of -60 mV. The jail cell has also been impaled with a 2nd electrode called the stimulating electrode. This electrode is continued to a battery and a device that tin can monitor the amount of current (I) that flows through the electrode. Changes in membrane potential are produced past closing the switch and by systematically changing both the size and polarity of the battery. If the negative pole of the battery is connected to the inside of the prison cell as in Figure 1.3A, an instantaneous change in the amount of electric current volition period through the stimulating electrode, and the membrane potential becomes transiently more negative. This result should not exist surprising. The negative pole of the battery makes the inside of the jail cell more negative than it was before. A change in potential that increases the polarized country of a membrane is chosen a hyperpolarization. The prison cell is more polarized than it was normally. Utilize yet a larger battery and the potential becomes fifty-fifty larger. The resultant hyperpolarizations are graded functions of the magnitude of the stimuli used to produce them.

Now consider the case in which the positive pole of the battery is continued to the electrode (Figure 1.3B).  When the positive pole of the battery is connected to the electrode, the potential of the cell becomes more positive when the switch is airtight (Figure 1.3B). Such potentials are called depolarizations. The polarized state of the membrane is decreased. Larger batteries produce even larger depolarizations. Once more, the magnitude of the responses are proportional to the magnitude of the stimuli. Still, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential called the threshold. A totally new type of betoken is initiated; the activity potential. Notation that if the size of the bombardment is increased even more than, the amplitude of the action potential is the same as the previous 1 (Effigy 1.3B). The process of eliciting an action potential in a nervus cell is analogous to igniting a fuse with a rut source. A certain minimum temperature (threshold) is necessary. Temperatures less than the threshold fail to ignite the fuse. Temperatures greater than the threshold ignite the fuse but as well as the threshold temperature and the fuse does non burn down any brighter or hotter.

If the suprathreshold current stimulus is long plenty, however, a train of activeness potentials will be elicited. In general, the action potentials will continue to fire equally long as the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus (Effigy 1.4).

Activeness potentials are not only initiated in an all-or-aught fashion, but they are also propagated in an all-or-goose egg style. An action potential initiated in the prison cell trunk of a motor neuron in the spinal cord will propagate in an undecremented style all the style to the synaptic terminals of that motor neuron. Over again, the situation is analogous to a burning fuse.  In one case the fuse is ignited, the flame will spread to its end.

1.4 Components of the Activity Potentials

The action potential consists of several components (Figure 1.3B). The threshold is the value of the membrane potential which, if reached, leads to the all-or-nothing initiation of an action potential. The initial or rise phase of the action potential is called the depolarizing phase or the upstroke. The region of the action potential between the 0 mV level and the tiptop aamplitude is the overshoot. The return of the membrane potential to the resting potential is called the repolarization phase. There is also a phase of the action potential during which time the membrane potential can be more negative than the resting potential. This phase of the activity potential is called the undershoot or the hyperpolarizing afterpotential.  In Figure 1.4, the undershoots of the action potentials do not go more than negative than the resting potential because they are "riding" on the abiding depolarizing stimulus.

1.5 Ionic Mechanisms of Resting Potentials

Before examining the ionic mechanisms of action potentials, information technology is get-go necessary to understand the ionic mechanisms of the resting potential. The two phenomena are intimately related. The story of the resting potential goes back to the early 1900'due south when Julius Bernstein suggested that the resting potential (V_m) was equal to the potassium equilibrium potential (Due east_K). Where

The key to understanding the resting potential is the fact that ions are distributed unequally on the inside and outside of cells, and that cell membranes are selectively permeable to different ions. Thou⁺ is specially of import for the resting potential. The membrane is highly permeable to K⁺. In add-on, the within of the prison cell has a high concentration of K⁺ ([K⁺]_i) and the outside of the cell has a low concentration of K⁺ ([K⁺]_o). Thus, Thousand⁺ will naturally move past improvidence from its region of high concentration to its region of low concentration. Consequently, the positive K⁺ ions leaving the inner surface of the membrane get out behind some negatively charged ions. That negative charge attracts the positive accuse of the K⁺ ion that is leaving and tends to "pull information technology back". Thus, there will be an electrical force directed inward that will tend to counterbalance the diffusional strength directed outward. Somewhen, an equilibrium will be established; the concentration force moving G⁺ out will rest the electrical force holding it in. The potential at which that remainder is achieved is called the Nernst Equilibrium Potential.

An experiment to test Bernstein's hypothesis that the membrane potential is equal to the Nernst Equilibrium Potential (i.e., V_k = E_K) is illustrated to the left.

The Thousand⁺ concentration exterior 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 very close to this line. Moreover, because of the logarithmic human relationship in the Nernst equation, a change in concentration of K⁺ by a factor of 10 results in a lx mV change in potential.

Note, however, that there are some deviations in the figure at left from what is predicted past the Nernst equation. Thus, ane cannot conclude that 5_m = Eastward_K. Such deviations indicate that some other ion is also involved in generating the resting potential. That ion is Na⁺. The high concentration of Na⁺ outside the cell and relatively low concentration inside the cell results in a chemic (diffusional) driving force for Na⁺ influx. In that location is likewise an electrical driving force because the within of the prison cell is negative and this negativity attracts the positive sodium ions. Consequently, if the cell has a pocket-sized permeability to sodium, Na⁺ will move across the membrane and the membrane potential would be more depolarized than would be expected from the K⁺ equilibrium potential.

1.6 Goldman-Hodgkin and Katz (GHK) Equation

When a membrane is permeable to two dissimilar ions, the Nernst equation tin can no longer be used to precisely determine the membrane potential. It is possible, all the same, to utilize the GHK equation. This equation describes the potential across a membrane that is permeable to both Na⁺ and K⁺.

Note that α is the ratio of Na⁺ permeability (P_Na) to 1000⁺ permeability (P_K). Notation also that if the permeability of the membrane to Na⁺ is 0, then alpha in the GHK is 0, and the Goldman-Hodgkin-Katz equation reduces to the Nernst equilibrium potential for Thou⁺. If the permeability of the membrane to Na⁺ is very loftier and the potassium permeability is very low, the [Na⁺] terms become very large, dominating the equation compared to the [K⁺] terms, and the GHK equation reduces to the Nernst equilibrium potential for Na⁺.

If the GHK equation is applied to the same data in Figure one.five, there is a much better fit. The value of alpha needed to obtain this good fit was 0.01. This means that the potassium K⁺ permeability is 100 times the Na⁺ permeability. In summary, the resting potential is due non only to the fact that at that place is a high permeability to K⁺. At that place is also a slight permeability to Na⁺, which tends to make the membrane potential slightly more positive than it would have been if the membrane were permeable to One thousand⁺ alone.

one.7 Membrane Potential Laboratory

Click here to go to the interactive Membrane Potential Laboratory to experiment with the furnishings of altering external or internal potassium ion concentration and membrane permeability to sodium and potassium ions. Predictions are made using the Nernst and the Goldman, Hodgkin, Katz equations.

Membrane Potential Laboratory

Test Your Knowledge

• Question i
• A
• B
• C
• D
• E

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

A. Not change

B. Approach the new K⁺ equilibrium potential

C. Arroyo the new Na⁺ equilibrium potential

D. Approach a value of about 0 mV

Eastward. Arroyo a constant value of well-nigh +55 mV

If a nervus membrane suddenly became equally permeable to both Na⁺ and 1000⁺, the membrane potential would:

A. Not modify This respond is INCORRECT.

A change in permeability would depolarize the membrane potential since blastoff in the GHK equation would equal one. Initially, alpha was 0.01. Try substituting dissimilar values of blastoff into the GHK equation and calculate the resultant membrane potential.

B. Approach the new K⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Approach a value of about 0 mV

East. Arroyo a abiding value of nearly +55 mV

If a nervus membrane suddenly became every bit permeable to both Na⁺ and K⁺, the membrane potential would:

A. Non change

B. Approach the new Thousand⁺ equilibrium potential This answer is INCORRECT.

The membrane potential would approach the K+ equilibrium potential merely if the Na⁺ permeability was decreased or the K⁺ permeability was increased. Also in that location would be no "new" equilibrium potential. Changing the permeability does not alter the equilibrium potential.

C. Arroyo the new Na⁺ equilibrium potential

D. Approach a value of about 0 mV

E. Approach a constant value of most +55 mV

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

A. Not change

B. Approach the new Yard⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential This respond is Wrong.

The membrane potential would arroyo the Na⁺ equilibrium potential simply if blastoff in the GHK equation became very large (e.one thousand., subtract PK or increase PNa). Likewise, there would be no "new" Na⁺ equilibrium potential. Changing the permeability does not alter the equilibrium potential; information technology changes the membrane potential.

D. Arroyo a value of about 0 mV

Eastward. Approach a constant value of about +55 mV

If a nervus membrane suddenly became as permeable to both Na⁺ and Thousand⁺, the membrane potential would:

A. Not modify

B. Approach the new K⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Arroyo a value of nigh 0 mV This answer is CORRECT!

Roughly speaking, the membrane potential would move to a value one-half manner between East_G and E_Na. The GHK equation could be used to make up one's mind the precise value.

E. Arroyo a constant value of about +55 mV

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

A. Not change

B. Approach the new K⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Approach a value of about 0 mV

E. Arroyo a constant value of about +55 mV This answer is INCORRECT.

The membrane potential would not arroyo a value of nearly +55 mV (the judge value of E_Na) unless there was a big increase in the sodium permeability without a corresponding change in the potassium permeability. Alpha in the Goldman equation would need to approach a very loftier value.

• Question 2
• A
• B
• C
• D
• E

If the concentration of Thou⁺ in the cytoplasm of an invertebrate axon is changed to a new value of 200 mM (Note: for this axon normal [G]_o = 20 mM and normal [K]_i = 400 mM):

A. The membrane potential would become more negative

B. The M⁺ equilibrium potential would alter by 60 mV

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

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

E. An action potential would exist initiated

If the concentration of K⁺ in the cytoplasm of an invertebrate axon is changed to a new value of 200 mM (Note: for this axon normal [K]_o = twenty mM and normal [K]_i = 400 mM):

A. The membrane potential would become more than negative This answer is Wrong.

The normal value 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 changed from 400 mM to 200 mM, then the potassium equilibrium potential as determined by the Nernst equation, will equal nearly -threescore mV. Since the membrane potential is usually -60 mV and is dependent, to a large extent, on East_K, the modify in the potassium concentration and hence E_K would make the membrane potential more positive, not more negative.

B. The G⁺ equilibrium potential would alter by 60 mV

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

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

E. An action potential would be initiated

If the concentration of K⁺ in the cytoplasm of an invertebrate axon is inverse to a new value of 200 mM (Annotation: for this axon normal [1000]_o = twenty mM and normal [1000]_i = 400 mM):

A. The membrane potential would become more negative

B. The K⁺ equilibrium potential would change by sixty mV This respond is INCORRECT. The potassium equilibrium potential would not change past lx mV. The potassium concentration was inverse just from 400 mM to 200 mM. 1 can utilise the Nernst equation to determine the exact value that the equilibrium potential would modify by. It was initially about -75 mV and as a effect of the modify in concentration, the equilibrium potential becomes -lx mV. Thus, the equilibrium potential does not change by 60 mV, information technology changes by nearly 15 mV.

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

D. The K⁺ equilibrium potential would be virtually -eighteen mV

E. An action potential would be initiated

If the concentration of K⁺ in the cytoplasm of an invertebrate axon is changed to a new value of 200 mM (Note: for this axon normal [Thousand]_o = xx mM and normal [Grand]_i = 400 mM):

A. The membrane potential would become more negative

B. The K⁺ equilibrium potential would change past threescore mV

C. The K⁺ equilibrium potential would be well-nigh -60 mV This answer is Correct! This is the correct answer. See the logic described in responses A and B.

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

East. An activity potential would be initiated

If the concentration of K⁺ in the cytoplasm of an invertebrate axon is inverse to a new value of 200 mM (Note: for this axon normal [Grand]_o = 20 mM and normal [K]_i = 400 mM):

A. The membrane potential would go more than negative

B. The K⁺ equilibrium potential would alter by sixty mV

C. The G⁺ equilibrium potential would exist almost -threescore mV

D. The M⁺ equilibrium potential would be well-nigh -18 mV This answer is Incorrect. Using the Nernst equation, the new potassium equilibrium potential tin exist calculated to be -sixty mV. A value of -18 mV would be calculated if yous substituted [Grand]_o = 200 and [1000]_i= 400 into the Nernst equation.

E. An action potential would be initiated

If the concentration of K⁺ in the cytoplasm of an invertebrate axon is inverse to a new value of 200 mM (Annotation: for this axon normal [K]_o = 20 mM and normal [One thousand]_i = 400 mM):

A. The membrane potential would become more than negative

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

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

D. The K⁺ equilibrium potential would be virtually -eighteen mV

E. An activeness potential would exist initiated This answer is Wrong. The membrane potential would non depolarize sufficiently to reach threshold (about -45 mV).

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Source: https://nba.uth.tmc.edu/neuroscience/m/s1/chapter01.html

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