Which Step Of Nerve Or Muscle Firing Would Be Directly Affected By A Change In Extracellular K+ ?

Chapter 1: Resting Potentials and Action Potentials

Video of lecture

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

Get-go, the means in which individual neurons, the components of the nervous system, are wired together to generate behavior.
Second, the biophysical, biochemical, and electrophysiological backdrop of the individual neurons.

A good place to begin is with the components of the nervous organization and how the electric properties of the neurons endow nerve cells with the ability to process and transmit information.

1.1 Introduction to the Action Potential

Figure 1.i
Tap the colored circles (light stimulus) to actuate.

Theories of the encoding and transmission of information in the nervous organisation become back to the Greek physician Galen (129-210 Advertisement), who suggested a hydraulic machinery by which muscles contract because fluid flowing into them from hollow nerves. The basic theory held for centuries and was farther elaborated past René Descartes (1596 – 1650) who suggested that animal spirits flowed from the brain through nerves and and so to muscles to produce movements (Encounter this blitheness for mod interpretation of such a hydraulic theory for nerve function). A major paradigm shift occurred with the pioneering work of Luigi Galvani who constitute in 1794 that nerve and musculus could exist activated by charged electrodes and suggested that the nervous organisation functions via electric signaling (see this blitheness of Galvani's experiment). However, there was debate amidst scholars whether the electricity was inside nerves and muscle or whether the nerves and muscles were just responding to the harmful electric shock via some intrinsic nonelectric mechanism. The outcome was not resolved until the 1930s with the development of modern electronic amplifiers and recording devices that allowed the electrical signals to be recorded. One case is the pioneering work of H.Thousand. Hartline lxxx years ago on electrical signaling in the horseshoe crab Limulus . Electrodes were placed on the surface of an optic nerve. (Past 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 within of the nervus cell.) And so 1-s duration flashes of lite of varied intensities were presented to the heart; first dim low-cal, then brighter lights. Very dim lights produced no changes in the activeness, merely brighter lights produced modest repetitive fasten-like events. These spike-like events are chosen activity potentials, nerve impulses, or sometimes only spikes. Action potentials are the bones events the nervus cells use to transmit information from one place to some other.

1.ii Features of Action Potentials

The recordings in the effigy higher up illustrate three very important features of nervus action potentials. First, the nerve activity potential has a short duration (near 1 msec). Second, nerve activity potentials are elicited in an all-or-nothing fashion. Third, nervus cells code the intensity of information by the frequency of activity 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 activity potentials increases. In general, the greater the intensity of a stimulus, (whether it be a lite stimulus to a photoreceptor, a mechanical stimulus to the pare, or a stretch to a muscle receptor) the greater the number of activity potentials elicited. Similarly, for the motor system, the greater the number of action potentials in a motor neuron, the greater the intensity of the contraction of a muscle that is innervated by that motor neuron.

Action potentials are of groovy 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 key nervous system to the periphery. Consequently, it is necessary to sympathize thoroughly their properties. To answer the questions of how activity potentials are initiated and propagated, we need to record the potential between the inside and outside of nerve cells using intracellular recording techniques.

i.3 Intracellular Recordings from Neurons

The potential departure across a nerve cell membrane can exist measured with a microelectrode whose tip is and then small (nearly a micron) that it tin penetrate the cell without producing any damage. When the electrode is in the bath (the extracellular medium) there is no potential recorded considering the bath is isopotential. If the microelectrode is carefully inserted into the cell, at that place is a sharp alter in potential. The reading of the voltmeter instantaneously changes from 0 mV, to reading a potential difference of -60 mV inside the prison 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 cell to cell. Here it is shown to be -sixty mV, but tin range between -fourscore mV and -40 mV, depending on the item type of nervus cell. In the absence of whatsoever stimulation, the resting potential is more often than not constant.

It is likewise possible to record and study the action potential. Effigy 1.3 illustrates an example 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 cell has as well been impaled with a second electrode called the stimulating electrode. This electrode is continued to a battery and a device that can monitor the amount of current (I) that flows through the electrode. Changes in membrane potential are produced by closing the switch and by systematically irresolute both the size and polarity of the bombardment. If the negative pole of the battery is connected to the within of the cell as in Figure ane.3A, an instantaneous change in the amount of current will menstruum 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 cell more negative than it was before. A change in potential that increases the polarized state of a membrane is chosen a hyperpolarization. The jail cell is more polarized than it was usually. Use all the same 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 connected to the electrode (Figure 1.3B). When the positive pole of the battery is continued to the electrode, the potential of the cell becomes more positive when the switch is closed (Effigy 1.3B). Such potentials are called depolarizations. The polarized state of the membrane is decreased. Larger batteries produce even larger depolarizations. Over again, the magnitude of the responses are proportional to the magnitude of the stimuli. However, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential chosen the threshold. A totally new type of signal is initiated; the activeness potential. Note that if the size of the battery is increased even more, the aamplitude of the action potential is the same as the previous one (Figure 1.3B). The process of eliciting an action potential in a nerve jail cell is analogous to igniting a fuse with a heat 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 just as well as the threshold temperature and the fuse does non burn whatever brighter or hotter.

If the suprathreshold current stimulus is long plenty, nevertheless, a train of activity potentials volition exist elicited. In general, the action potentials volition go along to burn down as long every bit the stimulus continues, with the frequency of firing beingness proportional to the magnitude of the stimulus (Figure 1.4).

Action potentials are non just initiated in an all-or-zippo style, just they are also propagated in an all-or-nothing fashion. An action potential initiated in the prison cell body of a motor neuron in the spinal cord will propagate in an undecremented fashion all the manner to the synaptic terminals of that motor neuron. Again, the state of affairs is coordinating to a burning fuse. In one case the fuse is ignited, the flame will spread to its finish.

ane.four Components of the Activity Potentials

The activity 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 activeness potential. The initial or rise stage 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 peak aamplitude is the overshoot. The render of the membrane potential to the resting potential is chosen the repolarization phase. There is as well a phase of the activeness potential during which time the membrane potential tin be more negative than the resting potential. This phase of the action potential is called the undershoot or the hyperpolarizing afterpotential. In Figure 1.four, the undershoots of the action potentials do not become more negative than the resting potential because they are "riding" on the constant depolarizing stimulus.

1.5 Ionic Mechanisms of Resting Potentials

Before examining the ionic mechanisms of action potentials, information technology is first 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's when Julius Bernstein suggested that the resting potential (V_m) was equal to the potassium equilibrium potential (E_Chiliad). Where

The key to agreement the resting potential is the fact that ions are distributed unequally on the inside and outside of cells, and that jail cell membranes are selectively permeable to dissimilar ions. K⁺ is particularly of import for the resting potential. The membrane is highly permeable to K⁺. In improver, 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 1000⁺ ([K⁺]_o). Thus, K⁺ will naturally motility by improvidence from its region of high concentration to its region of low concentration. Consequently, the positive Chiliad⁺ ions leaving the inner surface of the membrane leave behind some negatively charged ions. That negative charge attracts the positive charge of the Thousand⁺ ion that is leaving and tends to "pull it back". Thus, there will exist an electric force directed inwards that will tend to counterbalance the diffusional forcefulness directed outward. Eventually, an equilibrium will be established; the concentration forcefulness moving K⁺ out volition balance the electrical force holding it in. The potential at which that balance is achieved is called the Nernst Equilibrium Potential.

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

The K⁺ concentration outside the cell was systematically varied while the membrane potential was measured. Besides shown is the line that is predicted past the Nernst Equation. The experimentally measured points are very close to this line. Moreover, because of the logarithmic relationship in the Nernst equation, a alter in concentration of 1000⁺ past a gene of ten results in a threescore mV alter in potential.

Note, however, that there are some deviations in the figure at left from what is predicted by the Nernst equation. Thus, 1 cannot conclude that V_yard = E_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 chemical (diffusional) driving force for Na⁺ influx. There is too an electrical driving force because the inside of the prison cell is negative and this negativity attracts the positive sodium ions. Consequently, if the jail cell has a small permeability to sodium, Na⁺ will motility across the membrane and the membrane potential would be more depolarized than would be expected from the K⁺ equilibrium potential.

one.6 Goldman-Hodgkin and Katz (GHK) Equation

When a membrane is permeable to 2 different ions, the Nernst equation tin can no longer exist used to precisely determine the membrane potential. It is possible, still, to apply the GHK equation. This equation describes the potential across a membrane that is permeable to both Na⁺ and Yard⁺.

Annotation that α is the ratio of Na⁺ permeability (P_Na) to K⁺ permeability (P_Thousand). Note also that if the permeability of the membrane to Na⁺ is 0, and so alpha in the GHK is 0, and the Goldman-Hodgkin-Katz equation reduces to the Nernst equilibrium potential for One thousand⁺. If the permeability of the membrane to Na⁺ is very high and the potassium permeability is very low, the [Na⁺] terms become very big, 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 1.five, there is a much better fit. The value of blastoff 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 not only to the fact that there is a loftier permeability to Thousand⁺. There is also a slight permeability to Na⁺, which tends to make the membrane potential slightly more than positive than it would have been if the membrane were permeable to K⁺ alone.

1.7 Membrane Potential Laboratory

Click here to get to the interactive Membrane Potential Laboratory to experiment with the effects 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 Noesis

Question 1
A
B
C
D
E

If a nerve membrane suddenly became equally permeable to both Na⁺ and Grand⁺, 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

E. Approach a constant value of about +55 mV

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

A. Non alter This answer is Wrong.

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

B. Approach the new Yard⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Arroyo a value of about 0 mV

E. Approach a constant value of about +55 mV

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

A. Not change

B. Approach the new K⁺ equilibrium potential This answer is Wrong.

The membrane potential would approach the K+ equilibrium potential only if the Na⁺ permeability was decreased or the K⁺ permeability was increased. Too there would be no "new" equilibrium potential. Changing the permeability does non change the equilibrium potential.

C. Arroyo the new Na⁺ equilibrium potential

D. Approach a value of nigh 0 mV

E. Arroyo a constant value of most +55 mV

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

A. Not change

B. Approach the new One thousand⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential This answer is INCORRECT.

The membrane potential would approach the Na⁺ equilibrium potential merely if alpha in the GHK equation became very large (east.one thousand., subtract PK or increase PNa). Besides, there would be no "new" Na⁺ equilibrium potential. Changing the permeability does not change the equilibrium potential; information technology changes the membrane potential.

D. Approach a value of about 0 mV

East. Arroyo a abiding value of about +55 mV

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

A. Not modify

B. Approach the new G⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Approach a value of nearly 0 mV This respond is Correct!

Roughly speaking, the membrane potential would move to a value half fashion between East_K and E_Na. The GHK equation could be used to determine the precise value.

Due east. Approach a constant value of almost +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 K⁺ equilibrium potential

C. Arroyo the new Na⁺ equilibrium potential

D. Approach a value of about 0 mV

Due east. Approach a abiding value of most +55 mV This answer is INCORRECT.

The membrane potential would non arroyo a value of most +55 mV (the approximate 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 high value.

Question 2
A
B
C
D
E

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 = 20 mM and normal [Chiliad]_i = 400 mM):

A. The membrane potential would become more than negative

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

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

D. The Yard⁺ equilibrium potential would exist about -xviii mV

Due east. 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 [Thou]_o = 20 mM and normal [K]_i = 400 mM):

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

The normal value of extracellular potassium is twenty mM and the normal value of intracellular potassium is 400 mM, yielding a normal equilibrium potential for potassium of nigh -75 mV. If the intracellular concentration is changed from 400 mM to 200 mM, and then the potassium equilibrium potential as adamant by the Nernst equation, will equal well-nigh -60 mV. Since the membrane potential is normally -threescore mV and is dependent, to a large extent, on E_{One thousand}, the change in the potassium concentration and hence East_K would make the membrane potential more positive, not more negative.

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

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

D. The Yard⁺ equilibrium potential would be nearly -18 mV

Due east. 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 (Note: for this axon normal [K]_o = 20 mM and normal [G]_i = 400 mM):

A. The membrane potential would become more than negative

B. The K⁺ equilibrium potential would modify by 60 mV This respond is Wrong. The potassium equilibrium potential would non change by 60 mV. The potassium concentration was changed only from 400 mM to 200 mM. 1 can apply the Nernst equation to determine the exact value that the equilibrium potential would change past. It was initially about -75 mV and every bit a consequence of the change in concentration, the equilibrium potential becomes -60 mV. Thus, the equilibrium potential does not alter by 60 mV, it changes by virtually fifteen mV.

C. The M⁺ equilibrium potential would exist about -60 mV

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

East. An activity 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 [K]_o = 20 mM and normal [K]_i = 400 mM):

A. The membrane potential would become more negative

B. The One thousand⁺ equilibrium potential would change by 60 mV

C. The K⁺ equilibrium potential would be virtually -60 mV This reply is CORRECT! This is the correct answer. Run into the logic described in responses A and B.

D. The K⁺ 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 changed to a new value of 200 mM (Note: for this axon normal [1000]_o = 20 mM and normal [Thousand]_i = 400 mM):

A. The membrane potential would get more negative

B. The Yard⁺ equilibrium potential would change by threescore mV

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

D. The Thou⁺ equilibrium potential would be about -eighteen mV This answer is INCORRECT. Using the Nernst equation, the new potassium equilibrium potential can be calculated to be -60 mV. A value of -18 mV would be calculated if you substituted [Grand]_o = 200 and [K]_i= 400 into the Nernst equation.

E. An activeness potential would be initiated

If the concentration of G⁺ in the cytoplasm of an invertebrate axon is changed 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 go more than negative

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

C. The G⁺ equilibrium potential would be virtually -60 mV

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

Eastward. An action potential would be initiated This answer is Incorrect. The membrane potential would not depolarize sufficiently to achieve threshold (virtually -45 mV).