Resting Potentials and Action Potentials (Section 1, Chapter 1) Neuroscience Online: An eBook for Neuroscience | Department of Neurobiology and Anatomy (2023)

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Despite the enormous complexity of the brain, it is possible to understand its function by paying attention to two main details:

  • First, the ways in which individual neurons, the components of the nervous system, interconnect to generate behavior.
  • Second, the biophysical, biochemical and electrophysiological properties of individual neurons.

A good place to start is with the components of the nervous system and how the electrical properties of neurons give nerve cells the ability to process and transmit information.

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1.1Introduction to Action Potential

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

Theories of the encoding and transmission of information in the nervous system go back to the Greek physician Galen (AD 129-210), who suggested a hydraulic mechanism by which muscles contract as fluid flows into them from nerve orifices. The basic theory lasted for centuries and was elaborated by René Descartes (1596 - 1650), who suggested that animal spirits flowed from the brain through the nerves and then into the muscles to produce movement (seethis animationfor the modern interpretation of such a hydraulic theory for nerve function). A major paradigm shift occurred with the pioneering work of Luigi Galvani, who discovered in 1794 that nerves and muscles could be activated by charged electrodes and suggested that the nervous system works through electrical signals (see this animation ofGalvani's experiment🇧🇷 However, there has been debate among scholars as to whether the electricity was within the nerves and muscles or whether the nerves and muscles simply responded to the harmful electrical discharge through some intrinsic non-electrical mechanism. The problem was not resolved until the 1930s with the development of modern electronic amplifiers and recording devices that allowed recording of electrical signals. An example is the pioneering work of H.K. Hartline 80 years ago on electrical signaling in the Limulus horseshoe crab. Electrodes were placed on the surface of an optic nerve. (By placing electrodes on the surface of a nerve, it is possible to get an indication of changes in membrane potential that are occurring between the outside and inside of the nerve cell.) presented for visualization; dim light first, then brighter lights. Very dim lights produced no change in activity, but brighter lights produced small events similar to repeating spikes. These spike-like events are called action potentials, nerve impulses, or sometimes just spikes. Action potentials are the basic events that nerve cells use to transmit information from one place to another.

1.2Characteristics of action potentials

The recordings in the figure above illustrate three very important features of nerve action potentials.First, the nerve action potential is short-lived (about 1 ms).SecondNerve action potentials are elicited in an all-or-nothing manner.Third, nerve cells encode the intensity of information by the frequency of action potentials. When stimulus intensity increases, action potential size does not increase. Instead, it increases the frequency or number of action potentials. In general, the greater the intensity of a stimulus (be it a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a muscle receptor), the greater the number of evoked action potentials. Likewise, for the motor system, the greater the number of action potentials in a motor neuron, the greater the intensity of contraction of a muscle innervated by that motor neuron.

Action potentials are of great importance for brain function, as they propagate information from the nervous system to the central nervous system and propagate commands initiated in the central nervous system to the periphery. Accordingly, it is necessary to fully understand its properties. To answer the questions of how action potentials are initiated and propagated, we need to record the potential between the inside and outside of nerve cells using intracellular recording techniques.

1.3Intracellular registers of neurons

Figure 1.2

The potential difference across the membrane of a nerve cell can be measured with amicroeletrodowhose tip is so small (about a micron) that it can penetrate the cell without causing damage. When the electrode is in the bath (the extracellular medium) no potential is recorded because the bath is isopotential. If the microelectrode is carefully inserted into the cell, there is an abrupt change in potential. The voltmeter reading instantly changes from 0 mV to reading a potential difference of -60 mV inside the cell relative to the outside. The potential that registers when a microelectrode is punctured through a living cell is called the resting potential and varies from cell to cell. It is shown here as -60 mV, but can range from -80 mV to -40 mV depending on the specific nerve cell type. In the absence of stimulation, the resting potential is generally constant.

It is also possible to record and study the action potential. Figure 1.3 illustrates an example where a neuron has already been impaled with a microelectrode (the recording electrode), which is connected to a voltmeter. The electrode registers a resting potential of -60 mV. The cell was also impaled with a second electrode called the stimulating electrode. This electrode is connected to a battery and a device that can monitor the amount of current (I) flowing through the electrode. Changes in membrane potential are produced by closing the switch and systematically changing the size and polarity of the battery. If the negative pole of the battery is connected to the inside of the cell, as shown in Figure 1.3A, an instantaneous change in the amount of current will flow through the stimulating electrode and the membrane potential becomes transiently more negative. This result should not be 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 calledhyperpolarization🇧🇷 The cell is more polarized than it normally was. Use an even bigger battery and the potential becomes even greater. The resulting hyperpolarizations are graded functions of the magnitude of the stimuli used to produce them.

Figure 1.3

Now consider the case where the positive pole of the battery is connected to the electrode (Figure 1.3B). When the positive pole of the battery is connected to the electrode, the cell potential becomes more positive when the switch is closed (Figure 1.3B). These potentials are calleddepolarizations🇧🇷 The polarized state of the membrane decreases. Larger batteries produce even greater depolarizations. Again, the magnitude of the responses is proportional to the magnitude of the stimuli. However, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential calledlimit🇧🇷 A new type of signal starts; the action potential. Note that if the battery size is increased further, the action potential amplitude will be the same as before (Figure 1.3B). The process of triggering an action potential in a nerve cell is similar to lighting a fuse with a heat source. A certain minimum temperature (threshold) is required. Temperatures below the limit fail to light the fuse. Temperatures above the threshold light the fuse just as well as the threshold temperature and the fuse does not burn brighter or hotter.

If hesupraumbralthe current stimulus is long enough, however, a train of action potentials will be elicited. In general, action potentials will continue to fire as long as the stimulus continues, the firing rate being proportional to the magnitude of the stimulus (Figure 1.4).

Figure 1.4

Action potentials not only initiate in all-or-nothing mode, they also propagate in all-or-nothing mode. An action potential initiated in the cell body of a motor neuron in the spinal cord will propagate uninterruptedly to the synaptic terminals of that motor neuron. Again, the situation is analogous to a blown fuse. Once the wick is lit, the flame will spread all the way.

1.4Components of Action Potentials

The action potential consists of several components (Figure 1.3B). Thelimitis the value of the membrane potential that, if reached, leads to the all-or-nothing initiation of an action potential. The initial or rising phase of the action potential is calleddepolarizing phaseor theupward stroke🇧🇷 The region of the action potential between the 0 mV level and the maximum amplitude is theexceed🇧🇷 The return of the membrane potential to the resting potential is calledrepolarization phase🇧🇷 There is also a phase of the action potential during which the membrane potential can be more negative than the resting potential. This phase of the action potential is calledunderestimateor thehyperpolarizing postpotential🇧🇷 In Figure 1.4, the bursts of action potentials do not become more negative than the resting potential because they are "riding" on the constant depolarizing stimulus.

1,5Ionic mechanisms of resting potentials

Before examining the ionic mechanisms of action potentials, it is first necessary to understand the ionic mechanisms of the resting potential. The two phenomena are closely related. The history of the resting potential dates back to the early 20th century, when Julius Bernstein suggested that the resting potential (Vmetro) was equal to the potassium equilibrium potential (Ek🇧🇷 Where

The key to understanding the resting potential is the fact that ions are unevenly distributed inside and outside cells and that cell membranes are selectively permeable to different ions. which+it is particularly important for resting potential. The membrane is highly permeable to K.+🇧🇷 In addition, the interior of the cell has a high concentration of K+([K+]eu) and the outside of the cell has a low concentration of K+([K+]o🇧🇷 So K.+it will naturally move by diffusion from its region of high concentration to its region of low concentration. As a result, the positive K+Ions leaving the inner surface of the membrane leave behind some negatively charged ions. This negative charge attracts the positive charge of K.+ion that abandons it and tends to "withdraw" it. Thus, there will be an inwardly directed electrical force that will tend to counteract the outwardly directed diffusion force. Eventually, a balance will be established; the concentration force that moves K+it will balance the electrical force holding it back. The potential at which this equilibrium is reached is calledNernst equilibrium potential.

Figure 1.5

An experiment to test Bernstein's hypothesis that the membrane potential equals the Nernst equilibrium potential (i.e., Vmetro= mik) is illustrated on the left.

La K+the concentration outside the cell was systematically varied while the membrane potential was measured. The line that predicts the Nernst equation is also shown. The experimentally measured points are very close to this line. Furthermore, due to the logarithmic relationship in the Nernst equation, a change in K concentration+by a factor of 10 results in a potential change of 60 mV.

Note, however, that there are some deviations in the figure on the left from what the Nernst equation predicts. Therefore, it cannot be concluded that V.metro= mik🇧🇷 Such deviations indicate that another ion is also involved in generating the resting potential. This ion is Na+🇧🇷 The high concentration of Na+outside the cell and a relatively low concentration inside the cell results in a chemical (diffusional) driving force for Na+influx. There is also an electrical driving force because the inside of the cell is negative and this negativity attracts positive sodium ions. Consequently, if the cell has a low permeability to sodium, Na+will move across the membrane and the membrane potential will be more depolarized than would be expected from K+equilibrium potential.

1.6Goldman-Hodgkin and Katz Equation (GHK)

When a membrane is permeable to two different ions, the Nernst equation can no longer be used to accurately determine the membrane potential. However, it is possible to apply the GHK equation. This equation describes the potential across a membrane that is permeable to both Na and+is that+.

keep in mind thatoneis the proportion of Na+permeability (PDo) a K+permeability (Pk🇧🇷 Note also that if the permeability of the membrane to Na+is 0, so alpha in GHK is 0, and the Goldman-Hodgkin-Katz equation reduces to the Nernst equilibrium potential for K+🇧🇷 If the permeability of the membrane to Na+is very high and the permeability to potassium is very low, [Na+] terms become very large, dominating the equation compared to [K+] terms, and the GHK equation reduces to the Nernst equilibrium potential for Na+.

If the GHK equation is applied to the same data as in Figure 1.5, the fit will be much better. The alpha value needed to obtain this good fit was 0.01. This means that potassium K+permeability is 100 times Na+permeability. In summary, the resting potential is not only due to the existence of a high permeability to K+🇧🇷 There is also a slight permeability to Na.+, which tends to make the membrane potential slightly more positive than it would be if the membrane were permeable to K+only.


Figure 1.6

1.7Membrane Potential Laboratory

give a clickhere to access the interactive membrane potential labexperience the effects of changing the external or internal potassium ion concentration and membrane permeability to sodium and potassium ions. Predictions are made using Nernst and Goldman, Hodgkin, Katz equations.

Membrane Potential Laboratory

test your knowledge

  • question 1
  • ONE
  • B
  • C
  • D
  • mi

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't change

B. Approaching the new K+equilibrium potential

C. Approaching the new Na+equilibrium potential

D. Approaching a value around 0 mV

E. Arrive at a constant value of approximately +55 mV

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't changeThis answer is INCORRECT.

A change in permeability would depolarize the membrane potential, as alpha in the GHK equation would equal one. Initially, the alpha was 0.01. Try substituting different alpha values ​​into the GHK equation and calculate the resulting membrane potential.

B. Approaching the new K+equilibrium potential

C. Approaching the new Na+equilibrium potential

D. Approaching a value around 0 mV

E. Arrive at a constant value of approximately +55 mV

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't change

B. Approaching the new K+equilibrium potentialThis answer is INCORRECT.

The membrane potential would approach the K+ equilibrium potential only if Na+the permeability has been reduced or the K+increased permeability. Furthermore, there would be no "new" equilibrium potential. Changing the permeability does not change thebalancepotential.

C. Approaching the new Na+equilibrium potential

D. Approaching a value around 0 mV

E. Arrive at a constant value of approximately +55 mV

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't change

B. Approaching the new K+equilibrium potential

C. Approaching the new Na+equilibrium potentialThis answer is INCORRECT.

The membrane potential would approach Na+equilibrium potential only if alpha in the GHK equation becomes very large (eg PK decreases or PNa increases). Furthermore, there would be no "new" Na+equilibrium potential. Changing the permeability does not change the equilibrium potential; changes the membrane potential.

D. Approaching a value around 0 mV

E. Arrive at a constant value of approximately +55 mV

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't change

B. Approaching the new K+equilibrium potential

C. Approaching the new Na+equilibrium potential

D. Approaching a value around 0 mVThis answer is CORRECT!

Roughly speaking, the membrane potential would move to an intermediate value between Ekit's mineDo🇧🇷 The GHK equation can be used to determine the precise value.

E. Arrive at a constant value of approximately +55 mV

If a nerve membrane suddenly became equally permeable to both Na and+is that+, the membrane potential will be:

a. don't change

B. Approaching the new K+equilibrium potential

C. Approaching the new Na+equilibrium potential

D. Approaching a value around 0 mV

E. Arrive at a constant value of approximately +55 mVThis answer is INCORRECT.

The membrane potential would not approach a value around +55 mV (the approximate value of EDo), unless there is a large increase in sodium permeability without a corresponding change in potassium permeability. The alpha in Goldman's equation would have to be close to a very high value.

  • question 2
  • ONE
  • B
  • C
  • D
  • mi

If the concentration of K+in the cytoplasm of an invertebrate axon changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

A. The membrane potential would become more negative.

B.He K+the equilibrium potential would change by 60 mV

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

D. O K+the 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 changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

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

The normal value for extracellular potassium is 20 mM and the normal value for intracellular potassium is 400 mM, resulting in a normal equilibrium potential for potassium of approximately -75 mV. If the intracellular concentration changes from 400 mM to 200 mM, then the potassium equilibrium potential, determined by the Nernst equation, will equal approximately -60 mV. Since the membrane potential is normally -60 mV and is highly dependent on Ek, the change in potassium concentration and therefore Ekwould make the membrane potential more positive, nOld testmentmore negative.

B.He K+the equilibrium potential would change by 60 mV

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

D. O K+the 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 changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

A. The membrane potential would become more negative.

B.He K+the equilibrium potential would change by 60 mVThis answer is INCORRECT.Potassium equilibrium potential would not change at 60 mV. The potassium concentration was simply changed from 400 mM to 200 mM. The Nernst equation can be used to determine the exact amount by which the equilibrium potential would change. Initially it was about -75 mV, and as a result of the change in concentration, the equilibrium potential becomes -60 mV. So the equilibrium potential doesn't change at 60 mV, it changes at about 15 mV.

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

D. O K+the 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 changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

A. The membrane potential would become more negative.

B.He K+the equilibrium potential would change by 60 mV

C.La K+the equilibrium potential would be around -60 mVThis answer is CORRECT!That's the correct answer. See the logic described in answers A and B.

D. O K+the 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 changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

A. The membrane potential would become more negative.

B.He K+the equilibrium potential would change by 60 mV

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

D. O K+the equilibrium potential would be around -18 mVThis answer is INCORRECT.Using the Nernst equation, the new potassium equilibrium potential can be calculated as -60 mV. A value of -18 mV would be calculated if you substitute [K]o= 200 e [K]eu= 400 in the Nernst equation.

E. An action potential would be initiated.

If the concentration of K+in the cytoplasm of an invertebrate axon changes to a new value of 200 mM (Note: for this normal axon [K]o= 20 mM y normal [K]eu= 400 mM):

A. The membrane potential would become more negative.

B.He K+the equilibrium potential would change by 60 mV

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

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

E. An action potential would be initiated.This answer is INCORRECT.The membrane potential would not depolarize enough to reach threshold (about -45 mV).

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