14. What Is the Response of Neurons to Stimulus Called? Ch 48 Neurons Reading Guide

Chapter 1: Resting Potentials and Action Potentials

---

Video of lecture

Despite the enormous complexity of the brain, it is possible to obtain an agreement of its office by paying attention to 2 major details:

• Kickoff, the ways in which individual neurons, the components of the nervous system, are wired together to generate behavior.
• Second, the biophysical, biochemical, and electrophysiological properties of the individual neurons.
  

A good identify to begin is with the components of the nervous organization and how the electrical backdrop of the neurons endow nerve cells with the ability to process and transmit data.

1.1 Introduction to the Action Potential

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

Theories of the encoding and transmission of information in the nervous organisation go back to the Greek physician Galen (129-210 AD), who suggested a hydraulic mechanism past which muscles contract because fluid flowing into them from hollow fretfulness. The basic theory held for centuries and was further elaborated past René Descartes (1596 – 1650) who suggested that animal spirits flowed from the encephalon through nerves and so to muscles to produce movements (See this blitheness for modern estimation of such a hydraulic theory for nerve role). A major image shift occurred with the pioneering work of Luigi Galvani who plant in 1794 that nerve and muscle could exist activated by charged electrodes and suggested that the nervous system functions via electrical signaling (see this animation of Galvani's experiment). However, there was debate among scholars whether the electricity was within nerves and musculus or whether the fretfulness and muscles were simply responding to the harmful electric shock via some intrinsic nonelectric machinery. The outcome was not resolved until the 1930s with the development of modern electronic amplifiers and recording devices that immune the electric signals to be recorded. One example is the pioneering 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 nerve cell.) Then 1-due south duration flashes of light of varied intensities were presented to the eye; first dim lite, then brighter lights. Very dim lights produced no changes in the activeness, but brighter lights produced small repetitive spike-like events. These fasten-like events are called activeness potentials, nervus impulses, or sometimes simply spikes. Action potentials are the basic events the nerve cells use to transmit data from one place to some other.

1.2 Features of Action Potentials

The recordings in the effigy above illustrate three very important features of nerve action potentials. Get-go, the nerve action potential has a brusque elapsing (about 1 msec). Second, nerve activeness potentials are elicited in an all-or-nothing 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 go larger. Rather, the frequency or the number of action potentials increases. In general, the greater 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 greater the number of action 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.

Activeness potentials are of great importance to the functioning of the brain since they propagate information in the nervous organisation to the central nervous organisation and propagate commands initiated in the central nervous organisation to the periphery. Consequently, information technology is necessary to empathize thoroughly their 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 nervus cells using intracellular recording techniques.

1.3 Intracellular Recordings from Neurons

The potential difference across a nerve jail cell membrane can exist measured with a microelectrode whose tip is so small (about a micron) that it tin 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 carefully 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 -sixty mV inside the jail cell with respect to the exterior. The potential that is recorded when a living cell is impaled with a microelectrode is chosen the resting potential, and varies from cell to cell. Here information technology is shown to exist -threescore mV, simply can range between -80 mV and -40 mV, depending on the item type of nervus cell. In the absence of any stimulation, the resting potential is generally constant.

It is also possible to record and study the action potential. Figure 1.iii 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 prison cell has likewise been 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) that flows through the electrode. Changes in membrane potential are produced by 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 jail cell as in Effigy ane.3A, an instantaneous change in the amount of current will menstruation 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 within of the prison cell more than negative than information technology was before. A change in potential that increases the polarized state of a membrane is chosen a hyperpolarization. The cell is more polarized than information technology was normally. Employ yet a larger battery and the potential becomes even larger. The resultant hyperpolarizations are graded functions of the magnitude of the stimuli used to produce them.

At present 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 connected to the electrode, the potential of the cell becomes more than positive when the switch is closed (Effigy 1.3B). Such potentials are called depolarizations. The polarized land of the membrane is decreased. Larger batteries produce even larger depolarizations. Once again, the magnitude of the responses are proportional to the magnitude of the stimuli. All the same, an unusual event occurs when the magnitude of the depolarization reaches a level of membrane potential chosen the threshold. A totally new type of bespeak is initiated; the action potential. Annotation that if the size of the battery is increased even more, the amplitude of the action potential is the same every bit the previous one (Figure 1.3B). The procedure of eliciting an activity potential in a nervus prison 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 every bit the threshold temperature and the fuse does not burn any brighter or hotter.

If the suprathreshold current stimulus is long enough, even so, a railroad train of activeness potentials will be elicited. In general, the action potentials will continue to fire equally long every bit the stimulus continues, with the frequency of firing being proportional to the magnitude of the stimulus (Figure 1.iv).

Action potentials are not only initiated in an all-or-nothing fashion, only they are also propagated in an all-or-nothing fashion. An action potential initiated in the cell torso of a motor neuron in the spinal cord will propagate in an undecremented way all the way to the synaptic terminals of that motor neuron. Over again, the situation is analogous to a burning fuse.  Once the fuse is ignited, the flame will spread to its finish.

1.4 Components of the Activity Potentials

The activity potential consists of several components (Figure i.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 stage or the upstroke. The region of the action potential betwixt 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 stage. There is also a phase of the action potential during which time the membrane potential can be more negative than the resting potential. This stage of the activity potential is called the undershoot or the hyperpolarizing afterpotential.  In Effigy 1.four, the undershoots of the action potentials do not get more negative than the resting potential because they are "riding" on the constant depolarizing stimulus.

i.5 Ionic 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 ii 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_k) was equal to the potassium equilibrium potential (Eastward_K). Where

The key to understanding the resting potential is the fact that ions are distributed unequally on the inside and exterior of cells, and that prison cell membranes are selectively permeable to unlike ions. K⁺ is particularly important for the resting potential. The membrane is highly permeable to One thousand⁺. In add-on, the inside of the cell has a high concentration of Yard⁺ ([1000⁺]_i) and the outside of the cell has a low concentration of K⁺ ([K⁺]_o). Thus, Yard⁺ will naturally motion past improvidence from its region of high concentration to its region of low concentration. Consequently, the positive M⁺ ions leaving the inner surface of the membrane exit behind some negatively charged ions. That negative charge attracts the positive charge of the K⁺ ion that is leaving and tends to "pull it back". Thus, in that location volition be an electrical force directed inward that will tend to weigh the diffusional force directed outward. Eventually, an equilibrium volition be established; the concentration force moving G⁺ out volition balance the electric strength property it in. The potential at which that residuum 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 = E₁₀₀₀) is illustrated to the left.

The K⁺ concentration outside 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 modify in concentration of Thousand⁺ by a factor of 10 results in a sixty mV modify 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_m = E_K. Such deviations indicate that another ion is too 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. There is likewise an electrical driving force considering the within of the cell is negative and this negativity attracts the positive sodium ions. Consequently, if the cell has a small permeability to sodium, Na⁺ will motion beyond the membrane and the membrane potential would exist more depolarized than would be expected from the 1000⁺ equilibrium potential.

1.6 Goldman-Hodgkin and Katz (GHK) Equation

When a membrane is permeable to ii different ions, the Nernst equation tin can no longer be used to precisely decide the membrane potential. It is possible, however, to employ the GHK equation. This equation describes the potential across a membrane that is permeable to both Na⁺ and K⁺.

Annotation that α is the ratio of Na⁺ permeability (P_Na) to K⁺ permeability (P_K). Note likewise 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 K⁺. If the permeability of the membrane to Na⁺ is very high and the potassium permeability is very depression, the [Na⁺] terms get 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 information in Figure 1.v, at that place is a much better fit. The value of alpha needed to obtain this skillful fit was 0.01. This ways that the potassium K⁺ permeability is 100 times the Na⁺ permeability. In summary, the resting potential is due non merely to the fact that there is 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 accept been if the membrane were permeable to K⁺ alone.

1.7 Membrane Potential Laboratory

Click here to go 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 Knowledge

• Question 1
• A
• B
• C
• D
• E

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

A. Not alter

B. Approach the new K⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Arroyo a value of about 0 mV

E. Arroyo a constant value of nigh +55 mV

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

A. Not alter This reply is Wrong.

A change in permeability would depolarize the membrane potential since alpha in the GHK equation would equal 1. Initially, alpha 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. Arroyo a constant value of nigh +55 mV

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

A. Non modify

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

The membrane potential would approach the K+ equilibrium potential just if the Na⁺ permeability was decreased or the G⁺ permeability was increased. As well there would be no "new" equilibrium potential. Changing the permeability does not change the 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 suddenly became equally permeable to both Na⁺ and Thousand⁺, the membrane potential would:

A. Non change

B. Arroyo the new K⁺ equilibrium potential

C. Arroyo the new Na⁺ equilibrium potential This answer is Incorrect.

The membrane potential would arroyo the Na⁺ equilibrium potential only if blastoff in the GHK equation became very big (e.g., subtract PK or increment PNa). Also, 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 almost 0 mV

E. Approach a abiding value of near +55 mV

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

A. Not change

B. Arroyo the new K⁺ equilibrium potential

C. Arroyo the new Na⁺ equilibrium potential

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

Roughly speaking, the membrane potential would move to a value half way between E_K and Due east_Na. The GHK equation could be used to determine the precise value.

E. Approach a abiding value of about +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 K⁺ equilibrium potential

C. Approach the new Na⁺ equilibrium potential

D. Approach a value of well-nigh 0 mV

E. Arroyo a constant value of well-nigh +55 mV This answer is INCORRECT.

The membrane potential would non approach a value of nigh +55 mV (the approximate value of E_Na) unless there was a large increase in the sodium permeability without a respective change in the potassium permeability. Alpha in the Goldman equation would need to approach a very high value.

• Question two
• 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 [Thou]_o = 20 mM and normal [K]_i = 400 mM):

A. The membrane potential would become more negative

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

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

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

East. An activity 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 (Note: for this axon normal [One thousand]_o = 20 mM and normal [M]_i = 400 mM):

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

The normal value of extracellular potassium is xx 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 inverse from 400 mM to 200 mM, then the potassium equilibrium potential as determined by the Nernst equation, will equal about -60 mV. Since the membrane potential is commonly -threescore mV and is dependent, to a large extent, on Due east_K, the change in the potassium concentration and hence Eastward₁₀₀₀ would make the membrane potential more positive, not more than negative.

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

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

D. The Thou⁺ equilibrium potential would be near -xviii mV

E. An action potential would be initiated

If the concentration of M⁺ in the cytoplasm of an invertebrate axon is inverse to a new value of 200 mM (Notation: for this axon normal [1000]_o = 20 mM and normal [Yard]_i = 400 mM):

A. The membrane potential would get more than negative

B. The K⁺ equilibrium potential would change by 60 mV This answer is INCORRECT. The potassium equilibrium potential would not alter by 60 mV. The potassium concentration was changed but from 400 mM to 200 mM. One can use the Nernst equation to determine the verbal value that the equilibrium potential would change by. Information technology was initially well-nigh -75 mV and as a result of the change in concentration, the equilibrium potential becomes -lx mV. Thus, the equilibrium potential does not change by 60 mV, information technology changes by about 15 mV.

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

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

Eastward. 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 [One thousand]_o = 20 mM and normal [K]_i = 400 mM):

A. The membrane potential would become more negative

B. The 1000⁺ equilibrium potential would alter by 60 mV

C. The K⁺ equilibrium potential would be about -sixty 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 exist about -xviii mV

E. An activeness potential would exist 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 = twenty mM and normal [K]_i = 400 mM):

A. The membrane potential would become more negative

B. The 1000⁺ equilibrium potential would change by sixty mV

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

D. The Thou⁺ equilibrium potential would be well-nigh -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 exist calculated if you substituted [Grand]_o = 200 and [K]_i= 400 into the Nernst equation.

E. An action potential would exist initiated

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

A. The membrane potential would become more than negative

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

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

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

E. An action potential would be initiated This respond is Incorrect. The membrane potential would not depolarize sufficiently to reach threshold (about -45 mV).

Donations to Neuroscience Online will help fund development of new features and content.

murtaghablencesee.blogspot.com

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

Share this post