After an Action Potential

Affiliate ii: Ionic Mechanisms and Activity Potentials

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ii.one Ionic Mechanisms of Activity Potentials

Voltage-Dependent Conductances

Na⁺ is critical for the action potential in nervus cells. As shown in Effigy 2.1, action potentials are repeatedly initiated as the extracellular concentration of Na⁺ is modified. As the concentration of sodium in the extracellular solution is reduced, the action potentials become smaller.

Effigy 2.2 shows the directly line predicted past the Nernst equation (assuming the membrane was exclusively permeable to Na⁺). There is a good fit between the data and the values predicted by a membrane that is exclusively permeable to Na⁺. The experiment gives experimental support to the notion that at the peak of the activity potential, the membrane becomes highly permeable to sodium.

Nonetheless, there are some deviations betwixt what is measured and what is predicted by the Nernst equation. Why? One reason for the deviation is the continued 1000⁺ permeability. If there is continued Chiliad⁺ permeability, the membrane potential will never reach its ideal value (the sodium equilibrium potential) because the diffusion of K⁺ ions tends to make the cell negative. This betoken can be understood with the aid of the GHK equation.

An action potential is bounded by a region bordered on ane farthermost by the K⁺ equilibrium potential (-75 mV) and on the other extreme past the Na⁺ equilibrium potential (+55 mV). The resting potential is -60 mV. Note that the resting potential is not equal to the K⁺ equilibrium potential because, equally discussed previously, in that location is a small resting Na⁺ permeability that makes the cell slightly more than positive than EK. In principle, whatever point along the trajectory of action potential can be obtained simply by varying alpha in the GHK equation. If blastoff is very large, the Na⁺ terms dominate, and co-ordinate to the GHK equation, the membrane potential will move towards the Na⁺ equilibrium potential. The height of the activeness potentials approaches only does non quite accomplish ENa, because the membrane retains its permeability to K⁺.

How is it possible for a jail cell to initially take a resting potential of -sixty mV and then, in response to some stimulus (a brief transient depolarization which reaches threshold), change in less than ane millisecond to having a potential of approximately +40 mV? In the 1950's, Hodgkin and Huxley, two British neurobiologists, provided a hypothesis for this transition. They suggested that the backdrop of some Na⁺ channels in nerve cells (and muscle cells) were unique in that these channels were usually closed but could be opened by a depolarization. This simple hypothesis of voltage-dependent Na⁺ channels goes a long way toward explaining the initiation of the action potential. Suppose a small depolarization causes some of the Na⁺ channels to open. The key point is that the increase in Na⁺ permeability would produce a greater depolarization, which volition lead to an even greater number of Na⁺ channels opening and the membrane potential becoming even more depolarized. Once some disquisitional level is reached a positive feedback or regenerative bicycle will exist initiated, causing the membrane potential to depolarize chop-chop from -60 mV to a value approaching the Na⁺ equilibrium potential.

In order to test the Na⁺ hypothesis for the initiation of the action potential, information technology is necessary to stabilize the membrane potential at a number of dissimilar levels and measure the permeability at those potentials. An electronic device known as a voltage-clamp amplifier can "clamp" or stabilize the membrane potential to any desired level and measure out the resultant current required for that stabilization. The corporeality of current necessary to stabilize the potential can then exist used to quantify membrane permeability. Hodgkin and Huxley clamped the membrane potential to diverse levels and measured the changes in Na⁺ conductances (an electrical term for permeability, which for the present discussion can be used interchangeably). The more than the cell is depolarized, the greater is the Na⁺ conductance. Thus, the experiment provided back up for the existence of voltage-dependent Na⁺ channels.

2.2 Na⁺ Inactivation

Figure ii.4 likewise indicates an important property of the voltage-dependent Na⁺ channels. Notation that the permeability increases chop-chop and then, despite the fact that the membrane potential is clamped, the permeability decays back to its initial level. This phenomenon is chosen inactivation. The Na⁺ channels begin to shut, fifty-fifty in the continued presence of the depolarization. Inactivation contributes to the repolarization of the activity potential. Even so, inactivation is non enough past itself to account fully for the repolarization.

2.3 Voltage-Dependent K⁺ Conductance

In addition to voltage-dependent changes in Na⁺ permeability, there are voltage-dependent changes in K⁺ permeability. These changes can be measured with the voltage-clamp technique too. The effigy shown to to a higher place indicates the changes in K⁺ conductance also as the Na⁺ conductance. There are ii important points.

Commencement, just as at that place are channels in the membrane that are permeable to Na⁺ that are usually closed but then open in response to a voltage, at that place are also channels in the membrane that are selectively permeable to Chiliad⁺. These Grand⁺ channels are normally closed, but open in response to depolarization.

2nd, a major difference between the changes in the Chiliad⁺ channels and the changes in the Na⁺ channels is that the One thousand⁺ channels are slower to activate or open. (Some M⁺ channels also do non inactivate.)  Note that the return of the conductance at the end of the pulse is not the process of inactivation.  With the removal of the pulse, the activated channels are deactivated.

2.4 Sequence of Conductance Changes Underlying the Nerve Action Potential

Some initial depolarization (e.thousand., a synaptic potential) will brainstorm to open up the Na⁺ channels. The increase in the Na⁺ influx leads to a farther depolarization.

A positive feedback cycle apace moves the membrane potential toward its peak value, which is close but non equal to the Na⁺ equilibrium potential. Two processes which contribute to repolarization at the pinnacle of the activity potential are and so engaged. Get-go, the Na⁺ conductance starts to decline due to inactivation. Every bit the Na⁺ conductance decreases, another feedback cycle is initiated, but this 1 is a downward cycle. Sodium conductance decreases, the membrane potential begins to repolarize, and the Na⁺ channels that are open up and not nonetheless inactivated are deactivated and close. 2d, the One thousand⁺ conductance increases. Initially, there is very piddling change in the K⁺ conductance because these channels are slow to open, but past the peak of the activeness potential, the Grand⁺ conductance begins to increase significantly and a second force contributes to repolarization. As the effect of these 2 forces, the membrane potential rapidly returns to the resting potential. At the fourth dimension it reaches -60 mV, the Na⁺ conductance has returned to its initial value. Nevertheless, the membrane potential becomes more negative (the undershoot or the hyperpolarizing afterpotential).

The key to understanding the hyperpolarizing afterpotential is in the slowness of the Chiliad⁺ channels. But equally the K⁺ channels are slow to open (activate), they are also slow to shut (deactivate). Once the membrane potential starts to repolarize, the K⁺ channels begin to shut because they sense the voltage. Yet, even though the membrane potential has returned to -threescore mV, some of the voltage-dependent K⁺ channels remain open. Thus, the membrane potential volition be more negative than it was initially. Eventually, these 1000⁺ channels close, and the membrane potential returns to -sixty mV.

Why does the cell go through these elaborate mechanisms to generate an action potential with a short duration? Recall how information is coded in the nervous organization. If the action potential was about one msec in duration, the frequency of action potentials could change from one time a second to a chiliad a 2d. Therefore, short action potentials provide the nerve cell with the potential for a large dynamic range of signaling.

two.v Pharmacology of the Voltage-Dependent Membrane Channels

Some chemical agents can selectively block voltage-dependent membrane channels. Tetrodotoxin (TTX), which comes from the Japanese puffer fish, blocks the voltage-dependent changes in Na⁺ permeability, simply has no effect on the voltage-dependent changes in K⁺ permeability. This observation indicates that the Na⁺ and K⁺ channels are unique; 1 of these tin be selectively blocked and not impact the other. Some other agent, tetraethylammonium (TEA), has no effect on the voltage-dependent changes in Na⁺ permeability, but it completely abolishes the voltage-dependent changes in Thou⁺ permeability.

Use these ii agents (TTX and TEA) to exam your understanding of the ionic mechanisms of the action potential. What effect would treating an axon with TTX accept on an activity potential? An action potential would not occur because an action potential in an axon cannot be initiated without voltage-dependent Na⁺ channels. How would TEA affect the action potential? It would be longer and would non have an undershoot.

In the presence of TEA the initial phase of the action potential is identical, but note that it is much longer and does not take an after-hyperpolarization. There is a repolarization stage, but now the repolarization is due to the process of Na⁺ inactivation alone. Note that in the presence of TEA, there is no change in the resting potential. The channels in the membrane that endow the cell with the resting potential are different from the ones that are opened by voltage. They are not blocked past TEA. TEA merely affects the voltage-dependent changes in K⁺ permeability.

2.half dozen Pumps and Leaks

Information technology is easy to receive the impression that there is a "gush" of Na⁺ that comes into the jail cell with each action potential. Although, at that place is some influx of Na⁺, it is minute compared to the intracellular concentration of Na⁺. The influx is bereft to make any noticeable change in the intracellular concentration of Na⁺. Therefore, the Na⁺ equilibrium potential does non modify during or subsequently an action potential. For any individual action potential, the amount of Na⁺ that comes into the cell and the corporeality of Grand⁺ that leaves are insignificant and have no outcome on the majority concentrations. All the same, without some compensatory mechanism, over the long-term (many spikes), Na⁺ influx and Thou⁺ efflux would begin to alter the concentrations and the resultant Na⁺ and K⁺ equilibrium potentials. The Na⁺-K⁺ pumps in nerve cells provide for the long-term maintenance of these concentration gradients. They keep the intracellular concentrations of M⁺ high and the Na⁺ low, and thereby maintain the Na⁺ equilibrium potential and the Thou⁺ equilibrium potential. The pumps are necessary for the long-term maintenance of the "batteries" so that resting potentials and action potentials tin exist supported.

2.7 Types of Membrane Channels

So far, two basic classes of channels, voltage-dependent or voltage-gated channels and voltage-contained channels, accept been considered. Voltage-dependent channels can be further divided based on their permeation properties into voltage-dependent Na⁺ channels and voltage-dependent Thousand⁺ channels. At that place are also voltage-dependent Ca²⁺ channels (see chapter on Synaptic Transmission). Indeed, there are multiple types of Ca^two+ channels and voltage-dependent One thousand⁺ channels. Nevertheless, all these channels are conceptually like. They are membrane channels that are normally closed and as a result of changes in potential, the channel (pore) is opened. The amino acid sequence of these channels is known in considerable detail and specific amino acid sequences take been related to specific aspects of channel function (eastward.g., ion selectivity, voltage gating, inactivation). A third major channel class, the transmitter-gated or ligand-gated channels, will be described afterward.

two.8 Channelopathies

Ion channel mutations have been identified as a possible cause of a wide multifariousness of inherited disorders.  Several disorders  involving muscle membrane excitability have been  associated with mutations  in calcium, sodium and chloride channels as well every bit acetylcholine receptors and have been labeled 'channelopathies'. It is possible that movement disorders, epilepsy and headache, too as other rare inherited diseases, might be  linked to ion channels. The manifestations and mechanisms of channelopathies affecting neurons are reviewed in Kullman, 2002.  The being of channelopathies may provide insights into the variety of cellular mechanisms  associated with the misfunctioning of neuronal circuits.

2.9 Absolute and Relative Refractory Periods

The absolute refractory menstruum is a menstruation of fourth dimension subsequently the initiation of ane action potential when information technology is impossible to initiate a second action potential no thing how much the cell is depolarized. The relative refractory period is a menses after one activity potential is initiated when information technology is possible to initiate a 2nd activity potential, but only with a greater depolarization than was necessary to initiate the beginning. The relative refractory catamenia can be understood at least in part by the hyperpolarizing afterpotential. Presume that an initial stimulus depolarized a cell from -lx mV to -45 mV in lodge to accomplish threshold so consider delivering the aforementioned xv-mV stimulus sometime during the afterward-hyperpolarization. The stimulus would again depolarize the cell but the depolarization would exist below threshold and bereft to trigger an activeness potential. If the stimulus was fabricated larger, however, such that information technology again was capable of depolarizing the cell to threshold (-45 mV), an action potential could exist initiated.

The absolute refractory menstruum tin can be explained by the dynamics of the procedure of Na⁺-inactivation, the features of which are illustrated in Figure 2.ten. Here, two voltage clench pulses are delivered. The first pulse produces a voltage-dependent increment in the Na⁺ permeability which then undergoes the process of inactivation. If the 2 pulses are separated sufficiently in time, the 2nd pulse produces a change in the Na⁺ conductance, which is identical to the first pulse. However, if the second pulse comes soon afterward the first pulse, and then the change in Na⁺ conductance produced by the second pulse is less than that produced past the starting time. Indeed, if the 2d pulse occurs immediately after the offset pulse, the second pulse produces no change in the Na⁺ conductance. Therefore, when the Na⁺ channels open up and spontaneously inactivate, it takes time (several msec) for them to recover from that inactivation. This process of recovery from inactivation underlies the absolute refractory catamenia. During an activeness potential the Na⁺ channels open and so they become inactivated. Therefore, if a second stimulus is delivered soon after the one that initiated the start spike, there will be few Na⁺ channels available to be opened past the second stimulus considering they have been inactivated by the first action potential. The accented refractory menstruum seems like a relatively unimportant phenomenon, merely actually information technology is essential to ensure unidirectional propagation of activeness potentials along axons.

2.10 Activeness Potential Laboratory

Click here to get to the interactive Action Potential Laboratory to examine the ways in which the action potential is effected by changes in the Na⁺ conductance, Chiliad⁺ conductance and equilibrium potentials for Na⁺ and Thousand⁺.

Action Potential Laboratory

Test Your Cognition

• Question ane
• A
• B
• C
• D
• Eastward

Drug X, when practical to a nerve axon, results in both a gradual subtract in the aamplitude of individual activeness potentials and a depolarization of the resting potential, both of which develop over a period of several hours. The drug is most likely:

A. Blocking the voltage-dependent Na⁺ permeability

B. Blocking the voltage-dependent K⁺ permeability

C. Blocking the (Na⁺ -G⁺) pump

D. Blocking the process of Na⁺ inactivation

Due east. Increasing the rate at which voltage-dependent changes in Yard⁺ permeability occur

Drug X, when applied to a nervus axon, results in both a gradual decrease in the amplitude of individual action potentials and a depolarization of the resting potential, both of which develop over a catamenia of several hours. The drug is about likely:

A. Blocking the voltage-dependent Na⁺ permeability This answer is Incorrect. Blocking the voltage-dependent sodium permeability would decrease the amplitude of the action potential, simply it would probably practice nothing to the resting potential. If information technology did anything to the resting potential, it would pb to a hyperpolarization, not a depolarization as is the case with drug X.

B. Blocking the voltage-dependent K⁺ permeability

C. Blocking the (Na⁺ -Thou⁺) pump

D. Blocking the process of Na⁺ inactivation

Due east. Increasing the charge per unit at which voltage-dependent changes in One thousand⁺ permeability occur

Drug 10, when applied to a nerve axon, results in both a gradual decrease in the amplitude of private action potentials and a depolarization of the resting potential, both of which develop over a menstruum of several hours. The drug is almost probable:

A. Blocking the voltage-dependent Na⁺ permeability

B. Blocking the voltage-dependent K⁺ permeability This answer is INCORRECT. The voltage-dependent potassium channels are generally not activated unless the membrane potential is fairly depolarized. Thus, blocking the voltage-dependent potassium permeability would have very fiddling, if whatsoever, effect on the resting potential. Also, blocking the voltage-dependent potassium permeability would take a tendency to perhaps increase the aamplitude (and elapsing) of the action potential rather than decreasing it.

C. Blocking the (Na⁺ -1000⁺) pump

D. Blocking the process of Na⁺ inactivation

Due east. Increasing the rate at which voltage-dependent changes in K⁺ permeability occur

Drug X, when practical to a nerve axon, results in both a gradual subtract in the aamplitude of private activeness potentials and a depolarization of the resting potential, both of which develop over a period of several hours. The drug is most probable:

A. Blocking the voltage-dependent Na⁺ permeability

B. Blocking the voltage-dependent Grand⁺ permeability

C. Blocking the (Na⁺ -Grand⁺) pump This answer is Correct! Blocking the sodium potassium pump leads to a gradual influx of sodium into the prison cell, and efflux of potassium out of the cell. These changes in concentration lead to a change in the equilibrium potential for potassium, as well equally for sodium. Every bit the equilibrium potential for potassium becomes more positive, the resting potential becomes more positive (i.due east., more than depolarized). Because of the sodium influx into the cell, the equilibrium potential for sodium is changed, namely, it is less positive. And considering the peak amplitude of the action potential is dependent upon the value of the sodium equilibrium potential, the acme aamplitude of the action potential would also decrease over time.

D. Blocking the process of Na⁺ inactivation

E. Increasing the charge per unit at which voltage-dependent changes in M⁺ permeability occur

Drug X, when practical to a nerve axon, results in both a gradual decrease in the amplitude of individual activeness potentials and a depolarization of the resting potential, both of which develop over a period of several hours. The drug is virtually likely:

A. Blocking the voltage-dependent Na⁺ permeability

B. Blocking the voltage-dependent K⁺ permeability

C. Blocking the (Na⁺ -K⁺) pump

D. Blocking the process of Na⁺ inactivation This answer is INCORRECT.

Blocking the process of sodium inactivation would bear upon primarily the repolarization stage of the action potential. In that location would be no modify in the resting potential. The only event would be that the activeness potential would have a greater duration than normal.

E. Increasing the rate at which voltage-dependent changes in Yard⁺ permeability occur

Drug 10, when practical to a nervus axon, results in both a gradual decrease in the amplitude of individual action potentials and a depolarization of the resting potential, both of which develop over a period of several hours. The drug is most likely:

A. Blocking the voltage-dependent Na⁺ permeability

B. Blocking the voltage-dependent Thou⁺ permeability

C. Blocking the (Na⁺ -K⁺) pump

D. Blocking the process of Na⁺ inactivation

E. Increasing the rate at which voltage-dependent changes in K⁺ permeability occur This answer is Incorrect.

Increasing the charge per unit in which voltage-dependent changes in potassium permeability occur would only bear on the duration of the action potential. Perhaps if there was an increment in the rate, at that place might also exist a slight decrease in the amplitude of the action potential, just at that place would be no modify in the resting potential.

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Source: https://nba.uth.tmc.edu/neuroscience/m/s1/chapter02.html

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