GENERATION OF ACTION POTENTIALS

1.         An action potential is a rapid, transient, self-propagating electrical excitation in the plasma membrane of a cell such as a neuron or muscle cell resulting from a series of rapid reversal in the membrane potentials.

2.         Each action potential begins with a sudden change from the normal resting negative potential to a positive membrane potential (depolarization) and ends with an almost equally fast change back to the negative potential.

3.         Hence, an action potential is a rapid, reversal depolarization of a neuron’s membrane near the point of stimulation that generates a nerve impulse.

4.         It allows for nerve impulses to be transmitted for long distances along the axons of neurons.

5.            Factors

a.            presence of  voltage-gated ion Na and K channels in the plasma membranes of neurons.

b.         changes in the membrane potential of the neuron (incoming stimuli) that will activate the ion channels, resulting in the reversal of membrane potential.

c.         the electrochemical gradients of Na and K ions that exist across the membrane.

6.         This is an all-or-none event because the magnitude of the action potential is independent of the strength of the incoming stimulus. An action potential will be generated as long the threshold potential is reached.

7.            Composition of Voltage-gated channels

a.         the all-or-none characteristic of the action potential is based on the behaviour of regulatory gates that cover the ion-specific channels present in electrically excitable cells.

b.         in nerve cells, there are two channels: one specific for Na ions and the other specific for K ions.

c.         General composition: the ion-specific channels are transmembrane integral proteins that form an aqueous pore through the membrane; composing of several thousand amino acids.  

d.         Each channel contains:

i.            Selectivity filters: give the channel its ion-selective characteristics.

ii.          Gating particles: open and close the channels.

e.            Regulation of Na channel:

i.          the Na channel has two gating particles.

ii.          an activation gate covers the extracellular side of the Na channel.

iii.         an inactivation gate covers the intracellular side of the Na channel.

iv.         both the activation and inactivation gates must be open for Na to flow through the Na channel.

v.         when the activation gate is open, the channel is activated.

vi.         when the inactivation gate is closed, the channel is inactivated.

vii.        at resting membrane potential, the activation gate is closed while the inactivation gate is open, hence preventing the influx of Na ions.

f.            Regulation of K channels:

i.          has only one gating particle; no inactivation gate.

ii.          the activation gate covers the the extracellular side of the K channels.

iii.         when the activation gate is open, the K channel is activated.

g.            Summary: Characteristics of gates and their response to depolarization

8.         Phases of an Action Potential

a.         Resting stage:

i.          this is the resting membrane potential before the action potential occur

ii.          the membrane potential is polarized during this stage because of the large negative membrane potential that is present.

b.            Threshold:

i.          a slight increase in the resting membrane potential leads to increased K efflux (through the K leak channels) and Cl influx, restoring the membrane potentials.

ii.          when the depolarization exceeds 7mV, the voltage-gated Na channels starts to open at an increased rate by a sudden conformational change in the activation gate, flipping it to the ‘open’ position.

iii.         this allows rapid inflow of Na ions, which causes still further rise of the membrane potential thus opening more voltage-gated sodium channels and allowing more influx of Na ions.

iv.         rapid depolarization occurs if the membrane potential is increased to a critical level, known as the threshold potential (a sudden increase in membrane potential of 15 to 30mV is required).

v.         this occurs when the number of K ions entering the fiber becomes greater than the number of K ions leaving the fiber.

c.            Depolarization (upshoot):

i.          once the threshold potential is reached, the remainder of the depolarization is rapid and spontaneous.

ii.          this is caused by the positive feedback vicious cycle which will continue until all the voltage gated Na channels have become activated.

iii.         rapid flow of Na ions into the cell causes a sudden rapid increase in the membrane potential of the cell.     

d.            Overshoot:

i.          in large nerve fibers, the membrane potential ‘overshoots’ beyond the zero level and becomes positive.

ii.          the portion of the action potential during which the membrane is positive is the overshoot.

iii.         the peak of the action potential is the overshoot potential.

e.            Repolarization (downstroke):

i.          the same increase in voltage also closes the inactivation gate of the Na channels which occurs a few 10,000ths of a second after the activation gate opens.

ii.          at the same time, the activation gate of the K channel opens completely (the increase in membrane potential causes a slow conformational opening of the gate).

iii.         this causes the rapid efflux of K ions down their electrochemical gradient.

iv.         the stoppage of influx of Na ions and the efflux of K ions subsequently leads to the rapid decrease in the membrane potential of the neuron.

f.            Undershoot:

i.          the membrane potential becomes more negative than its resting value at the end of the action potential.

ii.          this is the hyperpolarization phase of the action potential.

iii.         it is caused by the K activation gates, which remain open for several miliseconds after the repolarization process is complete.

iv.         this allows K ions to diffuse out of the nerve fiber, leaving an extra deficit of negative ions on the inside.

g.            Recovery Phase:

i.          Na channel: the negative membrane potential causes the activation gate to return to the ‘closed’ state and the inactivation gate to the ‘open state’.

ii.          K channel: closing of the activation gate stems the efflux of K ions out of the cell.

h.            Comparison of Phases of Action Potential in a typical neuron:

9.         Re-establishing Na and K ionic gradients after action potentials

a.         the transmission of each impulse along the nerve fiber reduces infinitesimally the concentration differences of Na and K between the inside and outside of the membrane.

b.         this is due to the influx of Na ions during depolarization and efflux of K ions during repolarization.

c.         for a single action potential, this effect is so minute that it cannot be measured.

d.            however, with time, it becomes necessary to re-establish the Na-K membrane concentration difference.

e.         this is achieved by the action of the sodium-potassium pump.

f.          the degree of activity of the pump is strongly stimulated when excess Na ions accumulate inside the cell membrane.

g.            therefore, the recharging process of the nerve fiber is rapidly set into motion whenever the concentration gradients of Na and K is dissipated.

h.         this pump requires energy for operation and in nerve cells, it accounts for 70% of metabolism.

10.       The time-dependent nature of the gating particles is essential for the production of the all-or-none action potential:

a.         if the inactivation gates close as fast as the activation gates does, there will be no influx of Na ions.

b.         if the K gates open as fast as the activation gates of Na channels, depolarization will not occur.

11.            Refractory Period

a.         a new action potential cannot occur in an excitable fiber as long as the membrane is still depolarized from the preceding action potential.

b.         the only condition that will re-open them is for the membrane potential to return either to or almost to the original resting membrane potential level.

c.            absolute refractory period:

i.          period during which a second action potential cannot be elicited, even with a strong stimulus.

ii.            corresponds to the period from the time the firing level is reached until repolarization is one-third complete.

iii.         this period for large myelinated nerve fibers is 1/2500 second; hence such a fiber can carry a maximum of about 2500 impulses a second.

iv.         during the upstroke, a second action potential cannot occur because the activation gates are opening as fast as possible.

v.         during the early portion of the downstroke, an action potential cannot occur because the inactivation gates of the Na channels are still closed.

d.         a nerve impulse can only flow in one direction as the action potential cannot be generated at the refractory region preceding it.

e.         relative refractory period:

i.          during this interval, a second action potential can be elicited if the stimulus is sufficent.

ii.          it begins from the absolute refractory period and ends at the start of the after-depolarization.

iii.         the stimulus must be greater than normal because some Na channels are still inactivated and more K channels than normal are still open.

iv.         for the same reason, the action potential elicited during this interval will have a lower upstroke velocity and a lower overshoot potential.

12.            Membrane as a battery

a.         The membrane is a powerful battery: a substantial potential difference of 70mV exists across a minute cell that we cannot even see with our naked eyes

b.         The membrane potential is an indication of the activity of the cell; active cells will have a higher potential: compare the potential of blood cells (-10mV) with neurons (-90mV).

c.         The excitability or sensitivity of a neuron is defined as the ease at which it depolarizes, or the rate of increase of potential till it reaches the threshold value.

d.         The K⁺ leak channels provide the cell a mechanism to control the sensitivity of the membrane to depolarization.

e.         The number of K⁺ leak channels that are open at any time is dependent on the potential inside the cell; at a more negative potential, more channels will close to stem the efflux of positive charges.

f.          At a higher membrane potential, more K⁺ leak channels are open; entry of Na ions will increase the potential at a slower rate due to the ‘leakage’ of positive charges – hence it is less excitable.

g.         At a lower membrane potential, less K⁺ leak channels are open; entry of Na ions will increase the potential at a greater rate as due to less ‘leakage’, positive charges are now able to accumulate quickly inside the cell – the membrane is more excitable.

h.         The membrane reaches its maximum excitability at the potential at which all K⁺ leak channels are closed, after which its excitability decreases as a greater concentration of positive ions is needed to increase the larger negative potential to threshold value.

i.          The Na channels are dependent on the voltage of the cell too; if membrane potential is not negative, the inactivation gate of the channel remains closed and the cell is in absolute refractory period as no amount of stimulus will excite it to threshold.

j.          At a lower potential, a high concentration of Na ions may increase the potential to threshold: with the sudden influx of a large number of positive charges, there is not enough time for the K ions to leave the cell through the K⁺ leak channels.

l.          As such, the positive charges are able to accumulate inside the cell, building the potential up until it reaches threshold – this is the relative refractory period.

m.        Hence, whether the membrane reaches threshold potential is also determined by the rate of increase of potential, or influx of Na ions.

13.       Effects on membrane due to extracellular concentration of ions

a.         The cell is very sensitive to changes in external K ion concentration as it is very permeable to it; hence a slight increase in its concentration will have a drastic result on the cell’s resting membrane potential.

b.         An increase in external K ion concentration will reduce the diffusion of K out of the cell causing an increase in the resting membrane potential.

c.         When the external K concentration is increased from 5mM to 8mM, the increase in membrane potential (due to less K ions diffusing out) will cause the inactivation gate of the Na channel to remain close, effectively preventing any depolarization.

d.         This cause muscle weakness as it cannot be stimulated; in the heart, the consequences are more serious with the heart muscles stop beating, leading to death within a few minutes.

e.         Hence, additional care should be taken when administering K ions intravenously to patients.

14.       Ratio of Na/K conductance

a.         during the resting state, the conductance of K ions is 50 to 100 times as great as the conductance for Na ions due to the greater leakage of K than Na ions through the leak channels.

b.         during the beginning of the action potential , the Na/K ratio increases a thousand fold due to the opening of the activation gates of Na channels, causing a rapid influx of Na ions.

c.         during the repolarization phase, Na/K ratio decreases rapidly due to the closing of inactivation gate of Na channels and the opening of activation gate of K channel, resulting in the efflux of K ions.

15.       Role of other ions during Action potential

a.            impermanent negative ions in Axon: as they can’t live the axon, deficit of positive ions will leave behind an excess of negative ions which confer negative charge in the fiber.

b.            Calcium ions:

i.          Ca pump pumps Ca ions from interior to exterior of cell membrane.

ii.            voltage-gated calcium channels which are slightly permeable to Na and Ca ions.

iii.         when they open, both Na and Ca ions flow to interior of fiber.

iv.         slow channels: requiring 10 to 20 times as long for activation as Na channels.

v.            numerous in both cardiac and smooth muscle.

vi.         in some types of smooth muscle, Na channels are absent and action potentials are caused by activation of slow Ca channels.

vii.            increased permeability of Na channels when there is a deficit of Ca ions.

16.            Stimulus Intensity is coded by the Frequency of Action Potentials

a.         A stimulus in the formed of a graded potential reaching the trigger zone will trigger a burst of action potentials.

b.         If the graded potential increases in strength, the frequency of action potentials fired increases.

c.         The amount of transmitter released at the axon terminal is directly related to the total number of action potentials that arrive at the terminal per unit time.

d.         An increase in signal strength will increase the neurotransmitter output, which in turn changes the magnitude of the graded potential in the postsynaptic cell.

17.       Role of Sodium-Potassium Pump

a.         In a single action potential, very few ions move across the membrane.

b.         The relative Na⁺ and K⁺ concentrations remain essentially unchanged.

c.         The tiny amounts of Na⁺ and K⁺ that cross the membrane during action potentials do not disrupt the normal concentration gradient until a thousand or more action potentials have been fired.

d.         The concentration gradients of Na⁺ and K⁺ in all cells is maintained over time by the sodium-potassium pump which plays no direct role in the action potential.

e.         Since the number of Na⁺ that enters the cell with each action potential is very small, a neuron poisoned with the pump inhibitor ouabain will continue to fire action potentials for an extended time before the Na⁺ concentration gradient changes.