This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Action Potentials
Communication between neurons in the nervous system often requires that messages travel long distances down axons without loss of information. Neurons use action potentials to do this. (1) The action potential is a brief, transient change in membrane potential that sweeps down the membrane of an axon. (2) The action potential is triggered by a depolarization of the membrane above action potential threshold. If a depolarization is not large enough to reach threshold, the neuron will not fire an action potential. (3) The action potential starts with a rapid depolarization of the membrane potential. This rising phase reaches a peak, and is then followed by a repolarization (falling phase) back to resting membrane potential and then an after hyperpolarization (undershoot) below rest. Page 5-1 (4) Following the firing of an action potential, there is a period called the absolute refractory period during which it is not possible to fire an action potential. This absolute refractory period is followed by a relative refractory period during which an action potential can occur, but only if the membrane is depolarized to a much higher level than the normal threshold. (5) All of these events can be understood by examining the behavior of ion channels and the flow of ions across the cell membrane. What causes the rising phase of an action potential? (1) Let's revisit the GHK equation that determines the membrane potential (at room temperature): Vm = 58 log ( pK[K]o + pNa[Na]o + pCl[Cl]i ) / ( pK[K]i + pNa[Na]i + pCl[Cl]o ) What happens if the permeability to potassium ions increases dramatically? What happens if the permeability to sodium ions increases dramatically? What happens if the permeability to chloride ions increases dramatically? Page 5-2 (2) What's the equilibrium potential for these three ions in neuron? Nernst equation:
Extracellular Intracellular (3) When scientists first thought about what ions might be responsible for the action potential, they immediately proposed that the rising phase might be due to sodium entry into the cell. Why was sodium an obvious choice? Na+ K+ 117 mM 3 mM 30 mM 90 mM ClA- 120 mM 0 mM 4 mM 116 mM Page 5-3 Experiments to test whether sodium really is driving the rising phase of the action potential Action potentials were recorded in a squid giant axon bathed in normal sea water (similar to the squid extracellular fluid) and then in solutions containing lower concentrations of sodium. (1) What will lowering external sodium concentrations do to the driving force on sodium ions? (2) If sodium is in fact the ion responsible for the rising phase of the action potential, how will this affect the action potential? What is responsible for the repolarization of the membrane (the falling phase of the action potential)? (1) The simplest possibility would be that this is due to the sodium channels closing, so that membrane potential would return to rest. (2) It turns out that sodium channels do in fact inactivate during the falling phase of the action potential, but this closing of the sodium channels is not enough to account for the very rapid fall in membrane potential. Simply closing sodium channels results in a prolonged depolarization that only gradually returns to resting membrane potential. (3) Closing of sodium channels also cannot explain the after hyperpolarization. Therefore at least one other ion must be involved. Potassium is the ion that is responsible both for the speed of the falling phase, and for the after hyperpolariztion. How to determine exactly what the inward sodium current and the outward potassium current are at different times during the action potential? Two problems must be solved. (1) Current flowing across a membrane will change the membrane voltage (by Ohm's law) which in turn will affect conductances through the voltage gated channels that underlie the action potential. This problem can be solved by using the voltage clamp technique, which holds membrane voltage at a fixed level allowing currents to be measured accurately. When a cell is clamped at a depolarized voltage above action potential firing threshold, an instantaneous outward current is seen, corresponding to charging up the capacitor of the cell membrane. (Intracellular positive ions go to the membrane when Vm changes from -65 to -9) Next, a fast inward current is seen, followed by a slow outward current. Page 5-4 Page 5-5 (2) The second problem is to separate the sodium current from the potassium current. This can be done by blocking one or the other current pharmacologically. (A) TTX (tetrodotoxin) which comes from the puffer fish, selectively blocks the voltage-gated sodium channel, leaving only the potassium current. (B) TEA (tetraethylammonium) selectively blocks the voltage-gated potassium channel, leaving only the sodium current. Currents carried by sodium and potassium in the action potential To understand more about how the action potential works, it is useful to look at IV curves for the peak sodium and potassium currents obtained by clamping the axonal membrane at different voltages. (1) If the channels underlying these currents were not voltage gated channels, what would you expect the IV curves to look like? I V (2) In fact, however, both the sodium and the potassium channels responsible for action potentials are voltage gated. Both channel types increase their mean open time as voltage becomes less negative (more positive). Therefore, the cells total conductance to the ions increases as voltage increases. What is the effect of this conductance change on an IV curve? The IV curve increases slope (increasing conductance) with increasing voltage. This is what we saw for the potassium current. (3) The behavior of the sodium current is much more complex. Between resting membrane potential and about +10 mV, the current increases rapidly (negative slope conductance). It then decreases rapidly to about +50mV and then becomes an outward current. Page 5-6 This behavior is due to the fact that as the membrane depolarizes, the conductance of the cell to sodium is increasing as more channels open, but the driving force on the sodium ions is decreasing. Between rest and +10mV the increase in sodium current is due to the increase in conductance having a greater effect than the decrease in driving force. Page 5-7 (4) Further complexity is added to our understanding of the action potential if we look at the effects of a prior step (pre-pulse, 30 ms) in membrane potential on the sodium current. This behavior is due to inactivation of sodium channels with prolonged depolarization. The more the membrane is depolarized, the more sodium channels open, but then inactivate. How many sodium channels are inactivated at rest? Time courses of opening sodium and potassium channels (1)When an axonal membrane is depolarized more, more voltage gated sodium channels open, and also more voltage gated potassium channels open. The conductance changes with voltage for the two channel types are very similar. Page 5-8 (2) However, the change in conductance with time is very different for the two channel types: Voltage gated sodium channels open immediately following depolarization, but then inactivate after less than a millisecond. Voltage gated potassium channels, on the other hand, open after a delay, and do not inactivate (at least in the squid-- there are voltage gated potassium channels in other species that do inactivate). Page 5-9 Molecular mechanism of sodium channel inactivation Voltage gated channel inactivation is caused by a special feature of the molecular structure of the channel, called the ball and chain. (1) When the channel is closed, the balls float freely. (2) When the channel is opened, the balls are attracted to the open pore of the channel, and one will enter and block the pore. A number of experiments support this model. For example: (1)When the balls are removed either by injecting pronase into the axon to enzymatically remove them, or by site directed mutagenesis, no inactivation of the channel occurs. (2) If, after removing the balls, a synthetic peptide consisting of the same amino acid sequence found in the balls is added to the inside of the axon, inactivation is restored. Page 5-10 How does what we have learned about voltage gated sodium and potassium channels explain the behavior of an action potential? (1) Threshold (a) Remember that at resting membrane potential the inward sodium current is equal to the outward potassium current. (b) If no voltage gated channels open, depolarization means that the potassium current will increase and the sodium current will decrease. Why is this? (c) In an axon, however, when the membrane depolarizes, voltage gated sodium channels open faster than voltage gated potassium channels. This increases the sodium current. (d) When the cell is depolarized, and enough voltage gated sodium channels have opened that the sodium and potassium currents are again equal, the axon has reached threshold. (e) If the axon depolarizes any more, the sodium current will be greater than the potassium current (due to opening of more sodium channels) and the process becomes a positive feedback loop with more sodium entering causing more depolarization, causing more sodium channels to open. (2) The rising phase of the action potential is caused by this positive feedback causing very rapid opening of many sodium channels, causing a large inward sodium current, pushing the membrane potential towards the sodium equilibrium potential. K
(a) Rest K
(b) Depolarization K
(c) Depolarization In Neuron K
(d) Threshold K
(e) Action Potential Page 5-11 (3) The peak of the action potential occurs at the peak of the sodium current. At this time, some voltage gated potassium channels are already open (as well as the leak channels, non-voltage gated potassium channels responsible for the resting membrane potential), so the peak approaches but does not reach the sodium equilibrium potential. (4) The falling phase of the action potential is due to the rapid decrease of the sodium current caused by sodium channel inactivation, combined with the establishment of the delayed increase in potassium current due to the slower activation of voltage gated potassium channels. (5) The undershoot is caused by the fact that there is still a large outward potassium current through the voltage gated potassium channels, such that the conductance to potassium is larger at this time than when the cell is at rest, forcing the membrane potential closer to the potassium equilibrium potential. 6) The absolute refractory period Throughout the falling phase of the action potential, another action potential cannot be fired because few if any voltage gated sodium channels are not inactivated, and therefore few if any are available to start another action potential. Additionally, there is still a very large potassium current that would work against the generation of a new action potential. 7) The relative refractory period Following an action potential, threshold remains high for a period of time as voltage gated sodium channels gradually recover from inactivation, and voltage gated potassium channels gradually close. **A GHK equation simulator website** http://www.nernstgoldman.physiology.arizona.edu/ ...
View Full Document
This note was uploaded on 09/01/2009 for the course NPB 100 taught by Professor Chapman during the Summer '08 term at UC Davis.
- Summer '08