The Action Potential I

Bridge from previous lecture

• resting potential (RP) depends on ratio of sodium and potassium conductances (g_Na/g_k)
  • we saw that membrane potential (E_m) is pulled toward the potassium equilibrium potential (E_K) as g_Na/g_k gets smaller and toward the sodium equilibrium potential (E_Na) as g_Na/g_k grows
• action potential (AP) produced by sequence of changes in g_Na/g_k
• key idea is that conductance of certain channels is not fixed
  • in voltage-gated channels, conductance depends on E_m and may do so in a time-dependent manner

Review of properties of the action potential - (what we will try to explain in terms of changes in the conductance of voltage-gated channels)

• AP is triggered by depolarization
• depolarization must exceed threshold value to trigger AP
• AP is all-or-none
• AP propagates without decrement
• AP involves reversal ("overshoot") of membrane potential
• AP is followed by refractory period

Mechanism of initiation

• define and illustrate positive feedback
  • positive feedback adds part of the output of a system to its input
    • (negative feedback) subtracts part of the output of a system from its input)
  • example of positive feedback: ignition of gas/air mixture in the engine of your car
    • spark from spark plug increases the temperature of nearby molecules of fuel and oxygen, causing them to react
    • the reaction gives off heat
    • some of this heat (output) further increases the temperature of the gas/air mixture (input)
    • this drives additional chemical reactions, which give off additional heat, which promote additional reactions, etc.
    • thus, explosions, exemplify positive feedback
  • note that this instance of positive feedback has a well-defined threshold
    • if spark is too weak, insufficient heat is given off (the heat liberated by the chemical reactions is less that the heat that conducts, convects and radiates away from the spark plug)
      • reaction dies out, and car doesn't start
  • relationship between membrane potential (E_m), sodium conductance (g_Na), and sodium current (I_Na) is one of positive feedback

• idea of voltage-gated conductance
• sodium activation: increase in g_Na due to depolarization
  • increase in conductance reflects a voltage-induced change in the shape of the sodium channel
    • displacement of a highly charged region called the "m gate"
    • this displacement acts as if to open a pore, thus the m gate is said to be opened by depolarization
• Where does the initial depolarization come from?
  • at initial segment, stimulus is the summed depolarizations produces by post-synaptic potentials
  • at other segments, stimulus is current sourced by approaching AP
  • in sensory neurons, depolarization is coupled to the action of a stimulus, such as the stretch of a muscle or the deformation of the skin
• if depolarization is too small, no action potential is triggered
  • why? Exiting potassium current exceeds entering sodium current.
    • recall that depolarization will increase I_K
    • if increase in I_Na < increase in I_K then E_m will return to resting value
• at threshold, I_Na = -I_K;
  • as soon as I_Na exceeds I_K, positive feedback sets in, and an action potential is initiated

Mechanism of termination

• Na channel "turns itself off"
• where is E_m headed before AP begins to turn itself off?
  • towards E_Na
• E_m never quite gets there
  • Why? sodium inactivation
    • sodium channel includes an h gate as well as an m gate
      • h gate closes as a result of depolarization
      • however, closing of h grate is slower process than opening of m gate
      • thus, g_Na increases transiently following a supra-threshold depolarization
• evidence that m and h gates are in different parts of channel
  • proteolytic enzyme disables inactivation (h gate), but only if applied intracellularly
  • effects of altering gene that codes for channel

Repolarization

• what brings E_m back to resting value?
• K⁺ efflux
  • in mammalian myelinated axon, via "leak"
  • in squid, via voltage-gated channel
• n gate opens slowly with depolarization
• at peak of action potential, there is an instant when E_m is not changing
  • at this instant, I_Na = -I_K
  • note that at peak of AP, there is a very large driving force operating on potassium and a much smaller driving force operating on sodium
  • thus, the opening of potassium channels has a large effect on the membrane potential, pulling it back towards E_K
• at end of AP, increased g_k has not yet dissipated
  • this contributes to "undershoot" (depends both on g_K and g_Na)
• the Na⁺/K⁺ pump is not responsible for repolarization
  • thousands of APs can be produced in large axon following poisoning of the pump
    • pump responsible for long-term maintenance of concentration gradients, not short-term changes in membrane potential

The refractory period

• during the absolute refractory period (ARP), Na⁺ channels are closed
  • thus, g_Na is too low for I_Na to exceed I_K at any E_m
• during the relative refractory period (RRP), some Na⁺ channels are open
  • increased g_K may contribute as well

Review state of the sodium channel (m and h gates) and potassium channel (n gate) during the action potential in squid giant axon

Review - explain six characteristics listed at outset in terms of channel properties

• AP is triggered by depolarization
  • because voltage-gated sodium channels are activated by depolarization
• depolarization must exceed threshold value to trigger AP
  • a depolarization of a certain magnitude is required to produce significant sodium activation
  • the depolarization must be large enough for the entering sodium current to exceed the exiting potassium current
    • only then is the positive-feedback cycle controlling sodium entry set into motion
• AP is all-or-none
  • the peak of the action potential is determined not by the depolarizing stimulus but rather by
    • the sodium equilibrium potential
    • the time course of sodium inactivation
      • the onset of inactivation prevents the peak of the action potential from reaching the sodium equilibrium potential
• AP propagates without decrement
  • because the AP in one region of the membrane causes the depolarization that triggers an AP in an adjacent region (unmyelinated axons) or node (myelinated axon)
• AP involves reversal ("overshoot") of membrane potential
  • peak of AP comes close to the sodium equilibrium potential, which is in the vicinity of +60 mV
• AP is followed by refractory period
  • absolute refractory period during which no stimulus can initiate another AP
    • due to sodium inactivation
  • followed by relative refractory period during which another AP can be initiated, but only by a larger stimulus than required in the resting state
    • recovery from sodium inactivation is not yet complete (more sodium channels are inactivated than during the resting state) and
    • in membrane regions containing voltage-gated or calcium-activated potassium channels, potassium conductance is elevated during the relative refractory period, making it more difficult to produce an entering sodium current that exceeds the exiting potassium current (the necessary condition for triggering a new AP)

Some tests of the gated channel model

• gating currents
  • the voltage-sensitive gates are charged regions of the channel molecules
    • movement of such regions constitutes an electrical current
      • such currents have been measured in careful experiments
        • all ionic currents through voltage-gated sodium and potassium channels were eliminated by administration of channel-blocking drugs
        • the membrane potential was changed in depolarizing and hyperpolarizing steps of equal size
        • the difference between the absolute values of the recorded currents was calculated
        • this difference represents the movement of the gating regions, which occurs only during the depolarizing step
• effects of intracellularly administered pronase - enzyme that "cuts" proteins
  • eliminates sodium inactivation, thus implying that region responsible for inactivation is in an intracellular portion of the channel (now known to lie in the region linking domains III and IV - see below)
• genetic manipulation of sodium channel protein
  • produce artificial channels with altered structure so as to infer structure-function relationships
  • identification of candidate voltage-sensing region
    • segment 4 of each domain
  • identification of candidate region underlying inactivation
    • in intracellular region connecting segments III and IV
    • result consistent with results of pronase experiment
• single-channel recording experiments
  • properties inferred from studies of axons have been confirmed from studies of tiny patches of membrane in which only a single channel may be open at a given time
    • as predicted by cellular-level studies of axons
      • single voltage-gated sodium channels activate quickly in response to depolarization and inactivate slowly
      • single voltage-gated potassium channels activate in response to depolarization, but do so more slowly than voltage-gated sodium channels

Created by P. Shizgal and K. Oda (Dept. of Psychology, Concordia University)