Properties of Acton Potentials and Specific Ion Channels Flashcards
Action Potential
- Membrane structures that produce the action potential are distributed equally along the membrane
- AP does not decrease in amplitude with distance as it propagates along the membrane
- No decrease in amplitude of the AP when it is propogated past a bifurication into two or more branches of a nerve fiber
- not like an electrical current reaching a junction
Changes in Membrane Electrical Properties during an AP
- Decrease in transmembrane electrical resistance or increase in membrane conduction at each membrane site along its direction of propagation
- no change in membrane capacitance
- transmembrane ion permeabilities must increase transiently with membrane depolarization in region of AP
- must be voltage-dependent
Effects of reduction of Extracellular Na+
Decreases:
- its rate of rise
- its amplitude
- its magnitude of positive overshoot
- its propagation velcity
Conclusions - AP results from rapid, transient incrase in Na permeability resulting in a rapid, transient inward Na current
- Only a small, chemically undetectable amount of positive charge needs to accumulate on the inside surface of the membrane to prodcue the positive oveershoot of the action potential
Voltage Clamp Method
Selectively fix the transmembrane Vm (or Em) at any predetermined value for an extended time period.
- Provides a means of measuring the time course of the transmembrane ionic currents generated
Action Potential Mechanism
- First, a local deplorization generated by a stimulus removes capacitative charge from the membrane must move Vm to the threshold potential
- This causes an initial, self-limiting, rise in both gNa and gK due to transient increase in the number of open Na+ and K+ channels (channel activaiton)
- The transient rise in gNa and gK is followed by a spontaneous reutrn to the resting levels due to an atomatic closing of open channels (channel inactivation)
- Ion conduction must faster for gNa
- Peak gNa occurs much earlier than that of gK
Threshold Potential
Vth: The level of depolarization of the Vm where the increasing inward positive current (iNa) just equals outward positive current (iK)
- Just beyond this level of depolarization, the inward positive current increases very rapidly, compared to the outward iK thus generating the action potential
Electrical Excitability
The ease of generating an action potential
- Proportional to the inverse of the difference between Vth and Vm
- Excitabiity is inversely proportion to 1/(Vth-Vm)
- The smaller the difference the greater the excitability
Stabilizing action of extracellular Ca2+
- An increae in [Ca2+]o can move Vth towards zero further away from Vm in the positive direciton, thus decreasing excitablilty
- A fall in Ca2+ extracellular will do the opposite
- [Ca2+]o can bind to negatively charged fixed acidic aa groups of membrane bound proteins on the ECF side of the membrane
- increases strength of the internal electrical field across the membrane requiring removal of more positive charge from ECF side in order to open a critical number of Na+ channels to reach threshold
Functional properties of voltage gated Ion Channels
- Ion selectivity
- Activation
- Inactivation
Transmembrane sequences in Channel domain
- The S4 sequence has a large number of positively charged lysine and arginine residues
- act as the voltage sensor that opens the channel in response to a depolarizing stimulus
- Extracellular link loop between S5 and S6 “pore region” determine ion selectivity
*The K+ ion is too large to move sufficiently close to the electrical field generated by the ring of specific, charged amino acids within the Na+ pore
- Thus the K+ wates of hydration cannt be removed and the K+ cannot permeate the Na+ channel
No ring exists within the K+ channel and the K+ channel is sufficiently large to allow passage of a hydrated K+, but not a hydrated Na+ because it significantly larger than a hydrated K+
