PSL300H1 Lecture Notes - Lecture 2: Potassium Chloride, Extracellular Fluid, Sodium-Potassium Alloy
PSL300
Lecture 2: Neurophysiology 2
Resting Membrane Potential
• Membrane is most permeable to K+ at rest, but Na+ and Cl- ions are also diffusing somewhat
• Another equation puts all of the ion species into consideration, which include K, Na, and Cl with their relative
permeability (K is the major player at rest)
o Actual membrane potential can be calculated from an expanded equation containing a term for each
diffusible ion species
Goldman Equation
• K is the major contributor due to the fact that at rest the channels are open
o The resting membrane potential is closer to the equilibrium potential of K+ rather than to Na+
• Fight between Na+ and K+
o K wants the membrane potential at -90 mV
▪ At rest the membrane is more permeable to K+ which is why the potential is closer to the ideal
K+ value
o Na wants the membrane to be at a more positive voltage at +50-60 mV
▪ Membrane is not that permeable to Na at rest
Na+ Equilibrium Potential
• Under certain circumstances, the permeability of Na+ can be dominant and much more than the K+ ion and MP
can change drastically
• If the membrane properties change to make the membrane most permeable to Na+, then there is a new Na+
current inward
• At equilibrium, there is a net cation accumulation
inside the membrane
• Membrane potential is positive inside with respect to
the outside: ENa+ = +60 mV
Na+ Channels
• To generate a signal, membrane increases its
conductance by opening a channel permeable only to Na+ ion
• This is a voltage-gated Na+ channel
• In normal resting MP, this Na+ channel is shut and little Na+ is entering the cell
o To open this Na+ channel we need to depolarize (removing the polarization) the membrane by a certain
amount – influx of positive charges
• The influx of Na and the K leaving through the channels allows the RMP to be -70 mV
• Voltage gated sodium channels are very important in the generation of action potentials
• Going from -70 mV to -55 the S4 wings will not be attracted to the membrane and will open the channels
allowing more Na into the cell and allowing the inside of the cell to be more positive leading to an action potential
o The cell becomes very positive very quickly
• Na+ channel inactivation then takes place
o 2 gates to the Na channels – an inactivation gate in addition to the S4 segment
• As soon as the cell hits -55 mV (which is the threshold of opening the Na channels)
• The ‘ball and chain’ inactivation gate will start to close automatically once the membrane is depolarized
• Once the membrane reaches +30 the inactivation gate will start to close
o Rapid depolarization halts due to inactivation gate closing
o To remove the inactivation block, the MP must go below the threshold level
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Document Summary
K+ value: na wants the membrane to be at a more positive voltage at +50-60 mv, membrane is not that permeable to na at rest. Na+ equilibrium potential: under certain circumstances, the permeability of na+ can be dominant and much more than the k+ ion and mp can change drastically. Na+ channels: to generate a signal, membrane increases its conductance by opening a channel permeable only to na+ ion, this is a voltage-gated na+ channel. Phases of an action potential: resting membrane potential, depolarizing stimulus, membrane depolarizes to threshold. 55 mv: suprathreshold will fire the same action potential as a normal stimulus. All of none principle: action potential from threshold and supra-threshold stimulus have the same magnitude, more frequent action potentials occur due to an increased stimulus. Information pertaining to stimulus intensity is coded by the changes in the frequency of the action potential.
