Copyright ©Mark Nelson, 2002. All rights reserved.
Chapter 4: The Electrical Potential of a Resting Neuron
What you need to know
(exam questions will be a drawn from this subset of material)

What is the physical basis of the membrane potential?  (p. 91)
    the membrane potential results from a separation of positive and negative charges across a cell membrane

What charge carriers contribute?  (p. 91)
    charged ions that can cross the cell membrane; if the ions can't cross the membrane, they don't contribute
    typical contributors are: Na+ , K + , Ca++, and Cl- ;

What three factors can induce an ion to cross the membrane?  (p. 91)
    (1) concentration differences, (2) electrical potential differences, (3) ion pumps
    NOTE: (1) and (2) can contribute only if there are open ion channels

Which of these three factors are passive (no ATP); which are active (require ATP)?  (p. 92)
    diffusion and electrical forces are considered passive; ion pumps are active

What conditions define the steady state for the membrane potential?  (p. 93)
    steady state occurs when, for each type of ion, the flux of ions  in one direction is balanced by an equal flux in the opposite direction
    in this case, the intracellular concentration for each ion type is neither increasing  nor decreasing (steady state)

When is a neuron's membrane potential  in steady state?  (p. 93)
    when it is NOT actively engaged in electrical communication, i.e. when it is in the resting state ;
    under these conditions, the steady state membrane potential is referred to as the resting potential

Which permeable ions have a HIGH intracellular concentration relative to the extracellular concentration?  (p. 95)
    K+

Which permeable ions have a LOW intracellular concentration relative to the extracellular concentration?  (p. 95)
   Na+, Cl - , Ca ++

What is the equilibrium potential for an ion?  (p. 97)
    the electrical potential across the membrane for which the electrical gradient exactly counterbalances the concentration gradient
    at the equilibrium potential, there is no NET movement of that ion across the membrane

What is the Nernst equation?  (p. 98)
    The mathematical formula for calculating the equilibrium potential for a particular type of ion 
          
          RT       [ion]out
   Eion = ---- ln ---------- 
          FZ       [ion]in

Define the various symbols and  terms in the Nernst equation.  (p. 99)
    see Table 4-2

What is a typical equilibrium potential for K+, for Na+?  (p. 98-99)
    EK ~ -75 mV; ENa ~ + 54 mV
    TYPO: the units of membrane potential are wrong in the equations in the right hand columns of  pages 98 and 99
    Membrane potential is measured in mV (not mM)!

If you increase the temperature of a neuron, do you change the magnitude of the ionic equilibrium potentials?  (p. 99)
    Yes, according to the Nernst equation, increasing the temperature increases the magnitude of the equilibrium potential
    (positive equilibrium potential get more positive; negative equilibrium potentials get more negative)

If you double the temperature of a neuron from 18oC to 36 oC, do you double the ionic equilibrium potentials?  (p. 99)
    No, temperature in the Nernst equation is in degrees Kelvin ( oK = 273 + oC);
    a change from 18oC to 36o C, would change the equilibrium potential by about 6 %  (291 o K to 309oK )

When ions move across the cell membrane, do the intracellular and extracellular concentrations change appreciably?  (p. 101)
    No, typically the change in concentration is insignificant (less than 0.0001%),
    changes in concentration due to ionic movement can (almost always) be ignored when using the Nernst equation

If a neuron's resting potential is more (positive/negative) than an ion's equilibrium potential, what will be the net effect?  (p. 101-102)
    If the ion is POSITIVELY CHARGED:
        if the resting potential  is more positive than the equilibrium potential,
            the net passive flux of those ions will be OUTWARD (net efflux)
        if the resting potential is more negative than the equilibrium potential,
            the net passive flux of those ions will be INWARD (net influx)

    If the ion is NEGATIVELY CHARGED:
        if the resting potential  is more positive than the equilibrium potential,
            the net passive flux of those ions will be INWARD (net influx)
        if the resting potential is more negative than the equilibrium potential,
            the net passive flux of those ions will be OUTWARD (net efflux)

    NOTE: for a neuron at rest (i.e. steady state) the TOTAL flux for each type of ion must be ZERO (by definition)
    the passive flux described above is counterbalanced by an active flux via ion pumps

If the membrane of a hypothetical neuron were permeable only to K + ions, what would be the neuron's resting potential?  (p. 103)
    In this case the resting potential would be equal to the K+ equilibrium potential

Real neuronal membranes, at rest, are predominantly permeable to which type of ion?  (p. 103-104)
    K+

What is the Goldman equation?  (p. 103-104)
    The mathematical formula for calculating the resting potential of a neuron, taking into account permeabilities of multiple ion types.
          
        RT       PK[K+]o + PNa[Na+]o+ PCl[Cl-]i
   Em = ---- ln ------------------------------ 
        F        PK[K+]i + PNa[Na+]i+ PCl[Cl-]o

What has a larger effect on the resting potential of a neuron, changing extracellular [K+] or extracellular [Na+]?  (p. 104)
    changing extracellular [K+] has a much larger effect because of ratio of PK :PNa is large (approximately 1 : 0.04)
    because the relative permeability to Na+ is small, changing extracellular [Na+] does not have much of an effect


What is hyperkalemia?  (Not in text)
    Hyperkalemia is a medical condition caused by higher than normal levels of potassium in the bloodstream.
    Almost all (98%) potassium in the body is found inside the cells (intracellular). Only about 2% occurs in the fluids outside of the cells (extracellular).
    If extracellular potassium levels get too high, bad things happen because the magnitude of the resting potential is reduced in nerve and muscle cells.
    Cardiac arrhythmia, cardiac arrest,  muscle weakness, ...