Review Questions

Diffusion is principally driven by three properties within cells. What are they?

Concentration differences, the charges on the ions, and the potential difference between the intracellular and extracellular environments

Define the Nernst potential in 2 words

electrochemical balance

what is the equation to calculate the Nernst potential?

61.5/z * log (c_o/c_i)

what is the equilibrium potential for Na⁺?

+67 mV

what is the equilibrium potential for K⁺?

-98 mV

what is the equilibrium potential for Cl^-?

-90 mV

what is the equilibrium potential for Ca²⁺?

+129 mV

Why is the Nernst potential for a K+ a negative value even though it has a positive charge?

It has a high intracellular concentration, which makes the c_o/c_i ratio less than 1. log(<1) = a negative number

what is the difference between the equation for the Nernst potential and the Goldman equation?

the goldman equation accounts for relative permeabilities of each ion and the nernst equation accounts for the charge of the ion.

why does potassium play a major role in the resting membrane potential?

Potassium has a higher permeability than sodium due to the presence of leak channels

For a positively charged ion, if the membrane potential is more negative than the equilibrium potential, then the direction of flux is ___ the cell

into

For a negatively charged ion, if the membrane potential is more positive than the equilibrium potential, then the direction of flux is ___ the cell

into

For a negatively charged ion, if the membrane potential is more negative than the equilibrium potential, then the direction of flux is ___ the cell

out of

For a positively charged ion, if the membrane potential is more positive than the equilibrium potential, then the direction of flux is ___ the cell

out of

The membrane is clamped at +70mV. what direction does a Ca²⁺ ion move?

into the cell

if an anion has a nernst potential of -90 mV and the voltage is clamped at +10 mV, which direction is the ion flowing?

into the cell

what is changing in this graph?

conductance

a hypothetical neuron which expresses sodium ion channels is grown in culture and is subjected to an experimental drug which blocks sodium channels. What happens to the Nernst potential following the administration of the drug?

the nernst potential does not change (because blocking the sodium channels affects permeability, which is not a factor in the nernst potential.

a hypothetical neuron which expresses sodium ion channels is grown in culture and is subjected to experimental conditions in which additional sodium is added to the extracellular environment. What happens to the Nernst potential following the addition of sodium?

the nernst potential will increase

how does resistance affect the time constant?

resistance and time constant are proportional  

how do we define the time constant?

the time it takes for a capacitor to charge up to 63% (or to discharge 37%)

what is the quantity and directionality of ions being pumped by the Na+/K+ pump?

3 Na+ out, 2 K+ in

what is the quantity and directionality of ions being exchanged by the Cl-/K+ exchanger?

1 Cl- out, 1 K+ out

what is the quantity and directionality of ions being exchanged by the Na+/Ca2+ exchanger?

3 Na+ in, 1 Ca2+ out

t/f: it requires energy to maintain the Na/K pump

true

t/f: it requires energy to maintain the Cl/K exchanger or the Na/Ca exchanger

false

rank the following ions by the size of their respective hydration shells: Na+, K+, Ca2+

Ca2+ > Na+ > K+

How come a K+ ion can’t pass through a Na+ channel?

it is too large to fit with a water molecule.

how come a Na+ ion can’t move through a K+ channel?

it is too large to fit while hydrated and too unstable without a water molecule

define the following terms: hypertonic, isotonic, hypotonic.

all refer to the solute concentration outside the cell.

hypertonic - high solute concentration

isotonic - same concentration as inside

hypotonic - low solute concentration

why is saline used instead of water for an IV?

saline is a solution of salt and water. plain water would be hypotonic to the inside of a blood cell, so if water were used, it would enter the blood cells by osmosis, leading to the cells bursting. saline prevents this.

what happens to the capacitance of a cell as the surface area of the membrane increases? when the space between the plasma membrane increases?

increase, decrease

how does the force between 2 charges change if the distance between them is cut in half?

4x

when we are at nernst potential of an ion, are we in a static or dynamic equilibrium state?

dynamic

t/f: the nernst potential of an ion takes only the extra/intracellular concentration of that ion into account.

false, the charge of the ion and the temperature are also taken into account.

if i permanently close all K+ channels, but leave all other channels alone, what will happen to my resting membrane potential?

the permeability of potassium has changed, so the membrane potential would move more towards the Na+ nernst potential.

someone takes a drug that causes Na/K pump to be destroyed, which direction does K+ move?

out of the cell

B

how are voltage and current related?

proportional (V=IR)

how are voltage and resistance related?

proportional (V=IR)

how are resistance and current related?

inverse (I=V/R)

how are capacitance and surface area related?

proportional

how are capacitance and distance related?

inverse

how are the time constant and resistance related?

proportional (t=rc)

how are the time constant and capacitance related?

proportional (t=rc)

how are force and distance between the charges related?

inverse (squared)

if no ions: ___ resistance, ___ conductance

high resistance, low conductance (ions are needed to carry charge)

if some ions: ___ resistance, ___ conductance

moderate, moderate

if many ions: ___ resistance, ___ conductance

low resistance, high conductance

if your circuit has a voltage of 10 and your resistance drops from 4 to 2 ohms, will your current increase or decrease?

increases

when series resistance increases, capacitance ___

decreases

myelination (increases/decreases) capacitance

decrease (because it increases the thickness)

in terms of the time constant equation, C is a more important property for ___

axons

in terms of the time constant equation, R is a more important property for ___

dendrites

resistance is proportional to the number of ___

open channels at rest

If we added myelination to the axon, would τ increase or decrease?

decrease

If we opened K+ leak channels, would τ increase or decrease?

decrease

in a neuronal circuit model, the battery represents the

concentration gradient

the longer side of the battery symbol pointing toward external

K+

the shorter side of the battery symbol pointing toward external

Na+

in the neuronal circuit model, the capacitor represents the

cell membrane

in the neuronal circuit model, the resistors represent the

channels

an arrow across a resistor indicates

variable resistance (which represents gated channels)

in the neuronal circuit model, what is gL?

leak channels (ungated, so no variability → no arrow)

ions move because of ___ difference

concentration difference

a positive Nernst potential means the ion will move ___ the cell

into

a negative Nernst potential means the ion will move ___ the cell

out of

how do you use the goldman equation for anions?

multiply the end result by -1

which has the highest conductance?

3, because it has 3 resistors (more channels, lower resistance, higher conductance)

on an IV plot, the slope =

conductance

what change on an IV plot: ion concentrations are changed

shift to left or right (no slope change bc no conductance change)

why is there a jump?

the jump represents a change in conductance due to the opening of ion channels (like during an action potential)

if nernst = 0, how will the ions move?

they will not because there is no potential difference.

as R increases, T (tau) ___

increases

what change on an IV plot: more channels open

slope increases