Exam 1 - Membrane Potentials

Question 1

what is a Coulomb?

Answer 1

measure of electrical charge of a particle

Question 2

what is Faraday constant?

Answer 2

total charge on a mole of any monovalent ion

Question 3

what is potential difference?

Answer 3

work required to move a unit of charge from one point to another = E (voltage)

Question 4

describe concentrations of the following ions inside and outside cells: Na+, Cl-, Ca2+, K+

Answer 4

Na+: high outside, low inside
Cl-: high outside, low inside
Ca2+: high outside, very low inside

K+: low outside, high inside

Question 5

what is ionic equilibrium potential?

Answer 5

an electrical potential difference that exactly balances the ionic concentration gradient

Question 6

what is the principle of electrical neutrality?

Answer 6

can’t have a cell carrying a charge -> product of anions and cations inside cell must equal product of anions and cations outside cell

Question 7

explain how it is possible to have a charge difference b/w intracellular and extracellular

Answer 7

phospholipid bilayer is very thin, so ions on one side can interact electrostatically with ions on the other side -> the negative charge inside the cell is not distributed evenly in the cytoplasm but rather localized at the inner face of the membrane -> equilibrium is established such that there is no net movement of ions across the membrane, leaving a charge difference between the two sides.

Question 8

what determines the rate of flow of an ion across the plasma membrane? 3 things

Answer 8

• concentration gradient
• voltage difference across the plasma membrane
• conductance of the ion channels

Question 9

what does the Nernst equation calculate?

Answer 9

Eion (equilibrium potential) = membrane potential at which there is no net movement of a given ion

Question 10

what is the Nernst equation?

Answer 10

Eion = (58/z)log([ion]out/[ion]in)

valid only for ions that diffuse - describes the equilibrium state

Question 11

what does the Donnan equilibrium state?

Answer 11

the products of the concentrations of the permeant ions of opposite signs on each side of the membrane must be equal at electrochemical equilibrium

Question 12

describe the action of the Na-K pump

Answer 12

• binds Na+ and ATP intracellularly
• cleaves ATP, uses energy to translocate bound Na+ out
• binds K+ extracellularly
• brings K+ in, returns to original configuration

3 Na+ out for 2 K+ in

this pump accounts for most energy consumed in a cell during resting state

Question 13

how does the Goldman equation differ from the Nernst equation?

Answer 13

Goldman takes into account both the concentration gradients of the permeant ions and the relative permeability of the membrane for each permeant species

Question 14

what are the relative ionic permeabilities at resting state

Answer 14

Pk: Pcl: Pna = 1: 0.45: 0.02

Question 15

what are the passive membrane properties?

Answer 15

• transmembrane resistance (resting)
• axial resistance (internal resistance)
• membrane capacitance

Question 16

if you apply the same amount of current to a small cell and a large cell, which one will get an AP first and why?

Answer 16

small one: E = IR -> for same amount of I, cell w/ higher R will have most change in E and therefore get an AP first

small cells have higher internal resistance than large cells

Question 17

what is the specific membrane resistance? how does it relate to internal resistance?

Answer 17

resistance of a unit area of membrane measured in units of ohm cm^2

• function of density of ion channels and their conductance
• *input resistance is inversely proportional to the square of the radius (internal resistance is a fxn of cell size)

Question 18

what does electrotonic potential decay mean?

Answer 18

maximum membrane depolarization occurs at the site of current injection (or synapse) and decays with distance because of membrane resistance

Question 19

what is the length constant, lambda? how can you increase lambda?

Answer 19

the distance over which an electrotonic potential decrements to 37% of its original peak value

increase lambda by increasing membrane resistance or decreasing internal resistance

Question 20

what does it mean to have a larger value of lambda?

Answer 20

the larger lambda, the less decay over length (aka the farther the current spreads down the nerve fiber) and therefore the greater chance for two or more synaptic potentials to summate

Question 21

effect of diameter on electrotonic spread

Answer 21

other properties being equal:

• synaptic potential will be larger in a small process (b/c larger internal resistance)
• spread of potential will be greater in a large process (b/c larger value of lambda)

Question 22

what is the membrane time constant? what is the equation for it?

Answer 22

the time necessary for the electrotonic potential to decrement to 37% of its original value

tau = (membrane resistance)(membrane capacitance)

when membrane resistance is high, tau is increased b/c injected current first must discharge the membrane capacitor before it can depolarize the membrane

Question 23

what does a larger value of tau mean?

Answer 23

the larger tau, the less decay over time and therefore the greater chance for summation