Resting Membrane Potentials

Question 1

What is the equation for Ohms law?

Answer 1

I=V/R

Question 2

Breakdown the ohms law

Answer 2

Current (I, in ampere)= driving force (volt)/ resistance (ohm)

g(conductance)= ease of flow, units (A/V)= Siemens

Conductance and resistance are reciprocal: 1/R= g

Hence the inverse if conductance can be substituted for resistance :
I= V x g

Question 3

Explain the water flow analogy for electric current

Answer 3

Electric current= water flow

Driving force(voltage)= water pressure

Resistance(Ohm)= pipe resistance to water flow

Bigger pipes have more conductance,equivalent to less resistance since they are the inverse of each other

Conductance for specific ions is thus key for generating membrane potential

Question 4

Ions are either______ or ______

Answer 4

Positively(cations) or negatively charged(anions)

Same charges repel each other, opposing charges attract each other

Question 5

What is charge equivalency?

Answer 5

A neutral solution contains equal,amounts of positive and negative ions

Question 6

What is a charge gradient?

Answer 6

When charges move across a membrane, a charge gradient is set up. Only a small fraction is required.

Due to this, even though we talk about a charge flux, the moving proportion is generally so small that it will not significantly change the concentration of ions ins8de or outside the cell

Question 7

What are the requirements and effects of charge separation?

Answer 7

• requires selective movement of specific charge across the membrane, e.g. only Na+ or only K+
• this leads to a charge difference and thus electrical potential (voltage, V) across the membrane
• requires a pathway for specific charges to move, e.g. selective ion channels

Question 8

What is required for resting membrane potential?

Answer 8

• a semi- permeable (blocking water and ion flow)
• a concentration gradient of ions across the membrane driven by Na-K ATPase
• a pathway for charged ions to flow across the membrane: selective channels

Question 9

How is a resting membrane potential formed?

Answer 9

Charge separation across the membrane via selective ion channels

Question 10

What is a resting membrane potent determined by?

Answer 10

• which ion conductance is dominant at rest

- the magnitude of the concentration gradient for that ion

Question 11

What were the measurements and typical values of a resting membrane potential?

Answer 11

• written as inside relative to outside, e.g. -50 mV means interior is 50 mV more negative than exterior
• usually for an epithelial (non-excitable) cell -50mV or more negative, for a neuron (excitable) -80mV
• specific value depends largely upon the type of ion channels expressed in a cell

Question 12

What is the steady state model for electrochemical equilibrium potentials?

Answer 12

Constant ion pumping (active, e.g. Na-K ATPase) and leakage (passive via channels) results in uneven distribution of key ions across the membrane

Question 13

What is net movement of each ion driven by?

Answer 13

Chemical gradient(concentration difference) and electrical gradient (charge difference), together refferred to as el3ctrochemical gradient

Question 14

When is overall net movement of an ion 0?

Answer 14

When the electrical and chemical gradients balance each other out.

Due to this, ions with uneven chemical gradients, will generate a specific electrical charge gradient

The electrical potential for this ion is called the equilibrium potential/Nernst equilibrium potential for the ion, and can be calculated by the Nernst equation

Question 15

What is the Vm?

Answer 15

Membrane potential-the net effect of all charge movements taking place across the membrane. Most affected by the ion species with the largest conductance across the membrane (and requires a concentration gradient of that ion)

Question 16

What is Eion?

Answer 16

The equilibrium potential for a single ion species (Ena-sodium equilibrium potential). Calculated potential that would be generated across the membrane if this ion species were the only conductance present, determined by the size of the ion gradient

Question 17

What are the implications of the Nernst equilibrium equation on fractional conductance?

Answer 17

Implications of Nernst equilibrium potentials

• If the cell only had a K+ conductance: Vm= -82 mV
• If the cell only had a Na+ conductance : Vm= +60mV
• If the cell only had a Cl- conductance: Vm= -66 mV

Most cells have a mix of these, Vm will be closest to the Eion with the largest fractional conductance . All other species will contribute according to their fractional conductance

Question 18

What is fractional conductance?

Answer 18

Membrane can be dramatically change conductance for a specific ions by opening and closing selective channels

-More open channels or more flow per channel - -> increased conductance

Question 19

The ion with the largest conductance across the membrane has the most influence on the resting membrane potential….

Answer 19

• most cells have a high K permeability a rest
• hence RMP is close to the K+ equilibrium potential in most cells
• changing extracellular K+ concentrations has strongest effect on RMP

All other ions contribute proportionally to their relative conductance

Question 20

How can over Vm for multiple species be calculated?

Answer 20

via Goldman -Hodgkin-Katz

Question 21

Can we predict if an ion will flow into or out of a cell?

Answer 21

Yes

-we need to know the cells membrane potential (Vm)

• what is the ions calculated equilibrium potential (Eion)?
  - if equilibrium potential is the membrane potential at which net flux is zero- then clearly when Vm and Eion do not match the ion isn’t in equilibrium and is thus in flux

   - therefore, the bigger the difference between Vm and Eion, the bigger the driving force for flux

Question 22

Explain what is the driving force

Answer 22

If the Nernst potential is the Eion where there is no net flux of the ion-then the further Vm is from the Eion the higher the flux of that ion will be

-Driving Force is the difference between the actual measured Vm and the calculated equilibrium potential for a particular ion
DF= Vm - Eion

Question 23

What are the result of DF for cations and anions?

Answer 23

Cation(+) with +ve DF= efflux

Cation (+) with -ve DF= influx

Anion(-) with +ve DF= influx

Anion(-) with -ve DF= efflux

Question 24

What is the reason many transport systems use sodium as co or anti transport?

Answer 24

There is a lot of energy in Na influx