4 Resting and Action Potentials Flashcards
Q: What does an AP allow?
A: nerves to electrically communicate from one part of body to another
Q: What is diffusion? Energy?
A: movement of molecules down concentration gradient from high to low concentration until it reaches a dynamic equilibrium (concentration is equal throughout vessel)
spontaneous, no energy input
Q: What does flux mean? Unit? What is it when a dynamic equilibrium has been reached?
A: numerical term
number of molecules that cross a unit area per unit of time (number of particles) ie. molecules.m^-2.s^-1
no net flux
Q: What is Ohm’s law?
A: V= I x R
Q: What is voltage? Units?
A: potential difference generated by ions to produce a charge gradient
volts
Q: What is a current? Units?
A: movement of ions due to a potential difference
amps
Q: What is resistance? Units?
A: barrier that prevents the movement of ions (current)
ohms
Q: How is a membrane potential measured? Which cells have a membrane potential? Average?
A: reference electrode is placed outside the cell. This is the zero volt level.
Another electrode is placed inside the cell. It measures a voltage difference that is negative compared with the outside (i.e. reference).
All cells have a membrane potential
-70mV
Q: Describe general cell membrane permeability. How can it allow ion movement? (2) Due to? (3)
A: Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments.
- can contain ion channels to allow ion diffusion
- membrane can selectively allow ions to cross the barrier by changing its permeability
-Permeable pores in the membrane (ion channels) open and close depending on trans-membrane voltage, presence of activating ligands or mechanical forces (stretch sensitive)
Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+) and movement across the membrane will occur when the concentration of the permeant species is different on one side of the membrane
Q: Scenario: 2 compartments- CASE 1
1: 0.15M NaCl
2: 0.15M KCl
Membrane between is impermeable
Why is the membrane impermeable?
What does this result in? (2)
What is the membrane potential?
A: No channels in the membrane
- So… no diffusion across the membrane despite concentration gradients (osmotic gradient is the same but not the ion types)
- No separation of charge
Membrane potential = 0 mV
Q: Scenario: 2 compartments- CASE 2
1: 0.15M NaCl
2: 0.15M KCl
Membrane between is permeable to K+ ions
What happens? (6)
A: 1. K+ crosses the membrane from compartment 2 to 1 and the direction of flux is driven by its concentration gradient
- get charge separation between the compartments:
- compartment 1 gains +ve charge
- compartment 2 gains -ve charge - no Na+ movement as membrane is impermeable
- like charges start repelling: movement of K+ into +ve compartment slows/reduces as it comes up against +ve potential that has built up
- reach point where concentration gradient pushing K+ from 2 to 1 is balanced by electrical gradient forming across the membrane -> some ions are even pulled back : called= EQUILIBRIUM POTENTIAL
- stable trans-membrane potential is achieved
Q: Scenario: 2 compartments- CASE 3
1: 0.15M NaCl
2: 0.15M KCl
Membrane between is permeable to Na+ ions
What happens? (4)
A: 1. Na+ crosses the membrane and the direction of flux is driven by its concentration gradient
- Charge separation occurs
Compartment 2 gains +ve charge
Compartment 1 becomes more –ve - Enough +ve charge accumulates in compartment 2 to prevent further net movement of Na+
- Electrochemical equilibrium is achieved when electrical force prevents further diffusion across the membrane
Q: Compare case 2 and 3:
1: 0.15M NaCl
2: 0.15M KCl
2: Membrane between is permeable to K+ ions
3: Membrane between is permeable to Na+ ions
What do they both have? but? why?
What can be reached in both cases? when?
A: In both cases a membrane potential exists, but its sign is opposite
The difference in sign arises because of the selectivity of the membrane
Case 2 permeable to K+
Case 3 permeable to Na+
In both cases the electrochemical equilibrium has been reached at which the concentration gradient exactly balances the electrical gradient = point of the equilibrium potential
Q: What is the equilibrium potential? What does it cause?
A: The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient
Q: How can you calculate the equilibrium potential? How can this be simplified and made useful? (4) How is value given?
A: nernst equation
………RT………[X2]
E = —— ln ——–
………zF……….[X1]
R = gas constant T = Temp. Kelvin Z = charge on ion -> -1 for Cl-, +2 for Ca2+ F = Faraday’s number -> charge per mol of ion ln = log to base e X2= ion on one side of membrane X1= same ion on other side of membrane
- Assume T = 37C
- Convert natural log to common log
- State E in mV
- Make compartment 2 the inside of the cell and compartment 1 the outside
……..-61…………[X]inside
E = —— log —————-
……….z………….[X]outside
in mV
Q: What are the 2 most important ions for the resting membrane potential of neurons? What are their concentrations extracellularly and intracellularly? At rest, how does their membrane permeability vary?
A: Na+ and K+
Na+: extra= 150mM, intra= 10mM
K+: extra= 5mM, intra= 150mM
K+ permeability»_space; Na+
Q: Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV). Typical Em is -70 mV. Why?
A: Membranes have mixed K+ and Na+ permeability
biological membranes are not uniquely permeable to only one ion
