Lecture 3 and 5 Biophysics of the Neuronal Membrane pt 1 and 2 Flashcards
briefly describe the structure/environment of neuronal membrane
lipid bilayer
transmembrane protein complexes form channels/pores with selective passing of ions
extracellular and intracellular fluid composed of water and ions dissolved
define ions
atoms/molecules that have lost or gained charge (electrons)
what’s a cation?
positive ion
what’s an anion?
negative ion
what unit is used to describe the concentration of solutes (ions) in a solution?
Mole
define a mole
a mole of substance is constituted by 6.022x10^23 of that substance
what is 1 Molar (M) solution?
1 mole of solute dissolved in 1 litre of solution
how will solutes travel in a concentration gradient with permeable membrane?
from an area of high to low concentration until there’s no difference in concentration
as well as concentration gradient, what other factor effects ion flow?
electrical charge
what is electrochemical equilibrium?
when the concentration gradient is opposite and equal to that of its electrical gradient
what does the Nernst equation calculate?
the equilibrium gradient of a given ion
what does the membrane potential (Vm) define?
the difference in potential between inside and outside the cell
write out the Nernst equation
Es = RT/ZF ln([s]out/[s]in)
define the terms of the nernst equation and their associated values
Es= equilibrium potential of ion (volts)
R= universal gas constant = 8.314 J/mol
T= temp = celsius + 273.15= K
Z- charge of ion e.g. +1 or -1
F= faraday’s constant- 96,485 C/mol
what does an Na+/K+ion pump do and why?
actively (using energy) pumps out 3 Na+ and in 2 K+, this overall pushes +1 out of neuron contributing to its negative potential
it maintains resting potential actively
what are the nernst/equilibrium potentials (Es) for K+ and Na+?
Ek ~ -80mV
ENa ~ +58mV
what 2 factors determine whether Na+ or K+ drive the neuronal membrane to their equilibrium potential?
permeability and concentration gradients
if Vm= Ek, what does this mean for the flow of K+?
there will be no net passive current of K+ ions- this would be the equilibrium potential of K+
what happens to the membrane with respect of K+ if Vm>Ek
the membrane has become depolarised
This will cause an efflux of K+ in order to restore Vm to Ek
What happens to the membrane with respect to K+ if Vm<Ek?
the membrane has become hyperpolarised
this will cause influx of K+ to restore Vm to Ek
define reversal potential
the Vm value which changes direction of net current of a particular ion
if Vm is not equal to Es, what 3 factors does the net current depend on?
Vm potential
Concentration gradient of ion
Membrane permeabillity for ion
which 2 ions mostly determine the permeability of resting membrane potential?
Na+ and K+
what are Na+ and K+ ‘trying’ to do do membrane potential?
push it to that of their equilibrium potential (-80mV for K+ an +58mV for Na+)
which ion has a higher permeability to the membrane and by how much
K+ has a higher permeability by around 100 fold
Why is Vm at rest closer to Ek than ENa?
because the membrane has much greater permeability for K+ ions
What does the Goldman-Hodgkin-Katz equation take into consideration which the Nernst equation doesn’t?
the ion permeability of more than one ion
(in addition to concentration gradient which Nernst does consider)
write out the Goldman-Hodgkin-Katz equation
check book/slides (make sure the Cl- is the right way round
what is the notation for ion conductance
gs
define gs?
the ability of a membrane to support current of a particular ion (S) through a particular ion channel
what 2 factors determine conductance?
permeability and the concentration of ions available
(highest conductance= high permeability and high conc. of ions)
write an equation to describe the relationship between conductance and resistance
g=1/R (they are inverse)
what is driving force?
the difference in equilibrium potential and and membrane potential
(Es-Vm)
write the equation for ohms law for ion species
Is= gs (Es-Vm)
do outward currents of ions have negative or positive Is?
negative
do inward currents have negative or positive Is?
positive
describe the passive membrane potential as an electrical circuit
membrane potential (Vm) causes current (I) through conductance (dependant on resistance of ion channels g=1/r)
ion pumps- current source- batteries
what determines the net flux of one ion?
its driving force and ionic conductance
what measurement is net flux of ions in
current
write a basic equation to calculate the net flux (current) of ions
Ix = gx x ∆voltage
what is the total flux of ions across membrane?
sum of all individual ion currents
write the equation for total flux of ions (current) across membrane
Itotal= INa+ Ik + ICl =
gNa (V-ENa) + gK (V-EK + gCl (V-ECl)
basically the total current is calculated from the sum of individual currents which itself is calculated from the conductance of each ion multiplied by the difference between membrane potential and equilibrium
is permeability a constant or a function of something else?
a constant
what are the relative permeabilities of the 3 major ion species?
PK= 1
PNa= 0.03
PCl = 0.1
when V=0, what is the net flow of ions?
0 - this is resting potential
How does a patch pipette attach?
it has a blunt tip with an opening, used to make a hole in membrane by suction, forming a vacuum with access to cytoplasm
what are the 2 recording models of patch-clamps?
current clamp and voltage clamp
describe a current clamp
constant current injected into neuron and voltage/response is measured
describe a voltage clamp
instead of having a fixed current, there is a fixed voltage and any detected change in voltage is quickly corrected with current input
what are the 2 types of voltage clamp?
two-electrode voltage clamp (TEVC)
single electrode voltage clamp (SEVC)
what is the difference between TEVC and SEVC?
TEVC has 2 electrodes, one which records voltage and the other which passes current when the other electrode detects voltage change
SEVC has one electrode which both records voltage and injects current
describe SEVC in vivo recordings in mouse primary visual cortex
voltage clamp used on mouse with short bursts of visual stimuli
during the stimulation series, there’s a significant change in the current being injected into the neuron, hence the visual stimulation is activating the neuron of the visual cortex
to observe EPSCs, what is the ideal voltage clamp?
-70/-80 mV
why is -70/-80 mV the ideal voltage clamp for EPSC observance
it is the reversal potential or inhibition (chloride)
it prevents spiking
to observe IPSCs, what is the ideal voltage clamp?
0-20mV
why is 0-20mV the ideal voltage clamp for IPSCs ?
it is reversal potential of excitation (potassium and sodium ions)
it prevents spiking
what is put in the patch solution to prevent spiking/make the neuron more like an RC-circuit
sodium channel blockers- prevents spiking
does visual stimulus invoke excitatory or/and inhibitory responses
both
can excitatory and inhibitory responses be recorded at the same time?
no- because the voltage needs to be clamped at different levels depending on response to be oserved
