Week 4 - neurones Flashcards
Describe the set up of the experiment to test neurones at resting state
- cell bated in saline bath
- one electrode recording
- second electrode referencing
- difference between recording and reference is membrane potential
What resting potential was recorded with the recording electrode in the cell?
- 70 mV
What two factors determine ion behaviour?
diffusion and electric fields
What happened when conc. of solutions are equal on both sides of a membrane?
flow left to right will equal flow right to left
K conc L = R
What happens when the solution starts with different concentrations?
- ions follow a diffusion gradient from high to low
What happens if the barrier is selectively porous to only potassium?
ions flow down a gradient unit repulsed by an electrical difference as chlorides cannot diffuse across the membrane, hence charge difference develops
What is Bernstein’s hypothesis?
- theory that resting potential arises from a conc. gradient of K+ ions across a membrane
- requires a high rising permeability of membrane ONLY to potassium
membrane potential is a perfect of unequal conc of potassium across a membrane, selective permeability of the membrane to potassium ions
What os the equilibrium potential of potassium (Ek)?
Ek = klog(K+ out/ K+ in)
Explain the equilibrium potential of potassium (Ek)
- with conc gradient of potassium ions, ions flow down conc gradient
- this continues until electrical repulsive force detects this flow
- electrical repulsive forces come from inability of other ions to move expect potassium
- when diffusion and electrical forces are equal, equilibrium reached
- determined by difference in potassium conc, measured in mV
How can we test Bernsteins hypothesis?
- squid giant axon is 100X larger than nearly all other axons
- cytoplasm can be manually extracted
- ionic comp shows highly concentrated in potassium
How does Bernstein’s hypothesis vary from the Ek ?
- Vm deviates positively away form EK
- calc predicted potential = -93mV
- measured values = -65mV
-> does not fully explain resting membrane potential
How do we test Bernstein’s hypothesis?
by comparing measured potential to theoretical potential, assuming everything comes form permeability of membrane to potassium
What else might need to be considered?
influx of different ions
Why can chloride not be an implication?
- would favour even more negative value because the equilibrium potential value for chloride is also negative (ECl+ -55mV)
is sodium a factor?
- positive equlilibrium potential ENa= +55mV
What would happen if membrane is equally permeable to both sodium and potassium ?
the membrane potential is the sum of the two
-> that doesn’t equal -65mV
What does it tell us that if Sa and K had the same permeability, it would not equal -65mV?
that there must be an unequal permeability
What is the Goldman, Hodgkin and Katz equation?
- more complex equation which counts for multiple ion interactions
of pK=1.0, pNA=0.04, pCl=0.45
Vm=-63mV
What would happen if this cell is placed into a new saline solution which has 10X the conc of potassium within ?
resting membrane potential ill become less negative as the diffusion gradient will flatten
What happens if this cell is placed into a new saline solution which has 1000X the conc of potassium within?
rising membrane potential will become positive as the diffusion gradient will invert
What is the nature of action potentials?
all or nothing
What is responsible for an action potential?
- sodium ions in extracellular side of membrane
- proposes sodium ions enter the cell
What happens when sodium is absent?
action potentials cannot occur in cultured neurones
How did Hodgkin and Huxley measure an action potential?
- placing an electrode inside the squid giant axon
- reference in extracellular medium
- neurone stimulated with simulator
- AP measured
explain what it means that Aps are all or nothing events
- stimulus of specific magnitude is required to create an action potential
- too small results in depolarisation but not an AP
-> threshold potential must be surpassed
is an action potential more than just depolarisation?
yes - neuron overshoots 0mV and hyperpolarises following repolarisation
what are the three stages of an action potential?
- rising phase (depolarisation)
- falling phase (repolarisation)
- (hyperpolarisation = dip)
- after hyper polarisation (refractory period )
How to test Overtons theory that sodium matters?
- normal seawater
2-5 replace NaCl with choline chloride
6,. return to normal seawater
What does Overtons theory test show?
- confirms hypothesis
- indicates that rising phase of AP is due to transient increase in MP to Na+
Using Goldmans equation, how many folds need to be increased for the MP to Na+ to account for the overshoot?
500 fold
Are ions basis for the falling phase of the AP?
- if due to return to resting state permeability of sodium - MP would return to -70mV but it after- hyperpolarises
- this is similar to equilibrium potential of K+ ions -> suggests falling phase may involve increase MP to K+ ions
How many fold increases would the Goldman equation conclude if MP to potassium accounts for after hyper polarisation?
10 folds
What is the basis of an action potential ion flow?
- an increase in pNA+ (depolarisation)
- increase in pK+ and decrease in pNa+ (repolarisation)
- return to resting state permeability
-> overall inward sodium current followed by an outward potassium current
How is the difference in permeability achieved?
- voltage gated Na+ channels and voltage gated K+ channels mediate the changes in membrane permeability to there respective ions
How to show action potential propagation?
- multiple electrodes along an axon show there is a traveling wave
Explain the refractory period
- no new action potential can be initiated
- ensures unidirectional action potentials
- prevents bounce back form axon terminal
- repents summation of action potential
What happens during the refractory period to the voltage gated Na+ channels that mediate rising phase of the AP?
transiently inactivated
How does action potential propagation work?
- current flows through activated path of membrane and depolarises adjacent patch
- historic path depolarised and refractory, adjacent patch reaches threshold, current flows and next path depolarises
- historic path depolarises and next path depolarises
-> ensures unidirectional flow
What are the AP parameters?
- amplitude - always invariant (all or nothing), varies between neurones
- frequency - variable but absolute refractory period determines max nr of APs per sec
- velocity: invariant for an individual neurone but variability between neurone’s
What is the equation for the action potential velocity?
u = k x deriv(d)
u= velocity
k= constant
d= Anton diameter
By how much does the diameter have to increase to increase the velocity by 10x
100x
what is myelination?
- mechanism for increasing the velocity of action potential propagation without increasing axon diameter
