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Physiology > Week 4 - neurones > Flashcards

Week 4 - neurones Flashcards

1
Q

Describe the set up of the experiment to test neurones at resting state

A
  • cell bated in saline bath
  • one electrode recording
  • second electrode referencing
  • difference between recording and reference is membrane potential
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2
Q

What resting potential was recorded with the recording electrode in the cell?

A
  • 70 mV
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3
Q

What two factors determine ion behaviour?

A

diffusion and electric fields

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4
Q

What happened when conc. of solutions are equal on both sides of a membrane?

A

flow left to right will equal flow right to left
K conc L = R

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5
Q

What happens when the solution starts with different concentrations?

A
  • ions follow a diffusion gradient from high to low
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6
Q

What happens if the barrier is selectively porous to only potassium?

A

ions flow down a gradient unit repulsed by an electrical difference as chlorides cannot diffuse across the membrane, hence charge difference develops

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7
Q

What is Bernstein’s hypothesis?

A
  • 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

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8
Q

What os the equilibrium potential of potassium (Ek)?

A

Ek = klog(K+ out/ K+ in)

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9
Q

Explain the equilibrium potential of potassium (Ek)

A
  • 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
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10
Q

How can we test Bernsteins hypothesis?

A
  • squid giant axon is 100X larger than nearly all other axons
  • cytoplasm can be manually extracted
  • ionic comp shows highly concentrated in potassium
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11
Q

How does Bernstein’s hypothesis vary from the Ek ?

A
  • Vm deviates positively away form EK
  • calc predicted potential = -93mV
  • measured values = -65mV
    -> does not fully explain resting membrane potential
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12
Q

How do we test Bernstein’s hypothesis?

A

by comparing measured potential to theoretical potential, assuming everything comes form permeability of membrane to potassium

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13
Q

What else might need to be considered?

A

influx of different ions

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14
Q

Why can chloride not be an implication?

A
  • would favour even more negative value because the equilibrium potential value for chloride is also negative (ECl+ -55mV)
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15
Q

is sodium a factor?

A
  • positive equlilibrium potential ENa= +55mV
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16
Q

What would happen if membrane is equally permeable to both sodium and potassium ?

A

the membrane potential is the sum of the two
-> that doesn’t equal -65mV

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17
Q

What does it tell us that if Sa and K had the same permeability, it would not equal -65mV?

A

that there must be an unequal permeability

18
Q

What is the Goldman, Hodgkin and Katz equation?

A
  • more complex equation which counts for multiple ion interactions
    of pK=1.0, pNA=0.04, pCl=0.45
    Vm=-63mV
19
Q

What would happen if this cell is placed into a new saline solution which has 10X the conc of potassium within ?

A

resting membrane potential ill become less negative as the diffusion gradient will flatten

20
Q

What happens if this cell is placed into a new saline solution which has 1000X the conc of potassium within?

A

rising membrane potential will become positive as the diffusion gradient will invert

21
Q

What is the nature of action potentials?

A

all or nothing

22
Q

What is responsible for an action potential?

A
  • sodium ions in extracellular side of membrane
  • proposes sodium ions enter the cell
23
Q

What happens when sodium is absent?

A

action potentials cannot occur in cultured neurones

24
Q

How did Hodgkin and Huxley measure an action potential?

A
  • placing an electrode inside the squid giant axon
  • reference in extracellular medium
  • neurone stimulated with simulator
  • AP measured
25
Q

explain what it means that Aps are all or nothing events

A
  • 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
26
Q

is an action potential more than just depolarisation?

A

yes - neuron overshoots 0mV and hyperpolarises following repolarisation

27
Q

what are the three stages of an action potential?

A
  • rising phase (depolarisation)
  • falling phase (repolarisation)
  • (hyperpolarisation = dip)
  • after hyper polarisation (refractory period )
28
Q

How to test Overtons theory that sodium matters?

A
  1. normal seawater
    2-5 replace NaCl with choline chloride
    6,. return to normal seawater
29
Q

What does Overtons theory test show?

A
  • confirms hypothesis
  • indicates that rising phase of AP is due to transient increase in MP to Na+
30
Q

Using Goldmans equation, how many folds need to be increased for the MP to Na+ to account for the overshoot?

A

500 fold

31
Q

Are ions basis for the falling phase of the AP?

A
  • 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
32
Q

How many fold increases would the Goldman equation conclude if MP to potassium accounts for after hyper polarisation?

A

10 folds

33
Q

What is the basis of an action potential ion flow?

A
  • 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

34
Q

How is the difference in permeability achieved?

A
  • voltage gated Na+ channels and voltage gated K+ channels mediate the changes in membrane permeability to there respective ions
35
Q

How to show action potential propagation?

A
  • multiple electrodes along an axon show there is a traveling wave
36
Q

Explain the refractory period

A
  • no new action potential can be initiated
  • ensures unidirectional action potentials
  • prevents bounce back form axon terminal
  • repents summation of action potential
37
Q

What happens during the refractory period to the voltage gated Na+ channels that mediate rising phase of the AP?

A

transiently inactivated

38
Q

How does action potential propagation work?

A
  • 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
39
Q

What are the AP parameters?

A
  • 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
40
Q

What is the equation for the action potential velocity?

A

u = k x deriv(d)

u= velocity
k= constant
d= Anton diameter

41
Q

By how much does the diameter have to increase to increase the velocity by 10x

A

100x

42
Q

what is myelination?

A
  • mechanism for increasing the velocity of action potential propagation without increasing axon diameter

Physiology (18 decks)

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