Week 4 - neurones

Question 1

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

Answer 1

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

Question 2

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

Answer 2

• 70 mV

Question 3

What two factors determine ion behaviour?

Answer 3

diffusion and electric fields

Question 4

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

Answer 4

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

Question 5

What happens when the solution starts with different concentrations?

Answer 5

• ions follow a diffusion gradient from high to low

Question 6

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

Answer 6

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

Question 7

What is Bernstein’s hypothesis?

Answer 7

• 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

Question 8

What os the equilibrium potential of potassium (Ek)?

Answer 8

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

Question 9

Explain the equilibrium potential of potassium (Ek)

Answer 9

• 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

Question 10

How can we test Bernsteins hypothesis?

Answer 10

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

Question 11

How does Bernstein’s hypothesis vary from the Ek ?

Answer 11

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

Question 12

How do we test Bernstein’s hypothesis?

Answer 12

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

Question 13

What else might need to be considered?

Answer 13

influx of different ions

Question 14

Why can chloride not be an implication?

Answer 14

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

Question 15

is sodium a factor?

Answer 15

• positive equlilibrium potential ENa= +55mV

Question 16

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

Answer 16

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

Question 17

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

Answer 17

that there must be an unequal permeability

Question 18

What is the Goldman, Hodgkin and Katz equation?

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

What is the nature of action potentials?

Answer 21

all or nothing

Question 22

What is responsible for an action potential?

Answer 22

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

Question 23

What happens when sodium is absent?

Answer 23

action potentials cannot occur in cultured neurones

Question 24

How did Hodgkin and Huxley measure an action potential?

Answer 24

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

Question 25

explain what it means that Aps are all or nothing events

Answer 25

- 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

Question 26

is an action potential more than just depolarisation?

Answer 26

yes - neuron overshoots 0mV and hyperpolarises following repolarisation

Question 27

what are the three stages of an action potential?

Answer 27

- rising phase (depolarisation) - falling phase (repolarisation) - (hyperpolarisation = dip) - after hyper polarisation (refractory period )

Question 28

How to test Overtons theory that sodium matters?

Answer 28

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

Question 29

What does Overtons theory test show?

Answer 29

- confirms hypothesis - indicates that rising phase of AP is due to transient increase in MP to Na+

Question 30

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

Answer 30

500 fold

Question 31

Are ions basis for the falling phase of the AP?

Answer 31

- 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

Question 32

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

Answer 32

10 folds

Question 33

What is the basis of an action potential ion flow?

Answer 33

- 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

Question 34

How is the difference in permeability achieved?

Answer 34

- voltage gated Na+ channels and voltage gated K+ channels mediate the changes in membrane permeability to there respective ions

Question 35

How to show action potential propagation?

Answer 35

- multiple electrodes along an axon show there is a traveling wave

Question 36

Explain the refractory period

Answer 36

- no new action potential can be initiated - ensures unidirectional action potentials - prevents bounce back form axon terminal - repents summation of action potential

Question 37

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

Answer 37

transiently inactivated

Question 38

How does action potential propagation work?

Answer 38

- 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

Question 39

What are the AP parameters?

Answer 39

- 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

Question 40

What is the equation for the action potential velocity?

Answer 40

u = k x deriv(d) u= velocity k= constant d= Anton diameter

Question 41

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

Answer 41

100x

Question 42

what is myelination?

Answer 42

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