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PN3313 > Neuronal function > Flashcards

Neuronal function Flashcards

1
Q

How does electrical signalling work?

A

The nervous system transmits information within individual nerve cells as rapidly changing voltages across the plasma membrane

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2
Q

What is the symbol and unit of charge?

A 
Symbol = Q 
Coloumbs = unit
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3
Q

What is Faraday’s constant?

A

10^5 C = 1 mol monovalent ion

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4
Q

Define current. What is its symbol and and unit?

A

Current is the flow of charge

Symbol = I, unit = Amps (A)

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5
Q

What is voltage?

A

The field strength generated by charge separation where potential difference (p.d) is the difference in field strength between two points in space (unit = volts (V))

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6
Q

What will low resistance result in?

A

High conductance and high current for a given voltage

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7
Q

What are the symbols and units of conductance and resistance?

A

conductance g, conductance siemens
resistance R, ohms
R = 1/g

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8
Q

What is ohms law?

A

V=I*R

V=I/g

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9
Q

What is a capacitor, and what acts as a capacitor in the neuron?

A

Capacitor = 2 conductors separated by an insulator

in the neuron this is the membrane

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10
Q

What effect do capacitors have on circuits?

A

‘store’ charge causing a voltage to develop across it until voltage on capacitor = applied voltage where charge stored for given V depends on capacitance:
V*C=Q (Q = charge)

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11
Q

What is passive conduction?

A
  • Response amplitude proportional to different stimulus strength
  • attenuates (decreases over length of axon)
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12
Q

What is active conduction?

A
  • Constant amplitude with frequency proportional to strength
  • No attenuation
  • Propagates with finite conduction
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13
Q

What are the advantages of active conduction?

A
  • Resistant to noise

- Can travel a long distance with no degradation

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14
Q

What are the disadvantages of active conduction?

A
  • Indirect coding so requires time to integrate
  • Limited frequency range, as if frequency is too low will take too long to integrate
  • “range fractionation” must be used to correct this
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15
Q

How doesw integration occur?

A
  • Inputs at dendrites are non spiking

- Integrated over soma which if abpve threshold can cause spike at the axon hillock

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16
Q

What are the two types of summation?

A

Spatial (across different points)

Temporal (adds sum of close spikes, or pairing with inhibitory input)

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17
Q

What is the resistor-capacitor (RC) model?

A
  • Phospholipid bi-layer (capacitor)
  • Ion channels (resistor/conductor)
    IN PARALLEL
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18
Q

What determines the final voltage at the point of injection in passive conduction?

A

How much current is being injected and the sum of the resistance (Ohms law) (V=IR)

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19
Q

Why does passive conduction voltage attenuate?

A
  • Current moves along membrane and gradually leaks out via ion channels causing a voltage
  • Gradually less current and voltage (in an exponential fashion)
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20
Q

What is the space length constant (λ)

A

Describes the how rapidly the voltage drops with distance
Voltage at point x = V max e^(-x/λ)
If λ is large voltage will drop slowly visa versa. The distance at which the voltage is around 37% of that of the point of current injection

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21
Q

What does λ depend on?

A
  • membrane resistance (if low, current will leak out more quickly)
  • internal resistance (if low current can spread more quickly)
  • Glial wrapping (increase)
  • Fatter axons (increase)
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22
Q

What does λ result in?

A

More integration

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23
Q

Why is there a delay in voltage response relative to current stimulus?

A
  • Because the phospho-lipid membrane is a resistor

- Time taken to reach half of its final voltage is the time constant (t) where t = R*C

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24
Q

What does a longer time constant result in?

A

More potential for temporal integration

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25
Q

What are the limitations of the space constant equations?

A
  • Assume infinitely long cable of constant diameter (not true for any neuron)
  • Can’t calculate voltage profile
  • Can use computer to stimulate this through “compartmental modelling”
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26
Q

What is the Nernst equation?

A

E (eq) = 58/z log (X outside/X inside)

27
Q

What does the Nernst equation determine?

A

The equilibrium potential = voltage at which ions would be exactly balanced across the membrane

28
Q

What is the equation for ion flux?

A

I = g (Em - Eeq) gives driving force V

29
Q

What is the resting membrane potential?

A

NOT equilibrium potential ~-60mV. Is a compromise between the K and Na equilibrium potentials, although membrane more permeable to K so more weighted towards that

30
Q

What are leakage channels?

A
  • mediate the resting potential

- low conductance (higher for K) that is unchanging (unaffected by voltage and ligands)

31
Q

What channels are responsible for action potentials?

A
  • Voltage dependent Na/K channels

- Open (high conductance) when membrane depolarises

32
Q

What is the difference in opening and shutting speeds between Na and K v-dependent channels?

A
  • Na open rapidly and shut automatically even if membrane is still depolarised
  • K open and shut slowly
33
Q

What is the absolute refractory period caused by?

A
  • Na channels take time to de-inactive until then cannot open
34
Q

What is the relative refractory period caused by?

A
  • K channels shut slowly causing excess K conductance, harder to cause spike (but still possible)
35
Q

Why are action potentials hard to investigate?

A

Action potentials involve change in voltage, but these change conductance and current. Cannot only vary one parameter

36
Q

What is a voltage clamp?

A
  • 1 electrode monitors voltage on membrane with differential amplififier
  • 1 electrode injects current proportional to injected change
37
Q

What have voltage clamp experiments revealed about the K current?

A
  • Slower to respond, must kick out positive current to counteract
38
Q

What have voltage clamp experiments revealed about the Na current?

A
  • Responds quickly to change and automatically turns itself off, even though it’s positive
39
Q

What drugs can be used to block Na and K channels?

A

Na - TTX

K - TEA (less potent)

40
Q

What effect do leakage channels cause in voltage-clamp experiments?

A

Small compared to other currents, must inject small ammount of positive current

41
Q

What effect is seen on the Na current if current is increased slightly?

A
  • Decreases more quickly but to the same level

- Due to more Na channels being opened but driving force remaining the same

42
Q

Why is the K equilibrium potential harder to determine?

A
  • As it is very negative and so at this negative voltage all channels are shut
  • Take advantage of slow shutting, look at changes in current after you have opened them (“tail current”)
43
Q

What effect does changing current level have on K channels?

A

See that membrane is more negative and so shut (but slowly)

44
Q

What happens when you clamp a synapse at resting potential?

A
  • Observe having to inject negative current
  • Conclude increase in positive ions on the post-synaptic membrane
  • Much faster, less capacitance
45
Q

What are the problems with voltage clamp experiments?

A
  • Voltage often differs along a neuron, want a good “space-clamp” where this isn’t an issue
46
Q

What are the 6 principles of the Hodgkin-Huxley model of action potentials?

A
  1. Currents in action potentials due to electrochemical gradients (no pumps)
  2. Na and K flow through seperate channels
  3. Channels are either open or shut with no intermediates
  4. Each channel has 1 or more gates
  5. Channel is only open if all gates are open
  6. Each channel has 1st order kinetics with voltage dependent rate constants
    i. e voltage gates open and close randomly, probability of being open is reliant on voltage
47
Q

What did Hodgkin-Huxley propose as the two types of Na channel gate?

A
  • Activation gate (m) opens quickly with depolarisation with 3 channels in series
  • Inactivation gate (h) closes with slowly depolarisation, 1 gate
    Sigmoid rise for activation, exponential drop for inactivation
48
Q

What happens when the voltage dependent constants alpha and beta are large?

A

Rapid response to changes in voltage

49
Q

What did Hodgkin-Huxley propose as the gate for K channels?

A

1 type of gate (n) which opens with depolarisation, 4 in series

50
Q

Describe an action potential in terms of the ion channel gates opening and shutting

A

Resting - m likely to be shut, h likely to be open
Depolarising stimulus- opens m gate quickly, h gate slow to respond, Na ion inflow
Repolarisation - h gate closes and n (K) gate opens, outflow of K

51
Q

Describe the absolute and relative refractory period in terms of ion channels opening and shutting

A

Absolute - second stimulus may open m gate but h gate stays shut as it is slow to respond
Relative - Na channel has returned to resting state by K channel is still open, excess conductance requires a stronger force to ellicit response

52
Q

In what sense were Hodgkin-Huxley accurate?

A

Amino acids with charged side chains can act as voltage sensors enacting conformational changes on ion channels

53
Q

In what sense were Hodgkin-Huxley innacurate?

A

Channels are not completely independent of eachother, probability of activation of one gate influences activation of other gates

54
Q

Describe the Na channels and their gates

A
  • alpha subunit is v-dependent formed from 4 repeating domains of of transmembrane alpha helixes
  • Pore lining proteins have negative amino acids interact with Na+
  • Sensors on S4 displace outwards with depolarisation opening the pore
  • h gate residue can block channel for inactivation
55
Q

Describe the K voltage dependent channel

A
  • Structurally similar but no activation gate
56
Q

How can channels be selectively permeable to Na or K?

A

Depend on hydration shells and not size

57
Q

How can you patch clamp a channel?

A
  • Electrode bonds to membrane with small patch with singular membrane
  • Can manipulate where outside membrane is, therfore control of conditions
  • Record current through singular ion channel
58
Q

What has been shown from single patch-clamp recordings?

A
  • Channels either open or shut

- Na and K currents made up of the summed conductance of Na and K channels opening and shutting

59
Q

How does the chinese white shrimp have the fastest conduction?

A
  • Fat fibre with thin axon and thick myelin sheath

- Conduct via penaid saltatory conduction, through fenestration node and submyelinic space

60
Q

Where do Ca channels occur?

A
  • Cell body (mediating intracellular signal)

- Synapses

61
Q

What do Ca channels do?

A

Tells neuron how active it is (increases with depolarisation)

  • Can function as pacemaker (sinoatrial node)
  • Can regulate metabolism, muscle contraction and gene expression
62
Q

Describe inactivating K channels

A
  • Known as A-type
  • Slow response to neuron in response to depolarisation
    e. g in inking, takes prolonged stimulus to provoke
63
Q

Describe Ca dependent K channels

A
  • Can produce hyperpolarising pauses as well as make neurons fire bursts of spikes
  • Does not need synaptic input

PN3313 (9 decks)

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