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Physiology, Neurobiology and Cell Signaling > Neurobiology 2 > Flashcards

Neurobiology 2 Flashcards

Electrical properties of neurones

1
Q

How can the membrane potential be measured?

A

Using membrane potential difference between two electrodes.

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2
Q

When was the first glass microelectrode found?

A

-Intracellular glass microelectrodes: cells are very small, so hard to get access inside.
-First discovered by Ling and Gerard (1949).

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3
Q

What is membrane potential at rest?

A

~ -70mV
This is resting potential.

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4
Q

What is hyperpolarising?

A

Making the membrane potential more negative.

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5
Q

What is depolarising?

A

Making the membrane potential more positive.

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6
Q

What does the resting membrane potential require?

A

-Intact cell (semi-permeable) membrane.
-Ionic concentration gradients and ionic permeabilities, particularly K+ ions.
-Over the long term: metabolic processes.

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7
Q

What did Julius Bernstein (1880s) discover?

A

-the ionic theory
-the Nernst equation
-semi-permeable membrane
-noted that in order to create potential, a membrane is needed- metabolic process is needed to create gradient

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8
Q

What is the ideal plasma membrane?

A

-Impermeable to Na+ ions
-Changing Na+ concentration will not affect resting potential

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9
Q

What ions are and aren’t permeable to membrane?

A

Permeable: Na+, K+ and Cl-
Impermeable: anions

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10
Q

What happens at equilibrium?

A

-There is a balance between K+ ions moving in and out of the cell, this occurs at the resting potential.
-Concentration and electrical gradient are equal.

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11
Q

What causes electrical gradient?

A

Caused by moving only K+ from one side to another.

Leaving behind Cl-, leaves a negative electrical gradient in opposite way to chemical gradient.

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12
Q

What does Ek mean?

A

The balance point/ the resting potential for ideal membrane.

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13
Q

How can you predict membrane potential changes with extracellular [K+] if membrane is only permeable to K+ ions?

A

Can use equation to predict what might happen.

Ek = equilibrium (conc gradient = electrical gradient)
no voltage, no difference

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14
Q

What is Ek and Em usually?

A

Ek ~ -80mV
Em ~ -70mV

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15
Q

Why is membrane potential usually less negative than Ek?

A

-Cell membrane not completely impermeable to Na+ (Na+ moves in).
-Na+ and K+ movements will change the membrane potential.
-Depolarise: membrane potential less negative.

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15
Q

What is the permeability of K+ and Na+?

A

K+ = 1
Na+ = ~0.01

Under resting conditions, membrane is less permeable to Na+ than K+

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16
Q

What maintains ionic gradients?

A

ATP dependent ion pumps

It is a very energy expensive process.

2K+ in and 3 Na+ out
ATP -> ADP +Pi (intracellular)

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17
Q

What is the action potential?

A

-Major mechanism of neuronal communication.
-Travels down axon to terminals.
-Does not decrement, due to myelin.
-Trigger transmitter release.

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18
Q

How is the action potential generated?

A

By spending so much ATP.

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19
Q

What are the stages of the action potential?

A

-70mV is resting membrane potential
Sub-threshold stimulus
Threshold stimulus
-55mV is threshold potential
-55mV to 0mV is depolarisation
Overshoot
0mV to -55mV is repolarisation
Hyperpolarisation
Resting membrane potential

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20
Q

What happens in repolarisation?

A

Action potential is switched off.

This is important as the action potential triggers a lot of neurotransmitter release.

24
What does the Na+ influx cause?
The rising phase of action potential.
24
What was observed in the squid axon (Hodgkin and Katz, 1949)?
Stimulus, squid axon, measurable membrane potential. Overshoot occurs when Na+ is 50%, strong driving force, more Na+, rate of depolarisation is more rapid.
25
What happens when Na+ channels open?
This allows the influx of Na+
26
What are voltage gated channels? How are they activated?
Transmembrane proteins. Selective for ionic species e.g. Na+, K+, Ca2+ ect. Activated by changes in voltage (depolarisation).
27
When do voltage-dated Na+ channels open/close?
Resting potential ~-70mV (channels closed) Depolarised to -30mV (channels open)
28
Why does Na+ move into cell when channels open?
Concentration gradient- inward: outside (120mM) -> inside (12mM) Electrical gradient- inward: positive ions -> inside (-40mV) Driving force = concentration gradient + electrical gradient
29
What initially depolarises neurones to open the voltage-gated Na+ channels?
Synaptic transmission: excitatory postsynaptic potentials (EPSPs)- summative Generator (receptor) potentials (sensory neurones)- pressure Intrinsic properties (e.g. pacemaker activity in heart) Experimental (e.g. electrical stimulation)
30
Is Na+ channel opening regenerative?
Yes. Na+ channels open, Na+ influx (positive ions move into the neurone), depolarisation. CYCLE Explosive process, this is why action potential occurs quickly.
31
Describe the threshold.
Action potentials have threshold. Once threshold has been crossed the action potentials are the same size, 'all or none'. Size = same, frequency = changes.
32
What is the effect of hyperpolarisation?
Loss of + charge. Further away from the threshold. Makes it harder to initiate an action potential.
33
What happens in depolarisation?
Na+ moves into neurone via voltage gated Na+ channels. Provide Na+ influx and depolarise membrane.
34
What is repolarisation?
Na+ channels close. K+ moves out of the neurone via voltage gated K+ channels. This gives rise to hyperpolarisation.
35
What happens in overshoot?
Voltage gated Na+ channels start to close. Voltage gated K+ channels open- delayed becuase if they open at the same time as the Na+ channels they would compete against each other. As a result, there could not be much action potential.
36
What happens to the ion flow during the action potential?
-Around threshold Vm, the membrane becomes much more permeable to Na+ ions. -This leads to depolarisation and further recruitment of VG Na+ channels. -Depolarisation results in VG Na+ channels inactivation (closure). -After a delay, VG K+ channels open. -Both contribute to the repolarisation of the membrane after the action potential. *Na+ and K+ ions increase to the point where channels close*
37
What is conductance?
The movement of ions.
38
What contributes to repolarisation?
Na+ channels close (inactive) VG K+ channels open (after delay) Conc gradient: outward (125 mM K+ inside, 5 mM K+ outside) Electric gradient: outward (membrane potential: positive) Therefore, K+ ions move out of the neuron (repolarise)
39
What is the ball and chain model? Explain how it works.
VG Na+ channel inactivation. -Positively charged activation gene keeps channel closed. -Depolarisation of membrane causes activation gate to swing out of the way, allowing Na+ ions to enter and cause further depolarisation. -The inactivation 'ball' rapidly enters the channel to block Na+ influx.
40
What is the point of the refractory perion?
Ensures the action potential travels in one direction. Can't keep supplying action potential all the time
41
Where are the action potentials initiated?
At the axon hillock. High density of VG channels, anchored by axon and microtubules.
42
What is the correlation between the diameter and the speed of action potential conduction?
Bigger diameter = fast conduction E.g. squid giant axon (350 um) 25 m/s cockroach giant interneurones (60 um) 7 m/s small sensory fibres (1 um) 1 m/s
43
What is the absolute refractory period?
Starts from when GV Na+ channels open and continues for ~1 ms. All / enough of VG channels are inactivated. During this time it is not possible to elicit another action potential. The ARP is due to VG Na+ channel inactivation.
44
What is the relative refractory period?
Continues for 2-3 ms after the ARP. Action potentials can be elicited, but requires stronger or longer stimulation. The increased K+ permeability during the RRP makes it harder to depolarise the membrane to activate VG Na+ channels and electit an action potential.
45
Why is it harder to elicit an action potential in RRP?
Because conductance of membrane to K+ are still high. Because conductance of K+ during action potential remains elevated for a period of time after action potential.
46
What are the energy requirements of action potentials?
Do not need immediate energy source to fire action potentials. -Over the long term: build up of internal Na+ and loss of K+ will lead to a loss of membrane potential. -Therefore, do need ATP in the long term (to run pumps). -Maintaining and restoring Na+ gradients may use up ~32% of the brain's ATP, with a further ~43% used in synaptic transmission.
47
What is saltatory conduction?
Myelination greatly accelerates action potential velocity Action potential doesn't go backwards because previous nodes are now in the refractory period, allows action potential to only go in 1 direction.
48
What is the role of myelin?
Concentrates points where action potential is generated to the gaps in between the myelin. Action potential initiated and Na+ diffused down the axon to depolarise next node.
49
Why does the myelinated axons (cat) have a higher conduction velocity than unmyelinated axons (squid)?
Myelinated axons- quicker conduction, due to high density of VG Na+ channels. Unmyelinated axons- flow of ions across the entire length of that axonal membrane, not efficient and makes it prone to failing. Don't maintain action potential for the length of that axon.

Physiology, Neurobiology and Cell Signaling (8 decks)

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