Nerve cells and excitability Flashcards
(52 cards)
1
Q
How does the concentration of potassium ions vary inside and outside the cell?
A
K+ is high inside the cell but low outside the cell.
2
Q
How can sodium or potassium be transported across the membrane?
A
Using sodium-potassium ATPase transporters that use ATP to transport the ions against the concentration gradient.
3
Q
What is the resting membrane potential and how is it measured
A
- The potential difference (mV) between two electrodes placed inside and outside the cell.
2, A sharp electrode in the soma records the electrical potential across neuronal membrane - ~-70 mV in most neurones
4, Depends on separation of charge across lipid bilayer membrane - Charge resides on ions inside and outside cell
- Permeability of membrane is crucial
4
Q
What is depolarisation?
A
- Increase in membrane potential.
2. The potential moving from RMP to less negative values
5
Q
Hyperpolarisation?
A
Decrease in membrane potential (more negative).
6
Q
Where are pyramidal neurones found?
A
The hippocampus.
7
Q
Where are Purkinje neurones found?
A
The cerebellum.
8
Q
What does the Nernst equation allow?
A
The equilibrium potential for any ion to be calculated.
9
Q
What is the Nernst equation?
A
- Eion = RT/zF x log [ion]outside/[ion]inside.
- F = faradays constant
- z=charge on ion
- T=absolute temperature
- R=gas constant.
10
Q
What is the refractory period?
A
When Na+ channels become inactivated as the membrane depolarizes and cannot be activated again until the membrane is repolarized.
11
Q
What are graded (local) potentials?
A
- Changes in the membrane potential that are confined to a small region of the membrane.
12
Q
Distribution of charged ions
A
- sodium potassium atp transporter maintains gradient, 2. pumps against concentration gradient
- Na+ out, 2 K+ in
- using active transport
13
Q
3 transporters
A
- active transporters
- ion channels- selectively permeable, ions diffuse down concentration gradient
- voltage gated channels: passive, selective, rapid
14
Q
what drives conformational change of channels
A
phosphorylation
15
Q
How can Vm (RMP) be calculated
A
- Goldman equation
- Vm= RT/F *(Pk[K+]out+ PNa[Na+]out+ PCl-[Cl-]in/Pk[K+]in+ PNa[Na+]in + PCl-[Cl-]out)
- Pion- relative permeability of membrane to ion
- [ion]out- concentration of ion outside the cell
- [ion]in- concentration of ion inside the cell
- F = faradays constant
- T=absolute temperature
- R=gas constant.
16
Q
repolarisation
A
potential moving back to RMP
17
Q
propagation
A
- movement of AP along axon
18
Q
How many states do Na+ VGC’s exist in
A
- 3
- open, closed, inactivated
- once inactivated can’t go back to being open, have to go back to being closed, then open
19
Q
how many states for K+ VGC
A
- 2
2. open and closed
20
Q
Sequence for an action potential ( Na+ VGC’s)
A
- Na+ open rapidly with depolarisation- influx of Na+ ions.
- Inactivation gate rapidly blocks Na+ permeability during continued depolarisation
Inactivated gates move to closed on repolarisation
21
Q
Sequence for an action potential ( K+ VGC’s)
A
- open slowly on depolarisation, K+ move out of cell, drawing +ve charge out
- close slowly on repolarisation
- K+ continue to move out till reach equilibrium potential for K+. no net movement
- At this point K+ VGC closed and Na+ inactivated
- Then pump establishes concentration gradient from the beginning
22
Q
Threshold
A
point at which AP is generated
determined by extent of depolarisation
23
Q
refractory period- why is it absolute?
A
- because Na+ VGC are inactivated- unable to be opened (unlike in a close state)
- so cannot generate another membrane because it would require the membrane potential to be at rest and Na+ VGC’s to be open
24
Q
refractory period- why is it relative?
A
- because the membrane is hyperpolarised until K+ channels close, so an AP can only be generated if stimulus is stronger than usual
25
What makes an AP move fast?
1. axon diameter: large axons offer less resistance to current because they have a larger diameter, so faster propagation.
2. In small axons the signal dissipates and leaks out of the membrane
3. myelin sheath: electrical insulation, current moves faster. Prevents signal leaking out.
4. Gaps in the myelin sheath are nodes of ranvier which have clusters of Na+channels, so saltatory conduction can occur-
5. AP jumps node to node and depolarisation only occurs where there's no myelin
26
What are graded potentials
1. graded based on changes in strength of stimulus - unlike APs- (e.g. pushing harder on finger generates a stronger potential)
2. changes in membrane confined to a small area
3. depolarisation/hyperpolarisation
4. decay rapidly
5. don't propagate
6. add to show summation
7. if threshold for VGSC is reached, an AP is generated
8. Confined to small region of the membrane
27
How does the concentration of chloride ions differ from the inside to outside of the cell?
Higher concentration outside the cell than inside the cell.
28
What is the equilibrium potential for K+?
-80mV.
29
What is the equilibrium potential for Na+?
+60Mv.
30
The larger the concentration gradient....
...the larger the equilibrium potential.
31
What is resting membrane potential close to and why?
The potassium equilibrium potential (EK) as the permeability of potassium is greater than for sodium.
32
What is the symbol for resting membrane potential?
Vm.
33
What is an action potential?
1. A large transient change (reversal) in membrane potential
2. ‘all or none’ responses
3. Very rapid (1–4 milliseconds)
4. Can repeat at frequencies of several hundred per second
5. Depend on several types of membrane ion channels.
6. Also known as spikes, nerve impulses, nerve discharges
34
What happens to K+ channels during depolarisation?
They open slowly.
35
What happens to K+ channels during repolarisation?
They close slowly.
36
What are the characteristics of an action potential?
1. All or nothing event
2. Threshold- must reach to be generated
3. Regenerative- once it has started it generates itself
4. Overshoot- go beyond 0 about +40
5. Undershoot- goes beyond resting potential
6. Refractory period- period of time before another action potential can be generated
7. Propagate
8. Non-decremental- doesn’t change amplitude as it goes along
37
What is the absolute refractory period?
1. AP cannot be evoked as VGNC channels are inactivated
2. cannot be activated again until the membrane is repolarised and resting state restored
3. Stop cells firing at too high frequency
38
What is the relative refractory period?
1. Membrane potential hyperpolarised by VGKC.
| 2. AP can be generated if the stimulus strength is strong enough overcome hyperpolarisation to reach threshold
39
What is saltatory conduction?
1. The propagation of an action potential involving nodes of Ranvier.
2. Stimulation exceeds threshold for an AP at Node of Ranvier A.
3. Local current along inside of axon without leak
4. Local current opens VGNC and generates an AP at node B.
5. VGNC inactivation and open VGKC prevent back-propagation to node A
6. Process repeated at node C - AP ‘jumps’ in one direction down axon at high speed without decrement
40
What are the characteristics of graded potentials?
1. They can be a depolarisation or hyperpolarisation
2. their size and duration is graded
3. they decay rapidly
4. they travel small distances
5. show summation.
41
What are the different things a channel can be
1. channels may be leak (open)
| 2. voltage or transmitter gated
42
Describe how ion channels are involved in ion transportation
1. Membrane-spanning proteins - connect the cytosol to the cell exterior
2. Leak channels - pores - permanently open
3. Flux of ions is passive, selective and rapid
4. Flux is bidirectional
5. Direction of flux determined by concentration and charge
43
Describe the electrochemical equilibrium
1. When K+ is high inside it will diffuse out – chemical gradient
2. Cl- ions cannot exit – no channels
3. Builds –ve charge inside vs outside – membrane potential
4. Internal negativity draws K+ back in – electrical gradient
5. When chemical = electrical no net flow of K+
6. Membrane potential at balanced flux = equilibrium potential
7. -58 mV for K+ for 10 fold concentration gradient
8. Most of internal negative charge is on large anions and proteins rather than Cl- ions
44
What is Vm dependent on
1. K+ gradient
2. Increasing [K+]out makes Vm more positive (depolarised)
3. Decreased chemical gradient
4. When [K+]out = [K+]in, Vm = 0
5. Non-linearity is due to Na+ and Cl- permeability
6. Greater at more negative potentials due to increased electrical gradient
45
How is excitability maintained
1. Maintaining concentration gradients for Na+ and K+ is essential for excitability
2. Achieved via active exchange via the Na-K transporter aka sodium potassium pump-
3. phosphorylation of pump results in a conformational change causing Na+ to be released and K+ binding
4. Dephosphorylation- induced conformational change releases K+ - Exchanges 2K in for 3K out
5. Electrogenic and contributes to Vm
46
What is the difference between active and passive current
1. Passive- is conducted but decays
| 2. Active- conducted and doesn't decay
47
Describe leak channels
1. permanently open pore
2. set the resting membrane potential
3. NOT involved in action potentials
48
Describe voltage-gated channels
1. closed at resting membrane potential
2. only open when membrane is depolarized from rest
3. change in voltage is the trigger for opening
4. return to resting potential closes channels
5. responsible for initiation (Na+) and termination (K+) of action potentials
49
Describe Sodium Voltage-gated channels
1. Na+ open rapidly with depolarisation
2. Inactivation gate rapidly blocks Na+ permeability
3. inactivated channels blocked during continued depolarisation
4. Inactivated channels move to closed state on repolarisation
50
Describe Potassium voltage-gated channels
1. K+- open slowly during depolarisation
2. channels remain open during depolarisation
3. close slowly on repolarisation
51
What is the sequence of events in an action potential
1. Resting potential VGNC and VGKC closed
2. Stimulus causes depolarization to threshold - VGNC open
3. Na+ flows in - membrane rapidly depolarises - more VGNC open - VGKC still closed
4. VGNC inactivated - Na+ entry slows - VGKC open - K+ flows out membrane repolarisation begins
5. VGNC move to closed state VGKC remain open - delayed hyperpolarisation
6. Resting potential restored
52
Describe the action potential propagation
1. Stimulation at A exceeds threshold for an AP.
2. Local current spreads further along axon
3. Local current exceeds threshold, opens VGNC and generates an AP at B.
4. VGNC inactivation and open VGKC prevent back-propagation
5. Local current spread repeats process at point C, allowing AP to flow in one direction down axon without decrement
