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Cellular Physiology > Dr. Bai section on ions, nerves, and electrical potential (lec 11-15) > Flashcards

Dr. Bai section on ions, nerves, and electrical potential (lec 11-15) Flashcards

1
Q

Common electrical units used:
Potential / voltage (V)?
Current (I)?
Resistance (R)?
Conductance (G)?

A

Units:
Volt (V) - typical range: mV (10^-3 V)
Ampere (A) - nA (10^-9 A) nano, pA (10^-12 A) pico
Ohm (horseshoe) - M (10^6) mega, G (10^9) giga
Siemens (S) - nS (10^-9 S) whole cell, pS (10^-12 S) channel

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2
Q

Define current.
What is the direction of current?

A

Movement of charges.
Movement of positive charges from high to low electrical potential (voltage).
Larger difference in electrical potential can cause movement to be faster.

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3
Q

Ohm’s law

A

I = V/R or V = IR

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4
Q

Conductance calculation

A

G = 1/R therefore I = GV

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5
Q

What are ion channels and what are characteristics of them?

A

Proteins that span the cell membrane to form pores, allowing ions to traverse the generally impermeable lipid bilayer.
Conduct ions, selective, and open/ closes under diff conditions - allow/ stop flow of certain ions.
Electrical activity of neurons (excitatory and inhibitory) is almost entirely through opening and closing of ion channels

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6
Q

Types of gating for channels (sensor for gating):

A
  1. Electrical (voltage-gated ion channels) - e.g. many Na+ channels, L- and T-type Ca2+ channels
  2. Chemical (ligand-gated ion channels) - e.g. nAChR, AMPAR, NMDAR, and GABAR (neurotransmitters)
  3. Mechanical (mechanically gated ion channels) - physical stretching of membrane.
    Bonus- also temperature gated but not required in this course.
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7
Q

What is the normal polarization of the cell membrane (charge on either side)?

A

Negative on the inside, positive on the outside.

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8
Q

How does gating channels work?

A

Subtle changes in conformation underlie opening of channel. In voltage gate, the sensor actually moves up (showing a small current) in order to open channel.

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9
Q

What are the physical properties that determine how fast ions can move through channels?

A

Size, charge, hydration, mobility: velocity (microns/s) or (V/cm).

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10
Q

What is the size, charge, hydration # and mobility of Potassium ions?

A

Size (radius): 1.33 A (Armstrong: 10^-10m)
Charge: +1
Hydration #: 2.9
Mobility: 7.6 (um/s or V/cm)

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11
Q

What is the size, charge, hydration # and mobility of Sodium ions?

A

Size (radius): 0.95 A (Armstrong: 10^-10m)
Charge: +1
Hydration # 4.5
Mobility: 5.2 (um/s or V/cm)
*Carry more water so slower than other ions.

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12
Q

What is the size, charge, hydration # and mobility of Chloride ions?

A

Size (radius): 1.81 A
Charge: -1
Hydration #: 2.9
Mobility: 7.9 (um/s or V/cm)

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13
Q

What do we know about hydration?

A

Hydration depends on charge and size (OR charge density: amount of charge per unit volume)
Hydration number determines ionic mobility in water.
Water acts as a dipole to stabilize ions
Hydration # is not static, it is dynamic (changing)

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14
Q

Hydration as a selectivity filter explanation

A

Carbonyl oxygens on peptide chain in the region of the selectivity filter replace water and stabilize each ion as it passes through the pore. e.g. these carbonyl oxygens are the perfect distance for K+ but not for the smaller Na+ in K+ channel - selective filter.

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15
Q

Number of subunits or domains in an ion channel:
Transmitter-gated ion channels
Gap junction hemichannels
Voltage-gated ion channels

A

Transmitter-gated ion channels: 5 (pentamer) - not absolute, also tetramers
Gap junction hemichannels: 6 (# can go up or down)
Voltage-gated ion channels: 4 (tetramer) - most, sometimes a monomer
*pore size differs - smaller with less subunits

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16
Q

Define homo-oligomeric

A

All subunits are the same making up the ion channel

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17
Q

Define hetero-oligomeric

A

There are different subunits making up the ion channel

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18
Q

Define oligo

A

More than one subunit in ion channel (e.g. dimer, trimer, tetramer…)

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19
Q

Which part of the nicotinic AChR contributes to the pore lining region?

A

TM2 (M2) of each subunit contributes to the pore lining region - interacts with ions - often determines pore qualities.
*All subunits have 4 transmembrane domains - TM1-TM4

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20
Q

Which part of the nicotinic AChR contributes to the binding of acetylcholine?

A

The two alpha subunits both contain bonding pockets and both require bonding to open channel.

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21
Q

What is the molecular structure of nicotinic AChR?

A

It is a pentamer hetero-oligomeric (but all subunits share similarities)
Subunits include: two alpha subunits, beta, gamma, and delta.

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22
Q

What is the selectivity filter and gating of nAChR (nicotinic AChR)?

A

Negatively charged residues form a selectivity filter for a cation channel - repel negative anions (non-selective cation channel).
Cross-section at the gate in middle of the membrane, showing van de Waal’s surfaces of the atoms - very small rotation that is enough to “close”.

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23
Q

What type of ion channel is the nAChR?

A

Chemical (ligand-gated ion channel) - neurotransmitter opens channel.

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24
Q

What is the structure of the voltage-gated K+ channels?

A

Homo-Tetramers (all same type of subunit).
Each subunit is comprised of 6 membrane spanning domains, with cytoplasmic amino- and carboxyl-terminal regions.

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25
Q

What part of the voltage-gated K+ channel is known as the pore loop? Where is it found?
What does it contain?

A

H5, between S5 and S6 transmembrane domains.
It contains the selectivity filter and is also the site for biding of various blockers e.g. TEA (tetra ethyl ammonium)

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26
Q

What part of the voltage-gated K+ channel represents the voltage-sensor region?

A

The S4 segment is full of charges and represents the voltage sensor region. Drives channel open or closed.

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27
Q

What is the structure K+ voltage sensors?

A

The K+ voltage sensors resemble charged ‘paddles’.
S4 voltage sensing regions contain + charged arginines.
A hinged movement gates the channel pore open or close (paddles move up to open channel).

28
Q

What is the structure of the voltage-gated Ca2+ and Na+ channels?

A

Only one subunit (monomer) that is made up of 4 domains (I-IV), each of which has the 6 membrane spanning domains.

29
Q

Where is the pore loop found in the voltage-gated Ca2+ and Na+ channels?

A

Between domains I and II

30
Q

Where is the voltage-dependent inactivation region found in the voltage-gated Ca2+ and Na+ channels

A

Residues between domains III and IV responsible for inactivation gate

31
Q

Describe the N-type inactivation (A-type K+ channel)

A

The inactivation depends on the amino terminus (N-terminus)

32
Q

Describe the ball and chain model of channel inactivation
How is the inactivation gate removed?

A

Depolarization leads to channel activation, followed by a delay with blocking of the pore by the N-terminus
The N terminus swings into the channel and binds to channel pore quite tightly - almost like ligand + receptor
Membrane hyper-polarization is required for the removal of inactivation.

33
Q

What are gap junctions?
How are they diff from other channels?

A

A special class of membrane channels that couple neighbouring cells together by creating pores from the cytoplasm of one cell to the cytoplasm of another.
Diff from other channels which were intracellular to extracellular - these are intracellular to intracellular of two cells.

34
Q

How many subunits make up a gap junction?
What are these subunits called?

A

All together it is called a decamer (12 subunits), made up of two hemichannels that are made of 6 subunits each (hexameric).
Each molecule/ subunit is called a connexin, so 6 connecins assemble to form hemichannel and 2 hemichannels from neighbouring cells dock together to form a functional gap junction.

35
Q

List 5 main properties of gap junctions - how are these diff from chemical synapses?

A
  1. Provide cytoplasmic continuity between cells, so current passes between cells, typically in both directions (electrical synapse) - diff from chemical synapse which only goes on way
  2. An electrical synapse has a very short synaptic delay (0.1 ms) - chemical synapse 10x slower, has synaptic delay of 1 ms
  3. Synchronous activation of a large # of cells in heart and smooth muscles.
  4. Channels are large enough to pass small molecules (<1kD) between cells, e.g. nutrients, metabolites, and signalling molecules
  5. Increased intracellular acidity (decreased pH) or Ca2+ close gap junctions
36
Q

Why do we need patch clamp?

A

Monitoring cell electrical activities
Recording under controlled conditions

37
Q

What is a patch electrode and a sharp electrode?

A

Sharp tip, usually glass but can also use metal, impales cell. Since tip is so small, has very little damage. Can record electrical potential, and can also inject current.
The difference between the two is just that the patch electrode has a slightly more blunt tip, but still very sharp.
*Forms ‘giga’ seal with membrane through gentle suction.

38
Q

What are the two types of recording from a patch clamp?

A
  1. Current-clamp (I-clamp) - records voltage (commonly in mV) - close to physiological conditions - injects current (positive to mimic excite - negative to mimic inhibitory)
  2. Voltage-clamp (V-clamp) - records current (commonly in pA, or nA) - voltage is under control, I= GV
39
Q

What are the 4 configurations of patch recordings?
Which can do both voltage and current clamp?
Which can do single channel recording?
Which have full control of membrane potential?

A
  1. Cell-attached patch recording (used if want intracellular content unchanged - but don’t know what resting membrane potential is with this clamp)
  2. Whole cell recording (only one which can do both voltage and current clamp - the rest can’t do current clamp)
  3. Excised inside-out patch (inside of membrane facing out)
  4. Excised outside-out patch (outside of membrane facing out) - might be best choice for ligand gated channel
    1, 3, and 4 can do single channel recording, 2 cannot.
    2, 3, and 4 have full control of membrane potential, 1 does not.
40
Q

Define the diffusion coefficient

A

Parameter to describe how fast an ion (or atom) can defuse through a membrane barrier.

41
Q

What are the two forces on an ion?

A
  1. Chemical force - depends on the concentration gradient and absolute temp (T)
  2. Electrical force - depends on the charge and electrical gradient (potential)
    *Total force = electrical force + chemical force
42
Q

When will diffusion of a certain ion stop?

A

Diffusion of an ion will “stop” when the chemical force is balanced by the electrical force resulting in an electrochemical equilibrium (balanced movement)

43
Q

What is the Nernst equation? What do each of the variables represent?

A

Determines the equilibrium potential for membrane permeable only to one type of ion.
Ex= (RT/zF) ln ([X2]/[X1]) (measured in V)
Ek: equilibrium potential for X ion
R: universal gas constant (2 cal/ degree K x mole)
T: absolute temp (degrees Kelvin: 273 + C)
z: valence of ion
F: Faraday’s constant (# charges per mole: 96, 487 C/ mole, equivalent to 2.3 x 10^4 cal/ V x mole)
[X2]: concentration of X in compartment 2 or extracellular
[X1]: concentration of X in compartment 1 or intracellular
Example: For +1 charge ion
*Ex= 0.06 log ([X2]/[X1]) (in V)
*Ex= 60 log ([X2]/[X1]) (in mV)

44
Q

Define the equilibrium potential for an ion.
What are the common values for ion potentials of K+, Na+, and Cl-?

A

Dependent on ionic distribution, each cell type has its own equilibrium potential for each ion species. Common values are:
EK+ = -89~-100 mV
ENa+= +45~+60 mV
ECl-= -65~-75 mV

45
Q

State Ohm’s law for ion X
What are the conditions for each direction of current?

*Check with confusing current /+

A

Ix = Gx (Em - Ex), where Gx: conductance, Em: membrane potential, and Ex: equilibrium potential for ion x
When (Em-Ex) > 0, the current is outward (+)
When (Em-Ex) < 0, the current is inward (-)
When Em= Ex, the current is 0

46
Q

What is the slope of the line in a current-voltage relationship?

A

slope = G (conductance)

47
Q

What is Goldman equation?

A

Em= (RT/F) ln [(PnaNao + PkKo + PclCli) / (PnaNai + PkKi + PclClo)
e.g.
Pna = Na+ permeability
Nao = extracellular Na+ concentration
Nai = intracellular Na+ concentration
*membrane potential Em depends on 3 ions: Na+, K+, and Cl- permeabilities (P) - more common condition than in Nernst equation - Goldman equation reduces to the Nernst equation for only 1 ion.

48
Q

Define capacitance.
What is an example of a capacitance?
Why do we care about electrical capacitance?

A

A capacitance (C) is defined by its ability to store charges (Q) across an insulator.
Equal and opposite charges are stored across C.
Capacitance current (Ic) can increase (or decrease) the number of charges across C.
Cell membrane (membrane is thin) is an insulator and can store charges just like a capacitance.
Rather than a sharp drop or increase in voltage after current injection, the increase/ decrease instead occurs slowly, illustrating the membranes ability to store charge.

49
Q

What is the passive membrane model?

A

Electrical model of the membrane is a resistance and a capacitance in parallel.
Im = Ic + Ir
The ion channel is considered the resistance, while the membrane is the capacitance.
*At peak depolarization, no current goes through the capacitance, all goes through channel.

50
Q

How does input current effect output voltage?

A

Positive current depolarizes the membrane. Output voltage is slower than input current, slow to rise, and slow to fall.
Higher input current gives proportionally higher output voltage.

51
Q

Passive membrane properties

A

Graded
Depolarizing or hyper-polarizing
Not self generating
No refractory period
Linear properties

52
Q

Define action potential and its properties

A

A voltage-dependent response.
All or none
Self-generating after threshold is exceeded

53
Q

Describe the action potential features overshoot and undershoot.

A

Overshoot is the membrane depolarization that occurs after 0 mV, continued depolarization.
Undershoot is the after hyper-polarization that occurs after the repolarization, this is lower than the resting membrane potential.

54
Q

What are the absolute and relative refractory periods?

A

The absolute refractory period occurs while the membrane is repolarizing (returning to resting membrane potential) - no stimulus can cause an AP during this time. This is while the Na+ channels are inactivated and thus cannot be opened again.
The relative refractory period occurs after the absolute refractory period and during this time you need a stronger stimulus in order to stimulate an AP. At this point, sufficient Na+ channels have opened again - but have to overcome elevated K+ channels.

55
Q

What are the main differences between passive membrane and AP properties?

A
  1. Passive membrane is graded (not fixed amplitude) - AP is all or none (fixed amplitude)
  2. PM not self generating (no propagation), AP is self generating (propagate)
  3. PM is depolarizing or hyper-polarizing, AP is depolarizing followed by AHP (fixed profiles)
  4. PM does not have refractory period, AP has absolute and relative refractory periods
  5. PM has linear properties, AP has non-linear properties.
56
Q

What is the sodium hypothesis? Explain the experiment used to support this hypothesis.
(Hodgkin & Katz’s hypothesis)

A

The sodium hypothesis explains that the peak amplitude of an AP is Na+ mediated. The peak approaches the equilibrium potential of sodium ions, and this is responsible for the overshoot.
During an experiment, the Na+ concentration was gradually reduced (replaced by choline+ to keep osmolarity, larger molecule unable to go through Na+ channel). With the decrease in Na+ concentrations, see a reduction in the peak. Without any sodium, there is no depolarization whatsoever.

57
Q

How does the permeability of ions change during an action potential?

A

Rest: Pk= 1, Pna= 0.04, Pcl = 0.45,
Em= -60mV
Peak: Pk = 1, Pna= 20, Pcl = 0.45,
Em= +42 mV
AHP: Pk = 1.8, Pna= 0, Pcl = 0.45,
Em= -70 mV

58
Q

Which animal was used to study the voltage clamp

A

This technique was used with the squid giant axon. Because the axon is so large (almost 1 mm in size), they could perform voltage clamp by two electrodes, one to inject current and one to measure voltage.

59
Q

One way to stop the movement of Na+ across its channel is the use of choline to replace Na+ concentration. What is another method?

A

Block Na+ channels with Tetrodoxin (puffer fish toxin) - TTX

60
Q

What is the name of the current from K+ ions?

A

Delayed rectifier current (slower activation).

61
Q

How can you isolate for Na+ current?

A
  1. Subtract isolated K+ current recorded from total current.
  2. Block K+ current with tetraethylammonium (TEA) or 4-aminopyridine (4-AP)
62
Q

What are the characteristics of Na+ current?

A

Very rapid rise/ activation (activated within 1/2 ms)
Has decay, inactivation after only a few ms
Inward current (negative current)
Causes depolarization

63
Q

How would you describe the AP rise phase for the Na+ channel?

A

Positive feedback loop:
Initial trigger causes depolarization which opens Na+ channels, causing an inward flow / current of Na+, which further drives depolarization. (self-generating and rapid).

64
Q

How would you describe the AP repolarization phase for the K+ channel?

A

Gk a delayed negative feedback:
After depolarization there is a short delay and then K+ channels open, and there is an outward flow / current of K+, this causes hyper-polarization.

65
Q

How would you describe the AP repolarization phase for Na+ channel?

A

Na+ channel inactivation. The channel is inactivated, and flow of ions stops.

66
Q

What happens to an action potential when you block:
1. K+ channels
2. Na+ channels

A
  1. Prolong action potential - allows more neurotransmitter release (some use for this in clinical settings).
  2. No action potential, graded response instead.

Cellular Physiology (7 decks)

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