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Physiology I: Cellular Neuro > Lectures 5 & 6: Ionic Equilibria & Membrane Potentials > Flashcards

Lectures 5 & 6: Ionic Equilibria & Membrane Potentials Flashcards

1
Q

Electrochemical gradient/potential

A
  • Combination of forces acting to move a charged particle/ion
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2
Q

Concentration gradient/chemical potential

A
  • The driving force of concentration differences on the two sides of the membrane
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3
Q

The Nernst equation is valid only for

A
  • An ion at equilibrium
    OR
  • When the membrane is permeable to that ion only
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4
Q

Electrochemical equilibrium conditions

A
  • When the two forces act on an ion such that there is no net movement of the ion
  • The electrical force and the concentration force, are equal and opposite
  • Only under this condition can the Nernst equation be applied
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5
Q

The Nernst equation allows us to calculate

A
  • Magnitude of the electrical gradient (mV) that balances the chemical gradient, usually given in molarity (mM)
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6
Q

Electrochemical equilibrium definition

A
  • Balance achieved when diffusion of Na from side A down its concentration is opposed by the buildup if negativity on side A
  • At this point no more net movement of Na+ ions occurs
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7
Q

At electrochemical equilibrium there will be a

A
  • Separation of charge

- Equal numbers of charged particles will not exist on each side of the membrane

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8
Q

The size of the separation of charge will depend on

A
  • Concentrations of Na+ ions on side A and B at the end

- Can be determined using the Nernst equation if the [Na+]A and [Na+]B are known

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9
Q

In an excitable cell, such as a skeletal muscle cell, the resting measured membrane potential, Vm, is

A
  • (-90 mV)
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10
Q

Net driving force

A
  • NDF = electrical gradient - concentration gradient
  • NDF = membrane potential - equilibrium potential
  • NDF = Vm - Eion
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11
Q

For [Ko] > 10 mM, Vm behaves as though

A
  • Membrane is permeable only to K+
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12
Q

For [K+] o < 10 mM, Vm

A
  • Deviates from this simple relationship

- Some other ions must be contributing to determination of Vm

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13
Q

The Gibbs-Donnan equilibrium describes

A
  • The steady-state properties of a mixture of permeant and impermeant ions as exists in the cytoplasm of cells
  • This also encompasses the principle of electroneutrality
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14
Q

Electroneutrality states that

A
  • Any microscopic region of a solution must have an equal number of positive and negative charges
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15
Q

The presence of an impermeant ion on one side of a membrane leads to

A
  • An unequal distribution of the permeable ions

- Results in the generation of an electrical potential across the membrane

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16
Q

In cells, the permeant K+ and Cl- are nearly in

A
  • Electrochemical equilibrium across the plasma membrane - These two ions essentially obey the Gibbs-Donnan equilibrium
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17
Q

Sodium ions do not obey

A
  • Gibbs-Donnan equilibrium
  • Their movement causes changes in volume of cell to occur when the ion concentrations change either inside or outside the cell
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18
Q

NDF on Na+

A
  • Very large
  • Usually over 100 mV
  • Normally the cell is very impermeable to this ion
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19
Q

Normally, the NDF on K+ and Cl- ions

A
  • Is very small
  • Although the membrane is relatively permeable to these ions, the amount of these ions crossing the membrane is very small
20
Q

Na+ enters the cell by

A
  • Diffusion down its electrochemical gradient

- Usually pumped out of the cell, preventing the cell volume and the Gibbs-Donnan equilibrium from being disturbed

21
Q

Inhibition of the Na+/K+ ATPase causes

A
  • Disequilibration of the Gibbs-Donnan equilibrium

- Results to Na+ entry and accumulation (causes cell swelling)

22
Q

Ions in solution move across membranes under the influence of

A
  • An electrochemical gradient made up of two forces (concentration and electrical)
23
Q

Ionic diffusion ceases if

A
  • The concentration force can be balanced by the electrical force
24
Q

The Nernst equation allows the calculation of

A
  • Electrical force/equilibrium potential (Eion) that must be applied to oppose the concentration force
  • It only applies to one ion at a time
25
Q

Goldman-Hodgkin-Katz equation allows the calculation of

A
  • Membrane potential (Vm) based on ionic permeabilities
26
Q

Net driving forces can be calculated using

A
  • Vm and Eion

- These identify the nature, direction and size of the net force acting on the ion of interest

27
Q

The Gibbs-Donnan equilibrium identifies

A
  • How a membrane potential is generated when a solution containing some impermeant ions is separated form a solution containing only permeable ions
28
Q

The impermeant ions of major interest are

A
  • Proteins inside cells

- Some of the larger inorganic anions

29
Q

Two forces to consider with charged particles

A
  • Concentration/chemical (acts downhill)

- Electrical (attracting or repelling)

30
Q

Impermeable membrane characteristics

A
  • Equal numbers of charged particles on one side of membrane
  • No separation of charge
  • No Em recordable (Vm= 0)
31
Q

Non-selectively permeable membrane characteristics

A
  • Flux (J)(+) = Flux (J)(-)
  • Charged particles move equally easily in both directions until reach equilibrium
  • No separation of charge
  • No Vm recordable
32
Q

Selectively permeable membrane characteristics

A
  • Flux (J)(+)&raquo_space; Flux (J)(-)
  • (+) charged particles move until concentration force is equally opposed by electrical force
  • Electrochemical Equilibrium
33
Q

Electrochemical equilibrium is a form of

A
  • Diffusion equilibrium
  • Considers both charge and concentration of solute
  • Solute concentrations may not be equal on both of membrane
34
Q

Electrochemical equilibrium is achieved when

A
  • Movement of a charged solute down its concentration gradient is equally opposed by the increasing electrical gradient being generated due to its movement
35
Q

Nernst equation

A
  • Allows calculation of the equilibrium potential for an ion
  • This is the electrical force acting to stop NET diffusional movement of an ion across a membrane
  • The units of this force are millivolts (mV)
36
Q

Driving force

A
  • The resting membrane potential (Vm) of a cell does not equal any one ion’s equilibrium potential
  • This means that NONE of the ions are at equilibrium
37
Q

Vm is greatly influenced by

A
  • The ion with the largest permeability
38
Q

Gibbs-Donnan Equilibrium

A
  • The equilibrium reached by two solutions separated by a permeable barrier when one of the solutions contains impermeant ions
39
Q

According to Gibbs-Donnan equilibrium,

A
  • Regardless of the solution composition,
  • Equal numbers of (+) and (-) ions inside
  • Equal numbers of (+) and (-) ions outside
  • K+ and Cl- ions obey this principle across plasma membranes
40
Q

Na ions do not obey G-DE and in fact upset it because

A
  • NDF for K+ and Cl- in resting cells is quite small
  • NDF for Na+ in resting cells is very large
  • Na+ moves into cells driven by this NDF
41
Q

In G-DE, Na movement is accompanied by

A
  • Water
  • Hence, a volume change occurs
  • The cell counters this Na+ movement by pumping Na+ out, using the Na+/K+- ATPase (sodium pump)
42
Q

Charged particles, ions, in solution move

A
  • Across membrane driven by electrochemical gradient

- This gradient is made up of chemical and electrical forces

43
Q

The magnitude of the chemical force acting on an ion making it move down its concentration gradient can be calculated using

A
  • Nernst equation

- It is called Eion

44
Q

In order to look at the concentration forces acting on multiple ion simultaneously,

A
  • The GHK or Chord conductance equation is used
45
Q

Nernst and GHK equations yield

A
  • Membrane potential difference, Vm, acting across a cell membrane
46
Q

The NDF

A
  • Causes ions to diffuse down their electrochemical gradient

- Is used by cells for signaling purposes

47
Q

Gibbs-Donnan equilibrium describes a scenario when

A
  • Two solutions (one of which contains an impermeant ion) are separated by a membrane
  • Leads to the development of electrical potentials across membranes

Physiology I: Cellular Neuro (8 decks)

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