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ESA 1 - ICPP > Membrane Potential > Flashcards

Membrane Potential Flashcards

1
Q

What is the membrane potential of a cell?

A

The magnitude of an electrical charge (mV) that exists across a plasma membrane.
Negative at rest in all mammalian cells.

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2
Q

Give examples of resting cell membrane potentials.

A
  • Cardiac and skeletal muscle cells have largest resting potentials: -80mV to -95 mV.
  • Nerve cells: -50mV to -75mV.
  • Erythrocytes have the smallest: -9mV.
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3
Q

How is MP measured?

A

Using a voltmeter and microelectrode.

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4
Q

What are the 2 minimum essential factors in generation of the MP?

A
  • Asymmetric distribution of ions across the PM (i.e. Ion conc gradients)
  • Selective ion channels in the PM: K+, Na+ and Cl- channels are the most important for most cells
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5
Q

Describe the selective permeability of the plasma membrane.

A

Phospholipid bilayer has a hydrophobic interior so

  • is permeable to small uncharged molecules (O2, CO2, H2O, benzene)
  • but very impermeable to charged molecules (ions) - requires ion channels
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6
Q

What are the properties of ion channels?

A

Have an aqueous pore through which ions flow by diffusion in both directions down their chemical gradient.

  1. Selectivity: for one (or a few) ions species
  2. Gating: pore can open or close by a conformational change in the protein
  3. Rapid ion flow (microsecs)
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7
Q

Describe the ionic concentrations for a typical mammalian cell.

A

Intracellular Extracellular (plasma)

Na+ 10mM. Na+ 145mM
K+ 160mM. K+ 4.5mM
Cl- 3mM. Cl- 114mM
A- 167mV. A- 40mM

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8
Q

What dominates the membrane inonic permeability at rest?

A
  • Open K+ channels
  • When the K+ chemical gradient and electrical gradient are equal and opposite, there will be no net movement of K+ but there will be a negative charge across the membrane - the resting MP.
  • So resting MP arises due to selective permeability to K+.
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9
Q

What does the Nernst equation calculate?

A
  • The MP at which an ion will be in equilibrium, given the extracellular and intracellular concentrations of the ion.
  • E.g. Ek = K+ equilibrium potential = -95mV
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10
Q

What are depolarisation and hyperpolarisation?

A
  • Depolarisation: decrease in the size of the MP from its normal value - cell interior becomes less negative.
  • Hyperpolarisation: increase in the size of the MP from its normal value - cell interior become more negative.
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11
Q

What causes changes in MP?

A
  • Changes in the activity of ion channels - move the MP towards the equilibrium potential of those ions.
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12
Q

In which cells do K+ channels have an important effect on MP?

A
  • Cardiac muscle cells (-80mV) and nerve cells (-70mV): resting MP quite close to Ek but not exactly as membrane not perfectly selective for K+.
  • Skeletal muscle: many Cl- and K+ channels open in resting membrane so resting potential = 90mV, close to both Ek and Ecl.
  • Cells with lower resting MPs have lower selectivity for K+ - increased contribution from other channels. E.g. Smooth muscle cells (-50mV), erythrocytes have virtually no K+ selectivity (-9mV).
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13
Q

Opening of which channels leads to hyperpolarisation and depolarisation?

A
  • Opening Na+ or Ca2+ channels - depolarisation (Ena = +70mV, Eca = +122 mV).
  • Opening K+ or Cl- channels - hyperpolarisation (Ek = -95mV, Ecl = -96mV).
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14
Q

What is membrane conductance?

A

How permeable a membrane is to specific ions (i.e. The number of open channels for that ion).

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15
Q

Which equation describes the imperfect selectivity of membranes?

A

Goldman-Hodgkin-Katz equation (involves relative permeabilities to K+, Na+ and Cl-).

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16
Q

How can channel activity be controlled?

A
  • They are gated:
    1. Ligand gating - open/close in response to binding of a chemical ligand.
    2. Voltage gating - open/close in response to changes in MP.
    3. Mechanical gating - open/close in response to membrane deformation (e.g. Hair cells of the inner ear).
17
Q

What do ligand-gated channels give rise to?

A

Synaptic potentials

18
Q

Where do synaptic connections occur?

A

Between:

  • nerve cell - nerve cell
  • nerve cell - muscle cell
  • nerve cell - gland cell
  • sensory cell - nerve cell
19
Q

What is the difference between fast and slow synaptic transmission?

A
  • Fast (microseconds): the receptor protein is also an ion channel - transmitter binding causes channel to open.
  • Slow (milliseconds): the receptor and channel are separate proteins.
20
Q

Describe fast synaptic transmission at excitatory synapses.

A
  • Excitatory transmitters (e.g. ACh, glutamate, dopamine) open ligand-gated ions channels that cause membrane depolarisation (excitatory post-synaptic potential).
  • Channel can be permeable to Na+, Ca2+ or sometimes cations in general (e.g. nAChR).
  • Longer time course than an AP (1/2 msec) and graded by transmitter amount).
21
Q

How are the actions of inhibitory transmitters different to those of excitatory transmitters at synapses?

A

Inhibitory transmitters (e.g. Glycine, GABA) open ligand-gated channels that cause hyperpolarisation - more permeable to K+ or Cl-.

22
Q

What is summation?

A

Process that determines whether an AP will be triggered by the combined effects of excitatory and inhibitory signals from:

  • multiple simultaneous inputs (spatial summation)
  • repeated inputs (temporal summation)
23
Q

What are the 2 types of slow synaptic transmission?

A
  1. Direct G-protein gating: localised and relatively rapid (as G-protein doesn’t move far to get to channel). Involves GTP to GDP hydrolysis.
  2. Gating via an intracellular messenger: acts throughout the cell and can be amplified by an enzyme cascade. Requires more time as several intermediates, culminating in intracellular messenger or protein kinase activating channel.
24
Q

Which 2 additional factors can influence membrane potential?

A
  1. Changes in ion concentration - sometimes altered in clinical situations, can alter membrane excitability, e.g. In heart. Most important is extracellular K+ concentration.
  2. Electrogenic pumps (e.g. Na+/K+ ATPase) - in some cells, this contributes a few mV directly to MP, making it more negative (e.g. In erythrocytes).

ESA 1 - ICPP (18 decks)

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