Week 3 - The Resting Membrane Potential

Question 1

What is the membrane potential?

Answer 1

The electrical potential difference across the plasma membrane

• Always expressed as the potential inside the cell relative to the extracellular solution
• ~70mV

Question 2

How can you measure the resting membrane potential?

Answer 2

• Use a very fine micropipette that will penetrate the cell membrane
• It is filled with a conducting solution

Question 3

What is the range of resting membrane potentials in different cells?

Answer 3

• Animal cells: -20mV - -90mV
• Cardiac and skeletal muscle cells: -80mV - -90mV
• Nerve cells: -50mV - -75mV

Question 4

How can a membrane be selectively permeable?

Answer 4

By way of channel proteins
- Membrane-spanning transport proteins that allow ions to permeate
Characterised by:
- Selectivity: the channel lets only 1 ion species through
- Gating: channel can be opened or closed by a conformational change
- Rapid ion flow: always down the electrochemical gradient
So depending on which types of channel are open, the resting membrane can be selectively permeable to certain ion species

Question 5

What are the normal intracellular and extracellular concentrations of Na+, K+, Cl- and A- in a typical mammalian cell?

Answer 5

• Na+: I = 10mM, E = 145mM
• K+: I = 160mM, E = 4.5mM
• Cl-: I = 3mM, E = 114mM
• A-: I = 167mM, E = 40mM

Question 6

What is the potassium equilibrium potential? (Ek)

Answer 6

The membrane potential at which the electrical (anion) and diffusional forces balance one another and there is no net movement of K+

Question 7

What is the effect of changing Ek?

Answer 7

• The resting membrane potential is close to Ek, since open K+ channels dominate the resting permeability of many cells (but there are leaky Na+ and Ca2+ channels so it is slightly more positive)
• Hence changing Ek will change the resting membrane potential
• Increasing the extracellular concentration of K+ makes Ek more positive, and so changes the membrane potential in the same direction

Question 8

What is depolarisation?

Answer 8

A decrease in the size of membrane potential, so that the inside of the cell becomes less negative

Question 9

What is hyperpolarisation?

Answer 9

An increase in the size of membrane potential, so the the inside of the cell becomes more negative

Question 10

What is the relationship of membrane potential and cell signalling?

Answer 10

Changing membrane potentials underlie many forms of signalling between and within cells
E.g.
- Action potentials in nerve and muscle cells
- Triggering and control of muscle contraction
- Control of secretion of hormones and neurotransmitters
- Transduction of sensory information into electrical activity by receptors
- Post-synaptic actions of fast synaptic transmitters

Question 11

What is the effect of opening channels in the membrane for a particular ion?

Answer 11

The membrane permeability for that ion is increased

• So the membrane potential will move towards the equilibrium potential for that ion
• I.e. Opening Na+ and Ca2+ channels will depolarise cells
• Opening K+ and Cl- channels will hyperpolarise cells

Question 12

What is the effect on the membrane potential if channels for more than 1 ion species is open?

Answer 12

Each of the ion species will contribute to the membrane potential

• How important each ion is will depend on how easily it can get through the cell membrane relative to other ions
• This depends on the number of available channels and how easily they let the ion through

Question 13

What are the different gating mechanisms for channel opening?

Answer 13

• Ligand gating: the channel is opened (or closed) by binding of a chemical ligand, which may be an extracellular transmitter or an intracellular messenger
• Voltage gating: the channels opens or closes in response to changes in the membrane potential
• Mechanical gating: channels opens or closes in response to membrane deformation

Question 14

Where can synaptic connections occur?

Answer 14

Between:

• Nerve cell and nerve cell
• Nerve cell and muscle cell
• Nerve cell and gland cell
• Sensory cell and nerve cell

Question 15

What happens in fast synaptic transmission?

Answer 15

• The receptor is a ligand-gated ion channel
• Transmitter binding causes the channel to open
• Excitatory transmitters open ligand-gated channels that cause membrane depolarisation
• – Can be permeable to Na+, Ca2+ or several cations
• – The resulting change in membrane potential is called an excitatory post-synaptic potential
• – Transmitters include acetylcholine and glutamate
• Inhibitory transmitters open ligand-gated channels that cause hyperpolarisation
• – Permeable to K+ or Cl-
• – Causes an inhibitory post-synaptic potential
• – Transmitters include glycine and gamma-aminobutyric acid

Question 16

What happens at a synapse?

Answer 16

A chemical transmitter is released from the presynaptic cell and binds to receptors on the post-synaptic membrane

Question 17

What happens in slow synaptic transmission?

Answer 17

The receptor is not itself an ion channel, but it signals to the channel in 1 of 2 ways:

1. Within the membrane (rapid and localised)
2. Via an intracellular messenger (amplification by cascade, throughout cell)
   - Both involve a GTP-binding protein

Question 18

What factors can influence the membrane potential?

Answer 18

• Changes in ion concentration

- Electrogenic pumps (Na/K-ATPase sets the concentration gradient)