M&R Session 3 (Lecture 3.1)

Question 1

How can a membrane potential be measured?

Answer 1

1) Microelectrode (fine glass pipette) which penetrates the cell membrane.
2) Filled with conducting solution (KCl)
3) Measures membrane potential

Question 2

How are membrane potentials expressed?

Answer 2

In millivolts relative to the extracellular solution

Question 3

Why are membrane potentials set up?

Answer 3

The membrane is selectively permeable to different ions.

Question 4

How is the permeability of ions changed in membranes?

Answer 4

Opening or closing of ion channels, gated channels and concentration gradient

Question 5

How is the resting membrane potential set up?

Answer 5

Open K+ channels dominate the membrane ionic permeability at rest.
K+ effulx occurs due to chemical gradient so membrane becomes more negative.
No channels open for anions (A-) e.g. proteins.
Electrical gradient of K+ wants it to flow back into cell but when chem=elec, no net flow.

Question 6

State the electrical gradient equation.

Answer 6

VzF

Question 7

State the chemical gradient equation.

Answer 7

RT ln [K+o]/[K+i]

Question 8

What is the Nernst equation for potassium?

Answer 8

At equilibrium, the electrical and chemical gradient for K+ balance, so that there is no net driving force on K+ across the membrane.

V=RT/ZF ln [K+o]/[K+i]

Question 9

What is Ek equation.

Answer 9

Ek=61/z log [K+o]/[K+i] at 37 degrees C

Question 10

What would the membrane potential be if the cell was selectively permeable for potassium only?

Answer 10

At Ek i.e. -94.6mV

Question 11

Give examples where changing the membrane potential is necessary?V

Answer 11

1) Action potential in nerve and muscle cells
2) Muscle contraction
3) Secretion of hormones and NTs
4) Transduction of sensory info and electrical activity
5) Postsynaptic actions of fast synaptic transmitters

Question 12

What is depolarisation?

Answer 12

Decrease in size of the membrane potential from its normal value e.g. -70 to -50 mV

Question 13

What is hyperpolarisation?

Answer 13

An increase in the size of the membrane potential from its normal value e.g. -70 to -90 mV

Question 14

What does increasing the membrane permeability of one ion do?

Answer 14

Causes the membrane potential to move towards the equilibrium potential for that ion

Question 15

What does the GHK equation stand for and what does it allow?

Answer 15

1) Stands for Goldman-Hodgkin-Katz

2) Includes permeabilities of Na+, Cl- and K+ for a real cell which has imperfect selectivity for ions (i.e. >1).

Question 16

Give an example of a less selective channel and describe its actions.

Answer 16

1) nAChR

2) Have an intrinsic ion channel
3) Opened by Ach
4) Lets Na+ and K+ through but not A-
5) Moves membrane potential towards 0 mV (betweem Ena and Ek)

Question 17

Describe the three types of channels.

Answer 17

1) Ligand gated (opens/closes to ligand binding or channels open/close to secondary intracellular messengers)
2) Voltage gated (opens/closes in response to a voltage change in the membrane)
3) Mechanical gated (opens/closes in response to membrane deformation e.g. mechanoreceptors in carotid sinus)

Question 18

Describe fast synaptic transmission.

Answer 18

Receptor protein is also an ion channel e.g. nAChR

Question 19

What is an EPSP?

Answer 19

Excitatory post-synaptic potential.

Channels open causing the post synaptic membrane to depolarise e.g. Na or Ca influx.

1) Longer than AP (20ms compared to 1ms)
2) Graded with amount of transmitter
3) Transmitters include: ACh and Glutamate

Question 20

What is an IPSP?

Answer 20

Inhibitory post0synaptic potential

Inhibitory transmitters open ligand-gated channels that cause hyperpolarisation e.g. K+ or Cl-

E.g. Glycine or GABA

Question 21

Describe slow synaptic transmission.

Answer 21

Receptor and channel are separate proteins.

1) Direct G - protein gating (localised and quite rapid)
2) Gating via an intracellular messenger (throughout cell, amplification by cascade)

Question 22

What two other factors can affect membrane potential?

Answer 22

1) Changes in ion concentration (most important is [K+], can be altered e.g. hyperkalaemia.
2) Electrogenic pumps e.g. Na+ pump, contributes few mV directly to the membrane potential making it more negative.