5. The Resting Membrane Potential and Changing Membrane Potential

Question 1

How is the membrane potential expressed?

Answer 1

The potential inside the cell relative to the extracellular solution.

Question 2

What are three channel properties that effect the permeability?

Answer 2

Selectivity for one or a few ion species.
Gating so a pore can open or close by conformational change.
Rapid ion flow so ions move down their electrochemical gradient.

Question 3

What is a key ion in setting up the resting potential?

Answer 3

K+

Question 4

What influences the rate of K+ movement across the cell membrane?

Answer 4

The chemical gradient for K+ and the electrical gradient. The chemical gradient pulls K+ out to where the concentration is lower but the electrical gradient pulls K+ in to the negative inside of a cell.

Question 5

What is the equilibrium potential?

Answer 5

The electrical and chemical gradients for ion balances so there is no net driving force on the ion across the membrane, calculated using the Nernst equation.

Question 6

If the membrane were only selectively permeable to K+, what would its membrane potential be equal to?

Answer 6

The equilibrium potential of K+, -95mV.

Question 7

Why is the resting potential not -95mV?

Answer 7

Because the cell is still slightly permeable to ions other than K+. Na+, Ca2+ and Cl- can move into the cell via voltage sensitive channels that spontaneously open.

Question 8

What is the usual resting potential of cardiac tissue and nerve cells? Explain why they have the values they do.

Answer 8

Cardiac muscle -80mV and nerve cells -70mV. These are both close to the equilibrium constant of K+ (-90mV) but not exactly Ek so the membrane is predominantly selective for K+ but not perfectly selective.

Question 9

What is the usual resting potential of smooth muscle cells? Explain why they have the values they do.

Answer 9

-50mV, quite a lot lower than Ek so there must be increased contribution from other ion channels to the resting potential.

Question 10

What is the usual resting potential of skeletal muscle? Explain why they have the values they do.

Answer 10

-90mV because there are many Cl- and K+ channels open so the resting potential is close to both Ecl and Ek.

Question 11

Give two examples of how changing membrane potentials can be used in signalling between and within cells.

Answer 11

Action potential in nerve and muscles 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 12

Define depolarisation.

Answer 12

A decrease in the size of the membrane potential from its normal resting value. The cell interior becomes less negative.

Question 13

Define hyperpolarisation.

Answer 13

An increase in the size of the membrane potential from its normal value. The cell interior becomes more negative.

Question 14

How is a change in membrane potential achieved?

Answer 14

By changing membrane ion permeability, the more permeable the membrane is to an ion, the closer the potential moves towards that ions equilibrium potential.

Question 15

Which equation for membrane potential looks at the influence of all ions?

Answer 15

Goldman-Hodgkin-Katz equation.

Question 16

Give an example of a less selective ion channel at the neuromuscular junction.

Answer 16

Nicotinic acetylcholine receptors, activated by acetylcholine, lets through Na+ and K+ but not anions. This moves the potential towards 0mV - intermediate between Ena and Ek.

Question 17

What are the three types of gating used to control channel activity?

Answer 17

Ligand gating - channel opens or closes in response to binding of a chemical ligand.
Voltage gating - channel opens or closes in response to changes in membrane potential.
Mechanical gating - channel opens or closes in response to membrane deformation.

Question 18

What is fast synaptic transmission?

Answer 18

The receptor protein is also an ion channel. So transmitter binding causes the channel to open immediately.

Question 19

What are excitatory transmitters?

Answer 19

They open ligand-gated channels that cause membrane depolarisation. The change in membrane potential is excitatory post-synaptic potential.

Question 20

What are inhibitory synapses?

Answer 20

The open ligand-gated channels that cause hyper polarisation and an inhibitory post-synaptic potential, further away from the threshold.

Question 21

What are slow synaptic transmitters?

Answer 21

The receptor and channel are separate proteins so the response is slower.

Question 22

What are the two types of slow synaptic transmission?

Answer 22

Direct G-protein gating - localised and quite rapid (not as quick as fast synaptic transmission though). The receptor has 7 transmembranous domains and when the ligand binds, it releases the G protein which binds to the channel and opens it.
Gating via an intracellular messenger - throughout cell and amplified by cascade. The G protein is released and binds to an enzyme, the enzyme converts an inert substance into a signalling molecule and triggers a signalling cascade.

Question 23

What two other factors can influence membrane potential apart from channels?

Answer 23

Changes in ion concentration - most importantly K+ concentration.
Electrogenic pumps - like the Na+/K+ ATPase, this can contribute to the negative cell potential as each cycle moves out one positive charge.

Question 24

How does hyperkalaemia affect depolarisation?

Answer 24

K+ concentration outside is raised so the outward gradient for K+ is less so there is less outflow and the membrane is depolarised and closer to the threshold.

Question 25

How can membrane potential be measured?

Answer 25

Using a micro electrode (glass pipette), to record the potential relative to the inside of the cell.