SESSION 4

Question 1

What is the Nernst equation?

Answer 1

It gives the equilibrium potential of an ion:

Eion = RT/ZF In [ion] out/[ion]in

R is the Gas constant
T is the absolute temperature
F is faraday’s number
Z is the valency (+1 for K+)
[ion]out is the extracellular concentrations of the ion
[ion] in is the intracellular concentrations of the ion

Question 2

What is responsible for the unequal distribution of inorganic ions between the intracellular and extracellular fluid?

Answer 2

Selective permeable ion channels

Question 3

Resting cell membranes are selectively permeable to K+. Given the concentration gradient that exists across the plasma membrane, which direction would you expect K+ ion will move?

What effect will this have on the membrane potential and why?

Answer 3

As a result of the concentration gradient - more potassium inside the cell
You would expect K+ ions to move out of the cell

This will hyperpolarise the membrane as the inside of the cell will become more negative

Question 4

Extracellular concentration of K+ - 4.5
Intracellular concentration of K+- 160

Using the Nernst equation, calculate the K+ equilibrium potential (Ek) for the cells

Answer 4

61/1 log [4.5/ 160] = -94.6/ -95

Question 5

The actual membrane potential of a nerve cell, when measured by a micro-electrode, was found to be different to that calculated, why?

Answer 5

Potassium ions are not the only ions moving across the membrane, therefore they affect the value

Question 6

What contribution does the Na+ K+ ATPase make to the maintenance of the resting membrane potential ?

Answer 6

2 potassium ions in
3 sodium ions out
Pump brings the potassium ions back inside the cell
Essential for maintaining the electrical gradient

Question 7

Some neurotransmitters act to increase Cl- conductance in the postsynaptic cell. What are the consequences of an increased Cl- conductance for the membrane potential?

Answer 7

Chloride ions move from outside the cell to inside
This increased the negative charge within the cell
Resulting in hyperpolarisation

Question 8

During the initial phase of action potential in nerve and muscle plasma membranes the Na+ permeability increases.
What will happen to the membrane potential?

Answer 8

More sodium will enter the cell
This results in the cell become more positive
Therefore depolarisation takes place

Question 9

The membrane potential is restored rapidly to resting levels in nerve and muscle cells after an action potential.
How is this achieved?

Does the same thing happen in non- excitable cells?

Answer 9

Sodium channels close and potassium channels open therefore the charge within the cell will rapidly decrease- repolarisation

In non- excitable cells the same thing would happen

Question 10

During the heartbeat myocardial Ca2+ channels open and result in a substantial increase in Ca2+ permeability.
In which direction does the Ca2+ flow?

Answer 10

Calcium flows intracellularly
Calcium is released from T-tubules and rapidly increased in the cytoplasm
This calcium is required for initiating the muscle contraction

Results in depolarisation of the cell

Question 11

What would the clinical consequences be for a hyperkalemic patient with a plasma concentration of 7.5mM

Answer 11

This is an abundant amount of potassium outside the cell
There would therefore be a huge influx of potassium ions
Resulting in depolarisation

Potassium can be toxic in large amounts
This massive influx results in contractions
May even result in arrhythmia in the heart

Question 12

How can membrane potentials be measured?

Answer 12

Using a microelectrode

Skeletal and cardiac muscle have the largest resting potential

Nerve cells have resting potentials in the range -50- -75mV

Question 13

What are ion channels characterised by?

Answer 13

Selectivity:
Channel selective for Na+, K+, Ca2+, Cl-

Gating:
The channel can be open or closed by a conformational change in the protein molecule

High rate of ion flow down the electrochemical gradient

Question 14

Define potassium equilibrium potential (Ek)

Answer 14

The membrane potential at which there is no net movement of K+ ions

It can be calculated using the Nernst equation

Question 15

Describe how the resting potential is set up

Answer 15

At rest, the membrane has open k+ channels
K+ ions diffuse out of the cell, down the concentration gradient
Since anions cannot follow, the cell becomes negatively changed inside
Results in an electrochemical gradient–> potassium ions want to move both in and out of the cell- no net movement
So the system will come into equilibrium

Question 16

Describe the relationship between ion permeability and the resting potential

Answer 16

Open k+ channels dominate the resting permeability, so the RP is close to Ek (-95mV)- however the membrane is not perfectly selective

The dependence of the resting potential on K + permeability means that changing Ek will change the RP

Increasing K+ outside makes Ek more positive and so changes the membrane potential in the same direction

E.g. Cardiac (-80mV) and nerve cells (-70mV) –> RP is close to Ek
But not perfectly selective

Erythrocytes (-9mV) –> virtually no selectivity for K+

Skeletal muscle (-90mV) –> close to both Ecl and Ek

Question 17

Define depolarisation

Answer 17

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

Opening of Na+ or Ca2+ channels

Question 18

Define hyperpolarisation

Answer 18

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

Opening K+ or Cl- channels

Question 19

Define repolarisation

Answer 19

An increase in the membrane potential, so that the inside of the cell becomes less positive

Question 20

What are the three main ways in which channels are gated?

Answer 20

Ligand gating:
The channel is opened by binding of a chemical ligand
E.g. Channels at synapses that respond to a neurotransmitter

Voltage gating:
The channel opens or closes in response to changes in the membrane potential
E.g. Channels involved in action potential

Mechanical gating:
Channels open or close in response to membrane deformation
E.g. Channels in mechanoreceptors; carotid sinus stretch receptors

Question 21

Explain the difference between the fast synaptic transmission and the slow synaptic transmission

Answer 21

Fast synaptic transmission receptor is a ligand- gated ion channel
Two functions:
- bind its cognate receptor
- act as a ion channel

Slow synaptic transmission receptor is not an ion channel, but signals to the channel in one of two ways, both involving GTP binding protein:

1) direct G- protein gating:
- localised
- quite rapid
- G protein has to move-slow, but as it is localised= rapid

2) gating via an intracellular messenger
- throughout the cell
- amplified by cascade
- activates an enzyme causing a signalling cascade, activated the channel, number of intermediate= slow

Question 22

Define excitatory postsynaptic potential (EPSP)

Answer 22

Depolarising transmitter open channels
Channels selective for Na+ and Ca2+
Lead to excitation of cells - more likely to fire an action potential
Change in membrane potential is called EPSP

• longer time than action potential
• graded with amount of transmitter
• transmitter include: acetylcholine, glutamate and dopamine

Question 23

Define inhibitory postsynaptic potential

Answer 23

Hyperpolarise got transmitters 
Channels selective for K+ and Cl-
Lead to inhibition - less likely to generate an action potential 
Change in membrane potential 
Transmitters include: glycine and GABA

Question 24

Define membrane potential

Answer 24

Membrane potential is the magnitude of an electrical charge that exists across a plasma membrane and is always expressed as the potential inside the cell relative to the extracellular solution

Membrane potentials are measured in millivolts

Question 25

Explain the concept of selective permeability

Answer 25

The phospholipid bilayer:

• hydrophobic interior
• permeable to small uncharged molecules
• very impermeable to charged molecules

Ion channels

• proteins that enable ions to cross cell membranes
• have an aqueous pore through which ions flow by diffusion/ flow in both directions down their concentration gradient

Channel proteins:

• selectivity for one ion species- cation channels
• gating: the pore can open or close by a conformational change in the protein
• rapid ion flow down the electrochemical gradient

Question 26

Define conductance

Answer 26

The contribution of each ion to the membrane potential will depend on how permeable the membrane is to that ion
Real cell membranes have channels open fro more than 1 type of ion

The GHK equation (Goldman- Hodgkin- Katz) is used to measure the membrane potential

Question 27

Outline some of the roles of the membrane potential in signalling between cells

Answer 27

Example: the neuromuscular junction, motor neurone terminals release acetyl choline that binds to receptors on the muscle membrane

Nicotinic acetylcholine receptors:

• have an intrinsic ion channel
• opened by binding of acetylcholine (x2)
• channel lets Na+ and K+ through but not anions
• moves the membrane potential towards 0mV - intermediate between Ena and Ek

Question 28

Describe where synaptic connections occur

Answer 28

Nerve cell - nerve cell
Nerve cell - muscle cell
Nerve cell - gland cell
Sensory cell - nerve cell

Question 29

Describe two factors that can influence membrane potential

Answer 29

Changes in ion concentration

• most important is extracellular K+ concentration
• can alter membrane excitability, e.g. In the heart so a heart transplant can take place

Electrogenic pumps

• Na+ K+ ATPase- 3 sodium out and 2 potassium in
• makes cell more negative
• responsible for maintaining resting membrane potential

Question 30

Use the islets of langerhans to explain the clinical relevance of membrane potentials

Answer 30

Islets of langerhans- secretion of insulin

• glucose enter the cell through a channel
• mitochondria metabolise ADP to ATP- increasing the ratio
• ATP attaches to receptor preventing potassium leaving
• the membrane depolarises
• Calcium therefore enters
• the massive influx of calcium stimulates the excretion of insulin through vesicles, via exocytosis

Question 31

Describe the properties of cardiac ion channels

Answer 31

Selectivity- only permeable to a single type of ion
Voltage- as the cell depolarises or depolarises specific channels open and close
Time dependence- some ion channels are configured to close a fraction of a second after they open