The Resting Cell Membrane

Question 1

What is the membrane potential?

Answer 1

The electrical potential (voltage) difference across the plasma membrane. It is present in all cells.
This Membrane Potential provides the basis of signalling in the nervous system as well as in many other types of cells.
The resting membrane potential may be different in different cell types.

Question 2

How is membrane potential measured?

Answer 2

An electrode is placed outside the cell to measure extracellular voltage. Another microelectrode is placed inside the cell through the plasma membrane to record intracellular voltage. Difference between extra- and intracellular voltage is measured.
The microelectrode is a fine glass pipette
Tip diameter is <1um. It can penetrate cell membrane, and cell membrane forms a seal around it.
It is filled with a conducting solution (KCl), but this could also be a mimic of extracellular ion composition.

Question 3

How are resting membrane potentials expressed, and what are some key values relating to them?

Answer 3

Membrane potentials are always expressed as the potential inside the cell relative to the extracellular solution.
Membrane potentials are measured in millivolts (mV)
Animal cells have negative membrane potentials at rest that range from – 20 to – 90 mV
Cardiac and skeletal muscle cells have the largest resting potentials : – 80 to – 90 mV
Nerve cells have resting potentials in the range: – 50 to – 75 mV

Question 4

Why is selective permeability of the cell membrane important to membrane potential, and what makes the membrane selectively permeable?

Answer 4

Selective permeability is important for the creation of the resting membrane potential, as the resting membrane potential is created by the movement of ions across the cell membrane that results in an electrochemical gradient.
The phospholipid bilayer has a hydrophobic interior, which is permeable to small uncharged molecules, e.g. O2, CO2, H2O and ethanol, but very impermeable to charged molecules, such as ions.
Ionic permeability of the membrane is mediated by the presence of ion channels/ transporters. The types of ion channels present makes the membrane selectively permeable. Key properties of ion channels include:
1. Selectivity: for one (or a few) ion species.
Na+, K+, Ca2+, Cl-, cation channels.
2. Gating: the pore can open or close by a conformational change in the protein
3. Rapid ion flow: always down the electrochemical gradient

Question 5

What are the ionic concentrations for a typical mammalian cell?

Answer 5

Intracellular Extracellular (plasma)
Na+ ~ 10 mM Na+ 145 mM
K+ 160 mM K+ 4. 5 mM
Cl

Question 6

What is the role of potassium ions in the creation of the resting membrane potential?

Answer 6

For most cells, open K+ channels dominate the
membrane ionic permeability at rest. Non-voltage gated K+ channels are constitutively open on the cell membrane. The movement of K+ ions is therefore responsible for the creation of the resting membrane potential. As the membrane is impermeable to anions, this further increases the relative negativity inside the cell.
When the chemical diffusion gradient and the electrical gradient for K+ are equal and opposite, there will be no net movement of K+, but there will be a negative membrane potential. Thus the resting membrane potential arises because the membrane is selectively permeable to K+. The equilibrium potential for K+ is approximately -95mV. If a membrane is selectively permeable to K+ alone, its membrane potential will be at EK.
NB: The amount of K+ that move to set up the voltage is tiny.

Question 7

What are the equations for the electrical and chemical gradients?

Answer 7

Electrical gradient membrane= V z F
Where:
V = voltage
z = valency (+1 for K+)
F = Faraday’s constant
Chemical gradient= R T ln[K+]o/[K+]i
Where:
R = Gas constant
T = temperature in oKelvin
[K+]o, [K+]i = K+ concentrations on either side of the membrane

Question 8

What is the Nernst equation and what is it used for?

Answer 8

It is the equation that allows the equilibrium potential for a particular ion to be calculated.
Eion = RT ln [ion]out
ZF [ion]in
Where:
R is the Gas constant, T the absolute temperature, F Faraday’s constant and Z the valency (+1 for K+, -1 for Cl- etc, [ion]out and [ion]in are the extracellular and intracellular concentrations of the ion.
Working out the constants at 37°C, and changing the logarithm to base 10,
Eion = 61 log10 [ion]out Units are in millivolts.
Z [ion]in
You can write the Nernst equation for any ion: e.g. Na+, Cl-, Ca2

Question 9

What makes the resting membrane potential less negative than EK?

Answer 9

In the cell membrane at rest, Na+ and Ca2+ are normally closed, however they may transiently be open, causing ions to leak through down their concentration gradient into the cell. This makes the cell less negative, as ENa and ECa are much more positive than EK (approx. +70 and +122mV respectively).
In some cells, for example ventricular cardiac myocytes or skeletal muscle cells, Cl- also flows into the cell down the concentration gradient, which results in a lower resting membrane potential for these cells than other cells.

Question 10

Which cells have higher or lower resting membrane potentials than others?

Answer 10

Higher resting membrane potentials:
Cardiac muscle (-80 mV), nerve cells (-70 mV):
Resting potential is quite close to EK.
Not exactly at EK (less negative): membrane not perfectly selective for K+.
Skeletal muscle:
Many Cl- and K+ channels open in resting membrane.
Resting potential approx. -90 mV. Close to both ECl and EK.
Cells with lower resting potentials:
Somewhat lower selectivity for K+ : increased contribution from other channels, e.g. smooth muscle cells, -50 mV).

Question 11

Why is it useful for membrane potentials to change?

Answer 11

Changes in membrane potential 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
Postsynaptic actions of fast synaptic
transmitters

Question 12

What is the role of Na+K+ATPase in setting up the resting membrane potential?

Answer 12

The only input the sodium pump has in the creation of the resting membrane potential is that it creates the Na+ K+ concentration gradient that allows the generation of the resting membrane potential by the movement of ions.
Na+K+ATPase does not set RMP- cells can retain RMP for up to 8 days without it.

Question 13

What is depolarisation?

Answer 13

A decrease in the size of the membrane potential from its normal value.
Cell interior becomes less negative e.g. a change from – 70 mV to – 50 mV.

Question 14

What is hyperpolarisation?

Answer 14

An increase in the size of the membrane potential from its normal value.
Cell interior becomes more negative e.g. a change from – 70 mV to – 90 mV.

Question 15

How does ion permeability affect the membrane potential?

Answer 15

Membrane potentials arise as a result of selective ionic permeability. Changing the selectivity between ions will change membrane potential.
Increasing membrane permeability to a particular ion moves the membrane potential towards the Equilibrium Potential for that ion.
Opening K+ or Cl- channels will cause hyperpolarisation.
Opening Na+ or Ca2+ channels will cause depolarisation.
Thus changes in membrane potential are caused by changes in the activity of ion channels.

Question 16

What is the Goldman-Hodgkin-Katz equation and what is it used for?

Answer 16

The GHK (Goldman-Hodgkin-Katz) equation is
a theoretical treatment that fits real membranes quite well. It takes into account the relative permeabilities of potassium, sodium and chloride (PK, PNa, PCl)- the major contributors to resting membrane potential and depends on the number of open channels for each ion. It determines the overall membrane potential of a cell.
Real cell membranes have channels open for >1 type of ion.
How do we deal with membranes that are not perfectly selective for one ion species?
The contribution of each ion to the membrane potential will depend on how permeable the membrane is to that ion.

Question 17

Are all ion channels very selective?

Answer 17

No, some ion channels may not be as selectively permeable as others.
For example, at the neuromuscular junction, motor neurone terminals release acetylcholine, ACh, which binds to receptors on the muscle membrane.
Nicotinic Acetylcholine Receptors
Have an intrinsic ion channel
Opened by binding of acetylcholine
Channel lets Na+ and K+ through (and also a small amount of Ca2+), but not anions, meaning the membrane will not depolarise as much as if they were only permeable to Na+ due to the outflow of K+.
Moves the membrane potential towards 0 mV - intermediate between ENa and EK.

Question 18

How is ion channel activity controlled?

Answer 18

Channels can open and close: i.e. they are gated
Types of Gating:
1. Ligand Gating
The channel opens or closes in response to binding of a chemical ligand,
e.g. channels at synapses that respond to extracellular transmitters, or channels that respond to intracellular messengers (e.g. IP3 channels that respond to IP3 during a signalling cascade).
2. Voltage Gating
Channel opens or closes in response to changes in membrane potential, e.g. channels involved in action potentials, such as voltage gated K+ or Ca2+ channels.
3. Mechanical Gating
Channel opens or closes in response to membrane deformation, e.g. channels in mechanoreceptors: carotid sinus stretch
receptors, hair cells.

Question 19

What is synaptic transmission?

Answer 19

At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane.
Synaptic connections occur between:
nerve cell – nerve cell
nerve cell – muscle cell
nerve cell – gland cell
sensory cell – nerve cell
Symaptic transmission may be fast or slow.

Question 20

What is synaptic transmission?

Answer 20

At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane.
Synaptic connections occur between:
nerve cell – nerve cell
nerve cell – muscle cell
nerve cell – gland cell
sensory cell – nerve cell
Synaptic transmission may be fast or slow.

Question 21

What is fast synaptic transmission?

Answer 21

In fast synaptic transmission, the receptor protein is also an ion channel. Neurotransmitter binding causes the channel to open.

Question 22

What is an excitatory postsynaptic potential (EPSP)?

Answer 22

Excitatory transmitters open ligand-gated channels that cause membrane depolarization.
Can be permeable to Na+, Ca2+ or sometimes cations in general (nAChR).
The resulting change in membrane potential is called an Excitatory post-synaptic potential (EPSP).
An EPSP is smaller than an action potential (temporal and spatial summation of EPSPs to reach neuronal firing threshold can lead to an action potential), it has a longer time course than an action potential and it is graded with the amount of neurotransmitter released. Excitatory neurotransmitters include acetylcholine and glutamate.

Question 23

What is slow synaptic transmission?

Answer 23

Synaptic transmission in which the receptor and channel are separate proteins.
Two basic patterns:
-Direct G-protein gating- The receptor is a G-protein coupled receptor that when activated, activates an ion channel. The response is localised and quite rapid, although not as rapid as fast synaptic transmission.
-Gating via an intracellular messenger- The receptor is a G-protein coupled receptor that activates an enzyme, activating a signalling cascade, which causes an intracellular messenger or protein kinase to activate an ion channel. The response occurs throughout the cell, instead of being localised, and the response is amplified by the signalling cascade.

Question 24

Give two factors that can influence membrane potential.

Answer 24

1. Changes in ion concentration
   Most important is extracellular K+ concentration (~4.5 mM normally)
   Sometimes altered in clinical situations e.g. hyperkalaemia.
   Can alter membrane excitability, e.g. in heart.
2. Electrogenic pumps
   Na/K- ATPase moves 3Na+ out and 2K+ in per cycle.
   One positive charge is moved out for each cycle. In some cells, this contributes a few mV directly to the membrane potential, making it more negative.
   Indirectly, active transport of ions is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients.

Question 25

Give two factors that can influence membrane potential.

Answer 25

1. Changes in ion concentration
   Most important is extracellular K+ concentration (~4.5 mM normally)
   Sometimes altered in clinical situations e.g. hyperkalaemia.
   Can alter membrane excitability, e.g. in heart.
2. Electrogenic pumps
   Na/K- ATPase moves 3Na+ out and 2K+ in per cycle.
   One positive charge is moved out for each cycle. In some cells, this contributes a few mV directly to the membrane potential, making it more negative.
   Indirectly, active transport of ions is responsible for the entire membrane potential, because it sets up and maintains the ionic gradients.

Question 26

How does hyperkalaemia affect membrane potential?

Answer 26

The resting membrane potential will become more positive

The membrane potential is closer to the threshold for firing of an action potential