L4 Membrane potentials

Question 1

a) What is A membrane potential?

b) How is it expressed?

Answer 1

a) - Membrane potential is the electrical potential/voltage difference across their plasma membrane of a cell
b) - It is expressed as a voltage of the inside of the cell relative to the outside

Question 2

How is a membrane potential measured?

Answer 2

Using a microelectrode (a fine glass pipette)

- can penetrate cell membranes and is filled with conducting solution (KCL)

Question 3

a) What units are used to measure membrane potentials?
b) What is the normal range of animal cell resting membrane potentials?
c) What is the normal resting membrane potential of the following cell types:
i) Cardiac myocytes
ii) Neurones
iii) Skeletal muscle myocytes
iv) Smooth muscle myocytes

Answer 3

a) millivolts (mV)
b) -20mV to -90mV
c)
i) -80mV
ii) -70 mV
iii) -90 mV
iv) -50mV

Question 4

What does selective permeability have to do with the resting membrane potential?

Answer 4

• Membrane potentials are set up because the membrane is selectively permeable to different ions
• the permeability occurs due to channel proteins (membrane spanning proteins that allow ions to permeate)

Question 5

What are the characteristics of ion channels?

Answer 5

1. Selectivity: the channel lets through only one (or a few) ion types
2. Gating: the channel can be open or closed by a conformational change in the protein molecule
3. A high rate of ion flow that is always down the electrochemical gradient for the ion

Question 6

What are the a) intracellular and b) extracellular concentrations typically in an animal cells for the following ions:

i) Na+
ii) K+
iii) Cl-
iv) A- (anions other than chloride)

Answer 6

a)
i) About 12mM
ii) 155 mm
iii) 4.2 mM
iv) 167 mM

b)
i) 145 mM
ii) 4.5 mM
iii) 123 mM
iv) 40 mM

Question 7

a) What is the Resting membrane potential

b) How is it set up?

Answer 7

a) the voltage (charge) difference across the cell membrane when the cell is at rest.
b) ⇒ Membrane is selectively permeable to K+ at rest (open K+ channel)
⇒ K+ diffuse out of the cell (down [K+] gradient)
⇒ Anions cannot follow (channels not open)
⇒ Cell interior becomes negatively charged
⇒ Newly-created membrane potential opposes outward movement of K+
⇒ System reaches equilibrium

Question 8

What is potassium equilibrium potential?

Answer 8

• Potassium equilibrium potential (Ek) is the membrane potential when the system reaches equilibrium and there is no net movement of K+
• This can be calculated from the Nernst equation

Question 9

What does the nernst equation allow you to do?

Answer 9

Allows you to calculate the membrane potential at which K+ will be in equilibrium given the extracellular and intracellular K+ concentrations.

Question 10

a) Explain the relationship between Ek and resting membrane potential
b) What is significant about the relationship between Ek and membrane potential?

Answer 10

a) - Open K+ channels dominate the resting permeability of many cells, so the resting membrane potential is close to the Ek
b) The dependance of resting membrane potential on K+ permeability means that changing EK will change the RMP

Question 11

The Ek is -95 mV but the RMP in some cells, such as smooth muscle cells (-50mV) is less negative than this, why?

Answer 11

• The membrane is not always perfectly selective so the RMP is less negative than Ek
• Sometimes there is increased contribution from other channels such as na+ and ca2+

Question 12

Why is a change in membrane potentials significant for cells?

Answer 12

• underlies many forms of signalling between and within cells such as:
• -> AP in nerve and muscle cells
• -> triggering and control of muscle contraction
• -> control of secretion of hormone/neurotransmitters
• -> transduction of sensory information into electrical activity by receptors
• -> postsynaptic actions of fast synaptic transmitters

Question 13

Define:

a) Depolarisation

b) Hyperpolarisation

Answer 13

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

b) An increase in the size of the membrane potential from its normal value. The cell interior becomes more negative.
e. g. a change from -70mV to -90mV

Question 14

How are membrane potentials changed?

Answer 14

By changing the selectivity between ions i.e. increasing membrane permeability to a particular ion moves the membrane potential towards the equilibrium potential for that ion

Question 15

The opening of which ion channels will cause:

a) hyperpolarisation
b) depolarisation

Answer 15

a) Opening K+ or Cl- channels

b) Opening Na+ or Ca2+ channels

Question 16

What are the types of gating of channels?

Answer 16

1. Ligand gating:
   the channel is opened/closed by the binding of a chemical ligand (extracellular/intracellular)
   e.g. channels at synapses that respond to extracellular transmitters
2. Voltage gating:
   the channel is opened/closed by changes in the membrane potential
   e.g. channels involved in action potentials
3. Mechanical gating:
   channel opens or closes in response to membrane deformation e.g. channels in mechanoreceptors: hair cells or carotid sinus stretch receptors

Question 17

a) Synaptic connections occur between?
b) What happens at the synapse?
c) Distinguish between fast and slow synaptic transmission

Answer 17

a) nerve cells, nerve and muscle cells, nerve and gland cells, sensory and nerve cells
b) a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane

Question 18

What occurs in Fast synaptic transmission?

Answer 18

a) - The receptor is also a ligand-gated ion channel.
- This means it has two functions: 1. the ligand/transmitter has to bind to receptor and 2. it has to act as an ion channel (transmitter binding causes the channel to open)
- the transmitter can be excitatory or inhibitory

Question 19

a) What occurs at excitatory synapses? e.g.

b) What occurs at inhibitory synapses?e.g.

Answer 19

a) - Excitatory transmitters open ligand-gated channels that causes membrane depolarisation (memb potent becomes less negative)
- can be permeable to Na+ or Ca2+
- the resulting change in memb potential is called an Excitatory post-synaptic potential (EPSP)
- excitation of cells
e. g. Acetylcholine, glutamate

b) - inhibitory transmitters open ligand-gated channels that cause hyperpolarisation (memb potent becomes more negative)
- permeable to K+ or Cl-
- the resulting change in memb potential is called an Inhibitory post-synaptic potential (IPSP)
- inhibition of cells
e. g. Glycine and GABA

Question 20

What occurs in Slow synaptic transmission?

Answer 20

The receptor is not an ion channel but signals to channel via one of two ways (both use G-proteins:

1. Direct G-protein gating (localised)
2. Gating via an intracellular messenger or protein kinase (throughout cell, amplication by cascade)

Question 21

What other factors can influence membrane potential?

Answer 21

1. changes in ion concentration (e.g. extracellular k+ concentration and hyperkoelemia)
2. Electrogenic pumps e.g. Na/K-ATPase

Question 22

When channels for more than one ion are open, these ions will contribute to the membrane potential.

How does one deal with cell membranes that are not perfectly selective?

Answer 22

The GHK (Goldman-Hodgkin-Katz) equation is used to calculate the reversal potential across a cell’s membrane, taking into account all of the ions that are permeable through that membrane