Changing Membrane Potential

Question 1

Why is a change in the membrane potential important?

Answer 1

Because it underlies many forms of signalling between and within cells.

Question 2

Give examples of what a change in the membrane potential contributes to.

Answer 2

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 (ACh receptors e.g.)
Postsynaptic actions of fast synaptic transmitters

Question 3

Define depolarisation

Answer 3

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

Question 4

Define hyperpolarisation

Answer 4

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

Question 5

Explain what happens if there is a change in the membrane permeability of an ion.

Answer 5

The ion that has an increased permeability will move down its chemical gradient and either depolarise or hyperpolarise the membrane potential.

Question 6

How does membrane permeability of an ion relate to the equilibrium potential of that ion?

Answer 6

The higher the membrane permeability of an ion the more the cell membrane potential will shift towards the ions equilibrium potential.

Question 7

What happens if there is an increase in membrane permeability for K+ or Cl-?

Answer 7

The membrane potential will hyperpolarise because K+ will flow out of the cell, and Cl- will flow into the cell.

Question 8

What happens if there is an increase in membrane permeability for Ca2+ or Na+?

Answer 8

The membrane potential will depolarise because Ca2+, and Na+ will move down their chemical concentration gradient and flow into the cell.

Question 9

What does imperfect membrane selectivity entail and why is it important?

Answer 9

It means that membranes are not perfectly selective to one ion, but a lot of ions will move in and out of the cell. So you cannot measure only one ions membrane permeability and equilibrium potential in order to get an accurate reading of the cells membrane potential.

Question 10

What is the main ion that flows through a nicotinic acetylcholine receptor?

Answer 10

Sodium ions

Question 11

How does a nicotinic acetylcholine receptor open?

Answer 11

By acetylcholine binding to it.

Question 12

Is a nicotinic acetylcholine receptor perfectly selective to sodium?

Answer 12

No, it also lets potassium ions through as well as calcium ions, however no anions.

Question 13

What would a perfectly selective channel for sodium do to the membrane potential if it would be constantly open?

Answer 13

It would depolarise the membrane potential and shift it towards and very close to the equilibrium potential of sodium (+120 mV)

Question 14

What happens if a nicotinic acetylcholine receptor would be constantly open?

Answer 14

Since this type of channel is not perfectly selective to sodium ions, potassium ions would also flow into the cell to cancel out a bit of the depolarisation. So where as if it was perfectly selective for sodium ions and shift towards +120 mV, the nicotinic acetylcholine receptor moves the membrane potential towards 0 mV, an intermediate between the equilibrium potential of sodium ions, and of potassium ions.

Question 15

Give three subtypes of gated channels that control membrane potential. Give an example of each.

Answer 15

Ligand gating - Channel opens or closes in response to binding of a chemical ligand. Channels at synapses that respond to extracellular transmitters. Channels that respond to intracellular messengers (IP3).
Voltage gating - Opens or closes in response to a change in the membrane potential. Channels involved in action potentials.
Mechanical gating - Opens or closes in response to membrane deformation. Like mechanoreceptors - carotid sinus stretch receptors and hair cells.

Question 16

What types of synaptic transmissions are there. Give 4 examples.

Answer 16

Nerve cell to nerve cell
Nerve cell to muscle cell
Nerve cell to gland cell
Sensory cell to nerve cell

Question 17

What is fast synaptic transmission?

Answer 17

When the receptor protein is also an ion channel

Question 18

What is an excitatory synapse?

Answer 18

When an excitatory transmitter opens a ligand-gated channel that causes membrane depolarisation.

Question 19

What are the two main ions that are involved in excitatory synapses?

Answer 19

Na+ and Ca2+, however other cations can come in to play. Not K+.

Question 20

What is the resulting change in membrane potential due to excitatory synapses called?

Answer 20

Excitatory post-synpatic potential. (EPSP)

Question 21

Give two examples of transmitters involved in EPSP.

Answer 21

Actetylcholine and Glutamate.

Question 22

How does the amount of transmitters released relate to the depolarisation of the membrane potential?

Answer 22

The more transmitters released, the bigger the depolarisation.

Question 23

How does EPSP relate to action potential?

Answer 23

Since EPSP depolarise the membrane potential, it closes the gap to induce an action potential.

Question 24

How could a constant higher permeability of ions involved in EPSP be a problem?

Answer 24

Because the membrane potential is closer to the threshold for an action potential to occur, less is needed to induce an action potential and so cells can become over-excitable.

Question 25

What is an inhibitory synapse?

Answer 25

Inhibitory transmitters open ligand-gate channels that cause hyperpolarisation.

Question 26

What are the two main ions that are involved in inhibitory synapses?

Answer 26

K+ and Cl-.

Question 27

What is the resulting change in membrane potential due to inhibitory synapses called?

Answer 27

Inhibitory post-synaptic potential (IPSP)

Question 28

Give two examples of transmitters involved in IPSP.

Answer 28

Glycine and gamma-aminobutyric acid (GABA)

Question 29

How does IPSP relate to action potential?

Answer 29

Since IPSP hyperpolarise the membrane potential it takes the membrane potential away from the threshold of the action potential. This makes it hard for a signal to be induced.

Question 30

How could a constant higher permeability of ions involved in IPSP be a problem?

Answer 30

Since hyper polarisation occurs it can be hard for nerve cells for example to achieve the threshold of the action potential. So signals might not be induced and cells become ‘under-excitable’.

Question 31

What is a slow synaptic transmission?

Answer 31

The receptor and channel are two separate proteins.

Question 32

Give two types of slow synaptic transmission.

Answer 32

Direct G-protein gating. The receptor activates a G-protein which activates the channel.
Gating via an intracellular messenger. The receptor activates a G-protein which activates an enzyme that activates a signalling cascade. The signalling cascade (PKA e.g.) activates the channel.

Question 33

Give two examples of other factors that can influence membrane potential.

Answer 33

Changes in ion concentration

Electrogenic pumps

Question 34

Give an example of changes in ion concentration.

Answer 34

When the extracellular concentration of K+ increases.

Question 35

Give an example of changes in electrogenic pumps.

Answer 35

The Na+/K+ ATPase.

Question 36

What is hyperkalemia? Briefly describe the process of hyperkalemia and its complications.

Answer 36

When there is an elevated concentration of K+ extracellularly (blood plasma and interstitial fluid).
This means that less K+ will move out of the cell. So the membrane potential will then shift away from K+ the equilibrium potential and make the membrane potential less negative. This means that cells can become over excitable since the depolarisation brings the membrane potential closer to the action potential.
This can cause muscle pain, eventual skeletal muscle death (rhabdomyolysis) and tachyarythmia.