Electrical Signalling

Question 1

What is needed for ion movements?

Answer 1

Ion channels and transporters

Question 2

What does ion flow do?

Answer 2

Establishes concentration gradients
Generates membrane potential
Used for electrical signalling
Causes action potentials

Question 3

What are action potentials?

Answer 3

Transient alterations in membrane potential that propagate along axons

Question 4

What are the consequences of an excitable cell being activated?

Answer 4

Synpatic vesicle release (Neurons)
Hormone release (endocrine cells)
Contraction (muscle cells)

Question 5

What is the resting membrane potential?

Answer 5

A difference in charge across the membrane

Question 6

What happens with an impermeable membrane?

e.g. Concentrated potassium chloride concentration in the left chamber and a dilute potassium chloride concentration in the right hand chamber and an impermeable membrane between them

Answer 6

Voltmeter will have a zero read at equilibrium

There isn’t actually an electrical circuit here as this is impermeable so there’s no current flowing
No movement of ions and balanced on both sides

Question 7

What happens with a fully permeable membrane?

e.g. K/Cl flow, where the membrane allows full, free passage between ions with a concentrated solution on the left and a concentrated solution on the right and a fully permeable membrane between them

Answer 7

At equillibrium the voltmeter will read 0

Both the potassium and chloride ions will both flow from left to right until they reach equilibrium and the same concentrations

Question 8

What happens with a selectively permeable membrane?

e.g. (K/Cl) membrane only allows potassium ions through it- will not allow chlorine ions through it

Answer 8

There is a concentration gradient where potassium ions can flow down it but chlorine ions cant because this membrane isn’t permeable to them.
We end up with an excess of negative charge on the left and an excess of positive change on the right

Question 9

What is electrochemical equillibrium?

Answer 9

When two opposing forces or two competing forces balance each other out, then the system is said to be at an electrochemical equilibrium

Question 10

What is the effect of increasing extracellular K+?

Answer 10

As extracellular potassium concentration is raised, the membrane depolarises i.e. become less negative, tending towards zero
But
When similar experiments done with sodium, it had hardly any effect –> under equilibrium conditions the membrane is selectively permeable to potassium but not to sodium.

Question 11

What is the rising phase/depolarisation in an action potential?

Answer 11

Where the membrane potential is increasing by going towards and then above zero

Question 12

What is the overshoot phase in an action potential?

Answer 12

Where the potential goes above zero

Question 13

What is repolarisation in an action potential?

Answer 13

This is where the membrane becomes more negative after depolarisation

Question 14

What is the undershoot in an action potential and what does it cause?

Answer 14

Where the membrane potential becomes more negative that the original resting membrane potential
It causes hyperpolarisation

Question 15

Why is calcium important in neural transmission?

Answer 15

We need a calcium influx to get the fusion of synaptic vesicles and their release

Calcium is also often used as an on/off switch in biology

Question 16

What drives depolarisation?

Answer 16

An influx of sodium

With resting membrane potential, the membrane is more permeable to potassium than sodium
In the depolarising rising phase the permeability to sodium increases, and its an influx of sodium which is driving depolarisation

Question 17

What causes the action potential to return to RMP?

Answer 17

An efflux of potassium

So the permeability to sodium decreases and permeability to potassium increases so permeability to potassium is higher than sodium and this causes a fall in membrane potential. We get an efflux of potassium rather than the influx of sodium so everything normalises then we get back to the resting potential

Question 18

What do voltage-gated Na+ and K+ channels do in relation to action potentials?

Answer 18

Opening of voltage-gated Na+ channels causes depolarisation

Closure of Na+ and opening of K+ voltage-gated channels then causes repolarisation

The voltage-gated K+ channels remaining open after the action potential reaches resting level causes hyperpolarisation

Question 19

What do voltage-gated Na+ and K+ channels do in relation to action potentials?

Answer 19

Opening of voltage-gated Na+ channels causes depolarisation

Closure of Na+ and opening of K+ voltage-gated channels then causes repolarisation

The voltage-gated K+ channels remaining open after the action potential reaches resting level causes hyperpolarisation

Question 20

What does Tetrodotoxin (TTX) from a puffer fish do?

Answer 20

It blocks voltage-gated sodium channels

If depolarisation done in the presence of TTX you don’t get the inward flow of sodium voltage gates channels, you just get the out flow through the voltage gated potassium channels

Question 21

What does Tetraethylammonium (TEA) do?

Answer 21

It blocks voltage-gated potassium channels

If depolarisation is done in the presence of TEA you don’t get the outward flow because its been blocked, you only get the inward flow through the voltage gated sodium channels

Question 22

What are excitatory postsynaptic potentials?

Answer 22

They are depolarisations heading up towards zero that are not strong enough alone to cause an action potential

However

As soon as there is enough of these to raise the membrane potential above the threshold they will fire an action potential

Question 23

What is the most common cause of an excitatory post-synaptic potential?

Answer 23

Movement of sodium is the most common cause of EPSP
Sodium is higher on the outside than on the inside so if we get an influx its going to raise the membrane potential

Question 24

What is the most common cause of an inhibitory postsynaptic potential?

Answer 24

Movement of chloride is the most common cause of IPSP
Membrane is negative on the inside, you’ve got more chloride outside than inside, opening up a chloride channel will allow chloride to flow in, making it even more negative inside – occurs through GABA or glycine receptors

Question 25

Are action potentials always the same size?

Answer 25

Yes action potentials are always the same size regardless of the size of a stiumulus
BUT
A bigger stimulus can increase the frequency of action potentials, called spiking, where bigger potentials give rise to multiple action potentials referred to as a train of spikes

Question 26

What is the absolute refractory period?

Answer 26

The various confirmational changes involved in the open and resetting of the voltage gated channels takes some time so we have an absolute refractory period.
This means that if you get a second stimulus you wont fire a second action potential during this period
The fact we have an absolute refractory period does confer some advantage- it confers directionality of an action potential, it cannot bounce back

Question 27

What is a sub-threshold stimulus?

Answer 27

One not big enough to cause an action potential

Question 28

What is passive electrical flow in axons?

Answer 28

Passive current flow depolarizes the membrane potential in the adjacent region of the axon, once the current begins dissipating the membrane potential gets pregressively smaller

It demonstrates that axons are poor electrical conductors

There is also current leakage during passive flow which contributes to the diminishing effect

Question 29

What is active conduction in axons?

Answer 29

When stimulated with a supra-threshold stimulus, it has an action potential which is the same size all along, it doesn’t dissipate.

Active flow is far more efficient than passive flow is

Question 30

Is passive flow important?

Answer 30

Yes- its important during action potential propagation

Where the channels are open we get an influx of sodium and you get passive flow further along the axon which actually depolarises the membrane in this region, allowing voltage gated channels to open in the next region and hence this passive flow is important in propagating the action potential

Question 31

Where are channels and transporters concentrated?

Answer 31

At the nodes of Ranvier

Question 32

What is saltatory propagation?

Answer 32

It is the propagation of action potentials along myelinated axons from one node of Ranvier to the next