Membrane and action potentials

Question 1

What type of channels develop resting membrane potential?

Answer 1

Leak potassium and Leak Sodium channels

NOT the sodium potassium pump

Question 2

What ion is the membrane more permeable to at rest

Answer 2

Potassium. The Em (-65mV) lies closer to the equilibrium potential of potassium (-80mV)

Question 3

what is the equilibrium potential point

Answer 3

the point at which the chemical and electrical forces moving across the membrane is both equal and opposite

Question 4

What would happen is only K+ ion channels were open?

Answer 4

Chemical force at start leads to K+ efflux
[K+] inside > [K+] outside
K+ efflux results in membrane becoming more negative
this establishes a electrical force inside of cell becomes more negative
this leads to some K+ influx
at a sufficiently negative Em there is no net movement of K+ across the membrane as chemical and electrical forces become equal and opposite
this is the equilibrium potential around -80mV for K+

Question 5

Theory if Na+ only?

Answer 5

The chemical force would result in Na+ influx
Em becomes more positive
electrical force would be established that would push Na+ back out of the cell
when Em reaches a sufficiently positive value chemical and electrical forces become equal and opposite there is no net movement of Na+
ENa is +62mV

Question 6

Em is not equal to the E potential of an ion. why?

what is the effect on K+ and Na+

Answer 6

Em is not the Ek therefore forces on K+ are unequal
at -65mV chemical influence on K+ ([K+] inside is greater than outside therefore efflux) is larger than the electrical force that causes influx so net movement of K+ ions outside the neurone

For Na+ at -65mV both the chemical and electrical influence both cause Na+ influx as large difference in Na+ conc and there inside and the -65mV attracts positive Na+ ion the cell

Question 7

Why is the resting potential closer to K+ equilibrium potential than Na+ equilibrium potential

Answer 7

Membrane contains more K+ leak ion channels than Na+ leak channels. 40X more permable to K+ than Na+ therefore the Em lies closer to K+ EK than Na+ ENa

Question 8

Explain the establishment of the resting membrane potential

Answer 8

Na+ enter the cell due to chemical and electrical influence ie there is an ionic driving force driving

Na+ ion the cell
this makes the inside of the cell more +

This reduces the ionic driving force of K+
K+ move out of the cell down its concentration gradient as conc gradient > ion driving force that causes K+ influx
eventually Na+ influx= K+ efflux
This is the resting potential

Question 9

Why is there no significant change in concentration during the establishment of a membrane resting potential?

Answer 9

The movement of ions across the membrane is so small its effect on conc is negligible however over time it would have an effect on concentration

Na+/K+ maintain the ionic gradient and therefore the ionic driving force of Na+ and the efflux of K+ out of the neurone

Question 10

What is conductance?
Why is it a good measurement?
What is significant?

Answer 10

equivalent to permeability

easier to measure than permeability as it only takes into account the action of ion channels
is denoted as g

g is directly proportional to the no of open ion channels (permeability don’t have such as simple relationship)

Question 11

What are the stages of an action potential?

Answer 11

Depolarisation
Repolarisation
Hyperpolarization
Refractory period

Question 12

Why is Em -65mV

Answer 12

K+ efflux = Na+ influx

Na+ influx due to ionic driving force (electrical and chemical)

K+ efflux as ionic driving force trying ot move K+ in is less than the conc gradient causing k+ movement out of the neruone

Question 13

What is depolarisation?

Describe how it is achieved.

Answer 13

stimulus from synapse or generator potential causes a small amount of depolarisation

this causes some Na+ VG ion channels to open

Na+ influx as well as that from the leak channels

Increases the membrane potential more +
most overcome minimum threshold -55mV

if over the threshold potential more and more Na+ channels open causing rapid depolarisation

Question 14

Why does the Em approach the ENa during an action potential

Answer 14

gNa increases x1000
during the AP gNa is 25X the permeability to K+ during normal resting potential

more and more Na+ enter due to the opening of VG Na+ channels therefore the membrane potential approaches +40mV

Question 15

What are the two aspects of Repolarisation?

Answer 15

inactivation of VG Na+ channels

Opening of VG K+ ion channels at +40mV
gK increases
K+ influx

Question 16

Hyperpolarization? What occurs

Answer 16

Em approaches EK as gK maintained past the original Em

Eventually VGK+ ion channels close decreasing permeability
Allows time for VG Na+ channels to recover

EM returns to normal due to action of LEAK channels

Question 17

Summarise the events leading to depolarisation including the concept of threshold.

Answer 17

Initial stimulus causes depolarisation

Some Na+ influx through VG ion channels and through leak channels –> decrease on ionic driving force driving Na+ in the cell due to initial depolarisation and an increase on the ionic driving force moving K+ out

Therefore stimulus must cause enough depolarisation and opening of initial VG ion channels to overcome the reduced ionic driving force for Na+, and the increased ionic driving force on K+

Na+ influx must be greater than K+ efflux and reach the minimum threshold value to cause the graded opening of many VG Na+ channels that result in Depolarisation

ALL OR NOTHING RESPONSE

Question 18

Repolarisation

And Hyperpolarisation

Answer 18

As Em approaches ENa Vg Na+ channels are inactivated. gNA decreases

Vg K+ channels open
gK increases
K+ ion efflux out of the cell
Repolarization

Em approaches the resting potential Em is still high and goes past into hyperpolarization os VG K+ are still open

additional K+ efflux leading to Em approaching the EK

VG K+ eventually close Em returned to -65mV via LEAK channels

Question 19

How can the strength of a response be increased?

Answer 19

Increasing the firing frequency of action potentials

NB Magnitude cannot be changed

Question 20

What is the absolute refractory period?

Answer 20

No further action potentials can be generated by any stimulus:

most Vg Na+ ionic channels are still inactivated

Too many Vg K+ ion channels are open

Na+ influx must be greater than K+ efflux to overcome the threshold which is impossible here

Question 21

What is the relative refractory period?

Answer 21

Can get another AP but the stimulus must be larger as:

Na+ are still recovering from inactivation

K+ ion channels are now closing

Now possible for Na+ influx to be greater than K+ efflux however stimulus must be large enough to open VG Na+ ion channels and overcome the higher threshold value due to some Na+ still being inactivated and some K+ still closing

Question 22

Propagation of nerve impulse on an unmyelinated axon

Answer 22

Electronic spread allows propagation along the axon

Differences in ion concentration flow inside axoplasm

Positive ions move to area that hasn’t yet been depolarised –> depolarisation and opening of Vg Na+ ion channels leading to AP propagation

Na+ ions can flow backwards however the membrane is still in its refractory period and most of the Na+ are still recovering

Ensure unidirectionality

Question 23

Why is the refractory period so important?

Answer 23

Allows the neurone to recover to resting membrane potential before firing another action potential

ensures unidirectionality in the propagation of AP along axons

Question 24

Saltatory conduction (myelinated axons)

Answer 24

axolemma insulated by myelin sheet increasing the size of local circuits

Positive charge diffuses a longer distance to the Node of Ranvier where the axolemma is exposed.
Causes depolarisation and opening of Vg ion channels allowing AP propagation

Question 25

What is an advantage of myelinated axons

Answer 25

increases the size of local circuits inside the neurone leading to rapid transmission of action potential via saltatory conduction