Membrane and action potentials Flashcards
What type of channels develop resting membrane potential?
Leak potassium and Leak Sodium channels
NOT the sodium potassium pump
What ion is the membrane more permeable to at rest
Potassium. The Em (-65mV) lies closer to the equilibrium potential of potassium (-80mV)
what is the equilibrium potential point
the point at which the chemical and electrical forces moving across the membrane is both equal and opposite
What would happen is only K+ ion channels were open?
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+
Theory if Na+ only?
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
Em is not equal to the E potential of an ion. why?
what is the effect on K+ and Na+
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
Why is the resting potential closer to K+ equilibrium potential than Na+ equilibrium potential
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
Explain the establishment of the resting membrane potential
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
Why is there no significant change in concentration during the establishment of a membrane resting potential?
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
What is conductance?
Why is it a good measurement?
What is significant?
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)
What are the stages of an action potential?
Depolarisation
Repolarisation
Hyperpolarization
Refractory period
Why is Em -65mV
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
What is depolarisation?
Describe how it is achieved.
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
Why does the Em approach the ENa during an action potential
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
What are the two aspects of Repolarisation?
inactivation of VG Na+ channels
Opening of VG K+ ion channels at +40mV
gK increases
K+ influx
Hyperpolarization? What occurs
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
Summarise the events leading to depolarisation including the concept of threshold.
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
Repolarisation
And Hyperpolarisation
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
How can the strength of a response be increased?
Increasing the firing frequency of action potentials
NB Magnitude cannot be changed
What is the absolute refractory period?
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
What is the relative refractory period?
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
Propagation of nerve impulse on an unmyelinated axon
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
Why is the refractory period so important?
Allows the neurone to recover to resting membrane potential before firing another action potential
ensures unidirectionality in the propagation of AP along axons
Saltatory conduction (myelinated axons)
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
