Resting Membrane And Action Potential Flashcards
What is equilibrium potential?
At a particular voltage (equilibrium potential), chemical and electrical driving forces balance for a particular ion, such that net diffusion is zero
Where concentration gradient = electrical gradient
And net migration of the particular ion= 0
What determines the equilibrium potential?
Concentrations of ions in each side of the semi-permeable membrane
What is the resting membrane potential?
Membrane potential (Vm) due to
- Concentration gradient -which determines the equilibrium potential for that ion (point at which no ion flux occurs)
- Conductance -which specific ion channel is open at that time
What occurs due to K+ for membrane potential at rest?
- Concentration gradients of ions establish the equilibrium potential that then produce potentials (charges) across the membrane
- Near REST at specific K+channel is open, thus enhancing K+ conductance
- the exit of K+ from the cell pushes the Vm towards Eq potential of K+( -90 mV)
- Thus the resting membrane potential of nerve cell is primarily due to K+ and approximates -65 mV
What is the main determinant of RMP?
Conductance of potassium through membrane at rest is the main determinant of RMP
What sets up the chemical gradient of the resting membrane potential ?
ATPase pump
Why does Vm of resting membrane potential not reach -90 mV (or EK+)?
Due to small leak of other cations
How does the Na+/K+ pump contribute to the rest membrane potential?
Na+/K+ pump ONLY indirectly contributes to the resting membrane potential by maintaining the Na+ and K+ concentration gradients across the cell membrane
The direct contribution of the pump (3 Na+ pumped out of the cell for every 2K+ pumped into the cell) is small
Contrast membrane potential and equilibrium potential
Membrane potential- the potential difference between the inside and outside the cell
Equilibrium potential - the potential at which there is no flux of a given ion
What is conductance ?
Opening and closing of ion channels reflect gating
What is conductance?
Related to the flow of an ion through its channel
Describe the gates of the voltage gated Na+ channel
2 gates in this channel
Gates operate independently of each other and respond to membrane voltage
Describe the action of channel gating
Opening and closing of the voltage-gated ion channels (gating)- due to change in membrane potential
- Channels open for a short period of time
- gates assume either closed or open states- transitions are rapid. There may be more than one gate in a channel
- The more channels that ion open and allow passage of a given ion, the higher the conductance is for that ion
Explain the functioning of the Voltage gated Na+ channel
The gates do have different times of operation with depolarization. m gate is quicker than h gate
As the cell becomes de polarized ( more positive Vm) the activation gate (m) opens
- With depolarization the inactivation gate (h) closes but reacts more slowly than the activation gate (m)
- For current to flow, the activation gate opens before inactivation gate closes
Describe the functioning of delayed rectified K+ channel (to show gate sensitivity)
Delayed rectifier K+ channel- gate voltage sensitivity
Gates- time and voltage dependent activation
Voltage sensitive - potassium conductance (gk) increased by depolarization
Slow activation compared to Na+ channels
What is depolarization?
Membrane potential becomes less negative (the cell interior becomes less negative)
What is hyper polarization?
Membrane potential becomes more negative (the cell interior becomes more negative)
What is the action potential ?
Rapid depolarization, or upstroke, followed by repolarization of the membrane potential. Action potentials have stereotypical size and shape, are propagating , and are all-or-none. It is a phenomenon linked to excitable cells (I.e., nerve, muscle)
What is a threshold ?
Membrane potential at which the action potential may be inevitable and “all or none” depolarization occurs. “All or none” refers to presence versus absence of an action potential but not necessarily its magnitude
What are the changes in Na+ channel during the action potential?
- At the resting membrane potential, the activation gate closes the channel
- Depolarizing stimulus arrives at the channel
- With activation gate open, Na+ enters the cell
- Inactivation gate closes and Na+ entry stops.
- During repolarization caused by K+ leaving the cell, the two gates reset to their original positions
Then goes back to 1. This is important when discussing the refractory period
Explain the events of Action potential -threshold
- RMP- K+ channels are open at rest. Discussed prior.
- A signal causes membrane depolarization-opening of voltage gated Na+ channels and an influx of Na+ ions into the cell. Cell interior becomes more positive.
- Stimulus results in graded potential. If the stimulus is large enough (above threshold) it results opening of enough voltage-gated Na+ channels to generate an action potential
Explain the series of events that make up the depolarization in the action potential
- Na+ conductance of the membrane increases due to opening of voltage gated Na+ channels. Na+ conductance becomes higher than the K+ conductance
- The membrane potential is driven toward (but doesn’t quite reach) the Na+ equilibrium potential of +65 mV. Positive voltages form the overshoot
Explain the series of events of repolarization K+ of action potential
- Depolarization results in closing of the voltage gated Na+ channels
- Depolarization slowly opens K+ channels (delayed rectifiers). This increases K+ conductance to even higher levels than at rest.
K+ efflux from the cell results in the cell interior becoming more negative. Thus the membrane repolarizes
Explain the series of events in hyperpolarization
- The K+ conductance remains are higher than at rest for some time after closure of the Na+ channels.
- During this period, the membrane potential is driven closer to the K+ equilibrium potential
- This is where the membrane is hyper polarized I’m comparison to the normal resting membrane potential (after-hyperpolarization)
Why does membrane potential swing between Na+ and K+ equilibrium points?
Due to opening and closing of Na and K ion channels closing and opening at different times
Describe Na+ conductance
- Na+ channels are fully open and the Na+ conductance has peaked hence the Vm approaches ENa
- Na+ channels begin to inactivate and the Na+ conductance falls
- Vm doesn’t get to ENa as K+ channels have started to open and a small K+ conductance has formed pulling Vm down toward Ek
Describe K+ conductance
- K+ conductance starts to develop
- Not the Vm rapidly approaches Ek
- K+ channels are open/ Na+ channels closed- the Vm close to Ek which is below the resting membrane potential
Describe the absolute refractory period of the action potential
- Upstroke/depolarization- maximal number of Na+ channels open in response to the depolarization
- downstroke /repolarization- Na+ channels are closed. The activation gate open but inactivation gate closed, due to passage of time
Describe the relative refractory period.
The Na+ channel gates are reset.
However since the membrane is hyperpolarized, the membrane potential (Vm) is further away from the threshold.
-Thus a larger initial stimulus required to generate a subsequent action potential
Summarize the events of the absolute refractory period
- Rising phase/depolarization: Max # Na+ channels open
- Repolarization: Sodium channel inactivated (inactivation gate closed time- dependently)
Summarizes the events of the relative refractory period.
- Hyperpolarization increases distance from threshold
- Greater stimulus —> threshold for depolarization
What is the resting state of a voltage gated Na+ channel?
Resting state- activation gate closed and inactivation gate open- minimal flux of Na
What is the depolarization phase of the voltage-gated Na+ channel gates?
Activation gate open and inactivation gate open - full Na flux
What is the repolarization phase of the voltage gated Na+ channel?
Activation gate open but inactivation gate closed- minimal flux of Na (channel cannot be activated at this time - absolute refractory period)
What resets the voltage-gated Na+ channel to resting state?
Activation gate closes whilst inactivation gate opens- once reset the absolute refractory period is over
