Electrical properties of cells: Action Potentials Flashcards
Definition of action potentials
Membrane potentials of a specific cell location rapidly rises and falls
This depolarization causes adjacent locations to depolarize as well
Definition of resistive transmembrane currents
Ion flows through channel
Definition of capacitative transmembrane currents
Ion approaches 1 surface of membrane and another is expelled from the other side
Definition of saltatory conduction
Activation of Nodes of Ranvier from a distance
Action potentials can jump over the myelin sheath so the velocity of the action potential is faster
What are the stages in an action potential
Rest
Depolarization
Repolarisation
Hyperpolarisation
Properties of the action potential
Triggered by depolarization Threshold of depolarization required All of nothing principle Propagates without decrement At peak:Vm approaches Ena After AP, membrane is inexcusable during refractory period
Use of ion channels
AP due to current flow through Na, K voltage gated channels
Channels can be open/closed, highly selective
Probability of opening and closing determined by voltage across channel
Changing ion permeabilities
If a cell becomes permeable to an ion, will move down its electrochemical gradient, drive Vm => equilibrium potential
During AP, membrane becomes more permeable to Na+ first, then K+
Membrane driven to Ena and then to Ek
What happens when Vm is driven towards Ena
Depolarization of membrane
Na+ channels open fast
Na+ influx
Increases amount of depolarization
Positive feedback loop, prolonged depolarization causes Na+ channel inactivation
What happens when Vm is driven towards Rk
Depolarization of membrane
K+ channels open slow
K+ efflux
Repolarization of membrane
Why is there a threshold
At rest Pk, is much larger than Pna
Flow of K+ out can counteract flow of Na+ in
How much charge separation needed for 100mV
1picomole/cm^2 for 100mV
V few ions need to be separated with negligible consequences
How is the action potential propagated
Along axoplasm (Rlongitudinal) Across membrane (Rmembrane)
Describe the path of injected currents
Transmembrane currents either
Resistive, ion flows through channel
Capacitative, ion approaches 1 surface of membrane and another is expelled from the other side
Describe saltatory conduction and how myelination increases conduction velocity
Longitudinal resistance?
Membrane resistance?
Membrane capacitance?
As axon diameter increases, longitudinal R decreases
Increased membrane resistance (ion channels)
-current forced through axoplasm to next node
Decreased membrane capacitance
- charge not stored at membrane
- small change in ion conc => AP
Describe unmyelinated axons and their ability to conduct slowly’
Longitudinal resistance?
Membrane resistance?
Membrane capacitance?
Thinner axons, higher longitudinal R
Decreased membrane resistance
-current dissipates more quickly, voltage falls more rapidly
Increased membrane capacitance
- voltage changes more slowly
- large change in ion conc => AP
How is the resting membrane potential restored during hyperpolarisation?
The K channels close to prevent further efflux of K out of the cell.
The membrane is naturally leaky to both Na and K even when the channels are closed.
What is the difference between the absolute refractory period and the relative refractory period?
Absolute, during the depolarisation. No further AP can fire when Na are already open
Relative, during repolarisation. AP can fire but the stimulus has to be greater to overcome the efflux of K
Compare the flow of Na and K during the action potential and why
Na influx is faster as there is a inward force of -120mV
K efflux is slower as there is an outward force of +20mV
