Membrane depolarization and the action potential Flashcards
What are the factors that determine whether a presynaptic release will result in propagation of an action potential by postsynaptic cell?
Stimulus number
Stimulus magnitude
Excitatory/Inhibitory stimulus
Stimulus location
The release of a neurotransmitter causes a local membrane permeability and allows the flow of ion current and a small, localized depolarization. Will this always result in an action potential?
No, action potential will occur if there are voltage-gated channels in the area and only if the depolarization is large enough
The greater the flow of current the greater change in the what of depolarization?
Magnitude of depolarization
How is the flow of current related to the depolarization area?
Large flow of current = larger area of depolarization
If 2 cells receive the same stimulus, which cell are have a larger ara of depolarization?
The cell with the larger length constant
What is the spatial summation of non-propagating depolarization?
Overlapping zones of depolarization constructively reinforce one another resulting in greater depolarization
For an action potential to be propagated, what must be present in the zone of depolarization?
Axon hillock
What is decremental conduction of depolarization?
Postsynpatic membrane depolarizes in response to the presynpatic release of excitatory transmitter but propagation will occur depending on length constant of the cell
If a zone of depolarization overlaps with a high Cl- conductance, what is the result?
Reduced propagation of depolarization
What is temporal summation?
Post synaptic effects that occur close in time will build on each other to cause depolarization
What is the difference between spatial and temporal summation?
Spatial summation is 1 action potential occurring in 2 overlapping sections to create a depolarization
Temporal summation is 2 action potentials occurring in one section to cause depolarization
Signal propagation depends on the density of sensitivity of what channel? Where are these channels most dense?
Nav
Axon hillock
During an action potential, membrane potential increases initially due to what?
Electrotonic conduction
What is electrotonic conduction?
A depolarization zone is caused by a nearby depolarization without a change in membrane conductance
Why do Na+ channels open before K+ channels?
Na+ channels open at more negative potentials than K+ channels
At what point in the action potential is the K+ conductance increasing most rapidly?
Peak of action potential
What causes afterhyperpolarization?
K+ conductance is larger than during resting conditions
Why is hyperpolarization important?
Nav channels deinactivate
When K+ channels deinactivate?
At resting membrane potential
Does electrotonic depolarization rely on voltage-gated channels?
No
The magnitude of depolarization depends on what 2 factors?
Magnitude of depolarization at the source (current density) and distance from source (length constant)
Are voltage-gated Ca2+ channels located at the axon hillock?
Yes
Are Ca2+-activated K+ channels located at the axon hillock?
Yes
Describe the steps of depolarization at the axon hillock with short relative refractory period
Electrotonic depolarization opens Nav channels
Nav channels induce action potential
K+ channels open at peak of action potential
Ca2+ voltage-gated channels open (little effect)
Kv & Ca2+ channels inactivate
Exchangers & ATPase return resting membrane potential
How is calcium pumped out of the cell?
Na-Ca exchanger
Ca2+-ATPase
Describe the steps of depolarization at the axon hillock with long relative refractory period
Electrotonic depolarization opens Nav channels
Nav channels induce action potential
Ca2+ voltage-gated channels open
Ca2+-sensitive K+ channels open (hyperpolarization)
Nav channels deinactivate during hyperpolarization
Kca2+ channels have prolonged open probability as long as cytosol Ca2+ levels are elevated
Kca2+ channels causes prolonged hyperpolarization
Exchangers and ATPases return resting membrane potential
Do action potentials lose amplitude during spread from soma to terminal? What is this called?
No
Undiminished (reinforced) signal conduction
If an action potential occurs at the axon hillock, will it result in an excitatory response?
Yes
What are absolute refractory periods?
Duration during which an action potential cannot occur
What are relative refractory periods?
Duration during which it takes a stronger than normal stimulus to make an action potential
When does the absolute refractory occur?
During activation & inactivation of Nav channels
When does the relative refractory period occur?
During membrane hyperpolarization
Can action potentials move retrograde? Why or why not?
No because those ion channels are inactivated
If a nerve is stimulated between the soma and the terminus which way does the action potential propagate?
Both ways
What determines the length constant?
Ratio of membrane resistance to cytoplasm resistance
What 2 ways can the length constant be increased (better propagation)?
Decrease cytosolic resistance (increase diameter)
Increase membrane resistance (myelination)
