Action potentials and Graded Potentials Flashcards
How can a cell be hyperpolarized using a probe?
By injecting negative charge into the cell
T or F: one the threshold has been passes a graded potential can occur?
False - IF there are channels in that area, a full action potential will occur
T or F: a cell that has a resting potential of +40 mV is hyperpolarized at +35 mV
True - hyperpolarization is just relative to resting potential
What is typically the threshold potential?
10 - 15 mV more positive than resting Vm
T or F: different cells have different action potential durations
True - for average nerve its about 1 second
What happens when threshold potential is reached?
- only a slight depolarization needed (~-55mV)
- almost all Na+ open
- there is a chemical and electric pot. pulling Na+ into cell
Why does the maximum membrane potential not reach +60 mV, which would be the potential that Na+ would want to be at using the nernst eqn?
Because the permeability of Na+ is not 100%
What terminates the positive feedback loop where Na+ influx is greater than efflux?
The Na+ channels are inactivated
What is the state of the activation gate at resting Vm?
closed and channel is non-conductive
What happens in the rising phase of the action potential?
- Stimulus depolarization causes some Na+ to open causes Na+ influx
- Na+ influx causes more channels to open
What happens in the falling phase of the action potential?
- Na+ channels inactivate
- there is now K+ efflux (K+ channels open and it leaves)
What drives K+ out during the falling phase of the action potential?
The electrochemical gradient
What happens after hyperpolarization in the action potential?
- K+ permeability is greater at the end of the action potential than at the start
- K+ channels slowly close stopping K+ flow
What does TEA do to the action potential?
Blocks K+ channels so the cell is slow to depolarize and hyperpolarization can’t happen
What does TTX do to the action potential?
Blocks Na+ channels and you get absolutely no action potential
Found in japanese puffer fish
Can a second action potential occur in the rising or falling phase of a preceding action potential?
NO
What can a second action potential occur?
After inactivation of the Na+ channels - relative refractory period
- to illicit a response the stimulus must be larger than usual
Why is the relative refractory period important in the heart?
It allow a pause so that blood can be pumped in
What causes rickets and its side effects of cramps and seizures?
- hypocalecemia means not enough Ca2+ is available to bind the extracellular site on the Na+ channel
- this causes improper folding of the gates which causes them to open haphazardly
- this leads to cramps and siezures
How do cocaine and novocaine work?
drug binds to Na+ channel
Inhibit opening of channel
Reduced number of action potentials
lack of pain perception
What determines the peak of the action potential?
- Sodium permeability (approaches 100%)
- Note: potassium determines resting potential
What is the difference in fast rises in K+ (like KCl injection) and slow rises in K+ like Kout?
Fast rises generate crazy action potentials
Slow rises give you no action potential (see paper notes for why)
How do action potentials propagate?
- Electrode causes AP in nerve
- AP reaches +40 mV
- Adjacent area attracts + charge
- Adjacent region becomes slightly more +
- Voltage dependent Na+ gates open
What is the only way to increase the maximum voltage of an action potential?
Increase the Na+ gradient
What is a graded depolarization?
One that never reaches the threshold potential and fizzles out due to electronic spread
What is the space constant lambda?
distance at which graded potential has decreased by 63%
T or F: space constant is useful for comparing diseased nerves to healthy nerves.
True
What affect does myelin have on the space constant?
Increases it by increasing membrane resistance
—>this can also be done by increasing n. diameter
What is the equation for the space constant?
lambda = (dRm / 4Ri)^1/2
What is the time constant?
Time needed to change Vm by 63% of final value
What can a long time constant indicate?
Disease states
What does the time constant depend on?
tau = Rm Cm
- membrane resistance
- capacitive abilites (to store charge)
T or F: you want a high time constant when running from a cheeta?
False - high time const. means it take a lot of time to move the potential
Why do MS patients and 1 year old babies show slow clumsy uncoordinated movement?
lack of myelin
