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
