Week 1- Part 4- Action potentials. Flashcards
Action potential:
What is it?
When is it triggered?
What kind of phenomenon is it?
Signal within a neuron- down an axon.
When the neuron is stimulated/excited.
An all-or-nothing phenomenon- either a neuron fires and produces an action potential or it does not.
Continuation from action potential:
What needs to happen for it to happen?
What happens whether or not an action potential is triggered?
Threshold of stimulation needs to be reached- neuron “fires” (i.e., action potential triggered).
This is how your brain does stuff- either by triggering or not triggering action potentials in particular neurons at particular times.
Action potential trigger:
What happens when a neuron is resting?
What is the axon hillock?
How is the AP triggered?
Polarised- -70mV, inside compared to the outside.
Chunk of cell body where the axon originates.
Membrane in the region of the axon hillock becomes sufficiently depolarised- inside becomes depolarised relative to the outside- reaches the threshold of excitation (between -50mV and -65mV, depends on the type of neuron)- then AP triggered- the neuron will fire.
Explain how an AP is triggered further- give an example.
What happens if you do or do not get to the threshold?
Neuron- resting potential of -70mV + a threshold of excitation of -65mV- if the potential of the membrane in the region of the axon hillock is changed from -70mV to -65mV- this will trigger an action potential- if it gets it to somewhere like -66, this will not happen.
Threshold = action potential.
No threshold = no action potential.
Look at the picture on the slide of the action potential when it is recorded- Answer these questions:
What is the tiny little hump?
What happens after it becomes positive?
Excitatory postsynaptic potential (EPSP)- it is a little bit of stimulation- if it is enough to shift the membrane potential from -70mV to -65mV (threshold of excitation)- the neuron will fire + suddenly become positive.
Will go back to negative- goes back to normal.
Continuation with the picture:
What is the massive change in charge from?
What does it depend on?
Massive change in charge- from negative on the inside compared to the outside- to positive on the inside- then back to negative on the inside- this is an action potential which travels down the axon.
Cell membrane- does not let ions through- but has special channels which can let some through- some of these channels can open or close depending on the voltage of the membrane.
What are some special channels called?
When the neuron is at rest, how are they?
What happens when the neuron is slightly depolarised?
Why is it called voltage gated?
Voltage gated.
Sit there closed.
They can open.
Whether they are open or closed depends on the voltage.
Ionic basis of action potentials:
Explain the start of an action potential;
- What happens first?
- What happens second?
- What happens third?
What does this produce?
Look at the picture!!!
1) Membrane- region of the axon hillock- stimulated- so it is slightly less negatively charged on the inside- becomes depolarised- summation.
2) If it reaches -65mV (from -70mV)- special voltage gated sodium channels open- before this, remember that sodium is on the outside and wants to get in but can’t- sodium floods in.
3) Makes the inside of the cell positively charged compared to the outside- now it is +50mV on the inside compared to the outside.
The rising phase of the action potential.
How it gets back down again (back to normal):
What is the next phase called?
What is one of the first things that happens?
Now what has happened to the voltage of the membrane?
Repolarisation.
Voltage gated sodium channels close- become temporarily inactive- this stops the sodium rushing in- this happens quickly, after a millisecond.
Has changed- positive compared to the outside.
Continuation from how it gets back down again (back to normal):
What does this then open?
What have they got on the inside?
Another type voltage gated channel- potassium channels.
Got positively charged potassium ions on the inside- they are no longer attracted to the inside as it is no longer negative compared to the outside- so they diffuse down their concentration gradient out of the cell- happens rapidly.
Continuation from how it gets back down again (back to normal):
By leaving, what happens?
What is this phase called?
What is the definition?
By leaving- as they are positive- start to restore the negative charge on the inside.
Repolarisation.
Concentration gradient and change in charge leads to the outflow of K+ (potassium).
What happens if too much potassium rushes out?
What phase is it in now?
What happens as a result?
What is this the basis of?
What happens when the potassium channels close?
It overshoots and becomes too negative.
Phase- cell is now hyperpolarized- this is useful.
As it is now super negative on the inside compared to the outside- harder for it to be stimulated again- prevents an action potential from triggering another action potential, otherwise they would trigger themselves.
The refractory period (where it is too negative on the inside compared to the outside).
Slowly the sodium-potassium pump will start to put things back to its resting state (-70mV).
Refractory period:
What is the first part of the refractory period?
What is the other part of the refractory period?
What does it prevent?
Absolute- impossible to initiate another action potential, for the membrane to be stimulated again.
As it fades- Relative- harder to initiate another action potential.
Action potentials from triggering themselves + prevent the backward movement of APs down the axon (previous part is now refractory, cannot be stimulated again, it only goes one way, from the cell body to the synapse) + limit the rate of firing.
Continuation from the refractory period:
Continuation from APs not being allowed to move back, why does that happen?
One part of the axon becomes positive, depolarises, causes the gates to open etc- then the next part depolarises and so on- goes down the axon like this- stops them going back.
What things could cause a little bit of polarisation so that the cell crosses the threshold of excitation?
How long does this process happen?
Things like drugs and neurotransmitters.
Happens in milliseconds- because neurons can fire APs many times a second- each time, has to pump itself back to get to the resting potential again.
