W2 L2 (Action potential) Flashcards
Action potential
Short-term change and rapid change in Vm due to coordinated action of certain voltage-gated channels
What 2 things do actions potentials do?
- Code info 2. Trigger other neurons, muscles or glands
What happens during an action potential?
The inside and outside of the cell reverse in polarity ie. the inside becomes positive and outside negative.
How are action potentials like binary code or a light switch?
Action potentials are either on or off there is no in-between or graded response. For example, like in binary, they are a 0 or a 1.
Threshold
The strength of stimulus required for an action potential to be initiated.
What does a stronger stimulus entail with regards to action potentials?
It means that there will be more action potentials, not bigger ones (b/c it is not a graded response).
What is the duration of a neuron and heart action potential respectively?
Neuron: 1-5ms Heart: 300-400ms
What threshold must be reached for an action potential?
There is really a range depending on the cell between -20mV and -60mV. For this class assume the threshold to be -55mV.
What does it mean that action potentials travel in a non-decremental fashion?
It means that their intensity is constant and doesn’t diminish over their very short life.
Explain why the portion of the membrane that creates graded potentials doesn’t experience action potentials?
Typically, the portion of the excitable membrane where graded potentials are produced in response to a triggering event does not undergo action potentials. Instead, the graded potential, by electrical or chemical means, brings about depolarization of adjacent portions of the membrane where action potentials can take place.
Threshold potential
The critical potential that must be reached before an action potential is initiated in an excitable cell
AP diagram
What are transmembrane voltage sensors and what do they do?
They are positively charged amino acids meaning the sensor moves to open the gate of a channel when the inside is positive
Explain the steps of an action potential
- Resting State -The membrane is in a resting state (-60mV) with both channels activation gates closed. The inside is negative and the outside is positive.
- Activation/Triggering event - Depolarization proceeds slowly until an explosive depolarization (-55mV) happens.
- Depolarization (-55mV to approx 30mV) - Na+ enters the cell down its concentration and electrical gradient causing the Vm to go towards Vk.
- Repolarization (30mV to less than -60mV) -
Overshoot also give an example during an action potential
When a signal exceeds its steady-state value, for ex. when the action potential causes the Vm to go to +30mV instead of just 0mV.
What is another name for an action potential?
Spike
What happens to the membrane potential during an action potential?
Rapid reversal
What is a major factor in the shape of voltage-gated channels that is not a factor for other channels?
Voltage-gated channels are greatly affected by voltage changes in the fluid on both sides of them.
What are the two types of gates that are a part of voltage-gated channels?
- Activation gate
- Inactivation gate
Activation gate
Guards the channel by opening and closing like a hinged door.
Inactivation gate
A ball on a string configuration, the ball can block the door or permit entry.
What are the 3 conformations of a voltage-gated channel?
- Open and activated
- Closed but able to be activated
- Closed and unable to be activated
What does the open and activated conformation entail?
Both the activation and inactivation gates are open
What does the closed and able to be activated conformation entail?
Activation gate is closed and inactivation gate is open
What does the closed and unable to be activated conformation entail?
It means that the activation gate and inactivation gate are both closed.
Why is the open and closed description of K+ ion channels not exactly accurate?
Because most voltage-gated K+ channels are comprised of four individual subunits, there are no distinct activation and inactivation gates. Rather, it is the influence of the electrical field that changes the conformation of the subunits to determine if K+ may flow through the channel.
At the repolarizing stage of an action potential, the Na channels are inactivating, while at the after-polarization stage, the K channels are deactivating. Is there a difference between these two words, specific to the ion channels?
Inactivation is closure in the face of stimulus and for Na channels, this stimulus is the depolarization.
Deactivation is closure once a stimulus is removed. K channels repolarize the membrane and then shut off to rest.
Is the inactivation gate of the Na channel open or closed during the inactivation of the channel? Is it the opposite when the channel is open? Why are there two gates?
The inactivation gate is closed when the channel is inactivated but open when the channel is closed or open. There are two gates because the activation gate won’t close in the same way as the activation gate during a depolarization and maintained stimulus.
During depolarization, does the electrical gradient ever reach equilibrium since Na is flowing in? If it does, how does the positive feedback loop work?
No the Na channels shut off before it can reach the Vna (+60mV) and it repolarizes as more K channels open and Na shut before it gets out of hand
How does the duration of an external stimulus affect the size of the response of the nerve?
The longer the stimulus the more likely it is to overcome the threshold
What would happen if you stimulated an axon in the middle (even though this never happens)?
It would go in both directions
Can nerves conduct in both directions, is this ever likely?
Yes, however, this doesn’t happen as when a signal is sent in one direction the Na channels are still in a refractory state and can’t be stimulated
If current always spread in both directions from a node of Ranvier, then why does the signal only go in one direction?
This is because the current going backward only meets Na channels in the refractory period
What cells do refractory periods occur in?
Any cell that fires action potentials including neurons, muscle, and some glands
How many APs are fired during the refractory period?
Some, but nothing substantial enough to propagate an AP
Given that the leaking of current in a demyelinated neuron slows the conduction of the action potential, is it possible that so much current would leak out that there might not be enough to re-generate the action potential during its propagation toward the synapse?
If enough current is lost this can lead to a failure of the AP to propagate, this can lead to many serious health conditions
Which conducts faster a myelinated small diameter axon or a large unmyelinated axon?
The small myelinated is faster
In the lecture, you said that threshold rises during a relative refractory period (RRP) because there are not many Na channels available yet. However, would not one expect the threshold to NOT change even during RRP because the activation gate of the Na channel opens at a fixed voltage?
The threshold is a product of the probability that a sufficient amount of channels will be open. This means that during the refractory period there is a much lower chance that there will be sufficient channels to reach the threshold, hence the threshold rises.
Why do some sources indicate that an AP which occurs during the RRP is smaller than the regular AP?
Due to the higher threshold during a refractory period, there must be less Na channels available which lead to a smaller depolarization while the K channels still do their thing. Net result = Lower AP
