Action Potential

Question 1

1. Describe how the passive electrical properties of axons render them poor conductors of electrical signals over distances greater than a few millimeters.

Answer 1

axon by itself will have a dissipated signal. The myelin and the nodes of ranvier help propagate it.

Question 2

2. Describe the positions of the activation and inactivation gates in sodium channels during an action potential.

Answer 2

before: m is closed , h open
during: m is open, h is open
after: mi is open h is closed

Inactivation gate = h gate, which swings more slowly than the activation gate (m gate). At rest, m gate is shut and h gate is open. When membrane is depolarized, activation gate swings open and then very quickly afterwards, inactivation gate closes. Can’t go at same time or channel would never conduct because both gates have to be open for Na to flow through. H gates have delayed response to depolarization, but when they close, Na permeability declines. Then K channels open and we get faster repolarization as Vm brought back down to level of Ek.

Question 3

3. Describe why intracellular concentrations of sodium and potassium do not change much after a single action potential.

Answer 3

na/k pump resets plus dont need many ions to really affect membrane potential.

Question 4

4. Describe the role of the sodium/potassium pump during the action potential.

Answer 4

Excitatory stimulus  initial depolarization at receiving neuron  positive feedback between opening Na channels and more depolarization until we get an explosive depolarization  AP  as soon as this explosion happens we get Na channels being inactivated and K channels opening just slightly after  permeability of K increases and membrane rapidly depolarizes  K channels close, but respond with a delay to repolarization so you get an undershoot  Vm approaches EK  finally channels close and Vm reaches back to resting level. Enters a refractory period. Depending, this is generally the point where the Na/K pump will restore proper ion balances if necessary.
“resets concentration 3naout 2kin

Question 5

5. Describe the mechanisms underlying the refractory period of the action potential.

Answer 5

Right after an AP, axon can’t generate another one for a few milliseconds = refractory period. Absolute refractory period = time when no stimulus can evoke an AP; followed by relative refractory period where you need a stronger stimulus to evoke an AP. Generally due to the fact that inactivation gates require time to open after repolarization  no Na will be able to flow through. As well, after repolarization, takes a bit of time for all the K channel gates to close again so the higher K conductance in that period makes it harder for stimulus to depolarize the axon.

absolute h shut k open, AP will not
relative:h open and k open, AP will probs not happen

Question 6

6. Describe the mechanisms underlying accommodation of the action potential.

Answer 6

Neurons accommodate to long-lasting depolarization

If cell slightly depolarized for a long time,

• Na+ inactivation gates close (h)
• voltage-gated K+ channels open
• lessening Na+ currents -and increasing K+ currents. In this case, the threshold goes up which makes neuron becomes harder to excite.

Question 7

7. Define the threshold for an action potential.

Answer 7

A threshold amount of depolarization is required for the membrane to generate an AP. Threshold does not equal the point at which all of the Na channels are open  it’s actually the point at which Na and K currents are exactly equal and opposite. At threshold, if Na current (even momentarily) exceeds the outward K current  a bit more depolarization is produced  then a bit more and a bit more until you get an explosive AP. This is a positive feedback mechanism  more Na channels open, more depolarization, more channels open, more depolarization.
From that, it’s an all or none response. Then repolarization will occur because Na permeability declines back to resting level and K permeability undergoes a transient increase.
a membrane potential where you reach the point of no return.

Question 8

8. Describe the positive-feedback nature of the rising phase of the action potential.

Answer 8

If synaptic transmission relays an excitatory signal to the receiving neuron, we get a small initial depolarization  If inward current exceeds outward K current, we get a few more Na channels opening  more depolarization  causes more Na channels to open  eventually Na entry process will become explosive and produce an AP. This loop between conducting Na channels and increasing depolarization = positive feedback.
Consequently, after depolarization, K channels open in response to that voltage change  open slightly delayed after Na channels open. Allows for rapid repolarization and actually an afterhyperpolarization because K channels still open when Vm approaches resting potential so it keeps approaching EK. Thus, K channel behavior = negative feedback because depolarization causes them to open, which causes repolarization which makes them shut.

as some voltage gated Na open, na enters cell, depolarizes it, more happens. voltage dont react until threshold