1. how do the movements of sodium and potassium change during the action potential compared to during the resting membrane potential phase?

2. after you stimulate the neuron, why do you have to wait before stimulating it again?

3. what are the concentrations (in mm) of sodium (na⁺) ions inside and outside the cell at the end of the action potential, just before the ion levels start to return to normal? also, what are the concentrations of potassium (k⁺) ions inside and outside the cell at this same point?

4. do the ion - concentration differences between resting potential and the end of an action potential surprise you? why or why not?

5. which type of gated channel remains open longer? to determine this, use the sliding tool on the membrane potential vs. time chart and note when each gated channel opens and closes. pay special attention to the sodium gated channel, as it has a mechanism for locking that is different from potassiums.

6. considering what youve learned in this simulation, explain why a potassium and/or sodium deficiency could lead to fatigue, confusion, muscle weakness, seizures, or stroke.

Question

1. how do the movements of sodium and potassium change during the action potential compared to during the resting membrane potential phase?

2. after you stimulate the neuron, why do you have to wait before stimulating it again?

3. what are the concentrations (in mm) of sodium (na⁺) ions inside and outside the cell at the end of the action potential, just before the ion levels start to return to normal? also, what are the concentrations of potassium (k⁺) ions inside and outside the cell at this same point?

4. do the ion - concentration differences between resting potential and the end of an action potential surprise you? why or why not?

5. which type of gated channel remains open longer? to determine this, use the sliding tool on the membrane potential vs. time chart and note when each gated channel opens and closes. pay special attention to the sodium gated channel, as it has a mechanism for locking that is different from potassiums.

6. considering what youve learned in this simulation, explain why a potassium and/or sodium deficiency could lead to fatigue, confusion, muscle weakness, seizures, or stroke.

Accepted Answer

### Question 1
# Brief Explanations:
During resting membrane potential, the cell is at rest. The sodium - potassium pump maintains a high concentration of $K^+$ inside the cell and a high concentration of $Na^+$ outside the cell. The voltage - gated $Na^+$ and $K^+$ channels are closed (except for some leak channels, with $K^+$ leak channels being more permeable). So, there is very little net movement of $Na^+$ and $K^+$ across the membrane.

During the action potential:
- **Depolarization phase**: When a stimulus reaches the threshold, voltage - gated $Na^+$ channels open rapidly. $Na^+$ ions, which are more concentrated outside the cell, rush into the cell down their electrochemical gradient. This causes the membrane potential to become more positive (depolarize).
- **Repolarization phase**: After a short time (a few milliseconds), the voltage - gated $Na^+$ channels inactivate (a different state from closing). At the same time, voltage - gated $K^+$ channels open. $K^+$ ions, which are more concentrated inside the cell, flow out of the cell down their electrochemical gradient. This makes the membrane potential become more negative (repolarize) and move back towards the resting membrane potential.

In summary, during resting potential, there is minimal net movement of $Na^+$ and $K^+$ (mostly through leak channels). During action potential, $Na^+$ rapidly enters the cell (depolarization) and then $K^+$ rapidly exits the cell (repolarization).
# Answer:
During resting membrane potential, there is minimal net movement of $Na^+$ (little influx) and $K^+$ (little efflux) across the membrane (due to the sodium - potassium pump and leak channels). During action potential, $Na^+$ rapidly enters the cell (depolarization phase) and $K^+$ rapidly exits the cell (repolarization phase) through voltage - gated channels.

### Question 2
# Brief Explanations:
After stimulating a neuron, we have to wait because of the refractory periods. There are two types of refractory periods:
- **Absolute refractory period**: During this period, which corresponds to the time when voltage - gated $Na^+$ channels are inactivated (not just closed), the neuron cannot generate another action potential, no matter how strong the stimulus is. This is because the $Na^+$ channels need time to reset from the inactivated state to the closed state.
- **Relative refractory period**: After the absolute refractory period, the voltage - gated $Na^+$ channels have reset to the closed state, but the voltage - gated $K^+$ channels are still open (or slowly closing). The membrane potential is more negative than the resting membrane potential (hyperpolarization). A stronger - than - normal stimulus can generate an action potential during this period, but we still need to wait for the neuron to return to its resting state (so that the ion concentrations and channel states are back to normal) to be able to stimulate it with a normal stimulus and get a reliable action potential.

These refractory periods ensure that action potentials are propagated in one direction and that the neuron has time to recover and be ready for the next potential stimulus.
# Answer:
We have to wait because of the refractory periods (absolute and relative). During the absolute refractory period, voltage - gated $Na^+$ channels are inactivated and no action potential can be generated. During the relative refractory period, a stronger stimulus is needed, and the neuron needs time to return to its resting state (ion concentrations and channel states) for normal stimulation.

### Question 3
# Brief Explanations:
At the end of the action potential (just before ion levels start to return to normal, which is before the activity of the sodium - potassium pump restores the ion concentrations), the concentrations are as follows:

- **Sodium ions ($Na^+$)**:
    - Inside the cell: The concentration of $Na^+$ inside the cell has increased from the resting state (which is typically around 10 mM) to a higher value, usually around 15 - 20 mM (this is a rough estimate, as the actual increase is relatively small compared to the large concentration gradient, but there is a net influx of $Na^+$ during depolarization).
    - Outside the cell: The concentration of $Na^+$ outside the cell has decreased slightly from the resting state (which is typically around 140 mM) to around 135 - 138 mM (due to the influx of $Na^+$ into the cell during depolarization).
- **Potassium ions ($K^+$)**:
    - Inside the cell: The concentration of $K^+$ inside the cell has decreased from the resting state (which is typically around 140 mM) to around 135 - 138 mM (due to the efflux of $K^+$ during repolarization).
    - Outside the cell: The concentration of $K^+$ outside the cell has increased from the resting state (which is typically around 5 mM) to around 8 - 10 mM (due to the efflux of $K^+$ from the cell during repolarization).

It should be noted that these are approximate values, and the actual values can vary slightly depending on the type of neuron and the experimental conditions. The sodium - potassium pump (which uses ATP to pump 3 $Na^+$ out and 2 $K^+$ in) will then start to operate to restore the resting ion concentrations.
# Answer:
- Sodium ions ($Na^+$):
    - Inside the cell: ~15 - 20 mM
    - Outside the cell: ~135 - 138 mM
- Potassium ions ($K^+$):
    - Inside the cell: ~135 - 138 mM
    - Outside the cell: ~8 - 10 mM (approximate values, before the sodium - potassium pump restores concentrations)

### Question 4
# Brief Explanations:
The ion - concentration differences between resting potential and the end of an action potential may or may not be surprising, and here are the reasons:

- **Not surprising**:
    - We know that the action potential is a relatively short - lived event (milliseconds). The number of ions that move across the membrane during an action potential is a very small fraction of the total number of ions inside and outside the cell. For example, the resting concentration of $Na^+$ outside the cell is about 140 mM and inside is about 10 mM. During an action potential, only a small number of $Na^+$ ions (relative to the total) enter the cell. Similarly, for $K^+$, the resting concentration inside is about 140 mM and outside is about 5 mM, and only a small fraction of $K^+$ ions exit the cell during repolarization. So, the overall concentration differences (the bulk concentrations) do not change drastically. The membrane potential changes because of the movement of these ions, but the concentration gradient (the difference in concentrations) for $Na^+$ and $K^+$ across the membrane is still largely intact (since only a small number of ions move).
- **Surprising (for some)**:
    - Some people might expect that after the action potential, the ion concentrations would be very different from the resting state, especially if they think that a large number of ions are moving. But in reality, the action potential relies on the movement of a relatively small number of ions (compared to the total) to change the membrane potential. The sodium - potassium pump then works to restore the exact resting concentrations, but even before the pump acts, the concentration differences are not as large as one might initially think.

In general, for those who understand the scale of ion movement during an action potential (a small fraction of total ions), the relatively small change in ion concentrations at the end of an action potential (before the pump restores them) is not surprising. For those who are unaware of the scale, it might be surprising that the concentrations do not change more drastically.
# Answer:
It may or may not be surprising:
- **Not surprising**: The number of ions moving during an action potential is a small fraction of total ions. So, bulk ion concentrations (inside/outside differences) don't change drastically.
- **Surprising (to some)**: If one expects large - scale ion concentration changes, the relatively small change (before pump - mediated restoration) can be surprising.

### Question 5
# Brief Explanations:
To determine which gated channel remains open longer, we analyze the timing of the opening and closing of voltage - gated $Na^+$ and $K^+$ channels:

- **Voltage - gated $Na^+$ channels**: These channels open rapidly at the start of the action potential (depolarization phase) when the threshold is reached. However, they inactivate (a different state from closing) very quickly, usually within a few milliseconds. After inactivation, they cannot open again until they reset to the closed state.
- **Voltage - gated $K^+$ channels**: These channels open more slowly than the voltage - gated $Na^+$ channels, usually just as the $Na^+$ channels are inactivating (at the start of repolarization). They remain open for a longer period of time, often until after the membrane potential has repolarized and even hyperpolarized (the relative refractory period). They close more slowly than the $Na^+$ channels inactivate.

So, the voltage - gated potassium ($K^+$) channel remains open longer than the voltage - gated sodium ($Na^+$) channel. The sodium channel has a fast inactivation mechanism, while the potassium channel opens later and closes later.
# Answer:
The voltage - gated potassium ($K^+$) channel remains open longer. Voltage - gated $Na^+$ channels inactivate (a form of closing) quickly (milliseconds) after opening. Voltage - gated $K^+$ channels open later and close more slowly, remaining open through repolarization and into hyperpolarization.

### Question 6
# Brief Explanations:
- **Potassium deficiency**:
    - $K^+$ is crucial for maintaining the resting membrane potential and for the repolarization phase of the action potential. If there is a potassium deficiency (hypokalemia), the resting membrane potential becomes more negative (hyperpolarized) because the $K^+$ concentration gradient across the membrane is increased (if $K^+$ outside the cell is too low, or $K^+$ inside is too low). This makes it harder for the neuron to reach the threshold for an action potential. Neurons in the brain (controlling cognition) and motor neurons (controlling muscles) are affected. For example, in muscles, abnormal action potential generation can lead to muscle weakness or even paralysis. In the brain, it can lead to confusion or seizures (due to abnormal neuronal firing).
- **Sodium deficiency**:
    - $Na^+$ is essential for the depolarization phase of the action potential. In sodium deficiency (hyponatremia), the $Na^+$ concentration gradient across the membrane is reduced (lower $Na^+$ outside the cell). This means that when voltage - gated $Na^+$ channels open, there is less of a driving force for $Na^+$ to enter the cell. Neurons may not be able to depolarize sufficiently to generate action potentials, or the action potentials may be weaker. In the brain, this can lead to confusion (due to abnormal neuronal communication) and seizures (due to abnormal firing patterns). In muscles, it can lead to muscle weakness (as muscle cells also rely on $Na^+$ for action potentials). In severe cases, if the sodium deficiency affects the neurons that control vital functions (like in the brainstem), it can even lead to stroke - like symptoms or life - threatening conditions due to impaired neuronal function.

In summary, potassium and sodium deficiencies disrupt the normal generation and propagation of action potentials in neurons (and muscle cells, which are also excitable cells). This disruption in neuronal communication leads to symptoms like fatigue (due to muscle or neuronal dysfunction), confusion, muscle weakness, seizures, or stroke - like events (in severe cases where vital neuronal circuits are affected).
# Answer:
- **Potassium deficiency**: Disrupts resting potential/repolarization. Neurons/muscles struggle to generate action potentials, causing muscle weakness, confusion, seizures (abnormal firing).
- **Sodium deficiency**: Reduces $Na^+$ gradient for depolarization. Neurons can't depolarize properly, leading to confusion, muscle weakness, seizures (abnormal firing). Severe cases (vital neuron circuits) cause stroke - like symptoms. Both disrupt action potential - based communication, leading to listed symptoms.

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