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At resting membrane potential, why does a small amount of sodium leak into the cell instead of out? (Hint: describe both aspects of the electrochemical gradient)
At resting membrane potential, the extracellular and intracellular environment have certain abundant ions. Inside the cell, there is a higher concentration of potassium ions and organic ions while outside the cell is a higher concentration of sodium and chloride ions. Ions can pass through the membranes depending on the concentration gradients of ions across the membrane. Hence sodium will go through sodium leak channels to travel down its concentration gradient to go into the cell. Likewise, sodium will leak into the cell because there is a generally negative charge inside the cell, hence the electrical gradient allows sodium to leak into the cell.
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Define depolarization and hyperpolarization. Do they bring the membrane potential closer to or further from threshold?
Depolarization is when the membrane potential becomes more negative than the resting membrane potential while Hyperpolarization is when the membrane potential becomes more positive than the resting membrane potential. Depolarization brings the membrane closer to an action potential while Hyperpolarization takes the membrane further from an action potential.
3. Kevin is conscious when certain neurons in his brain are active—they depolarize and undergo action potentials. Describe the process of depolarization in a neuron starting with an initial stimulus to the point where threshold is reached.
A stimulus such as a neurotransmitter causes the membrane to depolarize at the axon hillock, leading to an increase in the membrane’s voltage to surpass the usual negative resting membrane potential. When the depolarization reaches a threshold value, -55Mv, voltage-gated sodium channels are opened, allowing an influx of sodium ions into the cell. At this point, the membrane potential rises rapidly to about +40mV which is the peak action potential.
What does Cole mean when he says that anesthesia “inhibits the neurons?”
When Cole says that anesthesia “inhibits the neurons”, he means that the neurons are not able to reach action potential, hence propagation of impulses is inhibited. Cole explains that anesthesia keeps the potassium channels open, inhibiting neurons, hence allowing the leakage of potassium out of the cell leading to hyperpolarization.
Is Cole correct when he assumes that leak potassium channels are different than voltage-gated potassium channels? Explain your answer (hint: how do these two channels behave?)
Cole is correct when he assumes that leak potassium channels are different than voltage-gated potassium channels. This is because leak channels allow the leaking of potassium ions out of the cell across down an electrochemical gradient. This can only occur when there are more potassium ions outside the cell than inside the cell. The voltage-gated potassium channels which depends on ATP to drive ions in or out of the cell. Hence, the channel utilizes energy in form of ATP to drive sodium and potassium ions in and out of the cell respectively.
6. If the anesthesia opens more potassium leak channels, why are Kevin’s neurons less likely to produce action potentials while he is under anesthesia?
If anesthesia opens up more potassium leak channels, that means that there will be more positive ions outside the cell than inside the cell. As a result, the membrane potential will be more negative than the resting potential. Additionally, the membrane will be hyperpolarized, making it difficult to reach the threshold for an action potential. The result is that the action potential does not develop.
7. Skeletal muscles also rely on action potentials for contraction. Similar types of potassium channels are found in skeletal muscle cell membranes and neuron cell membranes. Predict the effect of general anesthesia on Kevin’s skeletal muscle contraction during surgery.
Since skeletal muscles rely on the generation of an action potential for contraction, anesthesia will cause the muscles to relax completely. This is because when anesthesia keeps the potassium leak channels open, the membrane is hyperpolarized, making it difficult to reach a threshold for the generation of an action potential. The effect is that no action potentials are generated hence no muscle contraction.
8. (Hard question) In an alternative scenario, imagine that Kevin’s pre-op blood work showed that his extracellular potassium concentration is much higher than average. This condition is known as hyperkalemia and must be corrected before he can receive anesthesia and undergo surgery. One of the dangers of hyperkalemia is that it makes neurons and muscle cells more excitable. Why does elevated extracellular potassium have this effect? (Hint – consider what this would do to the electrochemical gradient in a resting cell)
An elevated extracellular potassium generates an electrochemical gradient that causes the influx of more potassium ions into the cell. The result is that the cell becomes depolarized. When a cell continues to be depolarized to the threshold, then an action potential is generated. This means that the neurons and the muscle cells will continue to generate and propagate action potentials. This way, Kevin would have been conscious during surgery and would be able to move his muscles too. This way, the surgery would not take place successfully.
