Download Clinical Case Activity Answers

Answer Key to Short Answer Questions for
“Going Under the Knife: A Case on Membrane Structure and Function”
1. At resting membrane potential, why does a small amount of sodium leak into the cell
instead of out?
At rest, the electrochemical gradient favors movement of sodium into the cell. The
positively charged sodium ions are attracted to the negatively charged ions and
compounds inside the cell. The sodium/potassium pump also creates a higher
concentration of sodium outside the cell so sodium will move down the concentration
gradient into the cell.
2. Define depolarization and hyperpolarization and their relationship to threshold.
Depolarization occurs when the inside of the cell becomes more positive and the
membrane potential moves toward threshold. If depolarization is sufficient, threshold
is reached and the cell will undergo an action potential. Hyperpolarization is the
opposite – the inside of the cell becomes more negative and the membrane potential
moves away from threshold; the cell is therefore less likely to undergo 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 of a neuron to
threshold.
A neuron will depolarize when more positively charged ions enter the cell, as when
sodium channels open and sodium enters the cell. This creates a local or graded
potential that can spread across the cell (plasma) membrane to the axon hillock, where
the change in membrane potential opens voltage-gated sodium channels. The
membrane potential at which the voltage-gated channels open is known as threshold
and the resulting feed-forward cycle of sodium flowing into the cell produces the rising
phase of the action potential.
4. What does Cole mean when he says that anesthesia “inhibits the neurons?”
Anesthesia will alter the membrane potential of the neurons and prevent them from
reaching threshold. Thus, they are “inhibited” from firing or being active.
5. Is Cole correct when he assumes that leak potassium channels are different than
voltage-gated potassium channels? Explain your answer.
Yes, Cole is correct. The voltage-gated potassium channels are located in the axon of
the neuron and are essential components in producing an action potential. They are
normally closed until the membrane potential reaches threshold; after a delayed
opening, they allow potassium to flow down its concentration gradient out of the cell
which repolarizes the cell and returns the membrane potential to resting conditions.
The “leak” potassium channels are found in all parts of the cell (plasma) membrane
and function to control resting membrane potential. Some of them are open at rest, but
not all. Opening more of these channels will increase the leak current of potassium
out of the cell and hyperpolarize it. Closing some of the open channels will have the
opposite effect.
6. If the anesthesia opens more potassium leak channels, why are Kevin’s neurons less
likely to produce action potentials?
Opening more channels will allow an increased amount of potassium to flow out of the
cell down its concentration gradient, hyperpolarizing the cell. In order to produce an
action potential, the membrane potential must become more positive in order to reach
threshold. Making the resting membrane potential more negative moves it away from
threshold and the cell is less likely to produce an action potential.
7. Suppose Kevin’s pre-op blood work indicates that his extracellular potassium
concentration is much higher than usual. This condition is known as hyperkalemia
and must be corrected before he can 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?
An increase in the potassium concentration outside of the cell will reduce the
concentration gradient across the cell (plasma) membrane. Therefore, the driving
force that causes potassium to leave the cell by way of the leak current is reduced and
the resting membrane potential becomes more positive. This moves the membrane
potential closer to threshold and a depolarizing signal that ordinarily wouldn’t typically
have much effect will now produce an action potential.
8. Similar types of potassium channels are found in skeletal muscle cell (plasma)
membranes. Predict the effect of general anesthesia on Kevin’s skeletal muscle
contraction during surgery.
Muscle cells rely on action potentials for contraction, and hyperpolarization by many
general anesthetics will inhibit muscle contraction. In some cases, the patient can be
completely immobilized. This is beneficial in that it prevents unexpected or reflexive
movement during the surgical procedure. However, it can also be detrimental because
it is much more difficult to know if the anesthesia is wearing off before the surgery is
completed.