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LAURENTIAN UNIVERSITY CASE STUDY: MEMBRANE POTENTIAL STRUCTURE AND FUNCTION BY Sydney Boileau (0387912)

Submitted to: Dr. Gagnon

In partial fulfillment of the Requirements for the course: Physiology I PHED 2506 - EL Department name Kinesiology and Health Science Due Date December 18, 2020

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. Positively charged sodium ions are attracted to the negatively charged ions and compounds inside the cell. The sodium/potassium pumps also creates a higher concentration gradient into the cell 1) Chemical force ( concentration gradient) → Na+ will diffuse into the cell because the [Na+] is higher outside of the cell than the inside 2) Electrical force → opposite charges attract; positively charged Na+ ions will flow into the negatively charged membrane inside 2. Define depolarization and hyperpolarization and their relationship to threshold Depolarization is a decrease in membrane potential: the inside of the membrane becomes less negative (moves closer to zero) than the resting potential. For instance, a change in resting potential from -70 mV to -65 mV is a depolarization. Depolarization also includes events in which the membrane potential reverse and moves above zero becomes positive. Hyperpolarization is an increase in membrane potential: the inside of the membrane becomes more negative ( moves farther from zero) than the resting membrane potential. For example, a change from -70mV to -75mV is hyperpolarization. As we will describe shortly, depolarization increases the probability of producing nerve impulses whereas hyperpolarization reduces this probability. Depolarization occurs when the inside of the cell becomes more positive and the membrane potential moves towards threshold. If depolarization is sufficient, a 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 the 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 threshold. A neuron will depolarize when more positively charged ions enter the cell, 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 potentials 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. Anesthesia hyperpolarizes the

neurons which causes the membrane potential to become more negative, which moves it away from threshold and makes it harder for the membrane to depolarize.

5. Is Cole correct when he assumes that leak potassium channels are different than voltage-gated potassium channels? Explain your answer? Yes, the voltage-gated potassium channels are located in the axon of the neuron and are all 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 depolarizes 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

LEAK channels (NONgated/passive; ALWAYS open) -

All along the neuronal membrane (dendrites, soma, axon) Helps to maintain resting membrane potential (RMP)

VOLTAGE- gated channels (opens in response to specific voltage) -

-

Axon hillock; along axon (entire axon IF unmyelinated; at nodes of Ranvier IF myelinated) Generates action potential (outgoing signals)

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, moving it away from threshold and the cell is less likely to produce an action potential. To conclude, -K+ will move out of the cell because the [K+] is higher on the inside and the inside of the membrane will become more negative than RMP (hyperpolarization). Moreover, the membrane potential will move farther away from the threshold, making it difficult to stimulate the cell.

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 correct before he can undergo surgery. One of the dangers of hyperkalemia is that it makes neurons and muscle cells more excitable. Why does elevated potassium have this effect? An increase in the potassium concentration outside the cell will reduce the concentration gradient across the cell (plasma) membrane. Therefore, the driving force that causes potassium to leave the cell by the way of 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 action potentials. 8. Similar types of potassium channels are found in skeletal muscles cell ( plasma) membranes. Predicts the effects 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 surgery is completed....