11.23 C

Question 1

What is the sodium channel activation curve?

Answer 1

a curve showing the relationship between membrane potential and open probability of sodium activation gates

Question 2

At what membrane potential do sodium activation gates begin opening?

Answer 2

roughly -50 mV

Question 3

Describe the kinetics of the sodium activation and inactivation gates.

Answer 3

the activation gates open rapidly and the inactivation gate closes slowly

Question 4

Define threshold.

Answer 4

the point at which the depolarizing current slightly exceeds the repolarizing current

Question 5

Describe the kinetics of the potassium delayed rectifier channel.

Answer 5

it opens and closes slowly, beginning at roughly -50mV

Question 6

What is an absolute refractory period?

Answer 6

a period of time during which a cell cannot generate an AP regardless of the strength of the depolarizing stimulus

Question 7

What is a relative refractory period?

Answer 7

a time during which inactive channels are being converted to resting channels and the cell can generate an AP but only if a stronger depolarizing stimulus is given than would normally be necessary

Question 8

The number of resting sodium channels is dependent on what two factors?

Answer 8

• the amount of time given between depolarizing stimuli
• the extent of repolarization such that the sodium inactivation gate has a high probability of returning to the open state

Question 9

Why would hypokalemia decrease the number of resting sodium channels and thus diminish the ability of the cell to generate a sodium current?

Answer 9

because it would diminish the flow of potassium ions and thus the repolarizing current, which sodium channels rely on in order to reopen their inactivation gate

Question 10

The “All or None” Law of action potentials is dependent on what two factors?

Answer 10

• sodium and potassium concentrations remain the same

- there has been sufficient time during repolarization to recover resting sodium channels

Question 11

What does the “All or None” Law of action potentials state?

Answer 11

that if a depolarizing stimulus reaches threshold, the resulting AP will have the same characteristics

Question 12

The magnitude of the depolarizing current during the upstroke of the action potential will determine what four things?

Answer 12

• threshold potential
• amplitude of the AP
• rate of rise of the AP
• conduction velocity

Question 13

What is conduction velocity?

Answer 13

how quickly an AP propagates down the tissue

Question 14

Threshold potential, amplitude of the AP, rate of rise of the AP, and conduction velocity are all dependent on what other aspect of the AP?

Answer 14

the magnitude of the depolarizing current during the upstroke

Question 15

Conduction velocity is dependent on what factor of an AP?

Answer 15

the magnitude of the depolarizing current during the upstroke of the AP

Question 16

Why does magnitude of the depolarizing current during the upstroke of an AP influence the conduction velocity?

Answer 16

because the greater the magnitude, the faster cations accumulate inside the cell and spread to adjacent regions

Question 17

What conditions will reduce sodium current?

Answer 17

• increased intracellular Na will decrease E(na)
• drug blockage of sodium channels
• reduced number of resting channels due to hyper or hypokalemia
• depolarizing shift of activation curve due to hypercalcemia

Question 18

What is the most common life threatening disorder associated with cancer?

Answer 18

hypercalcemia

Question 19

How does cancer lead to hypercalcemia?

Answer 19

malignant cells release factors that cause calcium to be released from the bone

Question 20

What are some symptoms of hypercalcemia?

Answer 20

• muscle weakness is the hallmark
• tiredness, confusion
• unstable gait
• increased urination and bone pain

Question 21

An individual presents with muscle weakness, tiredness, unstable gait, and increased urination. What is a likely diagnosis?

Answer 21

hypercalcemia

Question 22

What does hypercalcemia due to the excitability of neurons and muscle cells?

Answer 22

it reduces the excitability

Question 23

How does hypercalcemia reduce the excitability of neurons and muscle cells?

Answer 23

by shifting the sodium activation curve in a positive direction, thus making them less likely to open and for the cell to be less likely to reach threshold

Question 24

What effect does raising the resting potential of a cell have on excitability?

Answer 24

it will lower the excitability

Question 25

Why would raising the resting potential of a cell lower the excitability?

Answer 25

because more sodium inactivation gates are closed and there is a smaller sodium current