Chapter 05

Hall: Guyton and Hall Textbook of Medical Physiology, 12th Edition Chapter 05: Membrane Potentials and Action Potentials Test Bank

1. The diagram shows compound action potentials recorded percutaneously from a single site a certain distance from a stimulating electrode indicated by the arrow. Which peak is most likely to represent action potentials from small, non-myelinated motor neurons? A B

C

D Time

ANS: D 2. A 42-year-old woman decides to lose weight on a diet prescribed by an anorexic friend. She loses about 30 pounds in 45 days, but her serum potassium level falls to 2.1 mmol/L (normal: 3.5 to 5.0 mmol/L). Which of the following changes is most likely to occur in this young woman? A. Depolarization of the resting membrane potential B. Hyperpolarization of the resting membrane potential C. Increased intracellular potassium concentration D. Potassium equilibrium potential becomes less negative ANS: B 3.

An 22-year-old man is admitted to the emergency department after an automobile accident. He has not lost a large amount of blood, but he suffers from a severe crush injury to his right leg. He is conscious and exhibits flaccid paralysis. Electrocardiogram results show a shortened QT interval and an increase in the P-R interval (i.e., decreased duration of action potential and decreased conduction velocity). Laboratory results indicate that his serum potassium level has increased to a value of 6.5 mEq/L (normal: 3.5 to 5.0 mEq/L). This increase in serum potassium causes which of the following? A. A decreased net driving force for potassium from cells B. Decreased intracellular potassium concentration C. Hyperpolarization of the resting membrane potential D. More negative Nernst potential for potassium E. More negative resting membrane potential

ANS: A

4. An 88-year-old nursing home resident has diabetes mellitus and is taking diuretics for hypertension. His blood glucose concentration has recently been greater than 350 mg/dL. Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

Test Bank

5-2

He is disoriented and has had limited oral intake for about five days. His vital signs include a temperature of 100.5°F, a supine blood pressure and pulse of 150/90 mm Hg and 100 beats per minute, and an upright blood pressure and pulse of 130/88 mm Hg and 118 beats per minute. The findings on examination of the heart are normal, and his axillae are dry. The table shows serum Na+ and K+ values before and after the dehydration that has occurred in this patient. The resting membrane potential of a cardiac myocyte was about −70 millivolts before dehydration occurred. What is the resting membrane potential of a cardiac myocyte after dehydration occurred, assuming no change in membrane conductance or intracellular concentrations of Na+ or K+ ions?

A. B. C. D. E.

−100 millivolts −50 millivolts −60 millivolts −70 millivolts −90 millivolts

Serum Na Serum K+

+

Before 140 mmol/L 3.5 mmol/L

After 151 mmol/L 3.5 mmol/L

ANS: D 5.

A 44-year-old man is recovering from third degree burns over 50% of his body surface area. His serum potassium level is low (2.9 mEq/L). He exhibits postural hypotension and skeletal muscle weakness. If the equilibrium potential for K+ in a neuron is −105 mV and the equilibrium potential for Na+ is +55 mV, what would be an approximate value for the resting membrane potential for a neuron in this man? A. +20 mV B. +48 mV C. +80 mV D. −25 mV E. −48 mV F. −96 mV

ANS: F 6.

A 23-year-old man receives a crush injury to his right leg in an automobile accident. His serum potassium level is 6.4 mEq/L. The equilibrium potential for K+ in a motoneuron is −73 mV, and the equilibrium potential for Na+ is +59 mV. Which of the following best describes the resting membrane potential (in mV) for a typical motoneuron in this man? A. +62 B. +90 C. −13 D. −67 E. −84

ANS: D

7. A 56-year-old woman is referred to the neurology clinic for evaluation of her long-term problem of a stumbling gait and a tendency to fall. Her visual acuity also seems to

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

Test Bank

5-3

change periodically. Neurological findings suggest multiple sclerosis. This diagnosis is further supported by magnetic resonance imaging, which shows of areas of demyelination in the central nervous system. Other tests show the presence of oligoclonal bands in the spinal fluid. The loss of myelin from a nerve fiber has which of the following effects? A. Decreased conduction velocity of action potential or no action potential B. Decreased energy requirement for successful action potentials C. Increased conduction velocity of action potential D. Increased velocity of salutatory conduction ANS: A

X

8. The diagram shows an action potential for a nerve fiber. What is the threshold +20 potential for this neuron? A. −15 mV B. −50 to −60 mV Membrane Potential C. −60 to −70 mV 0 (mV) D. −70 to −80 mV E. −80 mV

W

Y

ANS: C V −20

Z Time (msec)

−40

−60

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

−80

A Test Bank

9.

A

5-4

In the diagram shown, Em represents the measured initial membrane potential for a hypothetical cell in vivo. In relation to this membrane potential, the equilibrium potentials of three ions (X, Y, Z) are represented. Pick the path most likely taken by the membrane potential when membrane conductance for ion Y is increased.

E

B y−

Time Time C EQ− B

ANS: B 10. In the diagram shown, Pmemb represents the initial membrane potential for a hypothetical cell. In relation to this membrane potential, the equilibrium potentials of three ions (Q, R, S) are represented. Pick the path most likely taken by the membrane potential when membrane conductance for ion Q and ion S are increased simultaneously by 1000-fold.

D E

x− =

Em C E

ER− = Pmemb

ANS: C

D

11. The diagram shows compound action potentials recorded percutaneously from a single site at a certain distance from a stimulating electrode (S) indicated by the ARROW. What can be concluded from the diagram

E

z+

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

+

A

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5-5

concerning action potential velocity for the various categories of neurons? A. W = X = Y = Z E − Q B. W > X > Y > Z C. W > Y > X D. Y > W > Z E. Z > Y > X > W

E

W S

ANS: B

X

10 ms

Y Time

Z

B

12. Five hypothetical neurons are shown below. Neurons A and B are myelinated, whereas neurons C, D, and E are not myelinated. Which neuron is most likely to have the fastest conduction velocity? A. B. E

C.

R− =

Em C

D. E. ANS: B 13. In the following graph, Em represents the measured initial membrane potential for a hypothetical cell in vivo. In relation to this membrane potential, the equilibrium potentials of three ions (Q, R, S) are represented. During resting conditions, the conductance for Q and S ions is the same, which is 1000 times greater than that of ion R. Pick the path most likely taken by the membrane potential when membrane conductance to ion Q is decreased to 0 mmho/cm2.

D

E

S+

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

Test Bank

5-6

ANS: D

14. A 63-year-old man is dehydrated after three days of alcohol abuse. His serum sodium level has increased from a normal value of 140 mEq/L to 147 mEq/L. The resting membrane potential of a typical neuron was −80 mV before dehydration. What is the most likely resting membrane potential of a typical neuron after the dehydration, assuming no change in resting sodium conductance? A. −72 mV B. −74 mV C. −80 mV D. −88 mV E. −98 mV ANS: C 15. A 24-year-old soldier suffers a burn injury over 60% of his body surface area, which leads to a decrease in the serum K+ concentration from a normal value of 4 mEq/L to a value of 2.5 mEq/L. Which of the following sets of changes best describe the resting membrane potential and the K+ Nernst potential in a cardiac myocyte after the injury?

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

E

Test Bank

A. B. C. D. E.

5-7

Resting membrane potential Less negative Less negative More negative More negative No change

K+ Nernst potential Less negative More negative Less negative More negative No change

ANS: D

X

16. The diagram shows an +20 action potential for a nerve fiber. Use the diagram to answer the question. Where on the curve does Membrane Potential 0 (mV) potassium conductance exceed sodium conductance? A. V and Z B. V, W, and X C. V, X, and Z D. W, X, and Y E. X, Y, and Z

W

Y

V Z

−20

ANS: A

Time (msec) 17.

Consider the action potential shown. Which of the following changes in membrane conductances for Ca+ and K+ best account for the prolonged plateau (assume normal conductances for both ions during resting conditions, i.e., only the changes in conductances are important)?

−40 mV

200 ms

0 −80

−60

Time A.

B.

++

Ca

and

K+

Ca++

and

K+

−80

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

Test Bank

C.

D.

5-8

Ca++ Ca++

and and

K+ K+

ANS: A 18.

A neuron from an extraterrestrial being has the intracellular and extracellular sodium and potassium concentrations shown in the table (in mM). During resting conditions, the sodium conductance is 100-fold greater compared to the potassium conductance. Which of the following (in mV) best describes the resting membrane potential (Vm) in this alien neuron (assume that Vm is dictated mainly by sodium and potassium conductances and that the Nernst equation applies)? A. +150 B. +55 C. 0 D. −55 E. −150

Extracellular Intracellular Na+ K+

100 10

10 100

ANS: B 19. The intracellular concentration of ion Z is 100 milliequivalents per liter (mEq/L), and the extracellular concentration is 5 mEq/L. The molecular weight of molecule Z is ~40 daltons. This relatively high intracellular concentration of molecule Z is best explained by which of the following? A. Exocytosis B. Extracellular assembly C. Facilitative diffusion D. Intracellular production E. Primary active transport F. Simple diffusion ANS: E 20.

Equilibrium potentials for Na, K, and Cl are shown. Assume that a cell is permeable to all three ions and has a resting membrane potential of −85 mV. Which of the following best describes the net movement of the various ions across the cell membrane by way of passive diffusion? Sodium Potassium Chloride A. Inward Inward Inward B. Inward Inward Outward

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.

ENa+ = +61 mV E K+ = −94 mV E Cl− = −72 mV

Test Bank C. D. E. F. G. H.

Inward Inward Outward Outward Outward Outward

5-9 Outward Outward Inward Inward Outward Outward

Inward Outward Inward Outward Inward Outward

ANS: D

Copyright © 2011, 2006 by Saunders, an imprint of Elsevier Inc.