Deck 5: Membrane Potentials and Action Potentials

Question 1

High-frequency stimulation of a skeletal muscle causes tetanic contraction because the intracellular concentration of which of the following substances increases and remains at high levels?

A)Adenosine triphosphate
B)Calcium
C)Potassium
D)Sodium
E)Troponin

Answer 1

B

Question 2

The diagram shows an action potential for a nerve fiber. What is the threshold potential for this neuron?



A. $- 15$ millivolts
B. $- 50$ to $- 60 \quad$ Potential millivolts
C. $- 60$ to $- 70$ millivolts
D. $- 70$ to $- 80$ millivolts
E. $- 80$ millivolts

Answer 2

C

Question 3

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

B.

C.

D.

E.

Answer 3

B

Question 4

A voltage-gated sodium channel opens, allowing sodium ions to move into a cell. Which of the following best describes the mechanism of transport involved in this process?

A)Facilitative diffusion
B)Simple diffusion
C)Primary active transport
D)Secondary active transport

Question 5

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

Question 6

Electrical stimulation of a dorsal root fiber that synaptically connects with a motoneuron is observed to cause an electrotonic depolarization of the postsynaptic motoneuron. Which of the following changes in ion channels is the most likely cause for this depolarizing potential?

A)Closing of ligand-gated cation channel
B)Closing of voltage-gated anion channel
C)Opening of ligand-gated cation channel
D)Opening of voltage-gated anion channel

Question 7

A 44-year-old woman has a serum potassium of 5.5 mEq/L (normal: 3.5 to 5.0 mEq/L) and a serum sodium of 140 mEq/L (normal: 135 to 147 mEq/L). Which set of changes best describes the K⁺ Nernst potential and resting membrane potential in a typical neuron in this woman compared with normal? (Assume normal intracellular concentrations of Na⁺ and K⁺.) $\begin{array}{lll} & \underline{\text { K+ Nernst Potential }} & \underline{\text { Resting Membrane Potential }} \\\text { A. } & \text { Lessnegative } & \text { Less negative } \\\text { B. } & \text { Lessnegative } & \text { No change } \\\text { C. } & \text { Less negative } & \text { More negative } \\\text { D. } & \text { More negative } & \text { Less negative } \\\text { E. } & \text { More negative } & \text { More negative } \\\text { F. } & \text { More negative } & \text { No change } \\\text { G. } & \text { No change } & \text { Less negative } \\\text { H. } & \text { No change } & \text { More negative } \\\text { I. } & \text { No change } & \text { No change }\end{array}$

Question 8

A neuron from an extraterrestrial being has the intracellular and extracellular sodium and potassium concentrations shown in the table (in millimolars). During resting conditions, sodium conductance is 100 fold greater compared with potassium conductance. Which of the following (in millivolts) best describes the resting membrane potential in this alien neuron?
(Assume that membrane potential is dictated mainly by sodium and potassium conductances and that the Nernst equation applies.)

A)+150
B)+55
C)0
D)-55
E)-150

Question 9

In the diagram shown, $\mathrm { E } _ { \mathrm { m } }$ represents the measured initial membrane potential for a hypothetical cell in vivo.
In relation to this membrane potential, the equilibrium potentials of three ions $( \mathrm { X } , \mathrm { Y } , \mathrm { Z } )$ are represented. What is the path most likely taken by the membrane potential when membrane conductance for ion $\mathrm { Y }$ is increased?

Question 10

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


A.

B.

C.

D.

Question 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 concerning action potential velocity for the various categories of neurons?


A. $W = X = Y = Z$
B. $W > X > Y > Z$
C. $W > Y > X$
D. $\mathrm { Y } > \mathrm { W } > \mathrm { Z }$
E. $Z > Y > X > W$

Question 12

A 29-year-old man has a serum potassium of 2.1 mEq/L (normal: 3.5 to 5.0 mEq/L) and a serum sodium of 125 mEq/L (normal: 135 to 147 mEq/L). Which of the following sets of changes best describe the resting membrane potential and K⁺ Nernst potential in a typical neuron in this man compared with normal? (Assume normal intracellular ion concentrations.) $$\begin{array} { l l l } & \underline{\mathrm { K } ^ { + } \text {Nernst Potential } }& \underline{\text { Resting Membrane Potential }} \\\text { A. } & \text { Less negative } & \text { Less negative } \\\text { B. } & \text { Less negative } & \text { No change } \\\text { C. } & \text { Less negative } & \text { More negative } \\\text { D. } & \text { More negative } & \text { Less negative } \\\text { E. } & \text { More negative } & \text { More negative } \\\text { F. } & \text { More negative } & \text { No change } \\\text { G. } & \text { Nochange } & \text { Less negative }\end{array}$$

Question 13

A physiology experiment is conducted in which Drug X is applied to a nerve axon. The drug causes a gradual decrease in the amplitude of individual action potentials and a gradual depolarization of the resting membrane potential. Drug X is most likely to inhibit which of the following?

A)Na+ inactivation
B)Na+-K+-ATPase pump
C)Voltage-gated K+ channels
D)Voltage-gated Na+ channels

Question 14

Equilibrium potentials for $\mathrm { Na } , \mathrm { K }$ , and $\mathrm { Cl }$ are shown. Assume that a cell is permeable to all three ions and has a resting membrane potential of $- 85$ millivolts. Which of the following best describes the net movement of the various ions across the cell membrane by way of passive diffusion?


$\begin{array}{lcl}\underline{\text { Sodium }} &\underline{ \text { Potassium }} &\underline{ \text { Chloride }} \\ \text { a. Inward } & \text { Inward } &{\text { Inward }} \\\text { b. Inward } & \text { Inward } & \text { Outward } \\\text { c. Inward } & \text { Outward } & \text { Inward } \\\text { d. Inward } & \text { Outward } & \text { Outward } \\\text { e. Outward } & \text { Inward } & \text { Inward } \\\text { f. Outward } & \text { Inward } & \text { Outward } \\\text { g. Outward } & \text { Outward } & \text { Inward } \\\text { h. Outward } & \text { Outward } & \text { Outward }\end{array}$

Question 15

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, unmyelinated motor neurons?

Question 16

A 24-year-old soldier sustains 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 2.5 mEq/L. Which set of changes best describes the resting membrane potential and the K⁺ Nernst potential in a cardiac myocyte after the injury? $\begin{array}{c}\begin{array}{lll} \underline{\text {Resting Membrane Potential}}\\\text {A. \quad Less negative}\\\text {B.\quad Less negative}\\\text {C. \quad More negative}\\\text {D. \quad More negative}\\\text {E.\quad No change} \end{array}\begin{array}{lll} \underline{\text {\(\mathrm{K}^{+}\)Nernst Potential}}\\ \text {Less negative}\\\text {More negative}\\\text {Less negative}\\\text {More negative}\\\text {No change}\end{array}\end{array}$

Question 17

A 63-year-old man is dehydrated after 3 days of alcohol abuse. His serum sodium level has increased from a normal value of 140 to 147 mEq/L. The resting membrane potential of a typical neuron was −80 millivolts 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 millivolts
B)−74 millivolts
C)−80 millivolts
D)−88 millivolts
E)−98 millivolts

Question 18

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 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

Question 19

In the diagram shown, $\mathrm { P } _ { \text {memb } }$ 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. What is the path most likely taken by the membrane potential when membrane
conductancefor ion $Q$ and ion $\mathrm { S }$ are increased simultaneously by 1000 -fold?

Question 20

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. He is disoriented and has had limited oral intake for about 5 days. His vital signs include a temperature of 100.5°F, a supine blood pressure of 150/90 mm Hg and pulse of 100 beats/min, and an upright blood pressure of 130/88 mm Hg and pulse of 118 beats/min. 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⁺? $$\begin{array} { | l l c | } \hline & \underline { \text { Before } } & \underline { \text { After } } \\\mathrm { Serum } ^ { \mathrm { Na } } { } ^ { + } & 140 \mathrm { mmol } / \mathrm { L } & 151 \mathrm { mmol } / \mathrm { L } \\\text { Serum } \mathrm { K } ^ { + } & 3.5 \mathrm { mmol } / \mathrm { L } & 3.5 \mathrm { mmol } / \mathrm { L } \\\hline\end{array}$$

A)-100 millivolts
B)-50 millivolts
C)-60 millivolts
D)-70 millivolts
E)-90 millivolts