Unit 2 Review Flashcards

1
Q

Which of the following best describes the changes in cell volume that will occur when red blood cells (previously equilibrated in a 280-milliosmolar solution of NaCl) are placed in a solution of 140 millimolar NaCl containing 20 millimolar urea, a relatively large but permeant molecule?

A) Cells shrink initially, then swell over time and lyse

B) Cells shrink transiently and return to their original volume over time

C) Cells swell and lyse

D) Cells swell transiently and return to their original volume over time

E) No change in cell volume will occur

A

B) A solution of 140 millimolar NaCl has an osmolarity of 280 milliosmoles, which is iso-osmotic relative to “normal” intracellular osmolarity. If red blood cells were placed in 140 millimolar NaCl alone, there would be no change in cell volume because intracellular and extracellular osmolarities are equal. The presence of 20 millimolar urea, however, increases the solution’s osmolarity and makes it hypertonic relative to the intracellular solution. Water will initially move out of the cell, but because the plasma membrane is permeable to urea, urea will diffuse into the cell and equilibrate across the plasma membrane. As a result, water will re-enter the cell, and the cell will return to its original volume.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the calculated osmolarity of a solution containing 12 millimolar NaCl, 4 millimolar KCl, and 2 millimolar CaCl2 (in mOsm/L)?

A) 16

B) 26

C) 29

D) 32

E) 38

F) 42

A

E) A 1 millimolar solution has an osmolarity of 1 milliosmole when the solute molecule does not dissociate. However, NaCl and KCl both dissociate into two molecules, and CaCl2 dissociates into three molecules. Therefore, 12 millimolar NaCl has an osmolarity of 24 milliosmoles, 4 millimolar KCl has an osmolarity of 8 milliosmoles, and 2 millimolar CaCl2 has an osmolarity of 6 milliosmoles. These add up to 38 milliosmoles.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the equilibium potential for Cl- across the plasma membrane of this cell? (Intracellular mM = 11 Cl-; Extracellular mM = 110 Cl-)

A) 0 millivolts

B) 122 millivolts

C) -122 millivolts

D) 61 millivolts

E) -61 millivolts

A

E) The equilibrium potential for chloride (Ecl-), a monovalent anion, can be calculated using the Nernst equation: Ecl- (in millivolts) = 61 x log (Ci/Co), where Ci is the intracellular concentration and Co is the extracellular concentration. In this case, Ecl- = 61 x log (11/110) = -61 millivolts.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the equilibrium potential for K+ across the plasma membrane of this cell?

A) 0 millivolts

B) 122 millivolts

C) -122 millivolts

D) 61 millivolts

E) -61 millivolts

A

E) The equilibrium potential for potassium (Ek+), a monovalent cation, can be calculated using the Nernst equation: Ek+ (in millivolts) = -61 x log (Ci/Co). Here, Ek+ = -61 x log (140/14) = -61 millivolts.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

If the membrane potential of this cell is -80 millivolts, the driving force is greatest for which ion?

A) Ca++

B) Cl-

C) K+

D) Na+

A

A) Quantitatively, the driving force on any given ion is the difference in millivolts between the membrane potential (Vm) and the equilibrium potential for that ion (Eion). In this cell, Ek = -61 millivolts, Ecl = -61 millivolts, Ena = +61 millivolts, and Eca= 525 millivolts. Therefore, Ca++ is the ion with the equilibrium potential farthest from Vm. This means that Ca++ would have the greatest tendency to cross the membrane through an open channel (in this particular example).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

If this cell were permeable only to K+, what would be the effect of reducing the extracellular K+ concentration from 14 to 1.4 millimolar?

A) 10 millivolts depolarization

B) 10 millivolts hyperpolarization

C) 122 millivolts depolarization

D) 122 millivolts hyperpolarization

E) 61 millivolts depolarization

F) 61 millivolts hyperpolarization

A

F) If a membrane is permeable to only a single ion, Vm is equal to the eqiulibrium potential for that ion. In this cell, Ek = -61 millivolts. If the extracellular K+ concentration is reduced 10 fold, Ek = 61 x log (1.4/140) = -122 millivolts, which is a hyperpolarization of 61 millivolts.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

The diagram shows the length-tension relationshp for a single sarcomere. Why is the tension development maximal between points B and C?

A) Actin filaments are overlapping each other

B) Myosin filaments are overlapping each other

C) The myosin filament is at its minimal length

D) The Z discs of the sarcomere abut the ends of the myosin filament

E) There is optimal overlap between the actin and myosin filaments

F) There is minimal overlap between the actin and myosin filaments

A

E) Tension development in a single sarcomere is directly proportional to the number of active myosin cross-bridges attached to actin filaments. Overlap between the myosin and actin filaments is optimal at sarcomere lengths of about 2.0 to 2.5 micrometers, which allows maximal contact between myosin heads and actin filaments. At lengths less than 2.0 micrometers, the actin filaments protrude into the H band, where no myosin heads exist. At lengths greater than 2.5 micrometers, the actin filaments are pulled toward the ends of the myosin filaments, again reducing the number of possible cross-bridges.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Simple diffusion and facilitated diffusion share which of the following characteristics?

A) Can be blocked by specific inhibitors

B) Do not require adenosine triphosphate (ATP)

C) Require transport protein

D) Saturation kinetics

E) Transport solute against concentration gradient

A

B) In contrast to primary and secondary active transport, neither facilitated dissusion nor simple diffusion requires additional energy and, therefore, can work in the absence of ATP. Only fac. diffusion displays saturation kinetics and involves a carrier protein. By defn, neither simple nor facilitated diffusion can move molecules from low to high concentration. The concept of specific inhibitors is not applicable to simple diffusion that occurs through a lipid bilayer w/o the aid of protein.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Excitation-contraction coupling in skeletal muscle involves all of the following events EXCEPT one. Which one is this EXCEPTION?

A) ATP hydrolysis

B) Binding of Ca++ to calmodulin

C) Conformational change in dihydropyridine receptor

D) Depolarization of the transverse tubule (T tubule) membrane

E) Increased Na+ conductance of sarcolemma

A

B) Excitation-contraction coupling in skeletal muscle begins with an excitatory depolarization of the muscle fiber membrane (sarcolemma). This depolarization triggers the all-or-none opening of voltage-sensitive Na+ channels and an action potential that travels deep into the muscle fiber via the T tubule network. At the T tubule-sarcoplasmic reticulum “triad,” the depolarization of the T tubule causes a conformational change in the dihydropyridine receptor and subsequently in the ryanodine receptor on the sarcoplasmic reticulum. The latter causes the release of Ca++ into the sarcoplasm and the binding of Ca++ to troponin C (not to calmodulin) on the actin filament.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

A single contraction of skeletal muscle is most likely to be terminated by which of the following actions?

A) Closure of the postsynaptic nicotinic acetylcholine receptor

B) Removal of acetylcholine from the neuromuscular junction

C) Removal of Ca++ from the terminal of the motor neuron

D) Removal of sarcoplasmic Ca++

E) Return of the dihydropyridine receptor to its resting conformation

A

D) Skeletal muscle contraction is tightly regulated by the concentration of Ca++ in the sarcoplasm. As long as sarcoplasmic Ca++ is sufficiently high, none of the remaining events – removal of acetylcholine from the neuromuscular junction, removal of Ca++ from the presynaptic terminal, closure of the acytylcholine receptor channel, and return of the dihydropyridine receptor to its resting conformation – would have any effect on the contractile state of the muscle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Which of the following statements about smooth muscle contraction is most accurate?

A) Ca++ independent

B) Does not require an action potential

C) Requires more energy compared to skeletal muscle

D) Shorter in duration compared to skeletal muscle

A

B) In contrast to skeletal muscle, smooth muscle can be stimulated to contract w/o the generation of an action potential. For example, smooth muscle contracts in response to any stimulus that increases the cytosolic Ca++ concentration. This includes Ca++ channel openers, subthreshold depolarization, and a variety of tissue factors and circulating hormones that stimulate the release of intracellular Ca++ stores. Smooth muscle contraction uses less energy and lasts longer compared to that of skeletal muscle. Smooth muscle contraction is heavily Ca++ dependent.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Which of the following best describes an attribute of visceral smooth muscle not shared by skeletal muscle?

A) Contraction is ATP dependent

B) Contracts in response to stretch

C) Does not contain actin filaments

D) High rate of cross-bridge cycling

E) Low maximal force of contraction

A

B) An important characteristic of visceral smooth muscle is its ability to contract in response to stretch. Stretch results in depolarization and potentially the generation of action potentials. These action potentials, coupled with normal slow-wave potentials, stimulate rhythmical contractions. Like skeletal muscle, smooth muscle concentration is both actin and ATP dependent. However, the cross-bridge cycle in smooth muscle is considerably slower than in skeletal muscle, which allows for a higher maximal force of contraction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

The resting potential of a myelinated nerve fiber is primarily dependent on the concentration gradient of which of the following ions?

A) Ca++

B) Cl-

C) HCO3-

D) K+

E) Na+

A

D) The resting potential of any cell is dependent on the concentration gradients of the permeant ions and their relative permeabilities (Goldman equation). In the myelinated nerve fiber, as in most cells, the resting membrane is predominantly permeable to K+. The negative membrane potential observed in most cells (including nerve cells) is due primarily to the relatively high intracellular concentration and high permeability of K+.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Calmodulin is most closely related, both structurally and functionally, to which of the following proteins?

A) G-actin

B) Myosin light chain

C) Tropomyosin

D) Troponin C

A

D) In smooth muscle, the binding of four Ca++ ions to the protein calmodulin permits the interaction of the Ca++-calmodulin complex with myosin light chain kinase. This interaction activates myosin light chain kinase, resulting in the phosphorylation of the myosin light chains and, ultimately, muscle contraction. In skeletal muscle, the activating Ca++ signal is received by the protein troponin C. Like calmodulin, each molecule of troponin C can bind with up to four Ca++ ions. Binding results in a conformational change in the troponin C protein that dislodges the tropomyosin molecule and exposes the active sites on the actin filament.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Which of the following is a consequence of myelination in large nerve fibers?

A) Decreased velocity of nerve impulses

B) Generation of action potentials only at the nodes of Ranvier

C) Increased energy requirement to maintain ion gradients

D) Increased membrane capacitance

E) Increased nonselective diffusion of ions across the axon membrane

A

B) Myelination of the axons of large nerve fibers has several consequences. It provides insulation to the axon membrane, decreasing membrane capacitance and thereby decreasing the “leakage” of ions across the cell membrane. Action potentials in myelinated axons occur only at the periodic breaks in the myelin sheath, call nodes of Ranvier. Voltage-gated Na+ channels are concentrated at these nodes. This arrangement both increases the velocity of the nerve impulses along the axon and minimizes the number of charges that cross the membrane during an impulse, thereby minimizing the energy required by Na+, K+–ATPase to re-establish the relative concentration gradients for Na+ and K+.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

During a demonstration for medical students, a neurologist uses magnetic cortical stimulation to trigger firing of the ulnar nerve in a volunteer. At relatively low-amplitude stimulation, action potentials are recorded only from muscle fibers in the index finger. As the amplitude of the stimulation is increased, action potentials are recorded from muscle fibers in both the index finger and the biceps muscle. What is the fundamental principle underlying this amplitude-dependent response?

A) Large motor neurons that innervate large motor units require a larger depolarizing stimulus

B) Recruitment of multiple motor units requires a larger depolarizing stimulus

C) The biceps muscle is innervated by more motor neurons

D) The motor units in the biceps are smaller than those in the muscles of the fingers

E) The muscles in the fingers are innervated only by the ulnar nerve

A

A) Muscle fibers involved in fine motor control are generally innervated by small motor neurons with relatively small motor units, including those that innervate single fibers. These neurons fire in response to a smaller depolarizing stimulus compared with motor neurons with larger motor units. As a result, during weak contractions, increases in muscle contraction can occur in small steps, allowing for fine motor control. This concept is called the size principle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Similarities between smooth and cardiac muscle include which of the following?

A) Ability to contract in the absence of an action potential

B) Dependence of contraction on Ca++ ions

C) Presence of a T tubule network

D) Role of myosin kinase in muscle contraction

E) Striated arrangement of the actin and myosin filaments

A

B) The strongest common denominator among smooth, skeletal, and cardiac muscle contraction is their shared dependence on Ca++ for the initiation of contraction. Cardiac and skeletal muscles exhibit several characteristics not shared by smooth muscle. For example, the contractile proteins in both cardiac and skeletal muscles are organized into discrete sarcomeres. Both muscle types also possess some semblance of a T tubule system and are dependent on the generation of action potentials for their contraction. Smooth muscle, in contrast, is relatively less organized, is uniquely regulated by myosin light chain phosphorylation, and can contract in vivo in the absence of action potentials.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

In a normal, healthy muscle, what occurs as a result of propagation of an action potential to the terminal membrane of a motor neuron?

A) Opening of voltage-gated Ca++ channels in the presynaptic membrane

B) Depolarization of the T tubule membrane follows

C) Always results in muscle contraction

D) Increase in intracellular Ca++ concentration in the motor neuron terminal

E) All of the above are correct

A

E) The neuromuscular junction is equipped with a so-called safety factor that ensures that every nerve impulse that travels to the terminal of a motor neuron results in an action potential in the sarcolemma. Given a normal, healthy muscle, contraction is also ensured. The voltage sensitivity of the Ca++ channels in the presynaptic membrane and the high concentration of extracellular Ca++ ensure an influx of Ca++ sufficient to stimulate the fusion of synaptic vesicles to the presynaptic membrane and the release of acetylcholine. The overabundance of acetylcholine released guarantees a depolarization of the postsynaptic membrane and the firing of an action potential.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Which of the following decreases in length during the contraction of a skeletal muscle fiber?

A) A band of the sarcomere

B) I band of the sarcomere

C) Thick filaments

D) Thin filaments

E) Z discs of the sarcomere

A

B) The physical lengths of the actin and myosin filaments do not change during contraction. Therefore, the A band, which is composed of myosin filaments, does not change either. The distance between Z discs decreases, but the Z discs themselves do not change. Only the I band decreases in length as the muscle contracts.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

A cross-sectional view of a skeletal muscle fiber through the H zone would reveal the presence of what?

A) Actin and titin

B) Actin, but no myosin

C) Actin, myosin, and titin

D) Myosin and actin

E) Myosin, but no actin

A

E) The H zone is the region in the center of the sarcomere composed of the lighter bands on either side of and including the M line. In this region, the myosin filaments are centered on the M line, and there are no overlapping actin filaments. Therefore, a cross-section through this region would reveal only myosin.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Tetanic contraction of a skeletal muscle fiber results from a cumulative increase in the intracellular concentration of which of the following?

A) ATP

B) Ca++

C) K+

D) Na+

E) Troponin

A

B) Muscle contraction is dependent on an elevation of intracellular Ca++ concentration. As the twitch frequency increases, the initiation of a subsequent twitch can occur before the previous twitch has subsided. As a result, the amplitude of the individual twitches is summed. At very high twitch frequencies, the muscle exhibits tetanic contraction. Under these conditions, intracellular Ca++ accumulates and supports sustained maximal contraction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Malignant hyperthermia is a potentially fatal genetic disorder characterized by a hyper-responsiveness to inhaled anesthetics and results in elevated body temperature, skeletal muscle rigidity, and lactic acidosis. Which of the following molecular changes could account for these clinical manifestations?

A) Decreased voltage sensitivity of the dihydropyridine receptor

B) Enhanced activity of the sarcoplasmic reticulum Ca++ATPase

C) Prolonged opening of the ryanodine receptor channel

D) Reduction in the density of voltage-sensitive Na+ channels in the T tubule membrane

A

C) As long as the ryanodine receptor channel on the sarcoplasmic reticulum remains open, Ca++ will continue to flood the sarcoplasm and stimulate contraction. This prolonged contraction results in heat production, muscle rigidity, and lactic acidosis. In contrast, factors that either inhibit Ca++ release or stimulate Ca++ uptake into the sarcoplasmic reticulum, or that prevent either the depolarization of the T tubule membrane or the transduction of the depolarization into Ca++ release, would favor muscle relaxation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Weightlifting can result in a dramatic increase in skeletal muscle mass. This increase in muscle mass is primarily attributable to which of the following?

A) Fusion of sarcomeres between adjacent myofibrils

B) Hypertrophy of individual muscle fibers

C) Increase in skeletal muscle blood supply

D) Increase in the number of motor neurons

E) Increase in the number of neuromuscular junctions

A

B) Prolonged or repeated maximal contraction results in a concomitant increase in the synthesis of contractile proteins and an increase in the synthesis of contractile proteins and an increase in muscle mass. This increase in mass, or hypertrophy, is observed at the level of individual muscle fibers.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Which of the following transport mechanisms is not rate limited by an intrinsic Vmax?

A) Facilitated diffusion via carrier proteins

B) Primary active transport via carrier proteins

C) Secondary co-transport

D) Secondary counter-transport

E) Simple diffusion through protein channels

A

E) Facilitated diffusion and both primary and secondary active transport all involve protein transporters or carriers that must undergo some rate-limited conformational change. The rate of simple diffusion is linear with solute concentration.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Assuming complete dissociation of all solutes, which of the following solutions would be hyperosmotic relative to 1 millimolar NaCl?

A) 1 millimolar CaCl2

B) 1 millimolar glucose

C) 1 millimolar KCl

D) 1 millimolar sucrose

E) 1.5 millimolar glucose

A

A) The term ‘hyperosmotic” refers to a solution that has a higher osmolarity relative to another solution. The osmolarity of a 1-millimolar NaCl solution is 2mOsm/L. The osmolarity of a 1-millimolar solution of either glucose or sucrose is only 1 mOsm/L. The osmolarity of a 1.5-millimolar glucose is 1.5 mOsm/L. These solutions are all “hypo-osmotic” relative to 1 millimolar NaCl. The osmolarity of a 1-millimolar KCl solution is 2 mOsm/L. It is “iso-osmotic” relative to 1 millimolar NaCl. Only 1 millimolar CaCl2, with an osmolarity of 3 mOsm/L is hyperosmotic relative to 1 millimolar NaCl.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Which of the following is primarily responsible for the changes in membrane potential between points B and D?

A) Inhibition of the Na+, K+ATPase

B) Movement of K+ into the cell

C) Movement of K+ out of the cell

D) Movement of Na+ into the cell

E) Movement of Na+ out of the cell

A

D) At point B in this action potential, Vm has reached threshold potential and has triggered the opening of voltage-gated Na+ channels. The resulting Na+ influx is responsible for the rapid, self-perpetuating depolarization phase of the action potential.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Which of the following is primarily responsible for the change in membrane potential between points D and E?

A) Inhibition of the Na+, K+ATPase

B) Movement of K+ into the cell

C) Movement of K+ out of the cell

D) Movement of Na+ into the cell

E) Movement of Na+ out of the cell

A

C)The rapid depolarization phase is terminated at point D by the inactivation of the voltage-gated Na+ channels and the opening of the voltage-gated K+ channels. The latter results into the extracellular fluid and repolarization of the cell membrane.

28
Q

The delayed onset and prolonged duration of smooth muscle contraction, as well as the greater force generated by smooth muscle compared with skeletal muscle, are all consequences of which of the following?

A) Greater amount of myosin filaments present in smooth muscle

B) Higher energy requirement of smooth muscle

C) Physical arrangement of actin and myosin filaments

D) Slower cycling rate of the smooth muscle myosin cross-bridges

E) Slower uptake of Ca++ ions following contraction

A

D) The slower cycling rate of the cross-bridges in smooth muscle means that a higher percentage of possible cross-bridges is active at any point in time. The more active cross-bridges there are, the greater the force is generated. Although the relatively slow cycling rate means that it takes longer for the myosin head to attach to the actin filament, it also means that the myosin head remains attached longer, prolonging muscle contraction. Because of the slow cross-bridge cycling rate, smooth muscle actually requires less energy to maintain a contraction compared with skeletal muscle.

29
Q

An experimental drug is being tested as a potential therapeutic treatment for asthma. Preclinical studies have shown that this drug induces the relaxation of cultured porcine tracheal smooth muscle cells precontracted with acetylcholine. Which of the following mechanisms of action is most likely to induce this effect?

A) Decreased affinity of troponin C for Ca++

B) Decreased plasma membrane K+ permeability

C) Increased plasma membrane Na+ permeability

D) Inhibition of the sarcoplasmic reticulum Ca++ATPase

E) Stimulation of adenylate cyclase

A

E) The stimulation of either adenylate or guanylate cyclase induces smooth muscle relaxation. The cyclic nucleotides produced by these enzymes stimulate cAMP- and cGMP-dependent kinases, respectively. These kinases phosphorylate, among other things, enzymes that remove Ca++ from the cytosol, and in doing so they inhibit contraction. In contrast, either a decrease in K+ permeability or an increase in Na+ permeability results in membrane depolarization and contraction. Likewise, inhibition of the sarcoplasmic reticulum Ca++ATPase, one of the enzymes activated by cyclic nucleotide-dependent kinases, would also favor muscle contraction. Smooth muscle does not express troponin.

30
Q

Which of the following best describes muscle B, when compared to muscle A?

A) Adapted for rapid contraction

B) Composed of larger muscle fibers

C) Fewer mitochondria

D) Innervated by smaller nerve fibers

E) Less extensive blood supply

A

D) Muscle B is characteristic of a slow twitch muscle (Type 1) composed of predominantly slow twitch muscle fibers. These fibers are smaller in size and are innervated by smaller nerve fibers. They typically have a more extensive blood supply, a greater number of mitochondria, and large amounts of myoglobin, all of which support high levels of oxidative phosphorylation.

31
Q

The delay between the termination of the transient depolarization of the muscle membrane and the onset of muscle contraction observed in both muscles A and B reflects the time necessary for which of the following events to occur?

A) ADP to be released from the mysosin head

B) ATP to be synthesized

C) Ca++ to accumulate in the sarcoplasm

D) G-actin to polymerize into F-actin

E) Myosin head to complete one cross-bridge cycle

A

C) Muscle contraction is triggered by an increase in sarcoplasmic Ca++ concentration. The delay between the termination of the depolarization pulse and the onset of muscle contraction, also called the “lag,” reflects the time necessary for the depolarizing pulse to be translated into an increase in sarcoplasmic Ca++ concentration. This process involves a conformational change in the voltage-sensing, or dihydropyridine receptor, located on the T tubule membrane; the subsequent conformational change in the ryanodine receptor on the sarcoplasmic reticulum; and the release of Ca++ from the sarcoplasmic reticulum.

32
Q

A 55-year-old woman visits physician because of double vision, eyelid droop, difficulty chewing and swallowing, and general weakness in her limbs. All these symptoms are made worse with exercise and occur more frequently late in the day. The physician suspects myastenia gravis and orders a Tensilon test. The test is positive.

The increased muscle strength observed during the Tensilon test is due to an increase in which of the following?

A) Amount of acetylcholine (ACh) released from the motor nerves

B) Levels of ACh at the muscle end-plates

C) Number of ACh receptors on the muscle end-plates

D) Synthesis of norepinephrine

A

B) Levels of ACh at the muscle end-plates

33
Q

A 55-year-old woman visits physician because of double vision, eyelid droop, difficulty chewing and swallowing, and general weakness in her limbs. All these symptoms are made worse with exercise and occur more frequently late in the day. The physician suspects myastenia gravis and orders a Tensilon test. The test is positive.

What is the most likely basis for the symptoms described in this patient?

A) autoimmune response

B) botulinum toxicity

C) depletion of voltage-gated Ca++ channels in certain motor neurons

D) Development of macro motor units following recovery from poliomyelitis

E) overexertion

A

A) autoimmune response

34
Q

A 55-year-old woman visits physician because of double vision, eyelid droop, difficulty chewing and swallowing, and general weakness in her limbs. All these symptoms are made worse with exercise and occur more frequently late in the day. The physician suspects myastenia gravis and orders a Tensilon test. The test is positive.

Which of the following drugs would likely alleviate this patient’s symptoms?

A) Atropine

B) Botulinum toxin antiserum

C) Curare

D) Halothane

E) Neostigmine

A

E) Neostigmine

35
Q

The diagrams depict rigid containers composed of two aqueous chambers, A and B, each containing a Na+ solution and separated by a Na+-permeable membrane. The panel on the left represents the distribution of Na+ ions at rest in the absence of any electrical potential. In this scenario, the concentration of Na+ ions in chamber A equals the concentration of Na+ ions in chamber B. The panel on the right illustrates the effect of a +60 millivolt potential applied across the membrane (chamber B relative to chamber A). Assuming a temperature of 37degC, which of the following expressions best describes the resulting distribution of Na+ ions between the two chambers?

A) [Na]a = 10[Na]b

B) [Na]a = 2[Na]b

C) [Na]a = 60[Na]b

D) [Na]b = 10[Na]a

E) [Na]b = 60[Na]a

A

D) [Na]b = 10[Na]a

(Using Nernst equation)

36
Q

Questions 36-38 The diagram illustrates the isometric length-tension relationship in a representative intact skeletal muscle. When answering the following three questions, use the letters in the diagram to identify each of the following.

So-called “active” or contraction-dependent tension.

A

B) the difference between total tension (trace A) and the passive tension contributed by noncontractile elements (trace C)

37
Q

Questions 36-38 The diagram illustrates the isometric length-tension relationship in a representative intact skeletal muscle. When answering the following three questions, use the letters in the diagram to identify each of the following.

The muscle length at which active tension is maximal.

A

E) “Active” tension is maximal at normal physiological muscle lengths.

38
Q

Questions 36-38 The diagram illustrates the isometric length-tension relationship in a representative intact skeletal muscle. When answering the following three questions, use the letters in the diagram to identify each of the following.

The contribution of non-contractile muscle elements to total tension.

A

C) Trace C represents the passive tension contributed by noncontractile elements, including fascia, tendons, and ligaments.

39
Q

Smooth muscle contraction is terminated by which of the following?

A) Dephosphorylation of myosin kinase

B) Dephosphorylation of myosin light chain

C) Efflux of Ca++ ions across the plasma membrane

D) Inhibition of myosin phosphatase

E) Uptake of Ca++ ions into the sarcoplasmic reticulum

A

B) Dephosphorylation of myosin light chain

40
Q

Questions 40 - 42: A 56-year-old man sees a neurologist because of weakness in his legs that improves over the course of the day or with exercise. Extracellular electrical recordings from a single skeletal muscle fiber reveal normal miniature end plate potentials. Low-frequency electrical stimulation of the motor neuron, however, elicits an abnormally small depolarization of the muscle fibers. The amplitude of the depolarization is increased after exercise.

Based on these findings, which of the following is the most likely cause of this patient’s leg weakness?

A) Acytylcholinesterase deficiency

B) Blockade of postsynaptic acetylcholine receptors

C) Impared presynaptic voltage-sensitive Ca++ channel

D) Inhibition of Ca++ re-uptake into the sarcoplasmic reticulum

E) Reduced acetylcholine synthesis

A

C) Impared presynaptic voltage-sensitive Ca++ channel

41
Q

Questions 40 - 42: A 56-year-old man sees a neurologist because of weakness in his legs that improves over the course of the day or with exercise. Extracellular electrical recordings from a single skeletal muscle fiber reveal normal miniature end plate potentials. Low-frequency electrical stimulation of the motor neuron, however, elicits an abnormally small depolarization of the muscle fibers. The amplitude of the depolarization is increased after exercise.

A preliminary diagnosis is confirmed by the presence of which of the following?

A) Antibodies against the acetylcholine receptor

B) Antibodies against the voltage-sensitive Ca++ channel

C) Mutation in the gene that codes for the ryanodine receptor

D) Relatively few vesicles in the presynaptic terminal

E) Residual acetylcholine in the neuromuscular junction

A

B) Antibodies against the voltage-sensitive Ca++ channel

42
Q

Questions 40 - 42: A 56-year-old man sees a neurologist because of weakness in his legs that improves over the course of the day or with exercise. Extracellular electrical recordings from a single skeletal muscle fiber reveal normal miniature end plate potentials. Low-frequency electrical stimulation of the motor neuron, however, elicits an abnormally small depolarization of the muscle fibers. The amplitude of the depolarization is increased after exercise.

The molecular mechanism underlying these symptoms is most similar to which of the following?

A) Acetylcholine

B) Botulinum Toxin

C) Curare

D) Neostigmine

E) Tetrodotoxin

A

B) Botulinum Toxin

43
Q

Point at which the membrane potential (Vm) is closest to the Na+ equilibrium potential.

A

Point D

44
Q

Point at which the driving force for Na+ is the greatest.

A

Point F

45
Q

Point at which the ratio of K+ permeability to Na+ permeability (PK/PNa) is the greatest.

A

Point F

46
Q

ATP is used directly for each of the following processes EXCEPT one. Which one is this EXCEPTION?

A) Accumulation of Ca++ by the sarcoplasmic reticulum

B) Transport of glucose into muscle cells

C) Transport of H+ from the parietal cells into the lumen of the stomach

D) Transport of K+ from the extracellular to intracellular fluid

E) Transport of Na+ from the intracellular to extracellular fluid

A

B) Transport of glucose into muscle cells

(Occurs through facilitated diffusion)

47
Q

In the experiment illustrated in diagram A, equal volumes of solutions X, Y, and Z are placed into the compartments of the two U-shaped vessels shown. The two compartments of each vessel are separated by semipermeable membranes (i.e., impermeable to ions and large polar molecules). Diagram B illustrates the fluid distribution across the membranes at equilibration. Assuming complete dissociation, identify each of the solutions shown.

A

B) The redistribution of fluid volume shown in diagram B reflects the net diffusion of water, or osmosis, due to differences in the osmolarity of the solutions on either side of the semipermeable membrane. Osmosis occurs from solutions of high water concentration to low water concentration or from low osmolarity to high osmolartiy. In diagram B, osmosis has occurred from X to Y and from Y to Z. Therefore, the osmolarity of solution Z is higher than that of solution Y, and the osmolarity of solution Y is higher than that of solution X.

48
Q

The force produced by a single skeletal muscle fiber can be increased by which of the following?

A) Decreasing extracellular K+ concentration

B) Increasing the amplitude of the depolarizing stimulus

C) Increasing the frequency of stimulation of the fiber

D) Increasing the number of voltage-gated Na+ channels in the sarcolemma

E) Increasing the permeability of the sarcolemma to K+

A

C) Increasing the frequency of stimulation of the fiber

49
Q

Trace A best describes the kinetics of which of the following events?

A) Movement of CO2 across the plasma membrane

B) Movement of O2 across a lipid bilayer

C) Na+ flux through an open nicotinic acetylcholine receptor channel

D) Transport of K+ into a muscle cell

E) Voltage-dependent movement of Ca++ into the terminal of a motor neuron

A

D) Transport of K+ into a muscle cell

(Only one subject to a Vmax associated with active transport event. The rest involve simple diffusion.)

50
Q

Trace B best describes the kinetics which of the following events?

A) Na+ dependent transport of glucose into an epithelial cell

B) Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell

C) Transport of K+ into a muscle cell

D) Transport of Na+ out of a nerve cell

E) Transport of O2 across an artificial lipid bilayer

A

E) Transport of O2 across an artificial lipid bilayer

(The only event involving simple diffusion as represented by Trace B)

51
Q

Trace A represents a typical action potential recorded under control conditions from a normal nerve cell in response to a depolarizing stimulus. Which of the following perturbations would explain the conversion of the response shown in trace A to the action potential shown in trace B?

A) Blockade of voltage-sensitive Na+ channels

B) Blockade of voltage-sensitive K+ channels

C) Blockade of Na-K “leak” channels

D) Replacement of voltage-sensitive K+ channels with “slow” Ca++ channels

E) Replacement of the voltage-sensitive Na+ channels with “slow” Ca++ channels

A

E) Replacement of the voltage-sensitive Na+ channels with “slow” Ca++ channels

52
Q

Which of the following perturbations would account for the failure of the same stimulus to elicit an action potential in trace C?

A) Blockade of voltage-sensitive Na+ channels

B) Blockade of voltage-sensitive K+ channels

C) Blockade of Na-K “leak” channels

D) Replacement of the voltage-sensitive K+ channels with “slow” Ca++ channels

E) Replacement of the voltage-sensitive Na+ channels with “slow” Ca++ channels

A

A) Blockade of voltage-sensitive Na+ channels

53
Q

A 17yo soccer player suffered a fracture to the left tibia. After her lower leg has been in a cast for 8 weeks, she is surprised to find that the left gastrocnemius muscle is significantly smaller in circumference than it was before the fracture. What is the most likely explanation?

A) Decrease in the number of individual muscle fibers in the left gastrocnemius

B) Decrease in blood flow to the muscle caused by constriction from the cast

C) Temporary reduction in actin and myosin protein synthesis

D) Increase in glycolytic activity in the affected muscle

E) Progressive denervation

A

C) Temporary reduction in actin and myosin protein synthesis

(This reversible reduction in muscle mass is called atrophy.)

54
Q

Smooth muscle that exhibits rhythmical contraction in the absence of external stimuli also necessarily exhibits which of the following?

A) “Slow” voltage-sensitive Ca++ channels

B) Intrinsic pacemaker wave activity

C) Higher resting cytosolic Ca++ concentration

D) Hyperpolarized membrane potential

E) Action potentials with “plateaus”

A

B) Intrinsic pacemaker wave activity

55
Q

A) Simple diffusion

B) Facilitated diffusion

C) Primary active transport

D) Co-transport

E) Counter-transport

Match each of the processes described below with the correct type of transport listed above (each answer may be used more than once).

  1. Ouabain-sensitive transport of Na+ ions from the cytosol to the extracellular fluid
A

C) Primary active transport

Ouabain inhibits Na+, K+ ATPase. This ATP-dependent enzyme transports three Na+ ions out of the cell for every two K+ ions it transports into the cell. It is a classic example of primary active transport.

56
Q

A) Simple diffusion

B) Facilitated diffusion

C) Primary active transport

D) Co-transport

E) Counter-transport

Match each of the processes described below with the correct type of transport listed above (each answer may be used more than once).

  1. Glucose uptake into skeletal muscle
A

B) Facilitated diffusion

Glucose is transported into skeletal muscle cells via insulin-dependent facilitated diffusion.

57
Q

A) Simple diffusion

B) Facilitated diffusion

C) Primary active transport

D) Co-transport

E) Counter-transport

Match each of the processes described below with the correct type of transport listed above (each answer may be used more than once).

  1. Na+ dependent transport of Ca++ from the cytosol to the extracellular fluid
A

E) Counter-transport

(Because ATP is required to maintain the Na+ gradient that drives this counter-transport, this type of transport is called secondary active transport.)

58
Q

A) Simple diffusion

B) Facilitated diffusion

C) Primary active transport

D) Co-transport

E) Counter-transport

Match each of the processes described below with the correct type of transport listed above (each answer may be used more than once).

  1. Transport of glucose from the intestinal lumen into an intestinal epithelial cell
A

D) Co-transport

Much like Na+-Ca++ counter-transport, the strong tendency for Na+ to move across the plasma membrane into the cytosol can be harnessed by transport proteins and used to co-transport molecules against their concentration gradients into the cytosol. An example of this type of secondary co-transport is the transport of glucose into intestinal epithelial cells.

59
Q

A) Simple diffusion

B) Facilitated diffusion

C) Primary active transport

D) Co-transport

E) Counter-transport

Match each of the processes described below with the correct type of transport listed above (each answer may be used more than once).

  1. Movement of Na+ ions into a nerve cell during the upstroke of an action potential
A

A) Simple diffusion

60
Q

Traces A, B, and C in the diagram summarize the changes in membrane potential (Vm) and the underlying membrane permeabilities (P) that occur in a nerve cell over the course of an action potential. Choose the combination of labels below that accurately identifies each of the traces.

A

E) Trace A exhibits the characteristic shape of an action potential, including the rapid depolarization followed by a rapid repolarization that temporarily overshoots the resting potential. Trace B best illustrates the change in Pna that occurs during an action potential. The rapid increase in Pna closely parallels the rapid depolarization phase of the action potential. Trace C best illustrates the slow onset of the increase in Pk that reflects the opening of the voltage-gated K+ channels.

61
Q

If the intracellular concentration of a membrane-permeant substance doubles from 10 to 20 millimolar and the extracellular concentration remains at 5 millimolar, the rate of diffusion of that substance across the plasma membrane will increase by a factor of how much?

A) 2

B) 3

C) 4

D) 5

E) 6

A

B) Net diffusion of a substance across a permeable membrane is proportional to the concentration difference of the substance on either side of the membrane. Initially, the concentration difference is 5 millimolar (10 millimolar - 5 millimolar). When the intracellular concentration double to 20 millimolar, the concentration difference becomes 15 millimolar (20 millimolar - 5 millimolar). The concentration difference has tripled, therefore, the rate of diffusion would also increase by a factor of 3.

62
Q

Which of the following pairs of aqueous solutions will exert equal osmotic pressures across a normal cell membrane once steady-state conditions have been established?

Solution A; Solution B

A) 10% albumin; 10% IgG

B) 100mmol/L NaCl; 100mOsm/L CaCl2

C) 300mOsm/L glucose; 300mOsm/L urea

D) 300mOsm/L glycerol; 300mOsm/L NaCl

E) 300mOsm/L glycerol; 300mOsm/L urea

A

E) Glycerol and urea are both permeant molecules, which means that both will diffuse through the cell membrane until the intracellular and extracellular concentrations are identical. Thus, during steady-state conditions the intracellular and extracellular osmolarity is 600mOsm/L (300 mOsm/L from urea and 300mOsm/L from glycerol). Choice A is not correct because albumin is a smaller molecule compared to IgG. This difference in molecular weight means that a 10% solution of albumin will contain more molecules per unit volume compared to a 10% solution of IgG and thus exert a greater osmotic effect. Choice B: A solution of 100mmol/L NaCl has an osmolarity of 200mOsm/L because Na and Cl dissociate. Thus, the osmolarity of solution A will be two times greater than solution B. Choices C and D: Both solutions have equal osmolarities; however, both urea and glycerol are permeant molecules (whereas glucose and NaCl are not), which means that urea and glycerol will diffuse into the cell and effectively cancel their osmotic effects accross the cell membrane.

63
Q

A 12 yo boy presents with a 4-month history of diminished vision and diplopia. He also experiences tiredness toward the end of the day. There are no other symptoms. On examination, the patient has ptosis of the left eye that improves after a period of sleep. Clinical examination is otherwise normal. There is no evidence of weakness of any other muscles. Additional testing indicates the presence of anti-acetylcholine antibodies in the plasma, a normal thyroid function test, and a normal CT scan of the brain and orbit. What is the initial diagnosis?

A) Astrocytoma

B) Graves disease

C) Hashimoto thyroiditis

D) Juvenile myasthenia gravis

E) Multiple sclerosis

A

D) Myasthenia gravis is an acquired autoimmune disease causing skeletal muscle fatigue and weakness. The disease is associated with (caused by) IgG antibodies to acetylcholine receptors at post-synaptic membranes of neuromuscular junctions. The major symptom is muscle weakness, which gets worse with activity. Patients often feel well in the morning, but become weaker as the day goes on. The muscle weakness usually causes symptoms of double vision (diplopia) and drooping eyelids (ptosis). The presence of anti-acetylcholine antibodies in the plasma is specific for myasthenia graveis and thus rules out the other answer choices. In addition, the normal CT scan of the brain and orbit specifically rules out the possibility of an astrocytoma (choice A), that is brain tumor, that could compress cranial nerves. Double vision commonly occurs in Graves disease (choice B); however, the thyroid test was normal (which also rules out Hashimoto thyroiditis, choice C). Multiple sclerosis (choice E) is commonly associated with a spastic weakness of the legs, but, again, the presence of anti-acetylcholine antibodies is specific for myasthenia gravis.

64
Q

The length-tension diagram shown here was obtained from a skeletal muscle with equal numbers of red and white fibers. Supramaximal tetanic stimuli were used to initiate an isometric contraction at each muscle length studied. The resting length was 20 cm. What is the maximum amount of active tension that the muscle is capable of generating at a preload of 100 grams?

A) 145-155 grams

B) 25 - 35 grams

C) 55-65 grams

D) 95-105 grams

E) Cannot be determined

A

C) The diagram shows the relationship between preload or passive tension (curve Z), total tension (curve X) and action tension (curve Y). Active tension cannot be measured directly; it is the difference between total tension and passive tension. To answer this question, the student must first find where 100 grams intersects the preload curve (passive tension curve) and then move down to the active tension curve. One can see that a preload of 100 grams is associated with a total tension of a little more than 50 grams. Note that active tension equals total tension minus passive tension, as discussed above. Drawing these three curves in a manner that is mathematically correct is not an easy task. The student should thus recognize that active tension may not equal total tension minus passive tension at all points on the diagram shown here as well as on USMLE diagrams.

65
Q

The sensitivity of the smooth muscle contractile apparatus to calcium is known to increase in the steady-state under normal conditions. This increase in calcium sensitivity can be attributed to a decrease in the levels of which of the following substances?

A) Actin

B) Adenosine Triphosphate (ATP)

C) Calcium-calmodulin complex

D) Calmodulin

E) Myosin light chain phosphatase (MLCP)

A

E) Smooth muscle is unique in its ability to generate various degrees of tension at a constant concentration of intracellular calcium. This change in calcium sensitivity of smooth muscle can be attributed to differences in the activity of MLCP. Smooth muscle contracts when the myosin light chain is phosphorylated by the actions of mysoin light chain kinase (MLCK). MLCP is a phosphatase that can dephosphorylate the myosin light chain, rendering it inactive and therefore attenuating the muscle contraction. Choice A: Both actin and myosin are important components of the smooth muscle contractile apparatus much like that of skeletal muscle and cardiac muscle, but these do not play a role in calcium sensitivity. Choice B: ATP is required for smooth muscle contraction. Decreased ATP levels would be expected to decrease the ability of calcium levels. Choice C: The calcium-calmodulin complex binds with MLCK, which leads to phosphorylation of the myosin light chain. A decrease in the calcium-calmodulin complex should attenuate the contraction of smooth muscle. Choice D: Again, the binding of calcium ions to calmodulin is an initial step in the activation of the smooth muscle contractile apparatus.

66
Q

The diagram shows the force-velocity relationship for isotonic contractions of skeletal muscle. The differences in the three curves result from differences in which of the following?

A) Frequency of muscle contraction

B) Hypertrophy

C) Muscle mass

D) Myosin ATPase activity

E) Recruitment of motor units

A

D) The diagram shows that the maximum velocity of shortening (Vmax) occrs when there is no afterload on the muscle (force = 0). Increasing afterload decreases the velocity of shortening until a point is reached where shortening does not occur (isometric contraction) and contraction velocity is thus 0 (where curves intersect X-axis). The maximum velocity of shortening is dictated by the ATPase activity of the muscle, increasing to high levels when the ATPase activity is elevated. Choice A: Increasing the freq. of muscle contraction will increase the load that a muscle can lift within the limits of the muscle, but will not affect the velocity of contraction. Choices B, C, and E: Muscle hypertrophy, increasing muscle mass, and recruiting additional motor units will increase the maximum load that a muscle can lift, but these will not affect the maximum velocity of contraction.

67
Q

A 24yo woman is admitted as an emergency to University Hospital following an automobile accident in which severe lacerations to the left wrist severed a major muscle tendon. The severed ends of the tendon were overlapped by 6cm to facilitate suturing and reattachment. Which of the following would be expected after 6 weeks compared to the preinjured muscle? Assume that series growth of sarcomeres cannot be completed within 6 weeks.

Passive tension; Maximal active tension

A) decrease; decrease

B) decrease; increase

C) increase; increase

D) increase; decrease

E) no change; no change

A

D) Stretching the muscle to facilitate reattachment of the tendons leads to an increase in passive tension or preload. This increase in passive tension increases the muscle length beyond its ideal length, which in turn leads to a decrease in the maximal active tension that can be generated by the muscle. The reason that maximal active tension decreases is that the interdigitation of actin and myosin filaments decreases when the muscle is stretched; the interdigitation of a muscle is normally optimal at its resting length.