Unit 2 Review Flashcards
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
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.
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
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.
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
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.
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
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.
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) 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).
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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+
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+.
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
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.
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
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+.
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) 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.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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) 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.
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
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.
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
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.
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
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.
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
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.
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
D) [Na]b = 10[Na]a
(Using Nernst equation)
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.
B) the difference between total tension (trace A) and the passive tension contributed by noncontractile elements (trace C)
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.
E) “Active” tension is maximal at normal physiological muscle lengths.
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.
C) Trace C represents the passive tension contributed by noncontractile elements, including fascia, tendons, and ligaments.
Point at which the membrane potential (Vm) is closest to the Na+ equilibrium potential.
Point D
Point at which the driving force for Na+ is the greatest.
Point F
Point at which the ratio of K+ permeability to Na+ permeability (PK/PNa) is the greatest.
Point F
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.
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.
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
D) Transport of K+ into a muscle cell
(Only one subject to a Vmax associated with active transport event. The rest involve simple diffusion.)
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
E) Transport of O2 across an artificial lipid bilayer
(The only event involving simple diffusion as represented by Trace B)
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
E) Replacement of the voltage-sensitive Na+ channels with “slow” Ca++ channels
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) Blockade of voltage-sensitive Na+ channels
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.
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.
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
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.
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
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.
