Exam II Muscles

What type of isotonic contraction extends your leg?

Eccentric; Eccentric lengthens, and concentric shortens

Upstroke of an action potential of cardiac muscle

Inward calcium current and inward sodium current

Upstroke of an action potential of skeletal muscle

Inward sodium current

In a _____ muscle contraction, fibers develop tension, but do not change length. In a _____ muscle contraction, fibers develop tension by shortening and then maintaining that tone

Isometric Isotonic

Type IIa muscle fibers 1. Fast or slow 2. Source of ATP 3. Mitochondria density 4. Size 5. Resistance to fatigue

1. Fast twitch oxidative/glycolitic 2. Cellular respiration 3. High 4. Medium 5. Medium

Type I muscle fibers 1. Fast or slow 2. Source of ATP 3. Mitochondria density 4. Size 5. Resistance to fatigue

1. Slow twitch oxidative 2. Cellular respiration 3. High 4. Small 5. High

Sequence of channel processes (opening, closing, etc. in cardiac action potentials) 1. Depolarization: 2. Eerly repolarization: 3. Pllateau: 4. Rapid repolarization: 5. Resting membrane potential:

1. VGNCs 2. Closure of VGNCs and opening of VGKCs 3. Opening of L-type VGCCs 4. Closure of L-type VGCCs and opening of slow K+ channels and rectifier K+ channels 5. Closure of slow K+ channels, and remaining open of K+ rectifier cells

Type IIb muscle fibers 1. Fast or slow 2. Source of ATP 3. Mitochondria density 4. Size 5. Resistance to fatigue

1. Very Fast twitch glycolitic 2. Anaerobic glycolysis 3. low 4. Large 5. Low

A 32-year-old male is training for the 'World's Strongest Man' competition and is training by repeatedly dead-lifting giant boulders. In order to lift them his muscles must produce great contractile strength through repetitive action potentials. Repetitive stimulation of a skeletal muscle fiber will cause an increase in contractile strength because repetitive stimulation causes an increase in what? A) The total duration of cross-bridge cycling B) The concentration of calcium in the myoplasm C) The magnitude of the end-plate potential D) The number of muscle myofibrils generating tension E) The velocity of muscle contraction

A - This question is directly describing tetanic contraction. An increase in stimulation to the skeletal muscle fiber will cause an increase in the activity of the DHP and therefore RyR receptors and cause to an almost continues release of calcium from the sarcoplasmic reticulum. This will lead to an increase in the concentration of calcium in the sarcoplasm (not myoplasm, myoplasm refers to the contractile components and the Ca2+ surrounds them, it is not 'in' them). This increase in calcium concentration causes the cross-bridge cycling to occur for a longer duration leading to the increase in tension occurring for a longer period of time.

In a series of afterloaded isotonic twitches, as the load is increased, the: A) Force developed by the muscle increases, and the shortening velocity decreases B) Force developed by the muscle increases, and the shortening velocity remains the same C) Force developed by the muscle decreases, and the shortening velocity increasing D) The maximum shortening velocity increases

A This is a statement of the relationship that is graphically represented in the force-velocity curve. It is the force that determines the velocity, not the other way around, and the shortening of a skeletal muscle is independent of loading conditions

What does the powerstroke release?

ADP and Pi

Myasthenia gravis

An autoimmune disease characterized by the presence of anti-acetylcholine receptor antibodies in the plasma. Overexertion can cause junction fatigue, and both a decrease in the density of voltage-sensitive Ca++ channels in the presynaptic membrane and botulinum toxicity can cause muscle weakness. However, these effects are presynaptic and therefore would not be reversed by acetylcholinesterase inhibition. Although the macro-motor units formed during reinnervation following poliomyelitis compromise the patient's fine motor control, they do not affect muscle strength.

E/C coupling of smooth muscle

AP opens VGCCs NTs and Hromones open IP3CCs Ca++ binds to calmodulin, MLCKs phosphorylate

E/C coupling of skeletal muscle

AP to T tubules Ca++ released from SR Ca++ binds to troponin

ATP is required for shortening of skeletal muscle because A) ATP forms covalent links between actin and myosin B) ATP is required to detach the myosin from actin during the crossbridge cycle C) ATP provides the energy for the "Power stroke" during the crossbridge cycle D) ATP provides the binding site for calcium to initiate the crossbridge cycle E) ATP provides the energy needed to change the orientation of the myosin head from 90 degrees to 45 degrees

B ATP binds to myosin, changing its affinity for actin so that the crossbridges detach. The power stroke occurs when the myosin is attached to actin. ATP does not bind actin to myosin, nor is it the binding site for calcium.

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

B Botulinum toxin inhibits muscle contraction presynaptically by decreasing the amount of ACh released into the neuromuscular junction. In contrast, curare acts post-synaptically, blocking the nicotinic ACh receptors and preventing the excitation of the muscle cell membrane. Tetrodotoxin blocks voltage-sensitive Na+ channels, impacting both the initiation and the propagation of action potentials in the motor neuron. Both ACh and neostigmine stimulate muscle contraction.

If a muscle is arranged so as to lift an afterload equal to one half its maximal isometric capabilities, the force it develops is determined by the: A) The initial passive length of the muscle B) Size of the afterload C) Strength of the stimulation D) Number of motor units activated E) The preload

B Muscle contracting against an afterload that is less than the maximum isometric force capable for the preload to which that muscle is set will develop isometric twitch force only up to the level of the afterload.

The role of myosin light-chain kinase in smooth muscle is what? A) Bind to calcium ions to initiate excitation-contraction coupling B) Phosphorylate cross-bridges, thus driving them to bind with the thin filament C) Split ATP to provide the energy for the power stroke of the cross-bridge cycle D) Dephosphorylate myosin light-chains of the cross-bridge, thus relaxing the muscle E) Pump calcium from the cytosol back into the sarcoplasmic reticulum

B Myosin light chain kinase, when activated by calcium-calmodulin, phosphorylates myosin light chains initiating the cross-bridge cycling and interaction of smooth muscle fibers.

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 muscle mass. This increase in mass, or hypertrophy, is observed at the level of individual muscle fibers.

A 25-year-old female is playing an intense game of tennis with her friend. She lunges quickly to hit a difficult shot and intensly contracts her lower-limb muscles in the process. What prevents a drop in muscle fiber ATP concentration during the first few seconds of intense contraction? A) Because cross-bridges are pre-energized, ATP is not needed until several cross-bridge cycles have been completed. B) ADP is rapidly converted back to ATP by creatine phosphate C) Glucose is metabolized by glycolysis, producing large quanities of ATP D) The mitochondria immediately begin oxidative phosphorylation E) Fatty acids are rapidly converted to ATP by oxidative glycolysis

B The initial ATP source used during muscle contraction is produced from the breakdown of creatine phosphate.

Due to high blood pressure, the heart increases its filling load to overcome the great afterload within the cardiovascular system. What is the physiological basis for this? A) A direct increase in intracellular [Ca2+] B) An increase in preload reduces afterload C) An increase in preload stretches the cardiac muscle fibers to a length where they can produce a greater amount of active tension D) An increase in preload causes the thin and thick filaments to be very squished together reducing the interaction between actin and myosin

C An increase in preload allows for the cardiac muscle fibers to stretch and the myosin and actin molecules to be in a more optimal alignment allowing for more active tension to be produced, thereby allowing for the heart to overcome a greater afterload.

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.

Blockade of calcium release from the SR of smooth muscle will: A) Prevent contraction of the muscle following activation of ligand gated calcium channels B) Prevent action potential generation in smooth muscle cells C) Not affect the tonic contraction of the muscle stimulated by activation of ligand gated membrane channels D) Not affect the contribution to the contraction of the muscle as stimulated by IP3 E) Result in a total relaxation of the smooth muscle

C Calcium activation for smooth muscle comes from the SR and the entry of calcium through the ECF via one or more of the types of calcium channels in the membrane. Simply opening the ligand-gated channels will contract the muscle tonically, first due to the influx of calcium through the channel and secondly by activating the latch state mechanisms of the cell.

Why is the latent period longer during an isotonic twitch of a skeletal muscle fiber than it is during an isometric twitch? A) Excitation-contraction coupling is slower during an isotonic twitch B) Action potentials propagate more slowly when a fiber is shortening, so extra time is required to activate the entire fiber C) In addition to the time for EC coupling, it takes extra time for enough cross-bridges to attach to make the tension in the muscle fiber greater than the load. D) Fatigue sets in much more quickly during isotonic contractions, and when muscles are fatigued the cross-bridges move much more slowly E) The latent period is longer because twitches only occur in slow (type I) muscle fibers.

C In order to produce muscle shortening (isotonic contraction), the amount of tension the muscle fiber needs to produce is greater than the load of the object. This will take more time to happen than to produce tension equal to that of the load of the object which would be what happens during the isometric contraction.

The amount of force produced by a skeletal muscle can be increased by which of the following? A) Decreasing the activity of nicotinic ACh receptor B) Decreasing extracellular Ca2+ Increasing the activity of acetylcholinesterase C) Decreasing the interval between contractions E) Greatly increasing the starting muscle length

C Initiation of contractions close together in time causes a buildup of intracellular [Ca2+] which leads to a greater force produced by the skeletal muscle.

Prokinetic agents aid in the ability for the smooth muscle of the lower-esophageal sphincter to maintain continued contraction without the breakdown of ATP. Which of the following steps are responsible for this? A) Unphosphorylated MLCs → Phosphorylated MLCs B) Phosphorylated MLCs → actinomysin with phosphorylated light chains C) Actinomysin with phosphorylated light chains → actinomysin without phosphorylated light chains D) Actinomysin without phosphorylated light chains → Unphosphorylated MLCs

C Latch State is caused by the dephosphorylation of myosin light chains by myosin light chain phosphatase while the myosin head groups are bound to actin.

The delay between the termination of the transient depolarization of the muscle membrane and the onset of muscle contraction observed in muscles reflects the time necessary for which of the following events to occur? A) ADP to be released from the myosin 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 depolarizing 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.

Which Smooth muscle cells are initiated to contract by all of the following ways except which? A) Depolarization from a neighboring cell spread through connexon channels B) Norepinephrine binding to α1-adrenergic receptors C) Acetylcholine binding to nicotinic acetylcholine receptors D) The movement of a bolus of food through the small intestine

C Nicotinic AChR are the activating receptors in skeletal muscles and at autonomic ganglion, but not in smooth muscle cells.

How does the force-velocity relationship in smooth muscles compare to that of skeletal muscles? A) They are identical. B) Increasing the degree of phosphorylation of myosin light chains increases the shortening velocity of both muscle fiber types. C) The maximal shortening velocity in smooth muscles is much less than in skeletal muscles. D) The maximal active tension or force produced by smooth muscles is much less than skeletal muscles.

C The additional steps required to produce active tension in smooth muscles makes the shortening velocity greatly decreased compared with skeletal muscles. Additionally, the binding of ATP to the myosin head causing the disassociation from actin is a slower process in smooth muscles as well.

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) Acetylcholinesterase deficiency B) Blockade of postsynaptic acetylcholine receptors C) Impaired presynaptic voltage-sensitive Ca++ influx D) Inhibition of Ca++ re-uptake into the sarcoplasmic reticulum E) Reduced acetylcholine synthesis

C The normal miniature end-plate potentials indicate sufficient synthesis and packaging of ACh and the presence and normal function of ACh receptor channels. The most likely explanation for this patient's symptoms is a presynaptic deficiency—in this case, an impairment of the voltage-sensitive Ca++ channels responsible for the increase in cytosolic Ca++ that triggers the release of ACh into the synapse. The increase in postsynaptic depolarization observed after exercise is indicative of an accumulation of Ca++ in the presynaptic terminal after multiple action potentials have reached the nerve terminal.

If the calcium pumping ability of the sarcoplasmic reticulum was impaired, what would be the result? A) The muscles would relax more quickly because less calcium would be pumped B) Contraction would be slowed, but the muscle would relax normally C) The muscle would continue to develop force, but the relaxation would be slowed D) Activation would no longer be possible E) Cross bridges would not cycle in the muscle cell

C) The pump refers to the pumping out of calcium, whereas the voltage-gated calcium channels refer to the calcium entering the cell. This situation would not effect the incoming calcium to cause a contraction, but would effect the levels inside the cell that inhibit the relaxation.

Unfused vs. fused tetanus

Ca++ doesn't have time to go back to sarcolemma → summation of APs called "unfused tetanus" or a titanic contraction, which exceeds the peak twitch force. If this goes to 100% saturation → "fused" tetanus, or a sustained contraction occurs.

Three metabolic sources of ATP:

Creatine phosphate (first) Anaerobic metabolism/glycolysis (second) Aerobic metabolism/oxidative phosphorylation (third). In general, resting skeletal muscle relies on aerobic metabolism to maintain free-ATP levels in the sarcoplasm. In exercise, you also use the other too.

What is the relationship between the max velocity and max force between skeletal and smooth muscle? A) Skeletal has a faster max velocity and a stronger max force than smooth B) Skeletal has a slower max velocity and a stronger max force than smooth C) Skeletal has the same max velocity and a stronger max force than smooth D) Skeletal has a faster max velocity and a the same max force as smooth

D

At the muscle end plate, acetylcholine (ACh) causes the opening of A) Na+ channels and depolarization toward the Na+ equilibrium potential B) K+ channels and depolarization toward the K+ equilibrium potential C) Ca2+ channels and depolarization toward the Ca2+ equilibrium potential D) Na+ and K+ channels and depolarization to a value halfway between the Na+ and K+ equilibrium potentials E) Na+ and K+ channels and hyperpolarization to a value halfway between the Na+ and K+ equilibrium potentials

D Binding of acetylcholine (ACh) to receptors in the muscle end plate opens channels that allow passage of both Na+ and K+ ions. Na+ ions will flow into the cell down its electrochemical gradient, and K+ ions will flow out of the cell down its electrochemical gradient. The resulting membrane potential will be depolarized to a value that is approximately halfway between their respective equilibrium potentials.

Which of the following correctly characterizes type IIa skeletal muscle fibers? A) Few mitochondria and high glycogen content B) Low myosin ATPase rate and few surrounding capillaries C) Low glycolytic enzyme activity and intermediate contraction velocity D) High myoglobin content and intermediate glycolytic enzyme activity E) Small fiber diameter and fast onset of fatigue

D Fast-oxidative fibers are the type IIa fibers that are red with high amounts of myoglobin, but also have some glycolitic activity as well.

When comparing two types of twitch muscle fibers, muscle B will have a longer duration of action potential than muscle A. 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.

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 acetylcholine receptor channel, and return of the dihydropyridine receptor to its resting conformation—would have any effect on the contractile state of the muscle.

A 24-year-old 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 6 cm 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/Active tension) A) decrease decrease B) decrease increase C) increase increase D) increase decrease E) no change no change

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 interdigitation of actin and myosin filaments decreases when the muscle is stretched; the interdigitation of a muscle is normally optimal at its resting length.

How does the length-tension relationship in cardiac muscles compare to that in skeletal muscles? A) They are identical. B) There is much more passive tension associated with connective tissue in skeletal muscles. C) The tension curve due to the interaction between actin and myosin is much more broad in skeletal muscles. D) The resting length in cardiac muscles is below optimum to allow the heart to fill with blood before contracting with maximal tension. E) The amount of tension that skeletal muscles can produce is much greater than the amount that cardiac muscles can produce.

D The normal resting length in cardiac muscles is shorter which allows for the filling of the heart with blood to stretch the individual fibers to the length where they can produce the maximum amount of active tension.

13. 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+

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

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 that 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.

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

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.

How do you create a latch state

If the phosphate group comes off DURING the physical interaction (from the light chain phosphatase) the contraction enters rigor mortis state because affinity for ATP goes down.

Upstroke of an action potential of smooth muscle

Inward calcium current

E/C coupling of cardiac muscle

Inward Ca++ current during plateau Ca++ induced Ca++ release from SR Ca++ binds to troponin