Nerves and Muscles Physiology-Critical thinking
1. Describe the sequence of events following paralysis of the sodium pump. 1. Describe the sequence of events following paralysis of the sodium pump.
2. Explain what is happening at the level of individual sarcomeres when a person is using his or her biceps brachii muscle to hold a weight in a constant position. Contrast this with what is happening at the level of individual sarcomeres when an individual lowers the weight, as well as when the individual raises the weight.
3. Suppose that a poison causes death because of respiratory failure (the respiratory muscles relax but do not contract). Propose as many ways as possible that the toxin could cause respiratory failure.
4. A patient is thought to be suffering from either muscular dystrophy or myasthenia gravis. How would you distinguish between the two conditions?
5. Under certain circumstances, the actin and myosin myofilaments can be extracted from muscle cells and placed in a beaker. They subsequently bind together to form long filaments of actin and myosin. The addition of what cell organelle or molecule to the beaker would make the actin and myosin myofilaments unbind?
6. Design an experiment to test the following hypothesis: Muscle A has the same number of motor units as muscle B. Assume you could stimulate the nerves that innervate skeletal muscles with an electronic stimulator and monitor the tension produced by the muscles.
7. Harvey Leche milked cows by hand each morning before school. One morning, he slept later than usual and had to hurry to get to school on time. As he was milking the cows as fast as he could, his hands became very tired, and for a short time he could neither release his grip nor squeeze harder. Explain what happened.
8. Predict and explain the response if the ATP concentration in a muscle that was exhibiting rigor mortis could be instantly increased.
9. Shorty McFleet noticed that his rate of respiration was elevated after running a 100 m race but was not as elevated after running slowly for a much longer distance. How would you explain this?
10. Predict the shape of the length-tension curve for visceral smooth muscle. How does it differ from the length-tension curve for skeletal muscle?
11. Harry has a kidney condition that causes changes in his body’s electrolyte levels (concentration of ions in the extracellular fluid). As a result, he is exhibiting tachycardia, an abnormally fast heart rate. Which ion is involved, and how does a change in its concentration cause Harry’s symptoms?
12. Twenty neurons synapse with a single receptor neuron. Fifteen of the 20 neurons release neurotransmitters that produce EPSPs at the postsynaptic membrane, and the other five release neurotransmitters that produce IPSPs. Each time one of the neurons is stimulated, it releases enough neurotransmitter to produce a 2-mV change in potential at the postsynaptic membrane. If the threshold of the postsynaptic neuron is 10 mV, how many of the excitatory neurons must be stimulated to produce an action potential in the receptor neuron if all five inhibitory neurons are stimulated? (Assume that spatial summation occurs.)
13. In multiple sclerosis, there is intermittent and progressive damage to the myelin sheath of peripheral nerves. This results in poor motor control of the affected area. Why does destruction of the myelin sheath affect motor control?
14. What factor determines the maximum frequency of action potentials that could be conducted by an axon?
15. Many potent insecticides contain toxins, called organophosphates, that interfere with the action of the enzyme acetylcholinesterase. Ivan is using an insecticide containing organophosphates and is very careless. Because he does not use gloves or a dust mask, he absorbs some of the chemical through his skin and inhales a large amount as well. What signs would you expect to observe in Ivan as a result of organophosphate poisoning?
16. Linda’s father suffers an apparent heart attack and is rushed to the emergency room of the local hospital. The doctor on call tells her that he has ordered some blood work and that he will be able to tell if her father actually had an attack by looking at the blood levels of Creatine kinase (CK) and cardiac troponin. Why would knowing the level of CK and cardiac troponin help to indicate if a person suffered a heart attack?
17. Bill broke his leg in a football game, and after 6 weeks the cast is finally removed. As he steps down from the examination table, he loses his balance and falls. Why?
18. Jim Fitch decided that his physique left much to be desired, so he joined a local health club and began to “pump iron” three times weekly. After three months of training, during which he lifted increasingly heavier weights, he noticed that his arm and chest muscles were substantially larger. Explain the structural and functional basis of these changes.
19. When a suicide victim was found, the coroner was unable to remove the drug vial clutched in his hand. Explain the reasons for this. If the victim had been discovered three days later, would the coroner have had the same difficulty? Explain.
20. Muscle-relaxing drugs are administered to a patient during major surgery. Which of the two chemicals described next would be a good skeletal muscle relaxant and why? i. Chemical A binds to and blocks ACh receptors of muscle cells. Ii. Chemical B floods the muscle cells’ cytoplasm with Ca2+.
21. Michael is answering a series of questions dealing with skeletal muscle cell excitation and contraction. In response to “What protein changes shape when Ca2+ binds to it?” he writes “tropomyosin.” What should he have responded and what is the result of that calcium ion binding?
22. You have decided to manufacture a new sports drink that will help athletes, from football players to gymnasts. List at least four different ingredients you would include in your drink, and indicate why each is important for the athlete.
23. One way that scientists study muscles is to put them into a state of rigor by removing ATP. In this condition, actin and myosin are strongly linked but unable to move. On the basis of what you know about muscle contraction, predict what would happen to these muscles in a state of rigor if you (a) added ATP but no free calcium ions; (b) added ATP with a substantial concentration of calcium ions.
24. When curare, a South American Indian arrow poison, is placed on a nerve-muscle preparation, the muscle does not contract when the nerve is stimulated, even though neurotransmitter is still being released from the nerve. Give all possible explanations for the action of curare that you can think of.
25. Jason W. is waiting impatiently for the doctor to finish removing the cast from his leg, which Jason broke the last day of school 6 weeks ago. Summer vacation is half over, and he hasn’t been able to swim, play baseball, or ride his bike. When the cast is finally off, Jason’s excitement gives way to concern when he sees that the injured limb is noticeably smaller in diameter than his normal leg. What explains this reduction in size? How can the leg be restored to its normal size and functional ability?
26. Why does regular aerobic exercise provide more cardiovascular benefit than weight training does? (Hint: The heart responds to the demands placed on it in a way similar to that of skeletal muscle.)
27. Put yourself in the position of the scientists who discovered the sliding filament mechanism of muscle contraction by considering what molecular changes must be involved to account for the observed alterations in the banding pattern during contraction. If you were comparing a relaxed and contracted muscle fiber under an electron microscope, how could you determine that the thin filaments do not change in length during muscle contraction? You cannot see or measure a single thin filament at this magnification. (Hint: What landmark in the banding pattern represents each end of the thin filament? If these landmarks are the same distance apart in a relaxed and contracted fiber, then the thin filaments must not change in length.) How could you determine that thick filaments remain the same length?
28. Explain how the rate of firing of the muscle spindle receptors (primary and secondary endings) would change if (a) the gamma motor neurons are activated, but the alpha motor neurons are not activated and (b) the gamma motor neurons are not activated, but the alpha motor neurons are activated.
29. The rate at which the Na+–K+ pump operates is not constant but is controlled by a combined effect of changes in ICF Na+ concentration and ECF K+ concentration. Do you think the changes in both ICF Na+ and ECF K+ concentration following a series of action potentials in a neuron would accelerate, slow down, or have no effect on the Na+–K+ pumps in this cell?
30. Which of the following would occur if a neuron were experimentally stimulated simultaneously at both ends? a. The action potentials would pass in the middle and travel to the opposite ends. b. The action potentials would meet in the middle and then be propagated back to their starting positions. c. The action potentials would stop as they met in the middle. d. The stronger action potential would override the weaker action potential. e. Summation would occur when the action potentials met in the middle, resulting in a larger action potential.
31. Assume presynaptic excitatory neuron A terminates on a postsynaptic cell near the axon hillock and presynaptic excitatory neuron B terminates on the same postsynaptic cell on a dendrite located on the side of the cell body opposite the axon hillock. Explain why rapid firing of presynaptic neuron A could bring the postsynaptic neuron to threshold through temporal summation, thus initiating an action potential, whereas firing of presynaptic neuron B at the same frequency and the same magnitude of EPSPs may not bring the postsynaptic neuron to threshold.
32. You learned about rigor mortis in the Science. Perhaps you’re also a fan of crime scene shows as well. If so, you know that the onset of rigor mortis in a deceased person can be influenced by a number of factors. Consider the following: a. If a body was rapidly cooled after death, how would this affect the timing of rigor mortis? b. Discuss what factors, besides cooling, might delay or accelerate the onset of rigor mortis.
33. Rigor mortis is usually complete within one to two days after death (depending on environmental variables). Why would rigor mortis diminish after several days?
34. What causes a person with amyotrophic lateral sclerosis (Lou Gehrig’s disease) to die?
35. What is the rationale behind the use of "Calabar bean" as a lie detector by some
native tribes of West Africa?
36. Bob Canner improperly canned some homegrown vegetables. After eating the vegetables, he contracted botulism poisoning with symptoms that included difficulty swallowing and breathing. Eventually, he died of respiratory failure (his respiratory muscles relaxed and would not contract). Assuming that botulism toxin affects the neuromuscular junction, propose the ways that the toxin produces the observed symptoms.
37. A patient is thought to be suffering from either muscular dystrophy or myasthenia gravis. How would you distinguish between the two conditions?
38. Design an experiment to test the following hypothesis: Muscle A has the same number of motor units as muscle B. (Assume that you can stimulate the nerves that innervate skeletal muscles with an electronic stimulator and monitor the tension produced by the muscles.)
39. Explain what is happening at the level of individual sarcomeres when a person is using his or her biceps brachii muscle to hold a weight in a constant position. Contrast this with what is happening at the level of individual sarcomeres when a person lowers the weight, as well as when he or she raises the weight. 5. Predict the shape of an active tension curve for visceral smooth muscle. How does it differ from the active tension curve for skeletal muscle?
40. Predict the shape of an active tension curve for visceral smooth muscle. How does it differ from the active tension curve for skeletal muscle?
41. A researcher is investigating the composition of muscle tissue in the gastrocnemius muscles (in the calf of the leg) of athletes. She takes a needle biopsy from the muscle and determines the concentration (or enzyme activity) of several substances. Describe the major differences this researcher sees when comparing the muscles of athletes who perform in the following events: 100-meter dash, weight lifting, and 10,000-meter run.
42. Shorty McFleet noticed that his rate of respiration was elevated after running a 100-meter race but was not as elevated after running slowly for a much longer distance. How would you explain this?
43. High blood K+ concentrations cause depolarization of the resting membrane potential. Predict and explain the effect of high blood K+ levels on smooth muscle function.
44. Predict and explain the response if the ATP concentration in a muscle that was exhibiting rigor mortis could be instantly increased.
45. A hormone stimulates the smooth muscle of a blood vessel to contract. Although the hormone causes a small change in membrane potential, the smooth muscle contracts substantially. Explain.
46. Experiments were performed in an anatomy and physiology laboratory. First, the rate and depth of respiration for a resting student were determined. In experiment A, students ran in place for 30 seconds, immediately sat down and relaxed, and then had their respiration rate and depth measured. Experiment B was conducted in the same manner as experiment A, except that the students held their breath while running in place. What differences in respiration would you expect for the two experiments? Explain the basis for your predictions.
47. After learning about muscle fiber types in his anatomy and physiology class, Alex started to notice differences in the color of the turkey meat he ate for lunch. Some of the meat was very white and some of it was much darker. From the color of the meat, Alex guessed which muscles the bird used for maintenance of posture and/or slow movements, such as walking, and which muscles it used for quicker movements, such as running or flying. What type of muscle fiber predominates in white meat? In dark meat? Explain how the color of the meat relates to the function of the muscle
48. A child
eats a whole bottle of salt (NaCl) tablets. What effect does this have on
action potentials?
49. Some smooth muscle has the ability to contract spontaneously—that is, it contracts without any external stimulation. Propose an explanation for this ability based on what you know about membrane potentials. Assume that an action potential in a smooth muscle cell causes it to contract.
50. Assume that two nerve fibers have the same diameter, but one is myelinated and the other is unmyelinated. The conduction of an action potential is most energy-efficient along which type of fiber? (Hint: required ATP.)
51. Explain the consequences when an inhibitory neuromodulator is released from a presynaptic terminal and a stimulatory neurotransmitter is released from another presynaptic terminal, both of which synapse with the same neuron.
52. The speed of action potential propagation and synaptic transmission decreases with aging. List possible explanations.
53. Students in a veterinary school are given the following hypothetical problem: A dog ingests organophosphate poison, and the students are responsible for saving the animal’s life. Organophosphate poisons bind to and inhibit acetylcholinesterase. Several substances the students could inject include the following: acetylcholine, curare (which blocks acetylcholine receptors), and potassium chloride. If you were a student in the class, what would you advise to save the animal?
55. Strychnine blocks receptor sites for inhibitory neurotransmitter substances in the CNS. Explain how strychnine can produce tetanus in skeletal muscles.
56. Alcohol affects the central nervous system by enhancing the effect of GABA at its receptor. GABA binds to GABA receptors and opens ligand-gated Cl– channels. However, chronic consumption of alcohol makes the GABA receptor less sensitive to both alcohol and GABA, which increases alcohol dependence as well as alcohol withdrawal symptoms, such as anxiety, tremors, and insomnia. Benzodiazepines enhance the binding of GABA molecules to their receptors and thus are sometimes used to treat people with alcohol withdrawal symptoms. For synapses involving GABA, predict the effect of alcohol on the postsynaptic membranes; compare the effect of chronic alcohol consumption on the postsynaptic membranes in these synapses; and predict the effect of benzodiazepine treatment on the degree of polarization of postsynaptic membranes in people who are experiencing alcohol withdrawal symptoms.
57. The venom of many cobras contains a potent neurotoxin that binds to ligand-gated Na+ channels, causing them to open. Unlike ACh, which binds to and then rapidly unbinds from ligand-gated Na+ channels, the neurotoxin tends to remain bound to ligand-gated Na+ channels. How does this neurotoxin affect the nervous system’s ability to stimulate skeletal muscle contraction? How does it affect the ability of skeletal muscle fibers to respond to stimulation?
58. Epilepsy is a chronic disorder characterized by seizures that result when neurons in the brain produce excessive action potentials. Channelopathies are genetic disorders caused by mutations in ion channel genes, which result in ion channels that do not function normally. What kind of Na+ or Ca2+ channelopathies might contribute to epilepsy?