Medical Physiology 2005

Problem Set 4: Muscle:

Part I: Muscle Physiology

1. EC coupling involves transmission of a the muscle fiber action potential down the t-tubules, with subsequent release of Ca⁺⁺ from the sarcoplasmic reticulum. This process initiates muscle contraction in as little as several milliseconds, owing to short diffusion distances (i.e., the sarcoplasmic reticulum surrounds individual myofibrils). If this process did not occur, and the Ca⁺⁺ necessary for binding to troponin came solely from the extracellular medium (after all, there is a large inward driving force for Ca⁺⁺ entry into the fiber), how long would it take for the action potential to trigger contraction of the sarcomeres—especially in myofibrils near the center of the fiber? Assume that a “typical” fiber has a diameter of 100 mm, and also assume that the diffusion constant for Ca⁺⁺ is 10^-5 cm²/sec.

2. Electron micrographs from a newly discovered species show that the skeletal-muscle filaments are 1.7 mm in length, the bare zone is 0.3 mm in length, and the thin filaments (measured from end to end with the Z-disk in the middle) are 2.2 mm in length. From these measurements, accurately construct the active force length-tension relationship of the muscle.

3. As bones grow, muscle lengths adjust in order to always operate in the peak range of their length-tension curve. If the filament lengths remain constant throughout growth, and assuming that maximum force development of a muscle doesn’t change during a period of growth, what does this imply regarding number of sarcomeres, myofibrils, etc.? Repeated exercise of a muscle results in increased muscle growth via fiber hypertrophy (the number of fibers don’t change). What does this imply regarding number of sarcomeres, myofibrils, etc., resulting from exercise? If the maximum force development of a muscle should double as a result of strength training, then how would this affect the muscles diameter (assume the muscle is roughly cylindrical in shape)?

Part II: Drug Treatment of Clinical Neuromuscular Disorders: Problem Set

In normal individuals, the following is a true statement: A single end-plate potential (EPP) is always sufficient to elicit an action potential (and hence contraction) in a skeletal-muscle fiber. This is not the case, however, in individuals suffering from myasthenic gravis (from Greek mys plus asthenia: muscle weakness). The disease is an autoimmune disorder that affects »1 in 20,000 individuals. It starts in the thymus: thymus cells possess nicotinic-type ACh receptors (AChR), and developing immune cells exposed to AChR develop the ability to produce antibodies against AChR. These antibodies subsequently attack AChR at skeletal-muscle endplates, thereby reducing their number. Note that there is continual synthesis of new AChR at the endplate; problems occur, however, when the destruction rate exceeds the synthesis rate. Clinically, patients suffering from the disease present with significant muscle weakness.

Treatments strategies involve attempts to enhance cholinergic transmission at the endplate. Thymectomy (removing the source of the antibody producing cells) as well as immunosuppressant drug therapy (inhibiting the synthesis and release of the antibodies) is of limited effectiveness. However, neostigmine administered at carefully monitored doses is highly effective in treating myasthenia gravis.

4. Explain the electrophysiological reasons why patients lacking sufficient numbers of AChR exhibit muscle weakness.

5. Investigate the actions of neostigmine, and discuss in detail why it is effective in restoring neuromuscular transmission.

Malignant hyperthemia (MH) is a genetic disorder affecting »1 in 20,000 individuals. It’s an acute life-threatening disorder that typically shows up during surgery, and is caused by exposure to virtually all of the inhalation anesthetics (e.g., halothane, Ethrane and even ether). It is also exacerbated by paralytic agents (e.g., succinylcholine), that are typically administered to the surgical patient in order to inhibit withdrawal reflexes. MH manifests itself in the form of a dramatic increase in metabolic rate: body temperature rises at a rate of 1°C every 5 minutes, plasma CO₂ levels rise, O₂ levels drop, lactic acid levels rise (lactic acidosis), and there are dramatic increases in heart rate and blood pressure. There’s also a dramatic increase in muscle rigidity. If untreated, the patient will die from hyperthermia, acidosis and acute hyperkalemia (resulting from breakdown of muscle tissue, with the concomitant release of K⁺).

Treatment involves immediate cessation of surgery and anesthesia, with concomitant attempts to control the individual symptoms and ultimately lower the metabolic rate. For example, emergency control of the hyperthermia typically involves packing the patient in ice, administering cold i.v. fluids, or ice water via a nasogastric tube. Treatment of the acidosis involves administration of bicarbonate. But even if the patient survives the initial insult, one has to continue to closely monitor renal function, since muscle breakdown products (e.g., myoglobin) tend to cause renal failure. Thus, there is a high mortality rate (even today) among MH patients while they are in the operating room, and subsequently when they are in recovery.

MH causes uncontrollable intense muscle activity that is somehow triggered by the anesthetic agent(s). The increased activity does not seem to involve increased efferent activity via the motor neurons, but rather, it is caused by something that initiates muscle contraction directly. It has now been shown that the ultimate cause of MH is release of Ca⁺⁺ from the sarcoplasmic reticulum (SR). When this hypothesis was first proposed, it was not thought to be far-fetched, since a number of drugs are known to induce SR Ca⁺⁺ release, the best example being caffeine.

6. Why does an intense increase in muscle activity lead to hyperthermia? Why does it lead to lactic acidosis?

7. A breakthrough in the acute treatment of MH occurred with the discovery of dantrolene (with the concomitant cessation of anesthesia). Investigate the actions of dantrolene and come up with a plausible explanation of its life-saving action.

Botulism is a disease resulting from a high-molecular-weight protein (botulinum toxin or BOTOX) that is made by the anaerobic bacterium Clostridium botulinum. The toxin permanently poisons the individual nerve terminals of cholinergic neurons. The most common source of BOTOX is the consumption of poorly prepared canned foods. For example, home canned foods that have not been properly cooked prior to sealing provide a good medium for bacterial growth and toxin production (sealed jars exclude air).

Consuming foods contaminated with BOTOX and/or C. botulinum initially produces symptoms characteristic of paralysis of the autonomic nervous system. Subsequently, symptoms like muscle weakness result from paralysis of skeletal muscles. Ultimately, patients succumb to paralysis of the respiratory muscles. Should the patient be fortunate enough to survive, complete recovery requires many days or weeks.

BOTOX is used clinically in the treatment of certain uncontrollable muscle spasms, for example, strabismus and certain muscle “ticks.” Here, an extremely dilute isotonic solution of BOTOX is injected directly around the offending muscle. The injected BOTOX stays nearby the muscle (it’s a large 150 KDa protein that doesn’t readily diffuse), where it poisons the motor-neuron synapses, thereby functionally denervating the muscle. More recently, BOTOX has gained popularity as a cosmetic agent: numerous local injections near skin wrinkles are effective in relaxing muscles, thereby rendering the wrinkles less apparent.

8. Investigate the action of BOTOX, and explain how it’s effective in permanently inhibiting a particular synapse.

9. Patients receiving BOTOX therapy (either for strabismus or skin wrinkles) have to have the treatment repeated every several months. Why is this the case? After all, BOTOX inhibits a synapse permanently. (Hint: investigate such mechanisms as the growth of peripheral neurons, especially after denervation.)

Note: the following is a real experience of my wife when she, as a resident, was called to University Hospital ER to admit a patient to the medical service. The patient was discharged the next day.

You are serving time in your emergency room, and a patient presents suffering from vision problems, “cotton mouth,” and severe bradycardia (slow heart rate). Fortunately, he’s still conscious. It’s early spring and the weather is unusually warm. The man tells you that the previous day, he decided to spray his garden to control a newly discovered aphid infestation in the rose bed. In his garage, he found an old bottle of insecticide concentrate. In diluting the insecticide in his sprayer, he accidentally spilled some of the concentrate on his hand. He describes the concentrate as having a “chemical smell” and an oily consistency. He tells you his symptoms started to occur several hours after he finished spraying his garden. He doesn’t know the name of the bottle contents, other than he remembers that the label states that its “effective for aphids, scale, ants and other crawling insects.” He also says it’s a common product that he’s seen many times in garden stores. He noted that his roses look much better now.

10. What is the patient suffering from? What is the most probable causative agent, and how does it act physiologically? How did it happen? After all, he didn’t drink the stuff!

11. How would you treat the patient? Be sure to explain in detail how your treatment will act to alleviate his symptoms.