BISC 307 – Spring 2011 Exam 2 Answers 1. Which one of the following statements about Ca2+ release channels in the sarcoplasmic reticulum of skeletal muscle is false? (e) a. They are also called ryanodine receptors. b. They are opened by conformational changes in dihydropyridine receptors. c. They are similar to inositol triphosphate receptors in the smooth endoplasmic reticulum. d. They permit diffusion of Ca2+ ions from the lumen of the sarcoplasmic reticulum to the sarcoplasm. e. They play a minor role; contraction is mainly triggered by Ca2+ influx through voltage-gated Ca2+ channels in the sarcolemma. 2. A patient visited his doctor complaining of muscle weakness. Tests revealed the following results. Repetitive voluntary activity was associated with normal action potential activity in the motor nerve, although the patient’s muscles rapidly fatigued and force generation declined. Repetitive direct stimulation of muscle resulted in normally sustained force with no evidence of fatigue. Assuming the pathology occurs at a single site, where along the motor pathway is this patient’s problem? Briefly explain your reasoning, and suggest a reasonable diagnosis, i.e, name the patient’s disease, syndrome, or pathological condition. ANS: The patient’s problem is at the neuromuscular junction. The connection between the brain (motor cortex) and the spinal (alpha-) motoneurons must be normal, because voluntary motor activity elicited normal activity in the motor nerve. Stimulating the muscle directly led to normal muscular function. Therefore, everything from the sarcolemma to the force generating mechanism must also be normal. The only parts of the motor pathway left untested involve the neuromuscular junction, so there must be an abnormality in some aspect of synaptic transmission. A reasonable diagnosis (further tests would be needed) would be myasthenia gravis. [Note: There are other possible diagnoses, such as Lambert-Eaton myasthenic syndrome which impairs neurotransmitter release. However, diseases affecting the motoneurons themselves (such as amyotrophic lateral sclerosis, spinal muscular atrophy, etc.) or affecting muscle fibers (such as muscular dystrophy, etc.) would not be acceptable answers because they are not consistent with the test results.] 3. A human subject is asked to slowly increase the level of isometric force generated in a forearm muscle that is composed of a broad mixture of motor unit types. On each line below, place a mark in one of the blanks to define the characteristics of the last motor units to be recruited, i.e., those units that are activated when force is near maximal. a. size of motoneuron cell body (soma): _____ small _____ large b. force produced by the motor unit: _____ low _____ high c. number of mitochondria: _____ low _____ high d. duration of after-hyperpolarization of motoneuron action potential: _____ short _____ long e. level of myosin ATPase activity: _____ low _____ high ANS: a-large, b-high, c-low, d-short, e-high 5 pts. 10 pts. 5 pts.Page 2 of 5 4. Which one of the following statements is not characteristic of smooth muscles? (d) a. During cross-bridge cycling, ATP is required for myosin heads to detach from actin. b. They can produce graded contractions in response to graded depolarizations. c. Cytoplasmic [Ca2+] is kept low by Ca2+ pumps and Na+/Ca2+ exchangers. d. Their force-length relationship is similar to that of skeletal muscle. e. Contraction is triggered by activation of myosin light chain kinase. 5. Which one of the following statements comparing smooth muscle to skeletal muscle is false? (b) a. Smooth muscle uses less energy to generate a given amount of force and can maintain force for longer periods compared to skeletal muscle. b. Smooth muscle myosin has higher ATPase activity than skeletal muscle myosin c. Smooth muscle contraction is controlled by endocrine and paracrine hormones in addition to neurotransmitters. d. Both smooth and skeletal muscle cells have actin-myosin crossbridges, sarcoplasmic reticulum with calcium release channels, and a calcium signal that initiates contraction. 6. Which one of the following hormones originates in the anterior pituitary? (f) a. dopamine e. oxytocin b. growth hormone releasing hormone f. thyroid stimulating hormone c. somatostatin g. testosterone d. gonadotropin releasing hormone h. epinephrine 7. After years of suffering from embarrassing symptoms, a 51-year old man finally visited his doctor. He reported that his libido (interest in sexual activity) has been low for 8 years, he has been chronically fatigued for 3 years, is intolerant to cold, and growth of his beard and pubic hair has greatly slowed. For 20 years, his shoe and ring size has been progressively increasing, and his nose, lips, tongue, and jaw have also been growing. Blood tests revealed the following results: Result Normal Male Range Plasma cortisol 3 g/dl at 8:00 am 7-25 Serum T4 3 g/dl 4-11 Serum TSH < 1 U/dl 0-10 Serum testosterone 180 ng/100 ml 300-1100 Serum LH 1.5 mIU/ml 1-15 Serum GH 50 ng/ml < 5 Indicate whether each of the following statements is true or false by writing the word “True” or “False” in the blank (do not mark with T or F). ANS: a-F, b-F, c-T, d-T, e-T ________ a. The patient’s hypothyroid symptoms and low T4 levels are most likely due to a primary failure of the thyroid gland. ________ b. The symptoms related to low testosterone levels are most likely due to a primary failure of the testes. ________ c. From these results alone, it is not possible to determine with certainty whether the low cortisol levels are due to reduced ACTH secretion or reduced CRH secretion. 5 pts. 5 pts. 2 pts. 10 pts.Page 3 of 5 ________ d. Most likely, the problem that is causing abnormal GH levels is the cause of all the other hormone abnormalities. ________ e. Upon further examination, the doctor will likely find visual abnormalities, such as partial blindness. 8. Briefly define GLUT4 and the primary mechanism that regulates its presence in the plasma membrane. Then, explain how GLUT4 plays a role in the beneficial effects of exercise in type 2 diabetes mellitus. ANS: a) GLUT4 is an insulin-regulated, facilitated diffusion transporter for glucose that is found mainly in fat cells (adipocytes) and skeletal muscle fibers. b) In the absence of insulin, GLUT4 is stored in intracellular vesicles. Insulin stimulates insertion of GLUT4