BISC 307, Exam 2 Answers Spring 2015 1. Which one of the following is a characteristic feature of the sympathetic, but not the parasympathetic, nervous system? (h) a. most presynaptic axons are long b. ganglia are located in effector organs c. postganglionic neurons release epinephrine d. preganglionic neurons release acetylcholine e. all postganglionic neurons release acetylcholine f. preganglionic neurons release norepinephrine g. postganglionic neurons synapse on effector organs h. preganglionic neurons originate in the thoracic and lumbar spinal cord 2. Which one of the following statements about Ca2+ release channels in the sarcoplasmic reticulum of skeletal muscle fibers 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 in excitation-contraction coupling. Ca2+ influx through voltage-gated Ca2+ channels in the sarcolemma triggers contraction. 3. Which one of the following statements is not characteristic of smooth muscles? (b) a. Thin filaments contain troponin. b. Their force-length relationship is similar to that of skeletal muscle. c. Cytoplasmic [Ca2+] is kept low by Ca2+ pumps and Na+/Ca2+ exchangers. d. They can produce graded contractions in response to graded depolarizations. e. During cross-bridge cycling, ATP is required for myosin heads to detach from actin. 4. Which one of the following characteristics or components is shared by both skeletal muscle fibers and smooth muscle cells in adult mammals? (c) a. Ca2+ is released from the sarcoplasmic reticulum by IP3 binding to IP3 receptors b. innervation by sympathetic and parasympathetic postganglionic axons c. elevated intracellular [Ca2+] triggers excitation-contraction coupling d. regulation of contraction by myosin light chain kinase e. stretching an isolated muscle can trigger contraction f. thick and thin filaments arranged in sarcomeres g. voltage-gated Ca2+ channels in the sarcolemma h. cross-bridges can exist in a catch state i. electrical coupling between cells j. troponin2 5. In the figure below, the bottom trace shows action potentials recorded in situ from the soma of a motor neuron in the spinal cord of a healthy mammal. The upper trace shows the isometric force produced by that neuron’s motor unit. Answer each of the following questions. a. Judging from the appearance of the motor neuron action potential between times A and C, would you conclude that muscle fibers in this motor unit are more likely to rely on glycolysis for ATP production or on oxidative phosphorylation? Explain briefly. ANS: The very small after-hyperpolarization following the action potential suggests that this is a type IIb (or IIx) motor unit, not a type I motor unit. Type IIb motor units are more dependent on glycolysis. b. Between times B and C, the force trace is “bumpy,” but between times D and E, the force trace is smooth. Explain briefly. ANS: Between times B and C, the motor neuron is firing action potentials at a relatively low frequency, producing an unfused tetanus. That is, the interval between action potentials is long enough to allow the force of each twitch to decay between action potentials, producing the “bumpy” trace. Between times D and E, the motor neuron is firing at a much higher frequency. In this case, the interval between action potentials is too short to allow much decay from the peak force of each twitch, so the summed force is smoother. c. Between times F and G, the motor neuron is firing at a constant frequency, but motor unit force is declining. Briefly explain. ANS: Muscle fibers in the motor unit are fatiguing. d. It turned out that the motor neuron in this experiment had an input resistance of 1.0 M and a surface area of 400 m2. In the same animal, similar recordings were later made from another motor neuron with an input resistance of 2.0 M and its soma had a surface area of 200 m2. Compared to the first motor neuron, would it take more or less excitatory synaptic input to elicit an action potential in the second motor neuron? Explain briefly. Assume that the resting membrane potential and the threshold potential is the same in both neurons. ANS: It would take less synaptic excitation to elicit an action potential in the second motor neuron, for two reasons. First, the second motor neuron has a higher input resistance. By Ohm’s Law (V = IR), a given amount of excitatory current will produce a larger depolarization in a higher resistance neuron. Second, the second motor neuron has a smaller surface area, which will result in a lower capacitance. A Time 100 ms 0 Max Force Vm A B C D E F G3 given amount of excitatory current will depolarize a low capacitance neuron more rapidly than a high capacitance neuron. 6. Indicate whether each of the following statements is true or false by writing the word “True” or “False” in the blank. Do not write T or F. ANS: a-F, b-T, c-F, d-F ____________ a. Insulin inhibits Na-dependent amino acid uptake by cells. ____________ b. Growth hormone stimulates anabolism of protein and catabolism of fat. ____________ c. A person with type 1 diabetes who suffers a painful leg fracture should probably decrease his dosage of insulin. ____________ d. If the portal vein between the hypothalamus and the pituitary were destroyed but no other damage occurred, the release of all pituitary hormones would be disrupted. 7. You are shadowing an endocrinologist during her rounds in the hospital. You enter the room of a woman who was admitted from the emergency department with a wide array of symptoms. Blood tests reveal hyperglycemia. The results of a glucose tolerance test are normal. Other blood tests show that he has normal levels of epinephrine and thyroid hormone, but levels of cortisol are abnormally high and ACTH is abnormally low. He has (truthfully) says that he has not been taking any drugs or medications. Wanting to impress the doctor, you carefully consider these facts and suggest that ... (e) a. the patient has diabetes. b. the patient has Grave’s disease. c. the patient is suffering side effects from long-term glucocorticoid therapy. d. the patient has a pheochromocytoma (an over-secreting tumor of the