Exam 2 Answers BISC 307L – General Physiology March 6, 2006 NOTE: The given answers for the subjective questions are not the only ones possible. Grading for each of the subjective questions is done relative to other students’ answers, by one TA who applied uniform criteria. 1. Which one of the following statements about sensory receptors is true? (b) a. Tonic receptors are best suited for detecting changing stimuli, and phasic receptors are best suited for monitoring constant stimuli. b. Progressive activation of K+ channels can cause adaptation in a sensory receptor. c. Spontaneous activity in sensory neurons diminishes overall sensitivity because it creates background noise in the system. d. For touch-sensitive neurons in the somatosensory system, only those whose mechanoreceptive terminals are located in the skin have receptive fields. e. Information from touch-sensitive neurons in lumbar dorsal root ganglia ascends to the brain via tracts located in the anterior and lateral regions of the spinal cord. 2. How does the existence of multiple parallel channels enhance sensitivity in sensory systems? Be sure to discuss the role of spatial summation in your answer. ANS: Parallel channels enhance sensitivity in the following way. A weak stimulus may have only a small chance of producing an action potential in any particular receptor. Since most receptors are spontaneously and randomly active, an “extra” action potential, correlated in time with the stimulus, would be an ambiguous signal. If the CNS monitors many parallel channels at once, however, several “extra” action potentials, occurring simultaneously and synchronously with the stimulus, would be a clear signal. The basic mechanism by which the CNS monitors multiple channels is by convergence of the parallel axons onto a single postsynaptic neuron. Simultaneously arriving action potentials result in simultaneous epsps that sum spatially to bring the postsynaptic neuron to threshold. 3. Which one of the following is not a typical response to sympathetic nerve activity? (e) a. increased heart rate d. increased respiratory rate b. dilation of bronchioles e. increased salivary secretion c. vasoconstriction in intestines f. increased alertness 4. Write one number in each blank, to best match each of the steps in skeletal muscle contraction (on the left) with its functional significance (on the right). _______ a. Ca2+ pumping into SR 1. breaks the cross-bridge _______ b. binding of ATP to myosin 2. terminates cross-bridge cycling _______ c. release of Pi 3. causes depolarization of transverse tubules _______ d. binding of Ca2+ to troponin 4. causes rigor mortis 5. change in myosin conformation causing filaments to slide 6. forms the cross-bridge ANS: a-2, b-1, c-5, d-7 7. trigger that initiates cross-bridge cycling5. Shown below is a frog muscle contacted by stimulating electrodes attached to a stimulator that allows you to deliver single, 1 millisecond long pulses of current at any desired amplitude. You run 2 kinds of experiments, one under isometric conditions, and the other under isotonic conditions. Stim-o-matic a. When recording isometrically, what experimental conditions will yield the maximum force in response to a single, 1 ms stimulus? b. When recording isotonically, what experimental conditions will yield the most rapid velocity of shortening in response to a single, 1 ms stimulus? ANS: a) Stimulus strength should be high enough to stimulate all muscle fibers, and the muscle set at optimum length for force generation (length at which overlap of thick and thin filaments is optimal). b) Stimulus strength high enough to stimulate all muscle fibers, no (or minimal) initial load (force) on the muscle. 6. 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. Force is increased from the smallest possible level to the maximum over a period of ten seconds. In this situation, which one of the following statements is false? (d) a. The first motor units activated will be those that generate the smallest amounts of force. b. Action potentials in the somas of the last motor neurons activated will show little or no after-hyperpolarization. c. The last muscle fibers activated will have high levels of glycolytic enzymes in their myoplasm. d. Levels of myosin ATPase activity will be high in the first muscle fibers activated. e. The first muscle fibers activated will likely be those with the greatest number of mitochondria per unit volume of myoplasm. 7. In both skeletal muscle and smooth muscle, Ca2+ ions act as the trigger for contraction, but the sources, mechanisms, and fate of Ca2+ is different in some important ways. Compare and contrast the following aspects of Ca2+-dependent contraction in smooth muscle and skeletal muscle: a. sources of Ca2+ ions that trigger contraction. b. mechanisms by which Ca2+ ions trigger contraction. Page 2 of 5Smooth Muscle Skeletal Muscle Sources of Ca2+ Ca2+triggering mechs. 1) IP3-mediated Ca release from the SR, 2) influx through receptor-activated Ca channels in the plasma membrane, Ca binds to troponin, which causes a conformation change in tropomyosin that allows myosin to bind to actin. Ca binds to calmodulin to activate a myosin kinase that phosphorylates myosin. 3) influx through voltage-gated Ca channels in the plasma membrane. Ca comes entirely from the SR, following depolarization of the t-tubules by the action potential. 8. An increase in plasma glucose concentration promotes (c) a. lipolysis in adipose tissue d. breakdown skeletal muscle protein b. secretion of glucagon e. all of the above (a-d) c. synthesis of glycogen f. none of the above (a-d) 9. Fill in the blanks. a. The hormone ___________________________ inhibits the activity of neuropeptide Y-secreting neurons that stimulate the feeding center in the hypothalamus. b. Pancreatic β-cells are depolarized when uptake of glucose and amino acids closes a ______________________________________ channel. c. Pancreatic α-cells are inhibited by a rise in the plasma concentration of _____________________________. d. A person with untreated diabetes mellitus may experience metabolic acidosis due to the production of ________________________________. ANS: a: leptin; b: ATP-sensitive K+ channel (half-credit for K channel w/o mentioning ATP); c: glucose