EXAM 2 – POSTED ANSWER KEY 11. Briefly describe how a tonic mechanoreceptor in the skin can signal the intensity of a touch stimulus. ANS: Tonic touch receptors signal stimulus intensity by the frequency of firing of action potentials during the time the stimulus is applied. 2. How, in general, does averaging across multiple parallel channels increase the ability of sensory systems to detect weak stimuli? Be sure to include the role of spontaneous activity in your answer. ANS: Typically, sensory neurons are spontaneously active, even in the absence of a stimulus. A weak stimulus would cause only a slight increase in the probability of an action potential occurring in a single channel. For a single channel, this would be an ambiguous signal because the extra action potential would be difficult to detect on the background of spontaneous activity. Also, spontaneous action potentials in different channels occur asynchronously, such that only rarely do action potentials in different channels occur at the same time. Therefore, the simultaneous occurrence of even a single action potential in several parallel channels constitutes an unambiguous signal that a weak stimulus has occurred. 3. Which one of the following statements is true? a. Peptide transmitters are synthesized in the axon terminals of the neurons that release them. b. Acetylcholine is the main neurotransmitter released by sympathetic postganglionic neurons. c. Parasympathetic preganglionic axons leave the CNS in the thoracic and lumbar regions. d. The cell bodies of parasympathetic postganglionic neurons are located near their target cells. -ANSWER e. The parasympathetic innervation of the face passes through the superior cervical ganglia. 4. Stimulation of the _____________________________________ nerves to the heart would slow the rate of heart beating, while stimulation of the _______________________________ nerves to the heart would increase the rate of heart beating. ANS: parasympathetic / sympathetic 5. Indicate whether each of the following statements is true (T) or false (F) by writing the appropriate letter in the blank. ______ a. Dihydropyridine receptors are voltage sensors in the transverse tubule membrane. (T) ______ b. Type I skeletal muscle fibers contain large numbers of mitochondria. (T) ______ c. When a muscle fiber undergoes an isotonic contraction, the spacing between z-disks remains constant. (F) ______ d. Skeletal muscle fibers with more Ca2+ pumps in the sarcoplasmic reticulum will relax more quickly following a twitch. (T) 6. Will skeletal muscle fibers contract if you immerse them in a Ca2+-free solution and apply a direct electrical stimulation? Answer Yes or No, and then briefly explain. 3 pts. 7 pts. 3 pts. 4 pts. 8 pts. 4 pts.EXAM 2 – POSTED ANSWER KEY 2ANS: Yes, because the Ca2+ required for contraction is released internally, from the sarcoplasmic reticulum, and does not require Ca2+ in the bathing medium. 7. Briefly explain how a muscle can contract and lengthen at the same time, and give an example of such a contraction. ANS: If the load placed on a muscle exceeds the force the muscle generates, then the muscle will extend while contracting. There are many possible examples, but typical ones would be contractions of anti-gravity muscles in the legs when walking downhill or landing from a jump off a platform. 8. Explain how a drug that inhibits phosphatase activity in smooth muscle cells would affect contraction of those cells. ANS: In smooth muscle, phosphorylation of the myosin head is the trigger that allows cross bridge cycling and contraction. Dephosphorylation by myosin phosphatase terminates contraction. Thus, a drug that inhibited phosphatase activity in smooth muscle cells would likely strengthen and prolong contraction. 9. Which one of the following comparisons between single unit and multiunit smooth muscle is correct? a. Single unit smooth muscle receives mainly parasympathetic innervation, and multi unit receives mostly sympathetic innervation. b. Single unit smooth muscle cells have only muscarinic cholinergic receptors, and multiunit smooth muscle cells have only adrenergic receptors. c. In single unit smooth muscle, neurotransmitter is released from varicosities along the length of axons, and in multiunit smooth muscle, neurotransmitter is released only at the terminals (ends) of axons. d. Single unit smooth muscle cells are coupled by gap junctions, and multiunit smooth muscle cells are not coupled by gap junctions. -ANSWER e. Myosin light chain kinase activates contraction in single unit smooth muscle, and troponin-tropomyosin activates contraction in multiunit smooth muscles. 10. A normal human subject is given an injection of insulin. Name the hormone(s) that would likely show an increase in plasma concentration in response to this injection. Briefly explain why the plasma concentration(s) of the hormone(s) would increase. ANS: Credit for any two of the following: glucagon, growth hormone, epinephrine. The reason is that the insulin injection would cause hypoglycemia, which is a stimulus for secretion of all three of these hormones. The three hormones all raise plasma glucose concentration. 11. Immediately below are the names of 4 hormones. Below that is a list of 18 hormones that may or may not affect the release of the 4 hormones. In each blank, write a number or numbers to indicate which hormone(s) from the list stimulates or inhibits release of each of the 4 hormones. Do not guess, because points will be deducted for wrong answers! STIMULATES RELEASE INHIBITS RELEASE a. Growth Hormone ______________________ ______________________ b. Adrenocorticotropic Hormone ______________________ ______________________ 4 pts. 4 pts. 3 pts. 4 pts. 8 pts.EXAM 2 – POSTED ANSWER KEY 3c. Thyroid Stimulating Hormone ______________________ ______________________ d. Follicle Stimulating Hormone ______________________ ______________________ List o’ Hormones: (1) Inhibin (10) Estradiol (2) Cortisol (11) Thyrotropin Releasing Hormone (3) Insulin-Like Growth Factor-I (12) Antidiuretic Hormone (4) Growth Hormone (13) Triiodothyronine (5) Activin (14) Somatostatin (6) Tetraiodothyronine (15) Growth Hormone Releasing Hormone (7) Gonadotropin Releasing Hormone (16) Corticotropin Releasing Hormone (8) Prolactin (17) Testosterone (9) Gastrin (18) Glucagon ANS: a. Stimulate = 15; Inhibit = 14, 4, 3 c. Stimulate = 11; Inhibit = 6, 13