Biology 222 Exam 1, Fall 2009 Name_______KEY ______________ Page 1 This exam is double-sided. Put your first and last name on every sheet! Every sheet missing your name will lose 1 point. Answer all questions on pages 3-12. Equations you may find helpful At 20 °C Otherwise, where R = 8.314 J mol-1K-1, F = 96,492 C/mol, and T = (°C + 273) K or alternatively, terms can be rearranged as ix=gx(Vm-Ex) g= 1/R V=IR means ?proportional to? Vx = Vo * e (-x/) e ? 2.7 Conduction velocity: Table of log10 values log 10 [ out ]/[ in ] -3 0.001 -2 0.01 -1 0.1 0 1 1 10 2 100 3 10001) 20 points __________ 2) 20 points __________ 3) 20 points __________ 4) 20 points __________ 5) 20 points __________ Total __________ Leave this page blank1) (20 pts) Information carries over for parts (a) ? (c) a) (6 pts) A space mission to Mars has just returned to Earth but the lead scientist is unable to continue her work. Portions of her notebook have been damaged and you are responsible for reconstructing the data. Notation above the table says ?Properties of Martian neuron at 45 °C.? Calculate the equilibrium potentials for each ion and give the proper units. Show your work. = = =63(-2)=-126 mV =-63(1)=-63 mV =63(2)=+126 mV =63(-3)/(-2)=+94.5 mV =63(3)/2=+94.5 mV =63(-1)/2=-31.5 mV b) (4 pts) If the Martian neuron is only permeable to 2 ions and gx/gy = 1, which 2 ions would make its Vm = 0 mV? Which 2 ions would make its Vm = +31.5 mV? Refer to the table in part (a) and show your work. Na+ and K+ make Vm = 0 mV Ca2+ and Mg2+ or SO4-2 and Mg2+ make Vm = +31.5 mV K+ and Cl- also make Vm = +31.5 mV c) (2 pts) The notebook says that gK/gMg = 0.25. What is the Vm of the Martian neuron if it is permeable only to K+ and Mg2+? Show your work. == 0 mV Information carries over for parts (d) ? (g) d) (3 pts) A newly discovered single-cell Earth organism, Chlorita, has the following ionic composition in its native environment at 60 °C. Calculate the equilibrium potentials for each ion and give the proper units. Show your work. = = = 66(-1) = -66 mV =66(2) = +132 mV = -66(2) = -132 mV e) (2 pts) For Chlorita at rest, PCl = 0.25PNa and PK = 0. What is its Vm at 60 °C? Show your work. = -76.2 mV f) (2 pts) If only chloride channels were open at rest in Chlorita, what would be the direction of ion and current flow? Show your work. Vm ? ECl = driving force -76.2 ? (-132) = +55.8 mV Cl- flow is inward Current flow is outward g) (1 pt) You apply a chloride-channel blocking toxin to Chlorita. What is the value of Vm? At rest, Vm for Chlorita depends on PCl and PNa. Using the Goldman equation, PCl now is 0 after toxin treatment so Vm = ENa2) (20 pts) A________ nucleus ____________ B________ dendrite ___________ C_____ soma or cell body _______ D_____ axon hillock ___________ E________ myelin ____________ F_____ node of Ranvier ________ G_ axon terminal/terminal/boutona) (7 pts) Identify the parts of a neuron. G F E D C B A b) (4 pts) Maintenance of ionic concentration gradients is critical to neuronal function. What specific membrane protein is primarily responsible? Identify the ions regulated by this protein and state where each is highly concentrated (e.g. inside, outside). Na+, K+-ATPase, Na+, K+ pump Na+, outside K+, inside c) (2 pts) Ion channels are gated, meaning a stimulus causes channel opening. Give two examples of a stimulus that opens an ion channel. Depolarization, binding of a small molecule (ligand), covalent modification (phosphorylation), physical activation (stretch) d) (4 pts) A cell has channels selective for ion X. At a certain membrane potential, ion X has no net movement across the cell membrane. What is the name of this special potential? At this special potential, two different forces on the ion must be equal in strength, but opposite in direction. What are these forces? What is gX at this potential? The Nernst or equilibrium potential (EX) Diffusion and electrical potential gX > 0 (channels are open allowing ion movement but there is no net current flow at equilibrium) e) (3 pts) Hodgkin and Huxley received the 1963 Nobel Prize for their work on action potentials. Name the experimental technique they used in their studies of the squid giant axon. Briefly describe what they were able to measure using this technique. They used the voltage clamp technique to study the squid giant axon. By maintaining the cell?s membrane potential at different constant values, they were able to measure the ionic currents that were active during different phases of an action potential. 3 2 13) (20 pts) a) (8 pts) 4 Below is an action potential from a neuron. D A C B In the table below, answer the questions about each phase of the action potential. For the questions about ion channels, list the one state that describes the majority of the channels. b) (4 pts) For this question, use words rather than just an equation to explain your answer. What is driving force? What does the sign of its value indicate? Driving force is the difference between membrane potential and the equilibrium potential of a given ion. If it is a nonzero value, there will be current flow and the sign of the value indicates the direction of current flow (e.g. + means outward current, - means inward current). c) (2 pts) You record the responses of a neuron to two separate depolarizing stimuli. Stimulus 1 just barely exceeds action potential threshold and stimulus 2 is greater than stimulus 1. Will the neuron?s responses to the two stimuli be the same or different? Explain your answer. The responses will be different; the response to stimulus 2 will have a higher number of action potentials. (frequency coding of stimulus strength) d) (6 pts) The channelopathy PE (primary erythermalgia) is characterized by a S214T mutation of the Nav1.7 channel. What effect does the mutation have on the function of the channel? How would the firing properties of neurons with mutant channels compare to the properties of wild-type neurons? How would the mutant phenotype of PE-affected neurons influence a patient?s pain sensitivity? The mutation lowers the threshold for channel activation. Mutant neurons would be more likely to fire action potentials due to the lower threshold. This phenotypes would increase a patient?s sensitivity to pain (or cause innocuous stimuli to generate pain sensations).4) (20 pts) a) (3 pts) You discover a new toxin that causes an inactivating type of calcium channel to inactivate faster than normal. What effect would the toxin have on the duration of the calcium current? You decide to look at the activity of single channels. What might you observe if you compare channel openings with and without the toxin present? The toxin would decrease the duration of the current. Channel openings with toxin would be shorter in duration compared with channel openings without toxin. b) (6 pts) Your friend has been diagnosed with a channelopathy. The symptoms seem to mimic the effects of veratridine. What specific type of ion channels are likely to be involved? Suggest a possible treatment for the symptoms. It is likely that voltage-gated sodium channels are involved because veratridine is a toxin that acts on such channels. Because veratridine slows inactivation (makes current persistent), the channelopathy is probably characterized by persistent sodium currents. This is similar to PEPD. One possible therapy is the drug, carbamazepine, which can enhance inactivation of sodium channels. c) (4 pts) CIP or channelopathy-associated insensitivity to pain is referred to as a? loss-of-function? disorder. Briefly explain what is meant by this and how it relates to the effect of the mutations on the patient?s ability to sense pain. A ?loss-of-function? disorder involves mutations that cause a particular protein to stop functioning. In the case of CIP, mutations cause the protein to be made in a truncated (incomplete), nonfunctional form. The Nav1.7 protein is necessary in order for pain-sensing neurons to produce generator potentials. The neurons of CIP patients are unable to produce these potentials and therefore, cannot produce spikes that ultimately signal pain. d) (3 pts) You may want to refer to an equation to answer this question. In an unidentified animal cell, ion ?A? has a greater permeability than ion ?B?. If the cell is only permeable to those two ions, can you tell whether its Vm will be closer to EA or EB? If yes, what is the prediction; if no, explain why. What happens to Vm if the cell is permeable only to ion ?B?? Yes, using the Goldman equation. If PA > PB, then Vm is closer to EA. If the cell is only permeable to B, then PA = 0 and Vm = EB. e) (4 pts) A new drug is invented that prevents the removal of inactivation from voltage-gated sodium channels once they become inactivated. You apply the drug to a neuron and then depolarize it above threshold. What type of response would you expect to see? If you stimulate the neuron a second time, would the response be the same or different? The expected response would be an action potential. The response would be different; a second stimulus would be unlikely to elicit an action potential because the sodium channels would remain inactivated in the presence of the drug. 5) (20 pts) a) (6 pts) The behavior of action potentials has limits in terms of firing frequency and direction of conduction. What property of action potentials is responsible for these limits? Explain this property in terms of the behavior of a specific channel protein. Firing frequency and direction of conduction are limited because of the absolute refractory period. During this period, another action potential cannot be initiated because voltage-gated sodium channels are inactivated and waiting for a hyperpolarized membrane potential to remove inactivation (deinactivation). b) (2 pts) You are told that axon A has a diameter 1/100th that of axon B. How does the ? for axon A quantitatively compare with that for axon B? Briefly explain. ?A = 0.1?B or ?B = 10?A Because conduction velocity is proportional to ?, axon A will have a conduction velocity that is one-tenth the conduction velocity of axon B. (1 pt for one-tenth, 1 pt for slower) dA = 0.01dB ?B ? ?A ??==or dB = 100dA ?B ??== c) (3 pts) You inject positive current into a toad axon and measure a 0.77 mV depolarization at a distance 2? away. What was the amplitude of depolarization at your injection site? Show your work. Vx = Vo * e (-x/) 0.77 = Vo(2.7)(-2) Vo = 0.77/0.14 = 5.5 mV d) (2 pts) Fluorescently-labeled toxins are often used to study the distribution of ion channel proteins in cells. Give an example of a toxin that could be used to label channels highly concentrated in the nodes of Ranvier. tetrodotoxin, saxitoxin, veratridine, batrachotoxin, aconitine e) (3 pts) You inject negative current and hyperpolarize a toad axon by 6 mV. You record the membrane potential at 2 ? away from the injection site. What is the amplitude of the hyperpolarization at this site and what Vm will you record if Vm at the injection site is -60 mV? Show your work. Vx = Vo * e (-x/) Vx = 6*e(-2???) = 6*e-2 = 0.8 mV (amplitude of hyperpolarization) For Vm = -60 mV before injection, the Vm at 2? is: Vm = -60 - (0.8) = -60.8 mV For Vm = -60 mV after injection, the Vm at 2? is: Vm = -54 - (0.8) = -54.8 mV f) (4 pts) How does myelin affect the following parameters? Write your answer in the table below (increase, decrease, or no change). Ion Outside Inside Ex Na+ 1 100 -126 mV K+ 130 1.3 +126 mV Ca2+ 4 0.004 +94.5 mV Mg2+ 15 150 -31.5 mV Cl- 150 15 -63 mV SO4-2 0.2 200 +94.5 mV Ion Outside Inside Ex Na+ 5 50 -66 mV Cl- 100 1 -132 mV K+ 95 .95 +132 mV Which is greater? gK or gNa K+ leak channels: open or closed? Voltage-gated Na+ channels: open, inactivated, or closed? Voltage-gated K+ channels: open or closed? A gK open closed closed B gNa open open closed C gK open inactivated open D gK open closed closed Effect of myelin? ? increase gi no change gm decrease Conduction velocity increase
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