Unformatted text preview: Sample Midterm (some solutions and hints)
Part 1: (Multiple choice and fill in the blank) 1.1. A lateral giant neuron is connected to the axon of a motor giant neuron via an electrical synapse as shown in the diagram. The current flowing through this electrical synapse is sufficient to bring voltagegated sodium channels in the axon of the motor giant neuron above their threshold. In the motor giant neuron, DELETED FROM PRACTICE EXAM (CIRCLE THE CORRECT ANSWER) (a) action potentials will be generated and will propagate only in the direction toward the soma (b) action potentials will be generated and will propagate both toward the soma and toward the bouton (c) no action potentials will be generated. Action potentials can only be generated in the spikeinitiating zone of the hillock. (d) action potentials will be generated and will propagate only in the direction towards the bouton. Sample midterm (1997) 1.2 DELETED FROM PRACTICE EXAM
AB VD IC P PD LP PY Legend for figure shown above: The main groups of neurons involved in generation of the pyloric rhythms of the lobster stomach. Each large circle represents several individual neurons of a particular type. The initials represent the names that have been given to the different classes of neurons. Small solid circles represent inhibitory synapses; triangles represent chemical excitatory synapses and lines with resistor symbols represent excitatory electrical synapses. CIRCLE a connection that is specialized for synchronization. More than one correct answer Put a SQUARE around two neurons which when removed from the circuit would rhythmically oscillate if they had post-inhibitory rebound and if one of the neurons was stimulated with a brief excitatory input. More than one correct answer 1.3 You are Mother Nature and you want to design a fast neural circuit. In the space provided, list three properties you might put on the neurons or connections in the circuit to aid in this design (note: only list three; if you put more than three we will only look at the first three). Property 1 __________________ Property 2 ___________________ More than three correct answers Property 3 ____________________ voltage 1.4. In a voltage clamp experiment, the investigator controls the______________ (DO NOT PUT current CONCENTRATIONS) and records the ___________. Sample midterm (1997) Part 2: (Problem) Given: The threshold for firing an action potential is -45 mV. You pull off a patch of membrane from a giant squid axon to perform a voltage clamp experiment in a controlled environment. This patch of membrane only contains voltage gated sodium and voltage gated potassium channels. Once you have isolated the patch of membrane, you change the environment of your experimental system as follows: (1) You apply TEA to both the inside and outside of the patch (so now you only have VG-NA left) (2) You fill the pipette (the inside of the cell) with 50 mM of sodium acetate. You fill the bath (the outside of the cell) with 553 mM of sodium acetate. These concentrations are maintained for the entire experiment. NOTE: No other ions are in the bath or pipette. (3) You make the temperature of the bath 22 degrees Celsius such that 2.3*RT/F = 58.5 mV. (ENA is 61.1 mV) You now perform a series of voltage clamp experiments: You first voltage clamp the patch to -120 mV You then voltage clamp the patch to +120 mV Then, you voltage clamp the patch back to –120 mV Next, you voltage clamp the patch to +60 mV Again, you voltage clamp the patch back to -120 mV Next, you voltage clamp the patch to +0 mV Again, you voltage clamp the patch back to -120 mV Next, you voltage clamp the patch to -60 mV Next, you voltage clamp the patch to -120 mV All voltages are clamped for 10 seconds. 2.1 On the five current plots on the next page, draw the current you would observe when the voltage is clamped at +120mV, +60mV, 0mV, -60mV, and -120mV, respectively. Only draw the relative amplitudes of each current. DO NOT waste time trying to calculate the exact magnitude of the current for each curve. 2.2 Draw a schematic I-V curve for the channels. Although you are drawing a schematic, make sure you accurately portray the reversal potential. Sample midterm (1997) voltage 120 60 0 -60 -120 current time time time time time time I - 120 -60 +60 V
+120 2.3. Why did we clamp the voltage to -120 mV between each new recording. If we did not hyperpolarize the cell after we did a voltage clamp above t-hold, the VG-Na chnnels would get stuck in the inactivated state (after opening) and would be unable to get open again. GO TO THE NEXT PAGE Sample midterm (1997) Part 3: Given: You want to study a special K+ channel. You are recording from a cell that has many of these special K+ channels distributed all over the neuron. Initially, the cell’s resting membrane potential is –68.1 mV and these special K+ channels are closed. It has been discovered that the threshold to open/close these potassium channels is -68 mV. In addition, these channels will open only if a depolarizing voltage above -68 is MAINTAINED for 20 msec. 3.1. You stimulate the cell so that the cell will fire one single action potential. Will the special K+ channels located on the axon open during this action potential? No 3.2 Why or Why not? They will not open because depolarization has to be maintained for 20 msec. The undershoot of any action potential will not allow this. 3.3. You want to learn more about the function of this channel. Accordingly, you block all other voltagegated channels (so you can record the effects of these special channels without getting any action potentials). Assume that when all the special potassium channels are open, a 5 mV hyperpolarization results across the cell membrane. You now apply a current that at the steady state would depolarize the cell by 100 mV if all of these potassium channels remained closed (remember the resting membrane potential is –68.1 mV). The current injection starts at time t = 0 msec. and lasts for 30 msecs. The cell has a time constant of 10 msec. What will the voltage be across the cell membrane at time t = 50 msecs? Show your work and be sure to lay out your approach if you want partial credit. First you should show the channels open either using the equation to solve for the time or using the unified equation (they open at 0.01 msec after the current turns on so this should work for just about any time. (depending on the time you choose, you might also have shown that the neuron is above –68 mV 30 msec after you determined threshold was crossed. This is an on/off problem so you must create 2 steps: ΔV = 100*(1-e-50/10) – 100*(1-e-20/10) = 12.86 mV Ignoring the channels the final voltage would be 12.86+(-68.1) = -55.2 which is still above –68 so the channels stay open and we subtract 5 Vm = -55.2-5.0 = -60.2 mV DELETED FROM PRACTICE EXAM Sample midterm (1997) Part 4: (Fill in the blank and short answer) Refer to the picture below. dendrites X 4.1 Draw an X on the picture where an inhibitory synapse would be most effective. From your reading 4.2 Would an excitatory synapse also be most effective at this point? (Yes or No) Yes From passive electrical properties Why or Why not? Because this is where one would get the least attenuation due to passive electrical properties for excitation, and because either the above argument or you would get the best shunting for inhibition. 4.3 Draw an arrow to the most common target of synapses. Sample midterm (1997) Part 5: IMPORTANT: READ THE QUESTION AND USE THE NUMBERS GIVEN IN THIS PROBLEM! Given: Under normal conditions, ENa+ is between 40 mV and 70 mV Ek+ is between –90 mV and –70 mV Ecl- is between –50 mV and 0 mV The resting membrane potential is –70 mV You have isolated a neurotransmitter that will open a mystery receptor. Under normal conditions, when you apply this neurotransmitter onto the dendrites of a post synaptic neuron, a depolarizing post synaptic potential (PSP) occurs. The dendrites of this neuron only contain the mystery receptors. From this information, answer the following questions . 5.1 Is it possible that the mystery receptor is only permeable to potassium (Yes/No) Why? Had it been permeable only to K, when K channels opened the cell would hyperpolarize towards Ek (between –70 and 90 and because Ek is below RMP) It did not; therefore K+ is not possible. 5.2 Is it possible that the mystery receptor is only permeable to sodium (Yes/No) Why?. When a channel opens the membrane potential will go towards its reversal potential, Er. Er for a sodium channel is between 40 and 70 mV which is above RMP. Therefore it would depolarize. It did so this is still possible. 5.3 Is it possible that the mystery receptor is only permeable to chloride (Yes/No) Why? Same logic as 5.2 GO TO THE NEXT PAGE Sample midterm (1997) You now apply TTX and TEA to the cell and increase the extracellular sodium concentration until the resting membrane potential shifts from -70 mV to +2 mV (assume the cell does not burst or lyse). Under these novel conditions, when you apply neurotransmitter onto the dendrites of a post synaptic neuron, no voltage change across the cell membrane is recorded. Combining this information with the conclusions drawn from the recordings under normal conditions (your answers above), answer the following questions. 5.4 Is it possible that the mystery receptor is only permeable to sodium (Yes/No) Why? If it were only permeable to Na for Vm not to change ENa would have to equal Vm (at 2 mV). Raising extracellular Na would raise ENa, so the lowers ENa would have to be > 40 mV. Therefore this is not possible 5.5 Is it possible that the mystery receptor is only permeable to chloride (Yes/No) Why? If it were only permeable to Cl for Vm not to change ECl would have to equal Vm (at 2 mV) which is not possible because Ecl is < 2 mV. 5.6 Is it possible that the mystery receptor is permeable to both potassium and chloride ions (but not to sodium ions) (Yes/No) Why? . If it were permeable to both the reversal potential could be anywhere between –90 and 0 mV. The reversal potential measured of 2 mV is not in this range so it is not possible Sample midterm (1997) ...
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