NAME ______________________________________
BI 360 Final Exam
Fall 2015
N. Tublitz
This is an open book, open notes exam. Choose any 7 questions (out of 10) to answer. Each question is
worth 15 points. Typed answers will be appreciated but are not necessary as long as your handwriting is
legible.
You may make use of any reference material but may not discuss the questions with anyone.
You may take up to 24 hours to complete this test.
PLEASE PUT YOUR NAME ON ALL SHEETS.
The test must be returned to the Biology Department office (77 Klamath) by
1100 am Friday December 4th
(PLEASE NOTE THAT EXAMS TURNED IN AFTER 11am WILL NOT BE
GRADED AND WILL RECEIVE A FAILING GRADE)
This test will not be graded if the following is not signed:
"On my honor, I did not collaborate with any other person, including a fellow
student, during this exam."
Signature_____________________________________
Under no circumstances should your answers exceed the space given.
************************************************************************************
EXAM SCORE ____________________
EXAM GRADE ___________________
COURSE GRADE
____________________
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NAME ______________________________________
1. The brain of the fly Drosophila melanogaster has a cell with a resting potential of -70 mV and responds to
the neurotransmitter dopamine. Dopamine acts via a second messenger system to close a species of channels
which are usually open and which have a reversal potential of -58 mV.
A (5 pts). For this cell, [K+]out = 40 mM, [K+]in = 400 mM; [Cl-]out = 660 mM; [Cl]in = 66 mM. What are the
values for EK and ECl? Show your work.
(2.5 pts) EK = 58 log ([K+]out/[K+]in) = 58 log (40/400) = 58 (-1) = -58 mV
(2.5 pts) ECl = -58 log ([Cl-]out/[Cl-]in) = -58 log (660/66) = -58 (1) = -58 mV
B (5 pts). Consider the ligand-gated channels in this cell that are closed by dopamine. Do these channels
conduct chloride ions, potassium ions, both types of ions, or is there insufficient information? What
experiment(s) would you perform to confirm your answer?
There is insufficient evidence presented in this question to determine which ion is involved in
the dopamine response. To address this question, one would measure the amplitude of the
dopamine-triggered PSP before and after altering the external concentration of each of the
two ions. A difference between the PSP amplitudes in either manipulation would strongly point
to the involvement of that ion in the dopamine response.
C (5 pts). A different cell contains a set of ligand-gated potassium channels which are normally open at the
resting potential of -70mV and which are closed in the presence of dopamine. These channels have a reversal
potential of -90mV. If dopamine is applied to this cell at the resting potential, is it acting as an excitatory or
inhibitory transmitter? Explain your answer.
The action of dopamine on this cell is excitatory. When these K+ channels are open at -70mV,
K+ ions flow outward, down their concentration gradient. Closure of these channels by dopamine
transiently stops the outward K+ flow, resulting in a transient intracellular depolarization which
will make the membrane potential more positive. Thus dopamine is acting as an excitatory
transmitter on these cells.
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NAME ______________________________________
2. The gin trap reflex in Manduca displays the phenomenon of dishabituation. Define dishabituation and how
it might function in the gin trap reflex (5 pts). Use your knowledge of the gin trap circuitry and information
from other model invertebrate systems to postulate a cellular network (5 pts) and a molecular mechanism (5
pts) in Manduca that would account for the dishabituation of this behavior. Be as specific and detailed as
possible.
DEFINITION (5 pts): Dishabituation is the overriding of the cumulative effects of habituation
by a sensitizing stimulus. In the case of the gin trap reflex, a pinch to the body results in the
reappearance of the gin trap response previously habituated through repeated presentations
of a stimulus to the gin trap hairs.
CELLULAR NETWORK (5 pts): One possible hypothesis that would account for the
dishabituation of this behavior is that the pinch activates a sensory neuron which in turn
synapses onto the synaptic terminal of the gin trap interneuron, the second cell in the three
neuron circuit that makes up the gin trap reflex (sensory neuron-interneuron-motoneuron).
MOLECULAR MECHANISM (5 pts): To override the habituation in the circuit, the pinch
sensory neuron would have to release a transmitter onto the terminal of the interneuron to
cause enhancement of transmitter release at the interneuron-motoneuron synapse. One
hypothetical molecular mechanism that would account for this effect is an increase in internal
Ca2+ concentrations in the interneuron terminal caused by the transmitter released by the
pinch sensory neuron. Several other molecular mechanisms are possible.
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NAME ______________________________________
3. Explain the phenomenon of post-inhibitory rebound in neurons including a detailed molecular mechanism
(8 pts) and how it is used to produce cyclical motor patterns (7 pts) in some organisms.
POST-INHIBITORY REBOUND (8 pts): Post-inhibitory rebound (sometimes referred to as
“anodal break”) occurs when the membrane potential of a cell, after release from
hyperpolarization, depolarizes to a level above threshold and thus causes firing of one or more
action potentials. The molecular explanation for this phenomenon is that a smaller percentage
of voltage dependent Na+ channels are inactivated at hyperpolarizing potentials than at the
normal resting potential. Therefore, more voltage-dependent Na+ channels open upon the
release from hyperpolarization than are available to open at the normal resting potential,
leading to a relatively large Na+ influx which causes the membrane potential to rise above
threshold. When these Na+ channels close, the membrane potential will drop back to rest and
the cell will cease to fire action potentials.
POST-INHIBITORY REBOUND IN CYCLICAL MOTOR PATTERNS (7 pts): Cyclical motor
patterns are often generated by a central pattern generator consisting of two (or more) cells
connected to each other via inhibitory synaptic connections. In the simplest case of two cells
A and B, each connected to the other via an inhibitory synapse, activation of A will inhibit B
until A stops firing, in which case B will fire due to post-inhibitory rebound and inhibit A. When
B stops firing, A will depolarize due to post-inhibitory rebound and start firing action
potentials, leading to the next cycle of patterned activity.
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NAME ______________________________________
4. You are recording the activity at the neuromuscular junction of a striated fin muscle of a fish. Electrical
stimulation of the fin muscle motoneuron causes a large end plate potential (EPP) in the muscle. Focal
application of ACh from a pipette placed in the NMJ synaptic cleft produces a large EPP-like potential in the
muscle. Application of a novel sea anemone toxin blocks the stimulation-evoked EPP but not the ACh-evoked
muscle potential. Where in the neuromuscular junction is the site of action of this toxin (pre-synaptically, postsynaptically or in the synaptic cleft; 5 pts)? What is your reason for this answer (5 pts)? Propose a molecule
mechanism that accounts for the effect of the toxin on the neuromuscular junction (5 pts).
WHERE (5 pts): The sea anemone toxin must be acting pre-synaptically at the terminal of the
motoneuron.
REASONING (5 pts): This conclusion is based on the fact that the sea anemone toxin has no
effect on the ACh-evoked muscle response which means it cannot be affecting the ACh postsynaptic receptors on the muscle.
MOLECULAR MECHANISM (5 pts): The toxin’s mechanism of action could be by blocking ACh
release from vesicles, preventing vesicle fusion with the membrane, or affecting the voltage
dependent influx of calcium into the terminal.
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NAME ______________________________________
5. One of the classic methods to depolarize a cell is to increase the extracellular concentration of K+ ions.
A (7 pts). Explain exactly why this treatment depolarizes a cell.
The resting potential is primarily due to a resting K+ conductance (3 pts). The GoldmanHodgkin-Huxley equation unequivocally shows that a change in external concentration of K+
will increase the resting membrane potential of the cell. This is because the nerve cell
membrane is primarily permeable to K+ ions and increasing extracellular levels of K+ causes EK
to become more positive (4 pts).
B (4 pts). Increasing the extracellular concentration of Na+ does not have this effect. What effect is seen and
why?
The permeability of a neuron at rest to Na+ is much lower compared to K+. Increasing
extracellular Na+ will cause a minor depolarization however that effect will be slight if at all.
C (4 pts). Increasing extracellular K+ levels in the presence of TEA will have what effect on the resting
potential of the neuron? Why?
TEA blocks voltage dependent K+ channels, not the resting K+ conductance. Hence TEA will
not impact the depolarization of the resting membrane potential due to a rise in extracellular
K+ levels.
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NAME ______________________________________
6. Below is a recording from a squid giant neuron. Separate square pulses of hyperpolarizing current (-1 nA
=10-9 A) are injected into the cell for 20, 40 or 100 ms in duration and these generate the voltage and current
records presented below. For all questions, show your work.
A) What is the time constant (Tau) of the cell (in ms; 5pts)?
20 ms or 2 x 10-2 sec (time taken for the signal to drop 63%). In this example, the maximal
voltage in response to a -1 nA hyperpolarizing pulse is -100 mV. 63% of that change is -63 mV.
Hence the time constant is the time it takes for the voltage to drop 63 mV from the original
resting potential of -60mV or to -123 mV, which is approximately 20 ms.
B) What is the cell’s membrane resistance (Rm) in ohms (5 pts)?
1 x 108 Ohms or 100 megohms (obtained by solving V = IR where V = -100 mV or 1 x 10-1 V and
I = -1 nA or 1 x 10-9 A)
C) What is the cell’s membrane capacitance (Cm) in farads (5 pts)?
Cm = 2 x 10-10 Farads (obtained by solving τ= Rm Cm where
1 x 108 ohms)
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τ = 20 ms or 2 x 10-2 sec and Rm =
NAME ______________________________________
7. You are the chief medical officer on the Starship Enterprise traveling through space. A pointy eared
Vulcan comes in complaining of a headache. Before you can identify the possible cause(s), you proceed with
a complete neurological examination. As part of your examination, you ascertain that the resting membrane
potential of a typical Vulcan neuron is +50mV yet the internal and external concentrations of K+, Ca2+, Na+
and Cl- ions are exactly the same as humans. Provide a plausible explanation for this unusual resting
potential and devise an experiment to empirically test your explanation.
The resting potential of +50 mV in the Vulcan neuron must be due to a high resting
permeability of either Na+ and/or Ca2+ compared to K+ and Cl-. This hypothesis can be tested
by changing the external concentrations of Na+ and Ca2+, which should alter the resting
potential.
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NAME ______________________________________
8. What insights into neuroscience have you gained from reading The Man Who Mistook His Wife For A Hat
and viewing the Nova video? Include at least two specific examples from the book or video in your answer.
A correct answer to this question focuses on several general issues raised by the book and the
movie (e.g., neural deficits; plasticity; behavioral abnormalities) using two examples from the
book or movie to support your answer.
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NAME ______________________________________
9. Fill in the blanks:
RECEPTOR
POTENTIAL
SYNAPTIC
POTENTIAL/EPP
small or medium
small or medium
large
Duration (choose one
from brief, brief to
long, or long)
brief to long
brief to long
brief
Response to stimulus
(choose one from
graded, all or none, or
both)
graded
graded
Signal (choose one
from hyperpolarizing,
depolarizing or both)
both
both
depolarizing
Propagation (choose
one from passive,
active or both)
passive
passive
active
Amplitude (choose
one from small,
medium or large)
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ACTION
POTENTIAL
all or none
NAME ______________________________________
10. You undoubtedly studied one aspect of nervous system function you regarded as important but was not
covered on this exam. So that all that studying was not a total waste, pose a question that covers a topic from
any part of this course not addressed by other questions on this exam, and then answer your own question.
You’ll be graded both on the quality of the question (7.5 points) and on the quality of the answer (7.5 points).
In particular the question should not be too trivial (e.g., name two kinds of cells in the nervous system), and it
should not be a twist on one of the questions already on the exam Your question ought to reflect your judgment
of what a fair yet relatively difficult exam question should be.
Many correct answers are possible. Points were awarded on the basis of the originality and
quality of the experiment and answer.
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