C2006/F2402 ’11
Key to Review Questions for Exam #3
1. A-1. You would expect to find voltage-gated K+ channels in the electrocytes of (fresh water) electric fish.
A-2. You would expect to find ungated K+ channels in the electrocytes of (both) types of electric fish.
Explanation: (1). Only freshwater fish generate action potentials in their electrocyte membranes. Therefore
only freshwater fish will need voltage-gated K+ channels – they’ll need the channels to repolarize after an
AP. (2). Both types will need ungated K+ channels (leak channels) as these are needed to generate a RMP.
You need a baseline RMP to generate an EPSP (in both types of fish). You need voltage-gated channels for
EPSPs to lead to an AP (freshwater fish only). You need voltage-gated Na+ channels for the depolarization
phase and voltage-gated K+ channels for the repolarization phase of the AP.
B-1. The esterase should be found in (the synaptic cleft).
B-2. If you inhibit the esterase, the shock delivered by the electric ray will probably be (stronger or longer).
Explanation: Esterase is an enzyme that splits the NT (active) into acetate and choline (both inactive as
NTs). The enzyme is found in the synaptic cleft, which is where the NT is. If the enzyme is inhibited, the
average NT molecule will last (remain in the cleft) longer. Therefore there will be more NT to bind to the
receptors on the post-synaptic side, and the EPSP (& subsequent shock) generated in the electrocyte will be
larger.
C-1. The electrocyte is probably derived from which kind of muscle? (skeletal)
C-2. Assume all vertebrates (such as fish and humans) have a similar body plan. Then the cell bodies of the
neurons that innervate the electrocytes in fishes should be (in the CNS).
Explanation: The receptor here is a nicotinic acetyl choline receptor. That type of receptor is only found on
skeletal muscle.
The neurons that innervate skeletal muscle have their bodies in the spinal cord (CNS) and their axons in
the PNS – the axons extend from the CNS to the target muscle. (If you thought the electrocyte was derived
from smooth muscle, the cell body would be in a ganglion of the PS. One pt for this.)
For more on electric fish see http://www.sbg.ac.at/ipk/avstudio/pierofun/ray/eod.htm
2. A. The gene for this enzyme was probably transcribed in the (nucleus).
B. To reach its normal location, the protein would have to pass through (the OMM) (the IMM).
Explanation: Mitochondria contain DNA, so they can make some of their own proteins. However the mito.
DNA is very small, and only codes for a few proteins. (Mito. DNA codes for a few highly hydrophobic
transmembrane proteins in the IMM. Not for any matrix proteins.) Most of the proteins found in
mitochondria are encoded in the nucleus. The genes are transcribed in the nucleus, and the mRNA is
translated in the cytoplasm. The ribosomes translating the mRNA are not attached to the ER. The resulting
proteins are released into the cytosol, and are then transported into the matrix, crossing both the OMM &
IMM.
3. A. How should receptor cells in normal mice respond to sweet compounds?
A-1. The plasma membranes in the receptor cells should (depolarize).
A-2. Voltage-gated Na+ channels in the receptor cells should (neither).
A-3. The level of Ca++ in the cytosol of the receptor cells should _increase__.
Explanation (any two of the three was sufficient):
A-1. Cations are ions with a positive charge; monovalent ions have a charge of one. A channel for
monovalent cations will allow both K+ and Na+ to pass through. When the M5 channels open, Na+ will enter
the cell and K+ will leave. Both will go down their concentration gradients, but they won’t move to the same
extent. Since Na+ is very far from its equilibrium potential, lots of Na+ will enter the cell. Since K+ is very
close to its equilibrium potential, not much K+ will exit. Therefore the cell will become more positive inside
= depolarized.
A-2. There should not be any voltage-gated Na+ channels in the receptor cells since the cells do not fire
action potentials. The receptor cells should have only ligand-gated channels that generate graded (receptor)
potentials, and the amount of NT released should be proportional to the receptor potential. The second cell
in the afferent pathway, the sensory neuron, should contain voltage-gated Na+ channels because it is the first
cell in the pathway that can fire an AP.
A-3. Ca++ levels must be elevated to stimulate exocytosis and release of NT.
B-1. In the axon of the neuron, the interval between spikes should ________decrease______________
B-2. In the axon of the neuron, the size of the graded potential should _______stay the same at 0 (or STS)
B-3. In the dendrites of the neuron, the size of the graded potential should _____increase______________
Explanation (any 2 of the 3 was sufficient):
B-1. More stimulus means more NT released from the receptor cell, meaning more APs in the sensory
neuron per sec. A faster rate of firing of APs means less time between spikes.
B-2. There is no graded potential in the axon.
B-3. A graded potential is found on the dendrites, on the post synaptic side of a synapse. The more NT from
the receptor cells, the more channels open at the synapse in the sensory cells, and the more depolarization –
bigger EPSP.
C-1. The receptor proteins for sweetness are probably (metabotropic).
C-2. Which of the following proteins probably bind(s) to sweet compounds? (a GPCR).
C-3. Of the following choices, the M5 channel is most likely to be gated by (Ca++).
C-4. If sweetness detection uses a 2nd messenger, the 2nd messenger is most likely to be (IP3).
Explanation:
C-1. The sweet ‘tastant’ binds to the extracellular domain of the T1R protein = GPCR. If the T1R were an
ionotropic (direct) receptor, it would be a channel itself, and no additional channel would be required.
C-2. The activated GPCR activates a G protein. The tastant binds to the GPCR on the outside of the cell; all
the other substances are intracellular and inaccessible to the ‘first messenger.’
C-4. Since PLC is required, you assume the G protein in turn activates PLC. The PLC splits PIP2 in the
membrane, and releases IP3 (& DAG). You know you have activated the IP3 pathway since PLC is
required. (The cAMP pathway would require AC.)
C-3. Since IP3 usually opens channels for Ca++ (Ca2+) in the ER, the [Ca++] in the cytoplasm – really the
cytosol -- should rise, and that could gate (open) the M5 channel. Alternatively, IP3 itself, or DAG could
bind to and open the M5 channel. (These are not listed as options. See * below.) Opening of the channel
should depolarize the cell, and that should lead to release of NT. (Perhaps depolarization would open
voltage-gated Ca++ channels, leading to a rise in Ca++ and exocytosis.) If the G protein itself bound to and
opened the channel, you shouldn’t need PLC. There are more complex possible pathways possible, but these
are the obvious ones.
* Of the choices listed, Ca++ is the only reasonable candidate. However Ca++ itself causes exocytosis,
and so a rise in Ca++ might be expected to cause release of NT – in which case you wouldn’t need the M5
channel at all. However both the channel and PLC are required. Therefore it seems likely that the IP3 or
DAG or some factor ‘downstream’ from them -- something further in the pathway, other than Ca++ -- opens
the channel, that leads to depolarization, and that leads to a rise in Ca++ and subsequent NT release. Current
research will reveal what actually gates the channel. (It does not appear to be Ca++, although that is a
reasonable suspect.)
4. A. If the mice are missing the M5 channel, sensory neurons should fire when the mice drink water containing
(lemon juice).
B. If the mice are missing T1R receptors, they will NOT be able to distinguish (water & sugar solution).
Explanation: A. The M5 channel is required for detecting sweet and bitter tastes but not sour (lemon juice). The
B. T1R receptors are required to detect sweetness. Mice who have no T1R receptors should not be able to detect
sweetness – therefore they shouldn’t be able to tell the difference between a tasteless solution and a sweet one.
They can still detect sour and bitter, so they should still be able to tell water from Cyx solution (or from lemon
juice).
No points were awarded for the explanation. Anything reasonable was accepted, as this was just a way to be
sure you understood the experimental set up. However points were deducted for errors.
C-1. The simplest explanation of these results is that the two types of receptor proteins (T1R & T2R) -a. use different signaling pathways in their respective receptor cells. No, they use the same pathway – both
use the M5 channel and PLCß2.
b. are in cells that synapse on neurons connecting to different cells in the brain. This fits the ‘labeled line’
theory – each receptor is hooked up (by a chain of neurons) to a particular part of the brain. All signals coming
in are the same (APs) but it’s where they go in the brain that matters. If the ‘lines’ go to the sweetness center,
the mouse registers sweetness. Similarly for bitterness.
c. cause their respective receptor cells to release different NTs. No. They use the same pathway in the same
cells.
d. are in cells that synapse on the same neurons. No. If so, how would mice know which receptor was
tripped?
C-2. You would expect these genetically engineered mice to:
a. Have a preference for (Cyx solution) AND b. Have an aversion to (neither).
Explanation: Both receptors use the same pathways, so either type of receptor should work in either type of
cell. (Presumably they both use the same G protein.) If the T2R receptor binds its ligand, the cell with the
receptor will send a signal to the brain. If that cell is the one that usually has T1R receptors, the signal will
go to the sweetness center. The mouse will drink Cyx solution, which is normally considered to be bitter,
but the mouse’s brain will register sweetness. Therefore the mouse will prefer Cyx solution, just as it
normally prefers sugar solution.
If the mouse has no T1R receptors, it will find sugar solution to be tasteless. The mouse will not avoid
either type of solution – it will think the Cyx solution is sweet and the sugar solution is tasteless.
For the experiments with switched receptors, see
http://www.nature.com/nature/journal/v434/n7030/full/nature03352.html
For an overview of the work of the Zuker lab (at Columbia!) on sensory signal transduction, see
http://www.hhmi.org/research/investigators/zuker.html
5. A-1. Which of the following should be higher in females than in males? (number of spikes per min. in axon).
A-2. When the steroid is added to female fish, which of the following should increase? (time for pacemaker
potential to reach threshold).
Explanation: When steroid is added, the pacemaker potential should decrease – its slope should be
flatter. Therefore it will take the cells longer to reach threshold (which will be at the same value), and the
rate of firing spikes will decrease – the time between spikes will increase.
Answer to Problem 5, cont.
B-1. The effect of the steroid should be (blocked by inhibitors of transcription) & (blocked by inhibitors of
translation).
B-2. The steroid receptor is most likely to be synthesized on ribosomes that are (in the cytoplasm).
Explanation: Steroids generally bind to receptors that are TFs and the complex affects transcription, leading
to changes in translation. If the complex stimulates transcription, causing synthesis of, say, a protein that
slows opening of the channels responsible for the pacemaker potential, then blocking transcription or
translation of the mRNA should reduce the effect. When transcription or translation is blocked, less protein
should be made in response to steroid, and there would be less interference with the opening of channels &
less effect on the pacemaker potential.*
The receptor should be inside the cell in the nucleus or cytoplasm, so it should be made on free
ribosomes. Note that the question asks about the site of synthesis of the receptor protein, not the protein
made in response to steroid (which might be a membrane protein made on the ER).
* If the steroid inhibits transcription, say to prevent synthesis of channel protein, then the steroid effect will
be mimicked (in part) by blocking transcription or translation. In the female fish, the steroid could inhibit
synthesis of a protein to reduce the number of channels responsible for the pacemaker potential or increase
synthesis of a protein that interferes with the opening of the channels. Either one would decrease the slope
of the pacemaker potential.
Answers to Problem 6, Parts C-E.
C. The sympathetic branch of the ANS causes relaxation of the bladder wall muscle and contraction of the
internal sphincter. The PS has the opposite effect. What should cause relaxation of the bladder wall muscle by
the ANS?
C-1. The pre-ganglionic fiber should release (AcCh) AND
C-2. The post-ganglionic fiber should release (NE)
Explanation: The NT released by the pre-ganglionic fiber in the ANS is always AcCh, whether the synapse
is in the PS or the S. In this case, relaxation is caused by the sympathetic branch of the ANS, and it uses NE
as a neurotransmitter between the post-ganglionic fiber and the target cell (bladder).
D. Consider the synapses on the bladder wall muscle (1) and the synapses on the internal sphincter (2) from the
Sympathetic branch of the ANS.
D-1. The NT(s) at the two types of synapses is/are probably (the same) AND
D-2. The receptors at the two types of synapses are probably (different).
Explanation: Both synapses are made by post-ganglionic fibers of the same branch of the ANS. Therefore
both fibers should release the same NT.
Since the responses are different, something in the target cells must be different. Since both target cells are
smooth muscle, the most likely difference is in the receptor for the NT, and not in enzymes or structural
proteins that are modified in response to G proteins, 2nd messengers, etc.
E. What should cause relaxation of the external sphincter? (neither). The extrernal sphincter is skeletal muscle,
and relaxation of skeletal muscle occurs when stimulation stops. No NT is used to trigger relaxation.
7. A-1. In the short run, core body temperature should (decrease).
A-2. In the short run, the face should feel (hotter).
A-3. After the response, core body temperature should (rise).
A-4. The response to a HF is similar to what happens when someone (finishes with a fever).
Explanation of A:
A-1. Person responds by trying to cool off either through behavior (removing clothing, etc.) or by
physiological mechanisms (sweating, flushing, etc.)
A-2. Face feels hotter because of vasodilation -- smooth muscles around the arterioles relax, and blood flow
to face increases. The heat radiation from the blood near the surface makes the face feel hot.
A-3. Person has reduced core temperature below normal (due to erroneous signals), and now needs to
restore the temperature to normal.
A-4. When fever breaks, person feels hot and sweats and vasodilates to cool off. The same thing happens to
a person with a hot flash -- she suddenly feels hot, so her physiological response is the same -- sweating and
vasodilation. (Her behavioral response may also be the same -- she may throw off the blankets, take off a
sweater, etc.) This is the part you had to explain.
In the case of a fever & a hot flash, the physiological response is the same, but the mechanism behind the
'feeling hot' is different. In the case of a fever, the change from feeling normal to feeling hot is due to a shift
in the set point. People used to think the set point changed to cause a hot flash, but now they think
otherwise. See below. (You didn't have to explain how the effects of a shift in set point could cause a hot
flash, but it was okay if you did.)
B-1. The effectors causing the flush (redness of face) are (smooth muscle).
B-2. The regulated variable(s) here is (core body temperature).
B-3. The set point should be (the same for both types of women).
Explanation:
B-1. The smooth muscle relaxation is what is primarily responsible for the redness of the face. Sweat glands
help you cool off overall, but don't make your face red.
B-2. The negative feedback system here senses and regulates the core body temperature. The extent of
vasoconstriction and redness of face are 'controlled' -- that is, adjusted to keep the core body temperature in
line. The (face) surface temperature may be sensed, and that information may be used by the IC, but it is the
core body temperature that is maintained within narrow limits, not the surface temperature (which can vary
a lot).
B-3. The info page states that both types of women have the same average body temperature.
Answer to Problem 7, cont.
C. Estrogen and brain NE affect the frequency of hot flashes. Estrogen decreases them; NE increases them.
Measurements indicate that these substances affect the body’s thermo-neutral zone – defined here as the range
of temperatures that induce neither sweating nor shivering in the body.
C-1. In women who do not have hot flashes, the thermo-neutral zone is about 0.4oC. In these women, the
maximum core body temperature should be about ____37.2oC_______.
C-2. The thermo-neutral zone should be wider in (women without hot flashes).
C-3. How should NE affect the critical point for sweating? It should (lower) .
C-4. You have two women in a room; one has hot flashes (sometimes) and the other doesn’t. If you cool down a
room, who will start shivering first? (The one who does have hot flashes).
Explanation:
C-1. As defined here, the thermo-neutral zone is the distance between the critical point for sweating and the
critical point for shivering. In other words, it is 37oC ± 0.2oC. The thermo-neutral zone is not 37oC ± 0.4oC.
C-2 to C-4. The data indicate that women who have hot flashes have the same set point as those who don't,
but the women with hot flashes have a narrower range of tolerance for temperature changes. They are more
sensitive to changes in temperature, and both sweat and shiver more easily. (Therefore the critical point for
sweating is lower, and the critical point for shivering is higher.) A small change in temperature that would
not trigger a corrective action in a person without hot flashes will trigger corrective action in a person with
them. This has been shown by putting people in a room and heating or cooling it and seeing who sweats or
shivers and who doesn't with small temperature changes.
D. The majority opinion is that clonidine, used to treat hot flashes, is an agonist of a presynaptic receptor. In that
case,
D-1. Activation of these presynaptic α2-adrenergic receptors should (inhibit NE release) .
D-2. These receptors could be part of a negative feedback loop. Assume that normal levels of NE do not have
any effect on the presynaptic receptors. Then this feedback loop can be used to correct for (high NE).
Explain how the feedback loop works, and why someone might think clonidine is an antagonist.
Short explanation: The effects of clonidine negate those of NE, so you might think it should act as an
antagonist. But the information given indicates that clonidine works by activating an inhibitor, not by
kicking in an activator. Therefore it acts as an agonist. Clonidine mimics the effects of the normal ligand,
thought to be NE.
Full explanation: NE causes hot flashes; clonidine reduces them. NE and clonidine could operate
independently, but since clonidine affects adrenergic (NE) receptors, it seems reasonable that clonidine
works by its effects on NE. The obvious solution is that clonidine blocks post- synaptic NE receptors, and
reduces the signals that trigger a hot flash.* In this case, clonidine would be an antagonist of NE.
However it says that the majority opinion is that clonidine is an agonist of a pre-synaptic receptor. If
the drug acts on the presynaptic side of a synapse, then it should affect the release of NE. If it has the
opposite effect as high NE (in the synapse), it must lower the overall NE level, which means it must inhibit
the release of NE. If it is an agonist, then the normal ligand (presumably NE) should also inhibit release. So
it looks as if this is a negative feedback loop where high NE inhibits its own release, and clonidine mimics
that affect. (For this to work, clonidine must bind to the presynaptic receptors responsible for negative
feedback but not to the postsynaptic receptors responsible for synaptic transmission.)
*Note that hot flashes are triggered by events in the brain, not in the peripheral nervous system. So we are
talking here about synapses in the brain, not in the PNS. (Innervation from the sympathetic branch of the
PNS may kick in the effectors, and control the response, but they don't trigger the sense of being hot.)
For an article on the current understanding of hot flashes, see http://www.medscape.com/viewarticle/510409_3