NAME:_________________________
TA:_____________ SEC:_________
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7.012 Problem Set 8
Question 1
a) Pictured below is a generalized neuron. Label the parts.
1 is dendrites, 2 is cell body, and 3 in axon.
b) What is the purpose of an axon?
The axon conducts the action potential from the axon hillock to the nerve terminal
and thus relays the input received to the post-synaptic cell.
c) Name the specialized site where 2 different neurons interact __synapse__.
d) What 2 gradients exist across a nerve membrane?
A concentration gradient for ions (Na+, K+, Ca+=, Cl-), and a charge gradient.
e) At resting potential what gradient limits the exit of K+ ions from the cell?
The charge gradient.
f) Name two classes of protein molecules that allow ions to cross the membrane.
Ions are either carried by 1) transport proteins, or 2) they move through ion channels
or ion pumps.
g) List the two transmembrane proteins that create the resting potential. (Do not
include the Ca++ channel.) For each, indicate what ion(s) is moved and the direction of
movement. (towards the inside of the cell or towards the outside of the cell)
Transmembrane
Protein
1)ATP driven Na+/ K+ pump
2) resting K+ channel
ion(s) moved
Na+, K+
K+
direction of movement
Na+ out of the cell
K+ into the cell
Out of the cell
NAME:_________________________
TA:_____________ SEC:_________
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Question 2
Invertebrates make liberal use of electrical synapses, because many of their neurons
are connected by gap junctions.
a) What is a gap junction?
A pore or channel that spans the small gap between two different cells and directly
links the cytoplasm of the two cells.
b) In contrast, vertebrates more commonly use chemical synapses to signal from
neuron to neuron.
Why do signals travel slower in chemical synapses than in electrical synapses?
At a chemical synapse, the electrical signal must be converted to a chemical signal
and then back to an electrical signal. The gap junction allows the electrical signal to
continue directly to the second cell.
c) Influx of what ion plays a central role in converting an electrical signal to a
chemical signal?
d) Outline the steps involved in neuotransmitter release after the action potential
invades the terminal of the pre-synaptic cell.
Depolarization triggers the opening of voltage-gated calcium channels. Calcium
enters the cells and binds to and activates a calcium-dependent protein kinase. The
protein kinase phosphorylates synapsin which triggers fusion of synaptic vesicles and
release of neurotransmitters.
e) What would happen if the presynaptic neuron in d) were exposed to Ca2+ channel
blockers?
Without calcium entry into the cell, neurotransmitter release is blocked.
f) Suppose that neurons are removed from a mammal, placed in petri dishes with
medium, and allowed to form synapses with other neurons. Explain how the following
treatments would alter synaptic transmission in the dish.
Add tetrodotoxin to the medium (see page 992 in Purves et al.).
(BLOCK Na+ CHANNELS, NO ACTION POTENTIALS, NO SIGNALS TO
TRANSMIT)
Mutate the neurotransmitter receptors so that they are nonfunctional. (PREVENT
NEUROTRANSMITTERS FROM SIGNALING TO
POSTSYNAPTIC NEURONS)
Inhibit enzymes in the synaptic cleft that break down neurotransmitters.
(PROLONGS THE SIGNAL FROM ONE ACTION POTENTIAL, PREVENTING
SUBSEQUENT SIGNALS FROM BEING TRANSMITTED).
NAME:_________________________
TA:_____________ SEC:_________
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Question 3
Acetylcholine (abbreviated ACh) is one of the most common neurotransmitters in
both invertebrates and vertebrates. Its function is varied. At the neuromuscular
junction, acetylcholine is released from the terminal of a presynaptic motor axon. The
effect on the postsynaptic skeletal muscle cell is excitatory. In the heart, however,
acetylcholine can be inhibitory, so that it slows the heart rate.
a) How can the same neurotransmitter trigger the opposite responses in different cell
types?
TWO POSSIBILITIES: 1) DIFFERENT ACh RECEPTORS, or 2) DIFFERENT SIGNALING
MOLECULES DOWNSTREAM OF THE ACh RECEPTOR.
To study the function of ACh, one can isolate an entire neuromuscular junction. You
can study synaptic transmission by wrapping a wire around the presynaptic neuron,
stimulating it with an electrical current, and then measuring the effects of this
stimulation in the postsynaptic muscle cell. You can also apply ACh directly onto the
muscle cell to get the same effect.
b) In your studies you find that ACh receptors are localized to a specific point in the
muscle cell. (Applying ACh elsewhere on the muscle has no effect.) Where on the
muscle cell would ACh receptors need to be to function properly?
AT THE POINT WHERE THE NEURON SYNAPSES. THE MUSCLE CELL WILL BE
SENSITIVE TO ACh ONLY AT THIS POINT.
You can also use pharmacological agents (i.e. drugs) that alter ACh signaling to study
ACh function. The following drugs are used in your assays.
Curare, a toxin from a plant, used to coat poison darts, blocks ACh receptors
on muscles tissue.
neostigme, an active component in nerve gas, is an inhibitor of
acetylcholinesterase, the enzyme that breaks down ACh in the synapse.
c) On the graph below, label the plot that corresponds to the membrane potential of
the muscle cell after curare is added. Label the plot that corresponds to the membrane
potential of the muscle cell after adding neostigme
Question 4
NAME:_________________________
TA:_____________ SEC:_________
Neurons change their membrane potentials in response to stimuli that the cells
receive.
Any of the following signals would be expected to trigger a change in the membrane
potential of the corresponding neuron.
Signal
Fragrant and pleasing odor
Good vibrations in the air
Accidental pinprick
Activation of neuron
Corresponding Neuron
Olfactory receptor in the nose
Receptor in the ear
Pain receptors in the skin
Neuron next to the neighboring neuron
a) Below is a profile of an action potential. Label the steps, and describe the state for
both voltage-gated Na+ and K+ channels at each step.
Label Stage
A.
B.
C.
D.
Resting membrane potential
depolarization
repolarization
hyperpolarization
or refractory period
Voltage-gated Na+
channel
(open or
closed/inactive)
closed/inactive
open
closed/inactive
closed/inactive
Voltage-gated K+
channel open or
closed
closed
closed
open
open/closed
b) What would happen to the action potential if you could open voltage-gated
potassium channels at point B in the graph above?
As Na+ was moving out, K+ would be moving in and further depolarization of the
membrane would be inhibited.
c) In the graph above the stimulus that caused the action potential was just enough to
get the cell to reach threshold. What would happen if the cell receives another
stimulus of equal magnitude, but this time, at time point D?
The cell would fail to fire an ation potential. Because the cell is hyperpolarized, a
larger input stimulus is required to reach threshold.
Question 4 continued
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NAME:_________________________
TA:_____________ SEC:_________
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d) When the ion channels open, the membrane initially shows some extent of
depolarization, whose magnitude depends on the strength of the stimulus. At what
point is the magnitude of the depolarization independent of the strength of the
stimulus?
AFTER THE MEMBRANE HAS BECOME DEPOLARIZED TO THE THRESHOLD POTENTIAL.
e) The action potentials in invertebrates like squids and lobsters can be very fast (up to
100 m/s, which is over 200 mph), because the axons in these animals are very large.
Mammals do not have such large neurons, and must instead use myelin, an insulating
coat deposited by Schwann cells or glial cells, to increase action potential
propagation.
How is the propagation of an action potential different in a myelinated nerve vs. a
nonmyelinated nerve?
SCHWANN CELLS WRAP MYELIN AROUND SEGMENTS OF THE AXON. ION CHANNELS ARE
CONCENTRATED IN BETWEEN SCHWANN CELLS (IN NODES OF RANVIER), SO THAT IONS
FLOW
IN AND OUT OF THE AXON ONLY IN THESE NODES. THE ACTION POTENTIAL “JUMPS”
FROM
NODE TO NODE, SKIPPING INSULATED REGIONS. THIS SALTATORY CONDUCTION IS
FASTER
THAN CONTINUOUS PROPAGATION, AND PREVENTS IONS FROM LEAKING OUT OF THE
LENGTH OF THE AXON.
NAME:_________________________
TA:_____________ SEC:_________
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