Download Lab #6: Neurophysiology Simulation

Lab #6: Neurophysiology Simulation
Background
Neurons (Fig 6.1) are cells in the nervous system
that are used conduct signals at high speed from
one part of the body to another. This enables
rapid, precise responses to occur in order to
compensate for changes in the environment.
Neurons are able to send signals at high
speed due to their ability to generate and
conduct an electrical signal called an action
potential down the length of their axons. An
action potential is a brief reversal of the
membrane potential, so that for a brief interval at
a segment of the axon the intracellular fluid just
inside of the plasma membrane is more positive
than is the extracellular fluid just outside the
plasma membrane. This signal is typically
generated at the axon hillock of the neuron, and
requires the opening of voltage-gated ion
channels — specialized pore-like transmembrane proteins that open to allow ion
passage in response to changes in the relative
charge difference across the plasma membrane.
There are two different types of voltage-gated
ion channels important for the generation action
potentials: those specific for sodium ion (Na+),
and those specific for potassium ion (K+).
In the intervals between action potentials
(i.e., when the neuron is “resting”) the two types
of ions are kept at different concentrations
across the plasma membrane (Fig 6.2). Na+ is
maintained at higher concentrations outside the
cell than inside the cell. Conversely, K+ tends to
be accumulated at higher concentrations inside
the cell than outside the cell. The potential for
movement of these ions across the cell
membrane is thus influenced by the
Dendrites
Schwann Cells
Axon Terminals
Axon Hillock
Nucleus
Cell Body
Axon
Fig 6.1. Illustration of a neuron and its major
associated structures
Figure 6.2. Distribution of ions across the plasma
membrane during resting potential. Different font sizes
for Na+ and K+ indicate differences in relative
concentration.
concentration gradients for each ion. Moreover,
charge differences across the cell membrane
affect the potential for diffusion of these ions.
The interior of cells is typically more negatively
charged than is the outside of the cell, due to
negative charges on certain side-chains of the
amino acids of proteins inside the cell,
phosphorylated compounds (e.g., ATP), etc. As
a result, under resting conditions, there is a
strong electrochemical gradient favoring the
flow of Na+ into the cell, and a weak
electrochemical gradient favoring the flow of K+
out of the cell.
Ion concentrations are
maintained at relatively constant levels,
however, due to the normally low permeability
of the plasma membrane to Na+ and low-level
activity of the Na+/K+ pump, which pumps Na+
back out into the extracellular fluid and K+ back
into the intracellular fluid. The distribution of
charged particles across the cell membrane at
rest generates the resting potential of the cell
membrane, which is variable among different
neurons, but typically around -70 mV.
The membrane potential (the difference in
overall charge across the plasma membrane) of
the neuron can change if the relative difference
in charges across the membrane is changed. The
action potential is generated by just such a
redistribution of charged particles across the
membrane.
By opening large numbers of
voltage-gated channels, the permeability of the
membrane to Na+ and K+ is increased markedly,
allowing the ions to flow along their respective
electrochemical gradients from one side of the
membrane to the other.
Figure 6.3. Change in voltage-gated ion channels and
redistribution of ions during the depolarization phase of
the action potential
However, in order for voltage-gated ion
channels to open and allow this redistribution of
ions across the plasma membrane, the membrane
potential itself needs to be changed from resting
level by a minimum amount (threshold).
Changing the membrane potential to the
threshold level causes a redistribution of charged
areas within the protein itself, causing a shape
change in the channel and opening the passage
for the ion. The changes in membrane potential
needed to induce the voltage-gated ion channels
to open are typically due to the binding of
chemical signals (e.g., neurotransmitters) in the
extracellular environment to chemically-gated
ion channels in the dendrites and cell body of the
cell, which increase the permeability of the
membrane to certain ions. Physical factors such
as mechanical distortion of the plasma
membrane or extreme temperature changes, as
well as other chemical changes that may affect
the shape of proteins in the plasma membrane
(e.g. pH), can also alter the permeability of the
plasma membrane to certain ions. Moreover,
changes in the concentration gradients of the
ions themselves across the cell membrane can
alter the membrane potential, as the movement
of the ion across the membrane through fixed
open channels may be changed. In some cases,
the resultant change in charge distribution
depolarizes the membrane (moves the
membrane potential closer to 0 mV), and thus
moves the membrane potential towards the
threshold value. In other cases, the membrane
may become hyperpolarized (more negative,
further away from 0 mV), which typically
moves the membrane potential away from the
threshold value needed to open the voltage-gated
ion channels.
Figure 6.4. Changes in membrane potential during
an action potential.
An action potential begins when the plasma
membrane at the axon hillock is depolarized to
threshold. This induces the opening of the
voltage-gated ion channels (Figs 6.3 and 6.4).
The channels specific for Na+ open very quickly,
thus there is a rapid increase in the permeability
of the plasma membrane to Na+. Na+ rapidly
flows into the interior of the cell along its
electrochemical gradient, and drives the
depolarization phase of the action potential.
The membrane is fully depolarized to 0 mV, but
even then Na+ continues to flow into the interior
of the cell, so the fluid inside the cell becomes
more positive than the adjacent extracellular
fluid, and the membrane polarity is reversed
from normal resting levels. The membrane
potential rises to ~ +30 mV, but then the flow of
Na+ into the cell effectively stops – not because
Na+ has reached equilibrium, but because the
voltage-gated Na+ channels close at that
potential, cutting off the flow of Na+.
At approximately the same time the flow of
Na+ stops, the voltage-gated K+ channels, which
began opening at threshold but require more
time to open than do the voltage-gated
Na+ channels, begin to open in earnest (Figs. 6.4
and 6.5). Since it is now more positive inside
the cell than outside the cell, there is a strong
gradient favoring the flow of K+ out of the cell.
Thus K+ flows out of the cell, driving the
repolarization phase of the action potential. As
the positively charged K+ leaves the cell, the
interior of the cell becomes progressively more
Figure 6.5. Change in voltage-gated ion channels and
redistribution of ions during the repolarization phase of
the action potential.
negative, and the membrane potential moves
back towards the resting potential. Once the
membrane potential is repolarized below
threshold, the voltage-gated K+ channels close.
Although the resting potential has been restored,
the concentration gradients for Na+ and K+ are
now different from resting levels, with large
amounts of Na+ inside the cell and high amounts
of K+ outside the cell.
The Na+/K+ pump
restores the local concentration gradients back to
resting levels by pumping Na+ out of the cell and
K+ back into the cell.
Note that the action potential is an all-ornone response (Fig 6.6). The action potential
can occur only if the membrane is depolarized
enough to reach threshold and induce the
opening of voltage-gated ion channels.
Therefore, if the stimulus is not strong enough to
reach threshold, no action potential will occur.
If threshold is reached, however, a positive
feedback loop ensues that quickly leads to the
opening of all of the voltage-gated ion channels.
Thus, regardless of whether the membrane is
depolarized just to threshold or above threshold,
maximum permeability of the membrane to Na+
and K+ will be achieved. Moreover, since the
voltage-gated Na+ close at a specific potential as
well, the amplitude of the action potential is
always going to be the same – the difference in
voltage between the threshold and the potential
at which the Na+ channels close.
Once an action potential has started at the
axon hillock it quickly travels down the length
of the axon. Remarkably, the strength of the
action potential is maintained along the entire
length of the axon. This is because once sodium
enters the cell during the depolarization stage of
the action potential it quickly diffuses though the
intracellular fluid along its electrochemical
Figure 6.6. The “all or none” action potential of a
neuron. The plot at the top illustrates a series of stimuli
of progressively increasing intensity applied to the
neuron, whereas the plot at the top illustrates
corresponding electrical responses (action potentials)
from the neuron.
Notice that the neuron only
undergoes an action potential when stimulus intensity
is high enough to reach the threshold membrane
potential for the neuron, and that the amplitude of the
action potential (i.e., how much it depolarizes) does not
differ with stimulus intensity as long as the stimulus is
intense enough to reach threshold.
gradient. Lateral flow of Na+ inside the cell
down the length of the axon triggers the next
segment of the axon to be depolarized to
threshold, causing it to undergo an action
potential. This effect is repeated down the
length of the axon, with one segment of the axon
undergoing an action potential stimulating the
next to depolarize to threshold.
The speed at which the action potential
travels down the length of the axon is referred to
as its conduction velocity, and can be calculated
by dividing the distance an action potential
travels by the time it takes the action potential to
travel that distance.
Two major factors
influence conduction velocity:
1) Myelination – neurons with axons surrounded
by Schwann cells or oligodendrocytes
conduct action potentials in a manner called
saltatory conduction (Fig. 6.7).
Since
myelination prevents ion exchange between
the inside and outside of the cell to occur
Na+
Na+
Na+
A
Na+
Na+
C
Na+
Na+
Na+
B
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Figure 6.7. Saltatory conduction in myelinated axons.
A) The axon hillock reaches threshold, and Na+ flows
into the cell through voltage gated ion channels. B)On
inside the cell, Na+ rapidly diffuses to the next node.
Moreover, the flux of Na+ from the outside of the cell to
the inside of the node creates an electrochemical gradient
that draws Na+ away from the second node towards the
first, which also depolarizes the membrane at the second
node. The combined effect of these two actions caused
the next node to quickly depolarize to threshold and
undergo an action potential. C) Once voltage gated ion
channels at the second node open, the process repeats
with the third node.
over most of the length of the axon, the only
places where this ion exchange can occur
are in the nodes of Ranvier between
myelinated segments, where there are
particularly high concentrations of voltagegated ion channels. When the axon segment
is depolarized to threshold, Na+ rapidly
flows into the cell and diffused quickly
through the cytoplasm to the next node.
Moreover as Na+ flows into the cell at one
node, it creates a gradient favoring Na+ in
the extracellular fluid near the next node to
flow back to the previous node. This
reduces the charge difference between the
outside and inside of the cell at the second
node – depolarizing the cell membrane
towards threshold and evoking an action
potential at the next node. Therefore, the
action potential “jumps” quickly from one
segment to the next, and occurs more
quickly than if the entire length of the axon
was involved in ion exchange across its
membrane.
Fig 6.8 Plot of experimentally measured conduction
velocity as a function of axon diameter in mammalian
neurons. After Ruch TC, Patton HD (eds.) (1982):
Physiology and Biophysics, 20th ed., 1242 pp. W. B.
Saunders, Philadelphia
2) Axon diameter – Action potential propagation
requires the ability for Na+, once it enters the
cell, to be able to diffuse laterally down the
length of the axon. Thus, factors that
influence the movement of material within a
space with restricted diameter (i.e., current),
influence how quickly Na+ can move from
one axon segment to the next, and thus the
conduction velocity. The diameter of the
axon, thus, influences the ease by which Na+
can move from one axon segment to the
next. Small-diameter axons create more
resistance to current, as the area over which
Na+ can flow is restricted. Thus less Na+
can move per unit time from one segment to
the next, and it takes longer to accumulate
enough positive charges at the next segment
to reach threshold and evoke the action
potential. Conversely, large-diameter axons
facilitate the lateral flow of Na+. Conduction
velocity of a neuron, therefore, tends to be
proportional to the diameter of that axon
(Fig 6.8).
Effects of Drugs on Neurophysiology.
Many chemicals have neurological effects.
Although the specific effects of chemicals on
neurophysiology can differ considerably, there
area two primary ways in which neuron are
affected (Fig 6.9):
Synaptic
communication
Chemically-gated
channels
Action potential
propagation
Synaptic
communication
Voltage-gated
channels
Neurotransmitter
Release
Fig 6.9. Regions of a neuron where neurotoxins may
have an effect. Some neurotoxins affect action
potential propagation by influencing the voltage-gated
ion channels along the length of the axon (between the
dashed lines), whereas other toxins might affect
synaptic transmission of signals from one neuron to the
next by influencing either neurotransmitter release from
the synaptic terminal or by altering the ability of the
dendrites and cell body to bind neurotransmitter and
transduce it into an electrical signal.
1) Function of voltage-gated ion channels –
some chemicals influence action potential
generation by binding to portions of voltagegated ion channels and preventing their
function.
For example, tetradotoxin, a
poison produced by puffer fish, prevents
voltage-gated Na+ channels from opening
when threshold is reached. Another poison,
dendrotoxin (a component in the venom of
black mambas) prevents voltage-gated
K+ channels from opening, preventing
repolarization of an axon during an action
potential and thus greatly increasing the
refractory period for an action potential and
slowing conduction velocity.
2) Synaptic transmission of signals – the
primary means by which most neurons are
stimulated to undergo action potentials is
through the receipt of chemical signals
(neurotransmitters) that activate specific
pathways in the cell that lead to a change in
membrane potential. Different chemicals
can influence the signaling interactions
between one cell and another.
Some
chemicals
alter
the
amount
of
neurotransmitter released from a presynaptic
cell into the synaptic cleft (e.g.,
amphetamines stimulate dopamine release)
or the degradation of the neurotransmitter in
the synaptic cleft (e.g., cocaine blocks
norepinephrine and serotonin re-uptake by
presynaptic cells). Other chemicals bind to
the receptor proteins on the postsynaptic cell
that are responsible for triggering the graded
potentials that would normally determine
whether the neuron would undergo an action
potential. Some of these act as agonists –
they bind to a particular receptor protein on
the surface of the postsynaptic cell and
stimulate the metabolic pathway connected
to that receptor. Since they may supplement
the normal neurotransmitter that normally
activates that pathway, and may not be
degraded as easily as the normal
neurotransmitter, they amplify the effects of
the neurotransmitter (e.g., nicotine mimics
the effects of acetylcholine for certain ion
channels).
Conversely, others act as
antagonists – they bind to a receptor protein
but do not activate the metabolic pathway
connected to that receptor and prevent the
normal neurotransmitter from binding that
receptor, suppressing the normal effects of
the neurotransmitter (e.g., cobratoxin, found
in cobra venom, prevents acetylcholine from
binding to particular receptors).
Observing Action Potentials in Whole Nerves
The computer simulation we will run in this lab
mimics experiments that are conducted on
segments of whole nerves, not on individual
neurons. Importantly, this means that the action
potentials we will record are not action
potentials from a single neuron, but a compound
action potential, showing the cumulative action
potentials of the neurons within the nerve. This
is important because although many of the
neurons in this nerve have the same threshold,
some neurons have thresholds that are slightly
higher than others. Also, because of the way we
are stimulating the nerve (applying an electrical
shock to the outer surface of the nerve), not all
of the neurons in the nerve will receive she same
amount of stimulus. Thus, it is possible to vary
the amplitude of the action potential by varying
the strength of the stimulus. This does not
violate the all-or-none principle! Each neuron
is undergoing an action potential in an all or
none fashion. What is changing is the number
of neurons undergoing action potentials in
response to a stimulus of a particular strength.
Let us imagine that we are stimulating a
nerve segment with progressively more intensive
(i.e., higher voltage) electrical shocks (Fig.
6.10). There are ranges of stimulus intensity
that evoke different amounts of depolarization in
the compound action potential of a nerve. A
subthreshold stimulus is a stimulus with such a
low intensity that none of the neurons reach
threshold, therefore no compound action
potential is recorded. A threshold stimulus is
just intense enough to depolarize a few neurons
in the nerve to threshold and cause them to
undergo an action potential, so a very weak
compound action potential is recorded. If
stimulus intensity is increased above threshold,
the amplitude of the compound action potential
increases as more and more neurons undergo
action potential. We refer to this range of
stimulus intensities in which variable compound
action potential amplitudes can be produced as
submaximal stimuli. Eventually, we will apply a
stimulus intense enough to induce every neuron
in the nerve to undergo an action potential. That
stimulus intensity is referred to as the maximal
stimulus. If the intensity is increased above this
maximal stimulus, no further increase in
compound action potential amplitude will occur,
since all neurons in the nerve are already
undergoing action potential. We sometimes
refer to stimuli with intensities higher than a
maximal stimulus as supramaximal stimuli.
Figure 6.10. Variation in the amplitude of the
compound action potential of a whole nerve with
varying stimulus intensity. The upper plot depicts the
strength of electrical stimuli applied to the nerve, and
the lower plot depicts the amplitude of the resultant
electrical response by the nerve. Note that a threshold
stimulus here is a stimulus with the minimum amount
of intensity to generate a detectable compound action
potential. Increasing stimulus intensity above this
threshold value leads to the generation of bigger
compound action potentials until the maximal stimulus
is reached, whereupon all neurons in the nerve will be
undergoing action potential and the compound action
potential will reach its maximum amplitude.
Experimental Procedures
We will be running a simulation
of experiments conducted on
whole nerve segments using
PhysioEx™
(Benjamin
Cummings).
The software
should be loaded for you and
the screen illustrated in Fig.
6.11 should be displayed. There
are three different pieces of
equipment displayed on the
screen which you should note.
First, there is a nerve chamber.
A segment of a nerve has been
dissected and suspended over a
series of metal bars that act as
electrodes. The two electrodes
at the bottom are stimulating
Fig 6.11. View of the PhysioEx™ screen used for Experiment #1.
electrodes and are connected to
an electrical stimulator. The stimulator can be used to apply electrical current to the nerve at different
voltages and frequencies to try to elicit an action potential. The other set of electrodes are called
recording electrodes, and they are connected to an oscilloscope. Differences in charge between the two
recording electrodes (such as those caused by an action potential passing by) cause the line traced on the
oscilloscope screen to deflect. Thus, we can observe any action potentials forming in the nerve
Experiment 1. Stimulation of an Action Potential.
A. Electrical Stimulation – Observation of Threshold
Initially, the voltage on the electrical stimulator is set to 0.0 V. Increase the voltage to 1.0 V by
clicking on the + button next to “Voltage”. Then click on “Single Stimulus”. Notice that a flat-line
tracing is recorded on the oscilloscope, meaning no action potential was generated. Increase the
voltage by 0.3 V using the + button next to “Voltage” and click on “Single Stimulus” again. A
second tracing should appear in a different color. Repeat this procedure, increasing the voltage by 0.1
to 0.2 V increments until an action potential is recorded. Determine and record the minimum amount
of voltage needed to evoke the action potential. This value is the threshold stimulus. Record this
voltage on your data sheet.
B. Electrical Stimulation – Observation of Compound Action Potential and Determination of Maximal
Stimulus
Continue increasing the voltage above threshold by 0.1-0.2 V increments. Notice that as you increase
the stimulus strength, the amplitude of the action potential increases slightly (Fig 6.12). This is
because as you increase stimulus strength you are reaching the threshold of more individual neurons
in the nerve, more neurons undergo action potentials, and thus the compound action potentials
strength increases. Eventually, though, you will reach a point where no further increase in the
amplitude of the compound action potential will occur. This is because the stimulus strength is now
strong enough for all the neurons in the nerve to undergo action potential. The lowest stimulus
strength required to cause all the neurons in the nerve to undergo action potentials is called the
maximal stimulus. Determine and record this voltage.
Hint: try using the following
method to determine the
maximal stimulus: Once you
reach your threshold stimulus,
increase the voltage by 0.1-0.2
V and apply a stimulus. Then
click on the “Clear” button on
the lower right corner of the
oscilloscope.
Apply another
stimulus with the same voltage
as you had just applied, then
increase the voltage by 0.1-0.2
V and stimulate again. You
should only have two tracings
on your screen—one pink and
the other green. You should
still see two different tracings,
meaning that you have not
reached your maximum action
potential amplitude. Repeat
the procedure. Eventually, you
should reach a point where one
tracing completely overlaps the
other. When you see this, then
the lower of the two voltages
you applied is your maximal
stimulus strength.
Fig 6.12. A pair of compound action potentials recorded on the PhysioEx
oscilloscope. Notice that the green tracing (produced when the nerve was
stimulated at a higher voltage) has a higher amplitude than does the pink tracing.
This illustrates that through the submaximal range of stimuli strengths, action
potential amplitude will increase with increased stimulus intensity.
C. Chemical Stimulation.
Click on the “Clear” button on the oscilloscope to remove previous tracings. Move your pointer over
to the dropper bottle on the left labeled “Sodium Chloride”, left click on the dropper and hold, then
drag the dropper over the nerve chamber and release the left mouse button to apply the NaCl solution
onto the nerve. Notice what happens on the oscilloscope. If we assume that the NaCl concentration
in this solution is greater than that normally found in extracellular fluid, explain why the application
of NaCl to the nerve caused this response.
Click the “Clean” button on the nerve chamber to wash the NaCl solution from the nerve. Then
apply a dropper from the bottle labeled “Hydrochloric Acid” to the nerve. Again, note the response
of the nerve, and provide an explanation for this response. Click on the “Clean” button of the nerve
chamber when you are finished.
D. Heat Stimulation
Place your pointer over the glass rod on the lower left portion of the screen, left click, and drag it
down to the heater just below it. Click on “Heat” to make it red hot. Left click on the heated rod,
drag it over the nerve chamber, and release the left click to apply the stimulus. Note the response of
the nerve on the oscilloscope, and provide an explanation for this response.
Hint: For parts C and D, think about how these substances might influence a) the difference in electrical
charge, ion gradients, and diffusion rates of ions across the cell membranes and b) the permeability of the
cell membrane to different ions, with specific emphasis on the membrane proteins needed to allow ions to
move from one side of the membrane to the other.
Experiment 2. Inhibitory effects
of drugs on action potential
generation.
Move your mouse over the
“Experiment” menu at the top of the
screen. Select “Inhibiting a Nerve
Impulse” by left clicking on that line
in the menu. A new screen will pop
up, which looks very similar to the
screen used in experiment one
except that on the left-hand side
there are now three bottles of drugs
with neurotoxic effects (see Fig.
6.13).
Set the voltage on the
electrical stimulator to a level that
will evoke an action potential (e.g.,
5 V).
Fig 6.13. View of the PhysioEx™ screen used for Experiment #2.
A. Effect of Ether (ethyl ether was the first chemical used as a general anesthetic).
Click the stimulate button on the stimulator and notice the action potential displayed on the
oscilloscope. Transfer a dropperful of fluid from the bottle labeled “Ether”. Click on the stimulate
button again. What is the effect on action potential generation (does an action potential occur in the
nerve once treated with ether)? Based on the effect you observe and the fact that you are using
electrical stimuli to stimulate a segment of nerve containing only axons, what do you think is the
target of action for this chemical
(i.e., synapse or voltage-gated
channels)?
B. Effect of Lidocaine (a substance
found in cloves used as a local
anesthetic and anti-arrhythmic
agent).
Stop the tracing. Click on the
“Clear”
button
on
the
oscilloscope to remove the
previous tracings, and click on
the “Clean” button above the
nerve chamber to wash off the
ether.
Click the stimulate
button on the stimulator and
notice the action potential
displayed on the oscilloscope.
This indicates that the ether
from the previous exercise has
been removed.
Transfer a
dropperful of fluid from the
bottle labeled “Lidocaine”.
Fig 6.14. A view of compound action potential recordings associated with
a nerve stimulated with maximal stimuli before treatment with a neurotoxin
(pink) and after treatment with a neurotoxin (green) in Experiment 2.
Click on the stimulate button once again to attempt to trigger a second action potential. What is the
effect on action potential generation (does an action potential occur in the nerve once treated with
lidocaine)? Based on the effect you observe and the fact that you are using electrical stimuli to
stimulate a segment of nerve containing only axons, what do you think is the target of action for this
chemical (i.e., synapse or voltage-gated channels)?
C. Effect of Curare (a potent neurotoxin produced in the skin of “poison arrow” frogs).
Click on the “Clear” button on the oscilloscope to remove the previous tracings, and click on the
“Clean” button above the nerve chamber to wash off the ether. Click the stimulate button on the
stimulator and notice the action potential displayed on the oscilloscope. This indicates that the
lidocaine from the previous exercise has been removed, and the nerve is once again functioning
properly. Transfer a dropperful of fluid from the bottle labeled “Curare”. Click on the stimulate
button once again to attempt to trigger a second action potential. What is the effect on action
potential generation (does an action potential occur in the nerve once treated with curare)? Based on
the effect you observe and the fact that you are using electrical stimuli to stimulate a segment of nerve
containing only axons, what do you think is the target of action for this chemical (i.e., synapse or
voltage-gated channels)?
Hint: In this last experiment, what does NOT happen is as important as what does happen. Some
neurotoxins may apparently have no effect on the nerve. Keep this in mind, though—we are stimulating
a segment of a nerve which contains only axon tissue (so no dendrites, cell bodies, or axon terminals). We
are also applying an electrical stimulus to the nerve to trigger the opening of the voltage gated ion
channels.
Experiment 3. Nerve Conduction Velocity.
Move your mouse over the “Experiment” menu at the top of the screen. Select “Nerve Conduction
Velocity” by left clicking on that line in the menu. A new screen will pop up displaying a somewhat
different setup than seen in the previous experiments (see Fig. 6.15). The stimulator is now at the top
right of the screen, with the
oscilloscope below it. In addition to
the nerve chamber, there is a “bioamplifier” that must be switched on.
To the left are three nerve segments
and an earthworm. You can sedate
the earthworm with the ethanol
above it if you so desire.
The setup on this screen will
allow you to measure how long it
takes for an action potential to travel
a set distance along a nerve, and
thus
determine
that
nerve’s
conduction velocity (velocity =
distance/time).
The recording
electrodes in the nerve chamber are
set 43 mm away from the
stimulating electrodes. Therefore,
conduction
velocity
can
be Fig 6.15. View of the PhysioEx™ screen used for Experiment #3.
calculated for each nerve by the
following equation:
Conduction velocity (m/sec) =
43 mm
time (msec)
Procedure
You will be determining the conduction velocity for three different nerves: a frog nerve (thin,
myelinated), rat nerve #1 (thin, unmyelinated), and rat nerve #2 (thick, myelinated). Select one of the
three nerves and place it in the nerve chamber. Set the voltage on the stimulator to an amount that
will give an action potential (e.g., 5 V). Click on the “Pulse” button to ready the stimulator, then
click on the “Stimulate” button. An action potential will be recorded on the oscilloscope. Click on
the “Measure” button on the stimulator, and then click and hold on the + sign next to “Time”. Notice
that a yellow line will scroll across the oscilloscope. Using the + and – buttons, position the line so
that it intersects the action potential tracing at the point just when the tracing starts to move above
baseline (Fig 6.16). Record the time displayed at the top right of the stimulator. Divide the distance
the action potential traveled
(43 mm) by this time to
calculate
the
conduction
velocity for this neuron, and
record this value on your data
sheet. Repeat this procedure
for the other two nerves.
Comparing the conduction
velocity of the rat nerve #1
(thin, unmyelinated) with that
of the frog nerve (thin,
myelinated), what can you
conclude about the effect of
myelination on conduction
velocity?
Comparing the conduction
velocity of the frog nerve
(thin, myelinated) with that of
the rat nerve #2 (thick,
myelinated), what can you
conclude about the effect of
axon diameter on conduction
velocity?
Fig 6.16 Recording of an action potential showing the use of the measurement
tool (yellow bar) in order to determine conduction time from the stimulating
electrodes to the recording electrodes.