LEB
01.07
Neurobiology: the nerve cell
Principle and task
To use a nerve function model to study the following
aspects of a nerve cell:
Equipment
Neurobiology Lab
PC, Windows® 95 or higher
65963.11 1
• INTRACELLULAR POTENTIAL AND ACTION POTENTIAL
•
•
Comparison between low and high threshold levels
Comparison between low and high stimulus levels
• MEMBRANE TIME CONSTANT AND LOW-PASS FILTERING
•
•
Membrane time constant
Low-pass filtering
• EXCITATORY SYNAPSE
•
•
•
•
•
Depolarisation
Temporal summation
Spatial summation
Synaptic amplification by terminal branches
Effect of decreasing stimulus
• HEBBIAN SYNAPSE
•
Synaptic learning and forgetting
Setup and procedure
INTRACELLULAR POTENTIAL AND ACTION POTENTIAL (FIG. 1)
Internet search keywords:
Intracellular potential, resting potential, action potential,
stimulus, nerve cell, neuronal stimulation.
Action potential arises by influx of sodium ions through
the sodium channels of the nerve cells. Stimulus movement along the axon occurs due to the consecutive influx
of sodium ions along its cell membrane.
The measurement method of this experiment differs from
that of the other experiments so that action potential can
be displayed (together with intracellular potential). In the
other experiments (with the exception of the experiments
dealing with the excitatory synapse) presynaptic signal
strength is shown instead, along with intracellular potential.
• INHIBITORY SYNAPSE
•
•
Hyperpolarization
Spacial inhibitory-excitatory summation
• VETO SYNAPSE
Typical experimental setup. To set up the experiments, use the corresponding setup drawings.
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Experiment setup according to Fig. 1 and Fig. 2:
Software: select the fast measurement mode (trigger
+25%, rising, data transfer to "Analog in 2", frequency
10 kHz, 1024 values, show channels "Analog in 1" and
"Analog in 2", X data: time, range ±10V for "Analog in 1"
and ±0.1V for "Analog in 2").
Fig. 2: Window for settings
The two experiments desribed here show the effect of the
threshold level of the Neurosimulator and the stimulus
level emitted by the operating unit.
Fig. 1: Experimental setup
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Neurobiology: the nerve cell
a) Comparison between low and high threshold levels
Graph: maximum stimulus intensity (turn knob of operating unit to the right) and low threshold (here: 0) creates
fast frequency of action potential (see Fig. 3).
b) Comparison between low and high stimulus levels
Perform the experiment comparing low and high stimulus
intensities with one another, while keeping threshold at 0
(turn threshold knob to left).
Graph: same stimulus intensity, but this time, increase
threshold level, therefore lower frequency of action potential (see Fig. 4).
Fig. 3
Fig. 4
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Neurobiology: the nerve cell
MEMBRANE TIME CONSTANT AND LOW-PASS FILTERING (FIG. 5)
Internet search keywords:
Resting potential, membrane time constant, low-pass filtering.
Fig. 6: Window for settings
Experiment setup (see Fig. 5 and Fig. 6):
Here, as in all the other experiments in which the intracellular potential is measured together with a stimulus level,
choose the normal measurement mode with the following
settings: get value every 2 ms, start and end measurement on key press, show channels "Analog in 1" and
"Analog in 2", X data: time, range ±10 V for both "Analog
in 1" and for "Analog in 2", select the following displays:
digital displays 1 and 2, diagram 1.
After clicking on button "Continue", an intracellular resting
potential of -7 V is shown, which is 100 times the resting
potential in a real nerve cell.
Fig. 5: Experimental setup
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LEB
01.07
Neurobiology: the nerve cell
a) Membrane time constant (see Fig. 7)
Graph: membrane time constant. Since nerve membranes
have capacitative properties and electrical resistance,
intracellular potential behaves like the charging and discharging of an electrical capacitor. In nerve cells, the time
constant is 10 to 50 ms, i.e. this amount of time is needed for the intracellular potential to reach 63% of its highest level.
Graph: low-pass filtering. Low stimulation frequency:
intracellular potential changes can be fully reproduced.
High stimulation frequency: individual stimulations are not
reproduced any more and the intracellular potential
remains unchanged.
b) Low-pass filtering (see Fig. 8)
This phenomenon is used to allow that fast and shortterm signals are decreased and that intense (slow and
long) signals can be transmitted = filtering of low-pass
signals.
Fig. 7
Fig. 8
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Neurobiology: the nerve cell
EXCITATORY SYNAPSE (FIG. 9)
Internet search keywords:
Depolarization, summation, temporal summation, spatial
summation, EPSP.
Setup is as in "Intracellular potential and action potential“.
Software parameters are as in "membrane time constant
and low-pass filtering“. Threshold = 0.
Depolarization (see Fig. 10)
Stimulations via excitatory synapses depolarize the cell
membrane of the intracellular potential, i.e. the voltage
gradient between inside and outside the nerve cell membrane becomes less negative.
Graph: medium stimulus level. One stimulus. The intracellular potential builds up briefly and degrades again, as
shown in "membrane time constant and low-pass filtering“.
Fig. 9: Experimental setup
Fig. 10
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Neurobiology: the nerve cell
b) Temporal summation (see Fig. 11)
Temporal summation makes use of the integrated loudspeaker of the operating unit (acoustic monitor). The signal helps find the level of stimulus at which no acoustic
signal is emitted when pressing the stimulus button very
briefly – calibrate carefully by turning the stimulus knob
counterclockwise and testing by pressing the button.
Now, without changing the level of stimulus again, press
the button for a longer time. The acoustic signal will again
sound. Pressing the knob for a longer time is identical with
multiple stimuli.
Graph: temporal summation: the stimulus is so low that
no action action potential is created. Only multiple stimuli
create action potential. No significant increase of intracellular potential.
d) Synaptic amplification by terminal branches
Again here, acoustic determination of action potentials.
As in the two previous experiments, the position of the
stimulus knob is determined at which no signal is emitted,
i.e. no action potential is created. Then an additional cable
(white) is attached to connect the excitatory synapse,
which is connected to the stimulus, with the second excitatory synapse. This step is repeated again with the third
excitatory synapse. Each time the signal gets more pronounced.
c) Spatial summation
Spatial summation again makes use of the integrated
loudspeaker of the operating unit again. As in the previous
experiment, the position of the stimulus knob is determined at which no signal is emitted, i.e. no action potential is created. This is done for a second stimulus channel
which is connected to a second synapse (i.e. two white
cables are now required instead of one). Then both stimulus buttons are pressed at the same time, creating action
potentials.
Fig. 11
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e) Effect of slowly decreasing stimulus (see Fig. 12)
Decreasing stimulus leads to decrease of action potential
frequency and reduction of depolarisation.
Left measurement: maximum stimulus S frequency of
action potential high and hyperpolarized intracellular
potential.
Right measurement: stimulus is reduced S frequency of
action potential and intracellular potential back to normal.
HEBBIAN SYNAPSE (FIG. 13)
Internet search keywords:
Hebbian synapse, synaptic learning, synaptic plasticity.
Fig. 12
Fig. 13
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Neurobiology: the nerve cell
Synaptic learning and forgetting (see Fig. 14)
In the anatomical sample the Hebbian synapse is located
at the end of dendritic spines. It is an excitatory synapse
with variable transmission behavior.
To perform the experiment, turn threshold button to left (0)
and turn stimulus buttons of channels 1 and 2 to 75%.
The first 6 spikes in the graph show: consecutive activation of the Hebbian and excitatory synapses.
The next 12 spikes are created by simultaneous activation
of the Hebbian and excitatory synapses and increase of
depolarization.
The next 3 spikes show activation of the Hebbian synapse
which is now above the level of initial activation.
The last 2 spikes show activation of the Hebbian synapse
after pressing the reset button to initiate synaptic forgetting: the Hebbian synapse has unlearnt the properties
which it learnt when coupled with an excitatory synapse.
Longterm potentation: activation of Hebbian synapse by
complementary excitatory synapse can last several minutes up to several hours. In the Neurosimulator activation
lasts about 10 minutes, unless the reset button is pressed.
INHIBITORY SYNAPSE (FIG. 15)
Internet search keywords:
IPSP, hyperpolarization.
Fig. 14
Fig. 15
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In the anatomical sample the location of the inhibitory
synapse is in the shaft of the dendrite of a nerve cell.
Effect similar to excitatory synapses, however, its effect is
inverse: the negative polarisation of the intracellular
potential increases further (> 70 mV) = hyperpolarization
= reduced excitation.
For the two experiments, wire stimulation channels 1 and
3 with the two inhibitory synapses and stimulation channel 2 with one excitatory synapse.
a) Hyperpolarization (see Fig. 16)
Graph: activation of stimulation channel 1 (which is connected to an inhibitory synapse). Hyperpolarization.
Thereafter return to resting potential.
b) Spacial inhibitory-excitatory summation (see Fig. 17)
Sequential activation of all three stimulation channels to
demonstrate spacial inhibition. At first activation of the
excitatory synapse (depolarization), thereafter inhibition in
two steps, by activating at first one, then the second
inhibitory synapse. Keep buttons pressed.
Setting up the experiment: medium setting for stimulation
channels 1 and 3, maximum setting for stimulation channel 2 (which is connected to the excitatory synapse).
Yellow cable which connects to computer interface must
be plugged into stimulation channel 2.
Fig. 16
Fig. 17
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LEB
01.07
Neurobiology: the nerve cell
VETO SYNAPSE (SEE FIG. 18)
Internet search keywords:
Veto synapse.
The veto synapse does not bring about a change of the
intracellular potential (so-called silent inhibition). Its effect
is only on the excitatory synapse to which it is attached
(so-called presynaptic inhibition or shunting inhibition). No
spatial summation.
Experimental setup: Connect stimulation channel 1 to
excitatory synapse and stimulation channel 2 to veto
synapse. The stimulation channel for the veto synapse is
connected to the interface to show in the graph when the
veto synapse is stimulated.
Graph: threefold activation of the veto synapse while the
excitatory synapse is continuously stimulated (Fig. 19).
Fig. 18
Fig. 19
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Neurobiology: the nerve cell
Phywe Series of publication • Laboratory Experiments Biology • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen