Derivation of nerve and muscle potentials by mechanical
stimulation at the front end of an earthworm
TEB
Related topics
Nerve and muscle potentials, positive feedback, synaptic depression, synaptic facilitation, conduction velocity in median and lateral giant nerve fibres
Principle and task
To work on the following themes by measuring nerve and muscle potentials:
• The difference in sensitivity of the front and rear ends
• The facilitation effect
• Conduction velocity and fibre diameter
• Synaptic depression
Equipment
1 Cobra3 Basic Unit
1 Power supply, 12 V
1 RS232 data cable
1 Cobra3 Universal Recorder software
1 Bio-amplifier
1 Earthworm experiment chamber
1 Stimulus bristle, triggering
2 Connecting cord, 32 A, l = 25 cm, red
2 Connecting cord, 32 A, l = 25 cm, blue
2 Connecting cord, 32 A, l = 25 cm, black
1 Crocodile clip, insulated, black
1 Adapter BNC-plug/4-mm-sockets
Aluminium foil
Earthworms
PC, Windows® 95 or higher
12150.00
12151.99
14602.00
14504.61
65961.93
65981.20
65981.21
07360.01
07360.04
07360.05
07276.05
07542.26
Fig. 1: Experimental set-up
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TEB
Derivation of nerve and muscle potentials by mechanical
stimulation at the front end of an earthworm
Set-up
— Connect the instruments as shown in Fig. 1.
— Place the earthworm experiment chamber on aluminium foil and remove the lid
— Connect the bio-amplifier AMPLIFIER IN to the pins or the sheet metal of the chamber, so that the +
electrode is 3 cm and the - electrode 4 cm away from the rear end of the worm. Fix the sheet metal
for earthing the worm between the lid of the chamber and the earthworm (Fig. 2)
— Connect the earthed socket of the bio-amplifier to the earthing socket of the chamber, and to the aluminium foil (with the crocodile clip)
— Connect the bio-amplifier AMPLIFIER OUT to Cobra3 ANALOG IN 2 (red to +, blue to -)
— Connect the stimulus bristle to the bio-amplifier TRIGGER IN
— Connect the bio-amplifier TRIGGER OUT to Cobra3 ANALOG IN 1: the red (or most central) jack of
the Adapter with the upper yellow jack (+) of Analog In 1; the black (or lateral) jack of the Adapter with
the white jack ( ) of Analog In 1
— Set the bio-amplifier to EMG, amplification 1000 times
Fig. 2: Earthworm experiment chamber
Procedure
— Call up the COBRA3 MEASURE programme in Windows
— Select the UNIVERSAL WRITER as measuring instrument
— Set the measurement parameters (see Fig. 3) and go to measurement with CONTINUE
Fig. 3: Measurement parameters
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PHYWE Systeme GmbH & Co. KG © All rights reserved
P4010211
Derivation of nerve and muscle potentials by mechanical
stimulation at the front end of an earthworm
TEB
— Wash and dry the earthworm and place it in the groove in the chamber. Prevent the earthworm from
crawling out of the chamber by appropriately positioning the lid and rubber bands, and the pieces of
sponge rubber at the two ends of the groove
Experiment 1:
— Insert the stimulus bristle through the hole in the chamber lid and weakly stimulate the front end of the
earthworm, save the result
Experiment 2:
— Click on the red point and go to measurement with Continue
— Repeat the measurement procedure, but this time with strong stimulation. To effect strong stimulation,
strongly dent the front end, or touch the tip of it. Wait at least one minute between each stimulation.
Save the result.
Experiment 3:
— Click on the red point and go to measurement with CONTINUE
— Stimulate the front end strongly and more frequently (several stimulations must follow in direct succession). Save the result.
Results and evaluation
— Experiment 1: The median giant fibre reacts to weak stimulation with a biphasic action potential (Fig.
4). This is followed 1-2 ms later by the nerve impulse of a giant motoneuron, which is connected to
muscle cells of the longitudinal musculature and releases subsequent muscle potentials.
To estimate the conduction velocity v, take the conducting time t as the distance between the maximum and minimum of the action potential (0.47 ms in Fig. 4). The stretch covered s, i.e. the distance
between the electrodes, is 1 cm. Using the equation v = s/t, the conduction velocity is found to be
21.3 m/s.
— Experiment 2: With infrequent but strong stimulation at the front end, the response can be amplified
by a positive feedback. An interneuron, which reacts excitably back on the median giant fibre, releases a second action potential about 10 ms later (Fig. 5).
— Experiment 3: With frequent and strong stimulation, a synaptic depression which suppresses reflexes
can be observed (Fig. 6). Muscle activity does not occur.
Fig. 4: Result with weak stimulation
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Derivation of nerve and muscle potentials by mechanical
stimulation at the front end of an earthworm
Fig. 5: Result with infrequent strong stimulation
Notes
— On Experiment 1: A single action potential is sufficient to release muscle activity. The front end is
more sensitive than the rear end, where several action potentials are required for muscle contraction.
The higher conduction velocity of the median fibres compared to that of the lateral fibres can be explained by the larger conducting cross-section and so to a quicker propagation of the excitation.
— On Experiment 2: The synapses between the giant motoneurons and the longitudinal muscles exhibit
the phenomenon of synaptic facilitation. This can be recognized from the fact that the muscle potentials are distinctly higher after the second action potential.
— On Experiment 3: Synaptic depression is biologically very meaningful, as otherwise no controlled
movement would be possible in soil (permanent stimulation from sand particles and stones).
Fig. 6: Result with frequent strong stimulation
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