The most common cause for poor data quality when performing the Earthworm Action Potential experiment is improper experimental setup. Check that the electrodes were configured as follows:
- Connect the stimulating electrodes to the two head-end pins as shown in the image below. The negative lead wire (cathode) should be posterior to the positive lead wire (anode).
- Place the three silver wires over the body of the worm, with the chlorided (blackened) ends in contact with the body.
- Connect the wires to the alligator-clip electrodes as shown. Fix the clips in position using modeling clay or similar material.
- The common (or earth) wire should be the closest to the stimulus pins, but its exact placement is not critical.
- Position the negative recording lead (white or R1) anterior to the positive lead (black or R2) and approximately 10 mm apart.
- Make sure that no wire is touching another wire.
NOTE: Be careful when positioning the wires to avoid damaging the worm. Silver wire is quite malleable, so it can be easily manipulated if you need to alter the electrode position (alternatively, position the wire by moving the alligator-clips around on the modeling clay).
If the Action Potential signal appears inverted, switch the positive and negative lead wires. While Action Potentials have no “true” polarity, we usually have them deflect upward to make it easier for students to measure.
Another source of recording issues can be the worm itself.
- Do not over or under anesthetizing the worm; it needs to still be a bit active, but not so much as to pull itself off the pins.
- Never use MS-222 as an anesthetic in worms since it destroys the action potentials. Ethyl alcohol (ETOH) is inexpensive and works well as an anesthetic with worms.
- Earthworms (of at least 60 mm in length) are needed, larger worms are better for this experiment. Retailers such as Walmart offer large night crawlers which works well for this experiment.
