#8: Earthworm Action Potential

Question 1

Earthworm Neural Anatomy

Answer 1

The major component of an earthworm neural system is the ventral nerve cord. It runs along the length of the worm and is located on the inter-ventral surface.
- Contains three giant axons and some smaller nerve fibers.
+ If we apply a voltage stimulus to the earthworm, the first fiber to get excited is the median giant axon.
+ There are two giant nerve fibers lying on each side of the median giant axon. They’re smaller in size and function as a single fiber because of the extensive cross connections between them.

Each giant fiber is made of large individual cells that are electrically coupled to each other through gap junctions. Bring experimental advantage of earthworm setup, since these fibers can be stimulated by electrodes placed on the body. A single action potential can be recorded with an earthworm, right on the surface.

Question 2

Intracellular vs Extracellular Recording

Answer 2

To study a generated action potential, one can measure a single transmembrane potential by inserting a glass pipetium into one cell and recording the potential changes with respect to extracellular referenced electrodes. This technique is called intracellular recording. Can be used to directly measure resting membrane potential or action potential of an excitable cell.

Extracellular recording is a more convenient method. No direct access to membrane potential or action potential, but instead places on electrode in close proximity to the excitable cell and will study the potential changes at the membrane surface. It does not see the changes across the cell membrane of the excitable cell.

Both recordings differ in results in several important ways.

1) Shape of waveform depends on placement of electrodes.
2) Information obtained.
3) Size and shape.

Question 3

Biphasic Waveform

Answer 3

Action potentials recorded both ways are biphasic. Means both have a positive and negative deflection. Each deflection generated for a different reason.

Question 4

Recorded Electrical Difference

Answer 4

What’s recorded in this experiment is the electrical differences between two electrodes placed along the axon. These two electrodes we sometimes refer to as negative and positive.

Before stimulus is delivered, both wires should be measured at a equal or similar voltage. So there’s no deflection recorded because the amplified takes the differences of these two inputs.

The situation does change when action potential travels along the nerve. When it reaches first recording electrode, this electrode becomes more negative compared to the distal electrode, allowing measurement of the difference. As action potential proceeds between both electrodes, the recording potential returns to baseline level. As the action potential passes through second electrode, a deflection of the same size, but opposite sign, will be recorded. Sign is negative because weight amplifier compares these two inputs.

Once action potential passes both electrodes, signal will return back to the baseline.

Question 5

Stimulus Artifact

Answer 5

When you stimulate the earthworm, there is a peak that shows up right after the stimulus on your graph. This is NOT an action potential, but a stimulus artifact. Always occur even with the smallest stimulating voltage that’s not high enough to generate an Action Potential.

May increase as you increase stimulating voltage.

Question 6

Finding Threshold of Median Giant Fiber

Answer 6

Start from a stimulus high enough to fire an AP.

Halve the stimulus, if you still get response, decrease it again by a half until you don’t see anything.

Then increase the final value by 0.05v till you find the threshold.