Biological Concepts

• Nerve Impulses

• Rapid Escape Reflex

• Ventral Nerve Cord

• Regeneration

• Giant fibers

Class Time

This activity will require about 20 - 30 minutes for the preliminary observations which may be done on the day prior to VNC transection or on the same day during extended periods. Another 30 - 40 minutes are required to carry out the transection and make observations immediately following. Short observations (10 - 15 minutes) will be required for the next 2 or 3 days.

Background Information

Eisenia is not as easily dissected as Lumbricus due to the small size, but it is very similar anatomically. An advantage to using Eisenia is that it can easily reproduce in the lab, which makes it very useful for developmental studies. The students should know about the structure and function of the VNC and conduction through the medial (anterior to posterior) and lateral (posterior to anterior) giant nerve fibers. The rapid escape reflex can be elicited by touching the anterior or posterior end of the worm with a blunt probe. Before attempting this transection, a general review of the annelid anatomy should be carried out.

Materials

• Redworms, commonly used in vermiculture and composting, are available from biological supply companies. Several should be available per student team of 2.

• Dissecting microscopes

• 1 or 2 pairs of very fine tipped stainless steel forceps

• Very fine tipped dissection scissors, the smaller the better

• Ice; some large flat blocks or ice packs

• Paper towels

• Spring water

• Containers for worms

• Blunt probes

• Student notebooks

Teacher Preparation

Large numbers of E. foetida should be available and easily maintained with organic material and potting soil. To avoid damage and to obtain good initial observations, loop the worm around a probe rather than use forceps to pick it up. The following is not a precise procedure because it is very difficult to isolate the nerve cord without practice and the use of microdissection scissors. However, because the redworm regenerates very rapidly (within a day or two) all other structures that may get damaged in the process will quickly be repaired. Eisenia is thermophillic and responds well to cooling. Prior to the lab, the worms may be placed in the refrigerator to slow them down instead of placing them in ice as described in step #1 of the student activity.

During this activity, students will be expected to ask questions, make predictions and decisions, and draw conclusions about how transection of the nerve cord will affect the redworm. This is also a good opportunity for students to learn the importance of keeping a lab notebook and making carefully documented observations.

Extensions/Variations

• Use Lumbricus instead of Eisenia.

• Compare the site of transection (anterior to posterior) and the recovery time.

• Discuss spinal cord injuries in humans with regard to the effects and treatment.

• Compare the ability of the redworm to regenerate the ventral nerve cord with the inability of humans to repair damage to the dorsal nerve cord.

• Discuss the mechanism for repair in Eisenia.

• Discuss the ethics of using animal models.

• Discuss the rapid reflex behavior in terms of proximate and ultimate mechanisms

• Observe the direction of the nerve impulse through the severed VNC by using the methods described by Jones and Little, Ion Flow in Neurons. Neurobiology Module, Woodrow Wilson National Fellowship Foundation, High School Biology Institute. 1996.

Resources

Drewes, Charles D., Vining, Elizabeth P. and Zoran, Mark J. 1988. Regeneration of rapid escape reflex pathways in earthworms. Amer. Zool. 28: 1077-1089.

About The Author

Carol Bernon is a biology teacher at Barnstable High School, 744 West Main Street, Hyannis, Massachusetts 02601. Use the e-mail address gbern@capecod.

NAME:___________________________ DATE: ___________ PERIOD _______

REGENERATION OF THE VENTRAL NERVE CORD IN THE REDWORM

Introduction

In the following activity, the common redworm, Eisenia foetida, is used as a model to study the effect of damage to the nerve cord and its subsequent regeneration. Review the anatomy of the annelid and take notes about the location and function of the ventral nerve cord. Look up any new vocabulary terms and familiarize yourself with those terms. Keep a lab notebook with a careful account of all observations. Date all entries and make sketches when appropriate.

Carefully remove a worm from the culture container by looping it around a probe. Rinse it with spring water and place it on a moistened paper towel. Make careful observations and describe how it moves. Locate the anterior, posterior, dorsal and ventral surfaces.

To examine the rapid escape reflex, touch the anterior end lightly with a probe and note what happens. Repeat the procedure by touching the posterior end. Was there any difference? Describe it. Touch the anterior end of the worm repeatedly with the probe and note the movement in response to the stimulus. Repeat this procedure with the stimulus directed at the posterior end. Note all observations carefully in the notebook.

Please carefully read the directions on the following page before beginning the activity.

Problem: What effect will cutting the nerve cord of the redworm have on its movement and rapid escape responses?

Materials: Redworms, ice, paper towels, forceps, dissection scissors, stereomicroscope (dissecting microscope), container for worms.

Procedure:

1. Prepare a bag of ice in a dish and cover it with a paper towel moistened with spring water. Place the worm on the towel to cool it for 5 - 10 minutes. This will slow the worm down.

2. Use a large piece of ice or an ice pack covered with moist paper towel. Place the worm ventral side up on the covered ice pack and examine it carefully using the dissecting microscope. Determine the area along the ventral surface where you will transect the ventral nerve cord (refer to the diagram and note the general location.)

3. Formulate questions about how the worm’s behavior will be affected if the nerve cord is cut at a particular site. Make a decision with your partner and choose a transection site.

4. When working in teams, one student can hold strips of moist paper towel across the worm just above and below the transection site. This keeps movement to a minimum. Using fine tipped forceps, carefully lift the skin from the ventral surface just above the area to be cut. Then, with the tip of the scissors, carefully snip through the ventral surface deep enough to cut through the ventral nerve cord without damaging the digestive tract. (Don’t worry if you do, they’ll recover!) Some damage to the ventral blood vessel will be unavoidable.

5. Note carefully and make sketches in your notebook to indicate exactly where the cut was made in relation to the anterior (or posterior) end. Include predictions about what affect this particular cutting site will have on the responses and recovery of the worm.

6. Remove the worm from ice and place on it fresh moist paper towel in a clean container. Allow it to return to room temperature (5-10 minutes).

7. In order to establish controls, discuss with your partner a procedure that might be carried out on other worms for comparison. Write down your ideas in your notebook. If you decide to carry out this procedure, present your idea(s) to your instructor for approval.

Results:

1. Observe the movement of the worms. If you successfully transected the neve cord, the worm should drag the portion of of the body below the cut site. Describe the movement in your notebook. Carefully make notes of any changes in rapid escape reflexes and other responses to touch. Compare these observations with the control worms and note carefully any differences.

2. Determine if and when regeneration and recovery of the ventral nerve cord has occurred.

Conclusions and Follow Up:

• How did your results compare with your predictions?

• What can you do to validate your results?

• Summarize this activity in the form of a lab report and include research on how the redworm is able to regenerate.

• What new questions do you have as a result of carrying out this activity?

• Design another lab to answer the above questions.

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