Ah, the Good Old Days, when “curiosity-driven” science wasn’t naughty.  Of course, one might not want to get too nostalgic when the woman whose work is the subject of this post is referred to as “Miss Lillian Russell” in the running head of the paper.  Apparently, gender and marital status were considered relevant in a scientific publication 90 years ago.

The primary goal of the Solar Sea Slug project is to study the neurobiological specializations of photosynthetic molluscs.  In order to chart the input and output pathways, we need a good roadmap of the central nervous system (CNS) and the nerves that connect it with the rest of the animal.  I had hoped that someone had generated at least a crude diagram, and had found some drawings of the nervous systems of Elysia viridis (Huber, 1993, J. Moll. Stud. 59:381) and E. crispata (Gascoigne, 1972, Trans. R. Soc. Edinburgh 69:137), but had not found much more.  I fully expected to map the peripheral nerves by myself, and we had even done some preliminary experiments with fluorescent labels for axons for Slug Club during spring semester.

As is often the case, the breakthrough came as I was burrowing through the references from one of the older papers.  Huber’s 1993 paper reviews the structure of the CNS of a wide range of gastropod molluscs, and includes a nice diagram of the central nervous system of E. viridis, but provides no information about the periphery.  I had skimmed the section describing the image, but had mostly focused on the figure itself.  He made a passing reference to earlier work by Russell (1929, Proc. Zool. Soc. Lond. 14:197), so, for the sake of completeness, I ordered the paper through inter-library loan, making it clear that I would like them to include any photographic plates, which are often in a section separate from the main body of the paper.

When I downloaded the paper about a day later, I was pleasantly shocked.  As part of a study of the taxonomy of the ascoglossans (= sacoglossans), Lillian Russell had performed a thorough, detailed analysis of the nervous system, nerves, and internal anatomy of E. viridis and a nudibranch, Aeolidia papillosa. She provided an excellent description of the fusion of about seven ganglia to produce the nerve ring that comprises the CNS, along with multiple excellent drawings of the CNS, the peripheral nerves and internal organs of E. viridis.

Plate 4 of Russell (1929). Detailed map of the ganglionic origins and terminations of peripheral nerves that innervate the body wall.

It is hard to know which image is my favorite, but I am very fond of the drawing below, which shows the ganglia in the head, the nerves that originate from them, and the terminations of the nerves in sensory organs such as the eyes and rhinophores.

Russell (1929) plate 1. Detail of the head, including the nervous system and internal organs. Highlights include the eyes (labeled as such), the cerebral ganglia (labeled 1 and 8a), and the rhinophorial nerve (labeled n. 1).

The drawing below, of the CNS and nerves removed from the animal, is also quite impressive, showing the individual ganglia that make up the CNS, and the origins of the many nerves that carry sensory information in, and motor commands out.

Russell (1929) plate 3. Isolated central nervous system and nerves. The ganglia that comprise the CNS can be seen clearly with the esophagus removed.

Aside from publishing a similar description of E. clarki (which probably only differs in slight details), and leaving a note in her will to let me know about it, I can’t imagine how she could have done any more to help me out.  Although I had intended to do the work myself, anatomical description of this quality requires considerable patience and artistic skill.  I am not very patient, nor am I a good artist.  Thanks to Russell’s hard work, I am free to do my behavioral experiments and electrical recordings, and be in my happy place.

So, why is this useful?  In a general sense, it gives a starting place for finding neurons and circuits that generate behavior.  If you want to know where sensory information goes in the nervous system, finding the nerve that carries the axons from the relevant sense organ or part of the body is a good start.  Same goes for finding the motor neurons that cause muscles in a certain part of the body to contract.  As a specific example, if we are trying to find how information about light gets to the brain, we first need to find the nerves that connect to the eyes or (hypothetically) other sense organs that detect light or photosynthetic activity. Then we can record monitor impulse activity in the nerves, or from the specific neurons with axons in those nerves.

This is a nice reminder about a few things.

1. Possibly most importantly, we truly are standing on the shoulders of others as we try to move science forward.  I hesitate to say “on the shoulders of giants,” because, for all I know, Lillian Russell was of average height.
2. You really never know when something you did will be useful to another scientist.  Ninety years ago (90!), nobody knew about kleptoplasty, and the study was carried out because nervous system structure was considered to be a good marker for evolutionary relationships.  It is not reasonable to argue that every experiment that can be imagined must be performed, but during this time in which research needs to be either “translational” or “transformative” in order to be funded, it would be nice to make a tiny bit of room for work that is simply of high quality.
3. It may be necessary to dig deep into the literature to avoid reinventing the wheel.  Many of us are guilty of looking over the figures of a paper and skimming the text, rather than reading every word.  Time is short and papers can be long.   Nonetheless, some of the most relevant information to people in the field is in the body of the text, rather than in figures or tables.  Who knows what else I will find as I work my way through the old literature.

The Solar Sea Slug project has progressed by leaps during the past half year or so.  In addition to finding the roadmap described above, and establishing a self-sustaining colony of E. clarki, some of the students in the Neurobiology Lab course helped me to work out procedures for recording from neurons in the CNS.  There are still a few improvements  to be made, such as more easily getting the electrode through the sheath that surrounds the ganglia, but we have a little real data.  Below is an action potential that was stimulated by injecting 1 nA of current into a large neuron in the abdominal ganglion (Pierce’s “Parker Cell?”).

Action potential recorded from a large neuron in abdominal ganglion of Elysia clarki. 4/19/18

How are we doing?

Self-sustaining culture: 

Neuroanatomical roadmap:

Dissection techniques: 

First intracellular recording:

This project is really starting to take off.