Biological Sciences 300/301, Smith College | Neurophysiology

Checklist for Lab 8:
Reading and Writing Assignment on
the Crayfish Swimmeret System.

UPDATED: March 17, 2016


Lab 8 is a prepared discussion of the crayfish swimmeret system. It includes a writing assignment that is due at the start of your lab afternoon. Our rule is that both you and your paper must be present in lab for your paper to receive a letter grade. Late or absent papers are graded P/F.

The online Web pages give background information including:

visual material on Crayfish Neuroanatomy,
Tips on Writing about Neuroscience, and
Reading Notes for one of the papers.
Before you read the two articles, carefully read through the online material and look at all of the online figures. This will give you an overview of the swimmeret central pattern generating system.


Two readings will be distributed in class to serve as the basis for our discussion. It is useful to discuss the readings with your classmates, but you must write your paper by yourself.

For background on the swimmeret system read:

Mulloney, B and C Smarandache-Wellmann (2012) Neurobiology of the crustacean swimmeret system. Progress in Neurobiology 96: 242-267. Read only the provided excerpt, pages 242-252. The full paper is available on our Moodle site, where you can see the color figures and browse in other sections.

To prepare for a detailed discussion of experimental material, read:

Tschuluun, N, WM Hall, and B Mulloney (2009) State-changes in the swimmeret system: a neural circuit that drives locomotion. J. Exp. Biol. 212: 3605-3611. This paper is also available on our Moodle site.

Read the Tschuluun et al. article very closely, paying particular attention to experimental details. The READING NOTES will help you understand the experimental methods. Try to imagine exactly how each step of the experiment was done, from dissecting the preparation through data-analysis. Ask yourself what point each figure is making, and how its components contribute to that point. This is the article we will discuss most closely in class.

TIPS: A recent class suggested these tips about aspects to emphasize in reading these two papers.

Mulloney review paper:
Section 1.1 and Fig. 1:  Focus on the abominal nervous system and swimmeret anatomy.
Fig. 2:  In addition to the swim cycle in B & C, make sense of the over-all pattern in D.
Fig. 3:  (A) shows the LN location; note where the neurons in B - D branch and make or receive synapses.
Section 2 (circuits):  Keep track of the big picture, and the central pattern generator components.
Fig. 4:  Depolarization of the interneurons will release more [inhibitory] transmitter; connect that concept to the changes in the RS and PS motorneurons' firing.
Section 3.1 and 3.2, and Fig. 6:  Keep track of the big ideas about command neurons, without being buried in the details.
Section 3.3:  The information about modulators is useful for planning lab projects, but not every crustacean example is essential.

Tschuluun et al.:
Fig. 2:  Note that the bottom trace shows currents, not potentials; translate these into depolarizations and hyperpolarizations, and work out how these line up with the PS bursts for a PSE (A) and an RSE (B) motorneuron.
Fig. 3:  Translate the resistance axis into [inhibitory] postsynaptic channels open, and see how that lines up with PSE and RSE bursts.
Figs. 4 & 5:  Note the time scale compared to Fig. 2, and understand that the cycling currents in Fig. 2 will be squished together in these figures and look like a thick line.

OPTIONAL RESOURCES are available to provide further details of the swimmeret system and its pharmacology:

Braun, G and B Mulloney (1993) Cholinergic modulation of the swimmeret motor system in crayfish. J. Neurophysiology 70 (6): 2391-2398. [PDF] (Detailed pharmacology of the muscarinic and nicotinic modulation of the CPG system.)

B. Mulloney, B (1997) A test of the excitability-gradient hypothesis in the swimmeret system of crayfish. J. Neuroscience 17(5):1860-1868. [PDF] (Additional experiments with the cholinergic agonist carbachol. Figure 2 is useful for carbachol dose information.)

Mulloney, B and C Smarandache (2010) Fifty years of CPGs: two neuroethological papers that shaped the course of neuroscience. Frontiers in Behavioral Neuroscience 4: article 45 (July 2010). (A review of central pattern generators for crustacean swimmerets and locust flight.)


1. Write a three-to-four page draft paper that you bring with you to lab.

In your paper, give a brief overview of the swimmeret system drawn from the background reading. You can either describe the system, or draw your own informative summary diagram with a detailed caption. Continue your analysis by addressing the following issues; try to make your discussion a continuous narrative about the swimmeret system rather than a series of answers to unconnected questions.

1. A classic diagram of a CPG model is shown above. At the top, unpatterned input excites (arrows) interneurons 1 and 2, which inhibit each other (circles). The interneurons in turn excite motor neurons (arrows at the bottom; the neurons are not shown). But in the swimmeret CPG circuit, the rhythmic depolarization and hyperpolarization of the motor neurons (to create bursts of spikes during depolarization) is attributed principally to inhibition. How does the evidence in the Tschuluun et al. paper support a principal role for inhibition in creating rhythmic firing of motor neurons?

2. Acetylcholine affects the motor neurons directly and also through synaptic inputs. How do Tschuluun et al. show this? Yet the swimmeret circuit diagram does not show a source of acetylcholine, a potentially puzzling omission. Searching for neurons that participate in rhythmic activity typically relies on finding neurons that demonstrate the rhythm, but what if the acetylcholine neurons were non-rhythmic? Would a neuron that releases acetylcholine steadily during swimmeret activity be compatible with the existing evidence?

Additional advice: Cite the sources of your information using the (author, date) format. Cite only the most important facts or ideas, not every detail. (Since there are only two articles, don't overdo the citations.) Do not cite the instructor or the website, although you may use information from that source. Consult Tips on Writing about Neuroscience for further advice, including warnings about frequently misused words. (Using a listed word incorrectly will be taken as a sign of extreme indifference to the quality of your work!)

Lab Projects

2. In lab, after discussing the project with your lab partner(s), write a single paragraph that briefly describes an experiment on the swimmeret motor system that will be your group's lab project. Also include a list of drugs that you will need and their approximate concentrations. Put all partner's full names on the page. The paragraph's purpose is to describe what you are thinking of doing as a first step in shaping a realistic project. It does not have to be your final plan.

In planning your project, assume that you will employ extracellular electrodes and recording equipment that you have already used in lab. Use carbachol or another agonist to elicit the swimmeret motor pattern. It is not necessary to give experimental details beyond the overall plan of the experiment. Your experiments do not have to be new. For example, trying to record simultaneously from both sides of a ganglion or both branches of N1 can be an interesting challenge. If you are thinking of a pharmacological experiment, limit yourself to one drug. You can also use a drug (such as carbachol) to initiate the rhythm while you investigate some other aspect (like the timing between ganglia). Keep your experiment simple. You'll want to be able to repeat experiments, and we have only a few weeks.

Abstracts of lab projects from recent classes are available on our Moodle site.


3. One week after the discussion, submit a final version of your paper. Your grade will reflect the quality of both your draft and your final versions.