Biological Sciences 300, Smith College | NeurophysiologyLab 8: Discussion of the crayfish swimmeret system.http://www.science.smith.edu/departments/NeuroSci/courses/bio330/labs/L8discussn.html REVISED: March 17, 2016 |
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NOTE: An updated version of this laboratory, along with the Anatomy Supplement and the Appendices, is now available in updated form in another course: BIO 330: Research in Cellular Neurophysiology. |
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Writing
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In place of an experiment, this week's laboratory will involve a discussion and a writing assignment based on readings about the crayfish swimmeret central pattern generator (CPG). The discussion will help us design a class project for the remaining weeks of the semester. The writing assignment and readings are described in detail in the Lab 8 Checklist. The information below will help you to understand the system. |
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Cross section of N1 showing bundles of motor and sensory axons. |
An abdominal ganglion, showing the lateral neuropil (LN) where the swimmeret motor neurons are located, the first roots (N1) carrying axons from the motor neurons, and examples of extracellular recordings from the return-stroke (RS) and power-stroke (PS) branches of the first root. Source: Mulloney B, Skinner FK, Namba H, Hall WM (1998) Intersegmental coordination of swimmeret movements: mathematical models and neural circuits. Ann N Y Acad Sci 860:266-80. The excitatory motor neurons release glutamate at the muscle cells, causing them to depolarize and contract. There are about 30 excitatory motor neurons in each RS and each PS group. Like other crustacean muscles, the swimmeret muscles also receive axons from two or three large inhibitory motor neurons. These neurons release GABA at the muscle cells, opposing depolarization and reducing contraction. When the excitors to one class of muscles are active, often the inhibitors to the antagonist muscle are also active.
In each pattern generating module, the two pairs of non-spiking local interneurons (1 and 2) inhibit their respective clusters of motor neurons and also each other. The circuit is an example of reciprocal inhibition, which guarantees that only one pair of local interneurons (1 or 2) and thus one set of motor neurons (RS or PS) will be active at one time. This establishes the pattern of alternating bursts of RS and PS spikes, leading to alternating forward and backward swimmeret movements. Interestingly, it is not yet known which neurons excite the motor neurons, since only inhibitory connections to motor neurons have been discovered from the local interneurons. The system lends itself to pharmacological intervention. Acetylcholine (and its analog, carbachol) turn on the swimming CPG, as does the peptide proctolin. Proctolin, a five-amino-acid peptide, is known to be contained in three of the five pairs of command neurons that activate the swimmeret system, so it is plausible that applying proctolin to a ganglion mimics the release of this transmitter from command neurons to activate the CPG system. It is also known that acetylcholine directly depolarizes RS and PS motor neurons by acting on muscarinic receptors. However, it is not yet clear what the source of that acetylcholine might be. At the bottom left of the figure, sensory feedback from stretch receptors and other sensory neurons is shown connecting (in an undefined way) to the motor neurons and local interneurons. It is likely that at least some of these sensory neurons release ACh as their transmitter, but this is unlikely to be the sole source of cholinergic excitation of the CPG circuits..
In experiments, the motor pattern can be recorded from the first roots of abdominal ganglia A2-A5 using suction or pin electrodes. Adding the acetylcholine mimic carbachol or the peptide proctolin to the bath can activate the CPG pattern. It is not yet certain where those drugs act on the swimmeret system, although (as mentioned) some command neurons contain proctolin, and motor neurons (and probably some interneurons) respond to acetylcholine. |
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Readings and the Writing AssignmentThe list of readings and detailed instructions for the writing assignment are given in the Lab 8 Checklist.
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Links |
Supplement: Anatomy of the Crayfish Nervous System. |
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© 2003 - 2016 by Richard F. Olivo. Permission is granted to non-profit educational institutions to reproduce or adapt this Web page for internal use provided that the original source and copyright are acknowledged. |