Understanding how neuronal networks enable animals, including humans, to make coordinatedmovements is a continuing goal of neuroscience research. The stomatogastric nervous system ofdecapod crustaceans and particularly the networks of the stomatogastric ganglion (STG), whichcontrol feeding functions, have significantly contributed to our present understanding of generalprinciples underlying rhythmic motor circuit operation at the cellular level, and has shed light onthe mechanisms of network homeostasis and plasticity.
Rhythmic behaviors include all motor acts that, at their core, involve a rhythmic repeating set ofmovements (e.g., locomotion, breathing, chewing and scratching). The circuits underlying suchrhythmic behaviors, central pattern generators (CPGs), operate on the same general principlesacross all nervous systems. These networks can generate rhythmic output in the completelyisolated nervous system, even in the absence of any rhythmic neuronal input, including feedbackfrom sensory systems. Although the details differ in each circuit, all CPGs use the same set ofcellular-level mechanisms for circuit construction. More importantly, CPG circuits are usuallynot dedicated to producing a single neuronal activity pattern. This flexibility results largely fromthe ability of many different neuromodulators to change the cellular and synaptic properties ofindividual circuit neurons. When the properties of circuit components are changed, the output ofthe circuit itself is modified.
The STG contains a set of distinct but interacting motor circuits. The value of this system hasresulted from its experimental accessibility, owing to the small number of large and individuallyidentifiable neurons and the use of several innovative techniques. Because of the manysimilarities between vertebrate and invertebrate systems, especially with regards to basicprinciples of neuronal function, invertebrate model systems such as the crustacean stomatogastricnervous system continue to provide key insight into how neural circuits operate in thenumerically larger and less accessible vertebrate CNS.
The stomatogastric cycle (STG cycle) examines mechanisms of generation, regulation andplasticity of rhythmic neural activity produced by CPG networks. The STG cycle exerciseshighlight fundamental features of the cellular basis of motor pattern generation and thecharacterization of dynamical neural systems. Students obtain hands-on experience with theprinciple that rhythmically active networks can continue to generate rhythmic motor patterns inthe isolated CNS, a defining feature of CPGs. We also focus on the fact that anatomically hard-wired circuits remain functionally flexible. All neural networks are malleable through the actionof neuromodulatory inputs and intrinsic homeostatic mechanisms, which modify time andvoltage-dependent properties of intrinsic membrane properties, and functional synapticconnectivity. Additionally, several key aspects of neuronal communication are studied,including properties of spike-mediated and graded synaptic transmission, short-term synapticplasticity and the input/output properties of electrical synapses.
We build on skills developed during Cycle I, emphasizing electrophysiological analysis of neuralnetwork activity, and its underlying ionic mechanisms, in an isolated nervous system. Weemphasize modern experimental tools and paradigms: intracellular recordings of multipleidentified neurons; extracellular recordings of identified neurons; single-electrode discontinuouscurrent clamp methods for current injection and recording; synaptic pharmacology defined withpharmacological agonists and antagonists; superfusion of neuromodulators and their release byidentified projection neurons; study of graded transmitter release, the dynamic clamp techniquefor determining the functional impact of intrinsic and synaptic currents in network function, andquantitative analysis of electrophysiological recordings.
Students spend the first week building their understanding of STG networks. For example,students will learn dynamical system techniques to describe activity network parameters, such asphase analysis, resetting, entrainment and phase response curves (PRCs). The two-electrodevoltage clamp technique is introduced to quantify time and voltage dependent properties ofmembrane channels and to clarify how ionic currents (which will be characterizedmathematically using the Hodgkin-Huxley framework) contribute to neuronal and networkfunction. These ionic currents will be manipulated using the dynamic clamp technique tounderstand the functional significance of mathematically defined ionic conductances. Thedynamic clamp technique will also be used to examine the role of synapses in the network, byartificially adding or removing synapses in the biological network.
The second week of the STG Cycle is dedicated to independent projects that reflectcontemporary research issues asking questions about mechanisms underlying neural networkactivity, and address specific principles of motor network function common to all animals. Theseprojects are expected, with guidance from faculty mentors, to gather new, previouslyunpublished data, much as preliminary experiments would accomplish in a research lab. Almostall projects in the STG cycle address unknown conceptual questions and many projects in recentyears have led to novel results.