Neural Flip-Flops III: Stomatogastric Ganglion
The stomatogastric ganglion (STG) is a group of about 30 neurons that resides on the stomach in decapod crustaceans. Its two central pattern generators (CPGs) control the chewing action of the gastric mill and the peristaltic movement of food through the pylorus to the gut. The STG has been studied extensively because it has properties that are common to all nervous systems and because of the small number of neurons and other features that make it convenient to study. So many details are known
... details are known that the STG is considered a classic test case in neuroscience for the reductionist strategy of explaining the emergence of macro-level phenomena from micro-level data. In spite of the intense scrutiny the STG has received, how it generates its rhythmic patterns of bursts remains unknown. The novel neural networks presented here model the pyloric CPG of the American lobster (Homarus americanus). Each model's connectivity is explicit, and its operation depends only on minimal neuron capabilities of excitation and inhibition. One type of model CPGs, flip-flop ring oscillators, is apparently new to engineering, making it an example of neuroscience and logic circuit design informing each other. Several testable predictions are given here, and STG phenomena are shown to support several predictions of neural flip-flops that were given in a previous paper on short-term memory. The model CPGs are not the same as the more complex pyloric CPG. But they show how neurons can be connected to produce oscillations, and there are enough similarities in significant features that they may be considered first approximations, or perhaps simplified versions, of STG architecture. The similarities include 1) mostly inhibitory synapses; 2) pairs of cells with reciprocal, inhibitory inputs, complementary outputs that are approximately 180 degrees out of phase, and state changes occurring with the high output changing first; 3) cells that have reciprocal, inhibitory inputs with more than one other cell; and 4) six cells that produce coordinated oscillations with the same period, four phases distributed approximately uniformly over the period, and half of the burst durations approximately 1/4 of the period and the other half 3/8. These variables cannot be controlled independently in the design, suggesting a similar architecture in the models and the STG. Some of the neural network designs can be derived from electronic logic circuit designs simply by moving each negation symbol from one end of a connection to the other. This does not change the logic of the network, but it changes each logic gate to one that can be implemented with a single neuron.