The crustacean stomatogastric nervous system has some particularly attractive features as a model system for neuroscience. Both their 14-cell pyloric network and 11-cell gastric mill network are anatomically separated from the rest of the nervous system and produce distinct motor patterns, allowing researchers to study the properties of each network individually. Here are three of the major findings from this research:
1) Many if not most neurons in the network have intrinsic modes of firing, including endogenous repetitive bursting, rhythmic oscillatory capability, bistability, post-inhibitory rebound, and spike adaptation. Real-life neurons do not just integrate and fire! These different modes of activity can be modulated by the opening / closing or activation / deactivation of various types of ion channels (i.e., types of Na, Ca, K, Cl). In order to model the neural networks accurately with respect to their known biological activity, researchers must incorporate these complex intrinsic properties of the component neurons. Knowledge of the synaptic connections alone is not enough.
2) The neural networks are subject to extensive biological modulation via injection of hormones, stimulation of input nerves, or some other manipulation. For example, the electrophysiological properties of a given neuron can be changed, inducing an intrinsic mode of firing, or the synaptic connections between neurons can be made stronger or weaker. Enough of these changes can produce a distinct motor output–there is no evolutionary need for a separate neural network for each motor output.
3) Individual networks that appear independent will interact with one another in non-trivial ways given the appropriate environmental cues. It is useful to consider each network as a unit central pattern generator segment that can aggregate with others to form more complex behaviors. This is true of other animals too. For example, Cramer and Keller (2006) found that microstimulation of the vibrissae representation in the motor cortex of rats in frequencies of 50–90 Hz led to evoked whisking frequencies of 5 to 15 Hz, suggesting that the stimulation activated a subcortical central pattern generator instead of allowing for direct motor control.
Research into simple neural networks such as the STNS and C. elegans is indicative of the progress in neurobio generally: Until we can accurately model these systems, there is little reason to suspect that we will be able to accurately model systems with even more neurons and connections.
Inspired by CalTech’s Question #10 for cognitive scientists: “Describe several main findings resulting from the study of the crustacean stomatogastric nervous system and their implications for the study and understanding of local circuit function in larger, more complex systems.”
Harris-Warrick RM, et al. 1992 Dynamic Biological Networks: The Stomatogastric Nervous System. Parts available on Google Books here.
Cramer NP, Keller A. 2006 Cortical Control of a Whisking Central Pattern Generator. J Neurophysiol doi:10.1152/jn.00071.2006 .