With its hundred billion neurons and quadrillion synapses, the human central nervous system(CNS) can seem intractably complex. Fortunately, there is a class of animals whose nervous systems and behaviors are much more easily understood. Invertebrates, such as sea slugs and worms, have on the order of only hundreds or thousands of neurons and their connections are extremely well stereotyped. This simplicity makes them amenable to experimentation and modeling, and has allowed scientists to understand the structure and function of their neural circuits.
In his review, Allen I. Selverston, Professor Emeritus at UCSD, asks if information gained from the study of invertebrates can be translated to our understanding of the human CNS. He focuses on a particularly well characterized type of circuit called Central Pattern Generators (CPG). CPGs are networks of neurons which produce rhythmic outputs in the absence of sensory feedback, and often control simple motor actions such as feeding or swimming. CPGs are not only found in invertebrates but vertebrates as well, where they control certain low level functions. An example of a CPG is the leech heartbeat network which is shown in the diagram below.
The study CPGs using electrical and chemical manipulation of their constituent neurons has led to three primary types of discoveries. First, it has revealed how a complex array of ion channels contributes to the distinct activity properties of individual neurons. Second, it has shed light on the types of synapses and how they are modulated and third, how circuits produce functional outputs.
Selverston uses these three types of analysis to explain how many different CPGs from the invertebrate world work. Unfortunately, he concludes that there are very few general principles for the design of these circuits that are transferable from model to model. Each CPG has its own evolutionary history that has crafted it into a bespoke circuit for the unique function that it serves. Moreover, the experimental methods used to study CPGs are unlikely to be effective in more complicated vertebrate systems because they cannot be probed with single cell techniques. This means that while the cellular and synapse level data may broadly applicable, the further study of invertebrate CPGs is unlikely to give us much insight into the human CNS.
Selverston’s review can be found here.
Leo Breston is a first year student in the Neuroscience Graduate Program. He is currently rotating in the Navlaka lab.