Neurons work in groups, firing together. How such groups combine to generate sensation, action, or thought is unknown. A tractable target for understanding the co-operative action of neurons is the locomotion of simple invertebrates, in which a few hundred or few thousand neurons control basic movement. Angela Bruno and Bill Frost (Chicago Medical School) recorded the activity of single neurons in about 10% of the sea-slug Aplysia's locomotion network while it executed its locomotion motor program. Together we have sought to build a complete understanding of how this ~1600 neuron network generates locomotion.
We found that the network was made up of small groups of neurons, in which each member fired in synchrony. Some of these "ensembles" were active in short bursts, driving a dedicated portion of the locomotion program, such as neck extension. Some were active continuously, containing neurons that were correlated with each other, but not directly with phases of the locomotion program.
We showed that these two classes of ensemble represented two different dynamical systems in this network. To our surprise, we found that these different dynamics, defined only by activity, mapped to physically separate, discrete regions of the network.
Our results show that the locomotion motor program has a corresponding modular organization in both dynamical and physical space. Even in simple invertebrates, neural motor programs are implemented by large, distributed networks containing multiple dynamical systems.
Paper: Bruno, A. M., Frost, W. N. & Humphries, M. D. (2015) Modular Deconstruction Reveals the Dynamical and Physical Building Blocks of a Locomotion Motor Program. Neuron, 86, 304-318. http://dx.doi.org/10.1016/j.neuron.2015.03.005
Preview: Brownstone, R. M. & Stifani, N. (2015) Unraveling a Locomotor Network, Many Neurons at a Time. Neuron, 86, 9-11. http://dx.doi.org/10.1016/j.neuron.2015.03.056
A Aplysia, yesterday (Image: Angela Bruno). B The Aplysia's pedal ganglion, containing its complete locomotion network. Neuron bodies are visible across the entire surface. (Image: Angela Bruno). C Types of dynamics in the locomotion motor program. Each row is the output of one neuron for 80 seconds of the program's duration, each dot is one spike. Neurons are grouped and colour-coded according to their detected ensemble. We arrange the ensembles to show the starkly different dynamics. For example, at the top, four ensembles (greens and purples) fire in overlapping bursts throughout. Fourth from bottom is a large ensemble (dark purple) that does not burst, yet shows high synchrony between individual neurons.