Adaptive Encoding of Coordinating Information in the Crayfish Central Nervous System

Locomotion is essential for an animals survival. This behavior can range from directional changes to adapting the motor force to the conditions of its surroundings. Even if speed and force of movement are changing, the relative coordination between the limbs or body segments has to stay stable in order to provide the necessary thrust. The coordinating information necessary for this task is not always conveyed by sensory pathways. Adaptation is well studied in sensory neurons, but only few studies have addressed if and how coordinating information changes in cases where a local circuit within the central nervous system is responsible for the coordination between body segments at different locomotor activity states.\n\nOne system that does not depend on sensory information to coordinate a chain of coupled oscillators is the swimmeret system of crayfish. Here, the coordination of four coupled CPGs is controlled by central Coordinating Neurons. Cycle by cycle, the Coordinating Neurons encode information about the activity state of their home ganglion as burst of spikes, and send it as corollary discharge to the neighboring ganglia. Activity states, or excitation levels, are variable in both the living animal and isolated nervous system; yet the amount of coordinating spikes per burst is limited.\n\nHere, we demonstrate that the systems excitation level tunes the encoding properties of the Coordinating Neurons. Their ability to adapt to excitation level, and thus encode relative changes in their home ganglions activity states, is mediated by a balancing mechanism. Manipulation of cholinergic pathways directly affected the coordinating neurons electrophysiological properties. Yet, these changes were counteracted by the networks influence. This balancing may be one feature to adapt the limited spike range to the systems current activity state.