title: Systematic shifts in the balance of excitation and inhibition coordinate the activity of axial motor pools at different speeds of locomotion.
abstract: An emerging consensus from studies of axial and limb networks is that different premotor populations are required for different speeds of locomotion. An important but unresolved issue is why this occurs. Here, we perform voltage-clamp recordings from axial motoneurons in larval zebrafish during "fictive" swimming to test the idea that systematic differences in the biophysical properties of axial motoneurons are associated with differential tuning in the weight and timing of synaptic drive, which would help explain premotor population shifts. We find that increases in swimming speed are accompanied by increases in excitation preferentially to lower input resistance (Rin) motoneurons, whereas inhibition uniformly increases with speed to all motoneurons regardless of Rin. Additionally, while the timing of rhythmic excitatory drive sharpens within the pool as speed increases, there are shifts in the dominant source of inhibition related to Rin. At slow speeds, anti-phase inhibition is larger throughout the pool. However, as swimming speeds up, inhibition arriving in-phase with local motor activity increases, particularly in higher Rin motoneurons. Thus, in addition to systematic differences in the weight and timing of excitation related to Rin and speed, there are also speed-dependent shifts in the balance of different sources of inhibition, which is most obvious in more excitable motor pools. We conclude that synaptic drive is differentially tuned to the biophysical properties of motoneurons and argue that differences in premotor circuits exist to simplify the coordination of activity within spinal motor pools during changes in locomotor speed.
authors.affiliation: Department of Neurobiology, Northwestern University, Evanston, Illinois 60625.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Kishore
authors.affiliation: Department of Neurobiology, Northwestern University, Evanston, Illinois 60625.
authors.forename: Martha W
authors.initials: MW
authors.lastname: Bagnall
authors.affiliation: Department of Neurobiology, Northwestern University, Evanston, Illinois 60625 david-mclean@northwestern.edu.
authors.forename: David L
authors.initials: DL
authors.lastname: McLean
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology*; D005260:Female; D053444:Inhibitory Postsynaptic Potentials / Q000502:physiology*; D008124:Locomotion / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009924:Organ Culture Techniques; D013550:Swimming / Q000502:physiology; D015027:Zebrafish
keywords: excitation; inhibition; locomotion; motoneurons; recruitment; spinal cord