Mechanisms that initiate spontaneous network activity in the developing chick spinal cord

Area of Science:

• Neuroscience
• Developmental Biology
• Neurophysiology

Background:

• Developing nervous systems exhibit spontaneous activity crucial for development.
• Rhythmic episodes of activity occur in the embryonic chick spinal cord, separated by long quiescent periods.

Purpose of the Study:

• Investigate the initiation mechanisms of spontaneous rhythmic activity in the embryonic chick spinal cord.
• Examine the role of motoneurons and R-interneurons in triggering these developmental episodes.

Main Methods:

• Recorded spontaneous ventral root depolarizations and interneuronal activity using calcium-sensitive dyes.
• Stimulated motor axons to prematurely trigger episodes and observed effects of pharmacological blockade.
• Performed whole-cell recordings and calcium imaging of R-interneurons and other interneurons.

Main Results:

• Pre-episode motoneuronal discharge preceded episodes, and motor axon stimulation could trigger episodes prematurely.
• Activating R-interneurons was crucial, as blockade of their inputs/outputs prevented premature episode triggering.
• R-interneurons fired high-frequency bursts and their region became active before other interneurons at episode onset.
• Cholinergic blockade slowed episode initiation and altered the spatial pattern of early network activity.

Conclusions:

• Motoneuronal activity, possibly from random interneuronal fluctuations, activates R-interneurons.
• R-interneurons initiate spinal network recruitment, likely due to their functionally excitatory output during development.
• This highlights an unusual but critical role for R-interneurons in orchestrating developmental network activity.