While motor neurons, which make their way from the spinal cord to muscles and organs, have always been considered to be passive brain cells that receive signals from interneuornal circuits, new research from the Karolinska Institutet suggests otherwise. A team of scientists has discovered a new direct signaling pathway where motor neurons can influence locomotor circuits that are responsible for creating rhythmic movements.
"We have now uncovered an unforeseen role of motor neurons in the elaboration of the final program for motor behaviour," Abdel El Manira, principal investigator of the study, said in a press release. "Our unexpected findings demonstrate that motor neurons control locomotor circuit function retrogradely via gap junctions, so that motor neurons will directly influence transmitter release and the recruitment of upstream excitatory interneurons."
Using zebrafish, which is a common animal model in neurobiology due to their transparency and the ease with which they are genetically manipulated, the researchers used a combination of methods to show that there is a direct connection between motor neurons and the interneurons that create swimming motions in the fish via electrical synapses, also called gap junctions. These gap junctions allow the direct connection of two neurons in order for them to transmit electric signals between each other. Using optogenetics, the team silenced the activity of specific motor neurons and found that they had a big impact on the locomotor circuit.
"This study represents a paradigm shift that will lead to a major revision of the long held view of the role of motor neurons," El Manira said. "Motor neurons can no longer be considered as merely passive recipients of motor commands - they are an integral component of the circuits generating motor behavior."
The findings were published in the Jan. 13 issue of Nature.
