Transmitted light image of a five-day old zebrafish larva. Image credit: Dena Goldblat (CC BY 4.0)
Some external stimuli, such as a painful touch or sudden head movements, can trigger automatic physical responses. These reactions are controlled by sensorimotor circuits which are comprised of three types of neurons. First, sensory neurons detect the external stimulus. They then pass the information to interneurons, which relay the signal to motor neurons that activate the muscles required to produce a prompt physical response.
Sensorimotor circuits form very early in life, but it remains unclear how the three types of neurons involved contribute to one another’s development. Previous research suggests that motor neurons send chemical signals ‘upstream’ to interneurons to help them mature. However, there is conflicting evidence both for and against this hypothesis.
To investigate this theory, Goldblatt et al. studied a sensorimotor circuit found in all vertebrates known as the gaze stabilization reflex. The sensory neurons in this circuit detect head movements and transmit this information, via interneurons, to the motor neurons that control muscles in the eye. This allows gaze to remain stable while the head moves.
Goldblatt et al. used a genetic tool to eliminate the motor neurons involved in the gaze stabilization reflex from zebrafish larvae. This prevented the motor neurons from sending chemical signals upstream to the interneurons and caused the zebrafish larvae to develop eyes that were permanently rotated outward.
However, further experiments revealed that the connections between sensory neurons and interneurons still developed normally despite the absence of motor neurons. The interneurons also expressed the appropriate set of genes for the stage of larval development tested. This suggests that interneurons involved in the gaze stabilization reflex can develop normally without chemical signals from motor neurons.
These findings are a major step towards understanding how sensorimotor circuits develop, and suggest that current models of this process may need to be revised. It remains to be seen whether other sensorimotor circuits can also develop without signals from downstream motor neurons. Gaining more detailed insights into how these circuits develop could also enhance our understanding of certain neurological conditions further down the line.
