Sunday, May 11, 2025
5/11/2025
Circuit and cellular mechanisms underlying hierarchical recruitment of motor neurons. - Project Summary A fundamentally important motor pattern for terrestrial animals is walking. Walking requires precise control of large numbers of motor neurons. In order to simplify the task of motor control, it has been proposed that motor neurons controlling similar movements are organized in a recruitment hierarchy such that motor neurons are progressively recruited in greater numbers as force requirements increase. A prevailing theory for the establishment of this recruitment hierarchy is the existence of a gradient of excitability among motor neurons controlling progressively greater forces. The relationship between recruitment and excitability is known as the size principle, and evidence for the size principle has been observed across species, from insects to humans. A key assumption of the size principle is that motor neurons receive homogenous inputs, and recruitment order is an intrinsic property of motor neurons, but this has been difficult to test experimentally. This is due to the anatomical complexity of premotor circuits, and the inability to measure the strength of synaptic connections. I will use Drosophila to address these challenges in three ways. First, I will use existing serial-section EM datasets to identify motor neurons controlling tibia-flexion, reconstruct their synaptic inputs, and map all upstream sensory inputs to determine if motor neurons controlling similar movements receive homogenous sensory input. Next, I will use electrophysiology and optogenetic stimulation of common sensory inputs to determine the relationship between anatomical and functional connectivity in motor neurons. Finally, I will determine the contribution of intrinsic morphological properties of motor neurons to their recruitment order using compartmental modeling. Understanding the structure of sensory input onto motor neurons controlling similar movements will provide a significant step forward in our understanding of how intrinsic cellular properties interact with premotor networks to achieve proper motor control.
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