# Circuit and cellular mechanisms underlying hierarchical recruitment of motor neurons.

> **NIH NIH F32** · UNIVERSITY OF WASHINGTON · 2020 · $69,115

## Abstract

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.

## Key facts

- **NIH application ID:** 10137601
- **Project number:** 1F32MH125443-01
- **Recipient organization:** UNIVERSITY OF WASHINGTON
- **Principal Investigator:** Brandon J Mark
- **Activity code:** F32 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $69,115
- **Award type:** 1
- **Project period:** 2020-09-16 → 2021-09-13

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10137601

## Citation

> US National Institutes of Health, RePORTER application 10137601, Circuit and cellular mechanisms underlying hierarchical recruitment of motor neurons. (1F32MH125443-01). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10137601. Licensed CC0.

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