Glutamatergic input varies with phrenic motor neuron size

Area of Science:

• Neuroscience
• Motor Control
• Skeletal Muscle Physiology

Background:

• The diaphragm muscle, like other skeletal muscles, uses an orderly recruitment of motor units for diverse motor behaviors.
• Orderly recruitment is typically attributed to motor neuron intrinsic properties, assuming uniform excitatory input.
• Differences in excitatory input to phrenic motor neurons (PhMNs) could also influence this recruitment pattern.

Purpose of the Study:

• To investigate whether presynaptic excitatory input distribution varies across diaphragm motor neurons of different sizes.
• To determine if glutamatergic input density correlates with phrenic motor neuron somal size.

Main Methods:

• Retrograde labeling of PhMNs in Sprague-Dawley rats using cholera toxin B (CTB).
• Confocal imaging and semiautomated processing to quantify presynaptic glutamatergic terminals near PhMN somata.
• Analysis of terminal density relative to PhMN somal surface area.

Main Results:

• A higher density of glutamatergic terminals (~10%) was observed on smaller PhMNs (lower somal surface area tertile).
• Larger PhMNs (upper tertile) exhibited a lower density of these excitatory inputs.
• This finding suggests differential excitatory synaptic input based on motor neuron size.

Conclusions:

• Differences in excitatory glutamatergic input to PhMNs contribute to the orderly recruitment of diaphragm motor units.
• This supports the size principle, where smaller, more excitable neurons are recruited first for low-force tasks.
• Larger neurons, receiving less input, are recruited later for high-force, expulsive behaviors.