Breathing requires precise control of respiratory muscles to ensure adequate ventilation. Neurons within discrete regions of the brainstem produce oscillatory activity to control the frequency of breathing. Less is understood about how spinal and pontomedullary networks modulate the activity of respiratory motor neurons to produce different patterns of activity during different behaviors (i.e., during exercise, coughing, swallowing, vocalizing, or at rest) or following disease or injury. Here, we use a chemogenetic approach to inhibit the activity of glutamatergic V2a neurons in the brainstem and spinal cord of neonatal and adult mice to assess their potential roles in respiratory rhythm generation and patterning respiratory muscle activity. Using whole-body plethysmography (WBP), we show that V2a neuron function is required in neonatal mice to maintain the frequency and regularity of respiratory rhythm. However, silencing V2a neurons in adult mice increases respiratory frequency and ventilation, without affecting regularity. Thus, the excitatory drive provided by V2a neurons is less critical for respiratory rhythm generation in adult compared to neonatal mice. In addition, we used simultaneous EMG recordings of the diaphragm and extradiaphragmatic respiratory muscles in conscious adult mice to examine the role of V2a neurons in patterning respiratory muscle activity. We find that silencing V2a neurons activates extradiaphragmatic respiratory muscles at rest, when they are normally inactive, with little impact on diaphragm activity. Thus, our results indicate that V2a neurons participate in a circuit that serves to constrain the activity of extradiaphragmatic respiratory muscles so that they are active only when needed.
Respiratory motor failure is the leading cause of death in spinal cord injury (SCI). Cervical injuries disrupt connections between brainstem neurons that are the primary source of excitatory drive to respiratory motor neurons in the spinal cord and their targets. In addition to direct connections from bulbospinal neurons, respiratory motor neurons also receive excitatory and inhibitory inputs from propriospinal neurons, yet their role in the control of breathing is often overlooked. In this review, we will present evidence that propriospinal neurons play important roles in patterning muscle activity for breathing. These roles likely include shaping the pattern of respiratory motor output, processing and transmitting sensory afferent information, coordinating ventilation with motor activity, and regulating accessory and respiratory muscle activity. In addition, we discuss recent studies that have highlighted the importance of propriospinal neurons for recovery of respiratory muscle function following SCI. We propose that molecular genetic approaches to target specific developmental neuron classes in the spinal cord would help investigators resolve the many roles of propriospinal neurons in the control of breathing. A better understanding of how spinal circuits pattern breathing could lead to new treatments to improve breathing following injury or disease.
Accessory respiratory muscles help maintain ventilation when diaphragm function is impaired. The following protocol describes a method for repeated measurements over weeks or months of accessory respiratory muscle activity while simultaneously measuring ventilation in a non-anesthetized freely behaving mouse. The technique includes surgical implantation of a radio transmitter and insertion of electrode leads into the scalene and trapezius muscles to measure electromyogram activity of these inspiratory muscles. Ventilation is measured by whole body plethysmography and animal movement is assessed by video and synchronized with electomyogram activity. Measurements of muscle activity and ventilation in a mouse model of amyotrophic lateral sclerosis are presented to show how this tool can be used to investigate how respiratory muscle activity changes over time and to assess the impact of muscle activity on ventilation. The described methods can easily be adapted to measure activity of other muscles or to assess accessory respiratory muscle activity in additional mouse models of disease or injury.
Our prior studies suggested that one subset of V2a neurons activates accessory respiratory muscles whereas another subset of V2a neurons actively prevents their activation at rest. However, since these studies altered V2a neuron excitability throughout the spinal cord and brainstem, it was not clear whether the V2a neurons that activate accessory respiratory muscles are located in the same region of the neuraxis as V2a neurons that prevent activation of accessory respiratory muscles at rest. Therefore, we used a Cre‐dependent AAV virus injected into a Chx10Cre/+ mouse to selectively target either the excitatory (Gq)‐ or inhibitory (Gi)‐DREADD receptor to cervical spinal V2a neurons on one side or both sides of the cord in order to alter V2a neuron excitability. Whole breath plethysmography (WBP) and electromyography (EMG) were recorded in conscious mice at rest to measure ventilation and respiratory muscle activity before and after altering V2a neuron excitability. We found that bilaterally increasing the excitability of cervical spinal V2a neurons increases accessory respiratory muscle (scalene) activity, diaphragm EMG peak amplitude, and ventilation. Although V2a neurons project ipsilaterally, increasing their excitability on one side of the cord is able to synchronously activate scalene muscles on both sides of the body. Bilaterally silencing cervical spinal V2a neurons also activates scalene muscle activity, but does not impair diaphragm function. Our results are consistent with the hypothesis that one subset of V2a neurons in the cervical spinal cord activates accessory respiratory muscles whereas another subset of cervical V2a neurons inhibits accessory respiratory muscles at rest. Support or Funding Information 1RO1NS112255T32NS5007453University of Cincinnati Dean’s Dissertation Completion Fellowship
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.