Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

Yoshio Shinozaki,Hideaki Yoshida,Masaya Nakamura,Kentaro Fukuda,Yasumasa Okada,Yoshiaki Toyama,Mieczyslaw Pokorski,Ryoma Aoyama,Shigefumi Yokota,Fumikazu Miwakeichi

doi:10.1007/s00429-018-1757-3

Abstract

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Highlights

The diaphragm is the principal inspiratory pump muscle that is essential to maintain adequate ventilation in mam‐ mals, and its dysfunction elicits respiratory failure (Poole et al 1997)
In voltage imaging with 5 ms sampling rate on the C4/5 transverse plane of the cervical spinal cord with normocap‐ nic cerebrospinal fluid (CSF) (n = 3), the depolarizing optical signal appeared in the ventral horn, rapidly expanding concentrically in the medial direction
We found that the inspiratory-related depolarizing region consisted of spatially distinct two subpopulations; one in the middle portion of the Central and medial regions are indicated with red and blue circles, respectively, b–d were recorded simultaneously ventral horn corresponding to the central motor column and the other in the medial portion corresponding to the medial motor column

Summary

Introduction

The diaphragm is the principal inspiratory pump muscle that is essential to maintain adequate ventilation in mam‐ mals, and its dysfunction elicits respiratory failure (Poole et al 1997). It is innervated by the phrenic nerves, and the anatomical localization of the phrenic motoneuron pool in the cervical spinal cord has been investigated in various ani‐ mal species Despite the abundance of the research on phrenic motoneurons, most studies are either purely anatomical or physiological at a single cellular level

Methods

Results

Conclusion