Morphological characterization of spinal respiratory phrenic motoneurons

Abstract

Objective To study the spatial distribution of respiratory-related phrenic motoneurons within the spinal cord and comparison of their distribution characteristics with those of deltoid motoneurons and intercostal motoneurons. Methods Using a retrograde tracing strategy from peripheral to central, cholera toxin β-subunit (CTB) or pseudorabies virus (PRV) was microinjected into the ipsilateral diaphragm, deltoid and intercostal muscles of adult mice (n=3) to retrogradely label the spinal motoneurons innervating the target muscles. Then the distribution characteristics of different spinal motoneurons were analyzed and compared. Results The results of CTB- and PRV-mediated retrograde tracing showed that phrenic motoneurons in adult mice were distributed in the Ph9 region in the anterior horn of the C3~C6 segments of the cervical cord, and there was no significant difference in the efficiency of labeling phrenic motoneurons with either retrograde tracer. The phrenic motoneurons were distributed in a more ventral medial location in the anterior horn of the same spinal cord segment compared to the non-respiratory related deltoid motor cholinergic neurons in the C5~C6 segments. In contrast to respiratory-related intercostal motoneurons in the thoracic cord, both sets of neurons were distributed in a longitudinal column along the rostro-caudal axis in the cervical and thoracic cord, respectively. However, phrenic motoneurons were relatively uniformly distributed in the number of cells in segments C3~C6, whereas intercostal motoneurons were more variable in the number of neurons in segments T1~T11 (P < 0.000 1). In addition, the distribution of phrenic motoneurons was gradually shifted towards the medial anterior horn of the spinal cord along the rostro-caudal axis, whereas respiratory intercostal motoneurons were more uniformly distributed in the lateral anterior horn of the spinal cord. Conclusion Phrenic motoneurons in adult mice are distributed in segments C3~C6 of the cervical cord, which are located in the more medial Ph9 region of the anterior horn as compared to non-respiratory related deltoid motoneurons in the cervical cord, and have a more even distribution pattern than respiratory intercostal motoneurons in the thoracic cord.

Keywords

• respiration
• phrenic motoneurons
• diaphragm
• spinal cord
• detoild motoneurons
• intercostal motoneurons