Anatomical architecture and responses to acidosis of a novel respiratory neuron group in the high cervical spinal cord (HCRG) of the neonatal rat

Role of Propriospinal Neurons in Control of Respiratory Muscles and Recovery of Breathing Following Injury

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.

The Neuroplastic and Therapeutic Potential of Spinal Interneurons in the Injured Spinal Cord

The central nervous system is not a static, hard-wired organ. Examples of neuroplasticity, whether at the level of the synapse, the cell, or within and between circuits, can be found during development, throughout the progression of disease, or after injury. One essential component of the molecular, anatomical, and functional changes associated with neuroplasticity is the spinal interneuron (SpIN). Here, we draw on recent multidisciplinary studies to identify and interrogate subsets of SpINs and their roles in locomotor and respiratory circuits. We highlight some of the recent progress that elucidates the importance of SpINs in circuits affected by spinal cord injury (SCI), especially those within respiratory networks; we also discuss potential ways that spinal neuroplasticity can be therapeutically harnessed for recovery.

The respiratory control mechanisms in the brainstem and spinal cord: integrative views of the neuroanatomy and neurophysiology

Respiratory activities are produced by medullary respiratory rhythm generators and are modulated from various sites in the lower brainstem, and which are then output as motor activities through premotor efferent networks in the brainstem and spinal cord. Over the past few decades, new knowledge has been accumulated on the anatomical and physiological mechanisms underlying the generation and regulation of respiratory rhythm. In this review, we focus on the recent findings and attempt to elucidate the anatomical and functional mechanisms underlying respiratory control in the lower brainstem and spinal cord.

Midcervical neuronal discharge patterns during and following hypoxia

Anatomical evidence indicates that midcervical interneurons can be synaptically coupled with phrenic motoneurons. Accordingly, we hypothesized that interneurons in the C3-C4 spinal cord can display discharge patterns temporally linked with inspiratory phrenic motor output. Anesthetized adult rats were studied before, during, and after a 4-min bout of moderate hypoxia. Neuronal discharge in C3-C4 lamina I-IX was monitored using a multielectrode array while phrenic nerve activity was extracellularly recorded. For the majority of cells, spike-triggered averaging (STA) of ipsilateral inspiratory phrenic nerve activity based on neuronal discharge provided no evidence of discharge synchrony. However, a distinct STA phrenic peak with a 6.83 ± 1.1 ms lag was present for 5% of neurons, a result that indicates a monosynaptic connection with phrenic motoneurons. The majority (93%) of neurons changed discharge rate during hypoxia, and the diverse responses included both increased and decreased firing. Hypoxia did not change the incidence of STA peaks in the phrenic nerve signal. Following hypoxia, 40% of neurons continued to discharge at rates above prehypoxia values (i.e., short-term potentiation, STP), and cells with initially low discharge rates were more likely to show STP (P < 0.001). We conclude that a population of nonphrenic C3-C4 neurons in the rat spinal cord is synaptically coupled to the phrenic motoneuron pool, and these cells can modulate inspiratory phrenic output. In addition, the C3-C4 propriospinal network shows a robust and complex pattern of activation both during and following an acute bout of hypoxia.

Anatomical changes of phrenic motoneurons during development

Although the phrenic motoneurons are relatively well-developed at the time of birth as compared to non-respiratory motoneurons, they show distinct anatomical changes during postnatal development. In the present review we summarize anatomical changes of phrenic motoneurons during pre- and postnatal development. Cell bodies of phrenic motoneurons migrate into the ventromedial region of the ventral horn of C3-C6 by E13-E14 in the rat. During development the sizes and surface areas of phrenic motoneurons are increased with changes in dendritic morphology.

The central respiratory chemoreceptor: where is it located?-Invited article

We review previous reports on the localization of the central chemoreceptor focusing on our studies that used various experimental techniques including lesioning (brainstem transection and removal of pia mater), analyses of neuronal responses to CO(2) by electrophysiological and optical recording, mapping of CO(2)-excitable neurons by c-fos immunohistochemistry and local acidic stimulation. Among these experimental techniques, voltage imaging with calculation of cross correlation coefficients between the respiratory output activity and each pixel, i.e., correlation coefficient imaging technique, enabled us to effectively analyze imaging data without empirical signal processing. The reviewed studies have indicated that the most superficial layer of the rostral ventral medulla, i.e., the surface portions of the nucleus retrotrapezoideus/parafacial respiratory group, nucleus parapyramidal superficialis and nucleus raphe pallidus, is important in central chemoreception. We suggest that one of the major respiratory rhythm generators, i.e., the preBötzinger complex, is not chemosensitive in itself or rather inhibited by CO(2). Based on our detailed analysis of c-fos immunohistochemistry, we propose a cell-vessel architecture model for the central respiratory chemoreceptor. Primary chemoreceptor cells are mainly located beneath large surface vessels within the marginal glial layer of the ventral medulla, and surround fine penetrating vessels that branch from a large surface vessel. Respiratory neurons in the rostral portion of the ventral respiratory group could be intrinsically chemosensitive, but their role in chemoreception might be secondary. Definitive identification of chemosensitive sites and chemoreceptor cells needs further studies.