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Neural plasticity reflects a long-lasting functional change based on prior experience and is certainly important in a wide range of adaptive responses. Remarkable progress has been made in understanding the basis of neuroplasticity, especially in an attempt to decipher the mechanisms underlying learning and memory. In recent years, increasing attention has focused on plasticity of respiratory neuromotor control. For instance, the importance of the pattern of afferent (sensory) input to the brain stem pattern generator(s) and its underlying role in neural plasticity is reflected by changes in the frequency and depth of breathing and/or the coordination between different respiratory muscle groups. Mechanisms underlying this neuroplasticity may include structural changes (e.g., an increase in the number of synapses) or changes in the strength of synaptic input, which may result from alterations in the target neurons (e.g., intracellular calcium mobilization or neuronal morphology) and/or neuromodulatory systems. Obviously, plasticity in respiratory motor control is complex and may involve multiple sites from the pattern generator to the motor output.

We see respiratory plasticity at work in a variety of conditions. Clearly, a person with obstructive sleep apnea who undergoes repeated bouts of hypoxemia through the night may depend on plasticity in respiratory motor control as an adaptive response to this underlying pathology. Certainly, for natives of mountainous regions, the hypoxia associated with high altitude may affect the control of respiration differently compared with individuals growing up at sea level. After thoracic surgery, the pain associated with deep inspiration may lead to plasticity in respiratory motor control, thereby altering the pattern of breathing. For individuals with spinal cord injury, neural plasticity offers great hope for some recovery of function, not the least of which is the recovery of rhythmic activation of the diaphragm. In each of these conditions, overriding physiological factors can influence the extent of plasticity in respiratory motor control that occurs, e.g., differences between young vs. old, men vs. women, trained athletes vs. sedentary individuals, to name just a few. With the invited mini-reviews and original articles of this Highlighted Topics series, the Associate Editors and I are pleased to further promote research in this important area of investigation.

In the first mini-review of this Highlighted Topics series, entitled "Neuroplasticity in respiratory motor control," Drs. G. S. Mitchell and S. M. Johnson define plasticity and related neural properties as they pertain to respiratory control. These authors also discuss potential sites, mechanisms, and known categories of respiratory plasticity. Recent evidence demonstrates considerable neuroplasticity in the respiratory control system, and, with their mini-review, these authors aim to provide a framework for understanding this physiological phenomenon.

Also in this issue, in a mini-review entitled, "Developmental plasticity in respiratory control," Dr. J. Carroll explores the development of the mammalian respiratory control system. Development of the respiratory system begins early during the gestational period; however, it does not reach maturation until several weeks or even months after birth. The relatively long maturation process of the respiratory system allows for lengthened interactions with the environment, both before and after birth. These environmental interactions may include experiences such as episodic or chronic hypoxia, hyperoxia, and drug or toxin exposures. Developmental plasticity occurs when such experiences, during critical periods of maturation, result in long-term changes in the structure or function of the respiratory neural control systems. Dr. Carroll explores the multiple sites and mechanisms by which neural plasticity can occur during development.

In the February issue, in a mini-review entitled, "Plasticity in the control of breathing following sensory denervation," Dr. H. V. Forster will summarize plasticity that follows denervation of several peripheral sensory mechanisms. Upregulation of alternate sensory mechanisms and upregulation of phrenic motoneuron responsiveness have been shown to mediate some of this plasticity in the control of breathing. This mini-review may stimulate future study into the extent to which plasticity occurs after lesions in the central nervous system and the role of central reorganization in plasticity after both peripheral denervation and central nervous system lesions.

Also in the February issue, in a mini-review entitled, "Plasticity of the respiratory pathways following spinal cord injury," Dr. H. Goshgarian will explore the significant capacity of the respiratory system for compensation and recovery after spinal cord injury. Dr. Goshgarian reviews the literature associated with the crossed phrenic phenomenon, the anatomic substrate over which it is mediated, and the plasticity associated with the enhanced expression of this respiratory reflex. Because respiratory insufficiency is the leading cause of death in humans with cervical spinal cord injuries, an understanding of the underlying mechanisms related to plasticity and recovery of the respiratory system after spinal cord injury is very important. Further research may lead to therapies directed toward resolving spinal cord injury-induced respiratory insufficiency in humans.

In the March issue, in a mini-review entitled, "Mechanisms underlying motor unit plasticity in the respiratory system," Drs. G. C. Sieck and C. B. Mantilla will explore plasticity of diaphragm motor units, which are the final common output of the respiratory motor control system. This mini-review first summarizes the great diversity of mechanical properties of diaphragm motor units, which are critically important in providing diversity in diaphragm mechanical output under a variety of motor behaviors. The authors then explore plasticity at each level of the motor unit, including phrenic motoneurons, neuromuscular junctions, and diaphragm muscle fibers. Potential mechanisms underlying plasticity at each level of the motor unit are reviewed, including activity- or inactivity-dependent structural and functional adaptations and the role of nerve- and muscle-derived trophic factors.

Also in the March issue, a mini-review entitled "Neural network plasticity in respiratory control," by Drs. K. F. Morris, D. M. Baekey, S. C. Nuding, T. E. Dick, R. Shannon, and B. G. Lindsey, will summarize evidence for changes in neural circuits of the ventrolateral medulla and brain stem midline following several interventions that affect breathing. The review describes altered firing rates, changes in effective connectivity, and patterns of impulse synchrony, all appropriate for roles in long-term facilitation, a memory expressed as enhanced respiratory motor activity after brief, intermittent hypoxia. The production of the cough motor pattern by reconfiguration of the network implicated in respiratory rhythm generation and long-term changes in central respiratory control after spinal cord injury are also discussed.

As with each of the thematic topics featured in the Highlighted Topics series of the Journal of Applied Physiology, the Associate Editors and I aim to stimulate research in what we feel are important areas of investigation. The mini-reviews and original articles featured in this series only scratch the surface of the complexities of plasticity in respiratory motor control, but we hope that we have provided a strong basis for continuing research in this area. We remain committed to ongoing publication of articles exploring neural plasticity, and we strongly encourage investigators working in this area to consider submitting their work to the Journal of Applied Physiology.