| Popis: |
A fundamental goal in systems neuroscience is to understand how behaviors are executed by the nervous system. Respiration is an essential behavior which we perform hundreds of millions of times over our lifespan. Given that deficits in respiration underlie a variety of human diseases and conditions, the neural mechanisms which control respiration are the subject of active investigation. Here, using an ex vivo model of mouse respiration combined with optogenetic and pharmacological approaches, I examined how central neural networks control phrenic motor neurons—which are the primary motor output associated with inspiration.Inspiratory bursts are generated in a medullary structure termed the preBotC, and relayed via descending bulbospinal projections to phrenic motor neurons, which reside in cervical spinal cord levels C3-5/6. Spinal cord injury (SCI) above cervical level 4 disrupts these descending fibers and leads to permanent loss of diaphragm function. In this body of work, I found that after complete cervical SCI, spontaneous and bilaterally coordinated phrenic bursting could be initiated by blockade of fast inhibitory synaptic transmission. Here, glutamatergic neurons were sufficient and necessary for induction of phrenic bursts. Direct stimulation of phrenic motor neurons was insufficient to evoke phrenic burst activity. Therefore, I identify a recurrent excitatory network which can direct phrenic motor bursting in the absence of medullary input. Transection and pharmacological manipulations indicated that this propriospinal network is dissociable from medullary circuits which generate bona fide inspiration, suggesting a novel non-respiratory function. Thus, these results demonstrate that phrenic motor neurons can be controlled by two independent premotor networks: the classical inspiratory network of the preBotC, and a second “latent” network which may initiate phrenic motor activity in physiological situations of increased network excitability.The preBotC is required for generation of inspiratory bursts, and therefore changes in inspiratory frequency are ultimately coordinated by the preBotC. How is inspiratory frequency controlled? In the simplest model, excitation increases inspiratory frequency and inhibition causes apnea. To test this model, I used an optogenetic approach to stimulate select populations of hindbrain neurons and characterize how they modulate inspiratory frequency. I found that stimulation of excitatory Phox2b-lineage, putative CO2-chemosensitive neurons dramatically increased frequency. Surprisingly, however, this effect was completely abolished by blockade of fast inhibitory synaptic transmission, indicating an essential role for inhibitory neurons in increasing inspiratory rate in response to elevated CO2. To investigate inhibitory mechanisms which increase inspiratory frequency, I stimulated Vgat+ neurons and found that phasic inhibition can actually drive increases in inspiratory frequency via a rebound-like mechanism. I further found that the parafacial oscillator, which generates expiration, likely acts as a source of phasic activity during stimulation of Phox2b+ neurons. These data argue that generation of inspiratory rhythm occurs in two functionally distinct modes—a low-frequency mode which is generated by excitatory circuits of the preBotzinger complex, and a high-frequency mode which requires inhibition. We propose a model whereby high frequency inspiratory bursts are generated as a product of inhibitory coupling between multiple excitatory oscillators. |
