Information processing by respiratory motoneurons

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Understanding how neurons transform input into output, i.e. how they integrate and process information, is fundamental to understanding brain function and how behaviour emerges from the activity of neural networks. Many neurons in the brain have multiple tasks. A major question in neuroscience is how are properties of neurons adjusted so that they can meet the diverse requirements of all these different tasks that are constantly changing. We study motoneurons of the tongue because they are easy to access, and because they play important roles in many different behaviours like breathing, suckling, swallowing, chewing, and even speech, so are well-suited for asking questions about how diverse responsibilities are managed. We will approach the problem from opposite ends of the behavioural spectrum. First, we will consider dreaming, or REM, sleep and explore mechanisms by which the brain essentially turns off motoneurons that control voluntary muscles, causing a type "paralysis" referred to as REM sleep atonia. This atonia is hypothesized to be protective in that it prevents us from acting out our dreams, but underlying mechanisms are not fully known and appear to differ between muscles, posing a question of fundamental importance. However, this question is also clinically relevant since loss of activity in the motoneurons that control the tongue during sleep is causally related to obstructive sleep apnea. Resolving the underlying mechanisms is important in the development of new treatments. At the other end of the spectrum we will explore how the complex branching structures (dendritic trees) of motoneurons through which they communicate with other neurons are organized to help them efficiently perform their different jobs. The geometry of the dendritic tree has long been recognized as a critical factor in determining how synaptic input is transformed into neuronal output, but how the brain utilizes the complex geometries of neurons to do this remains unclear. In this study we will use optical and electrophysiological methods that allow us to record from single motoneurons in brain slices from neonatal rodents that retain functioning neural networks that generate two rhythmic behaviours, breathing and suckling, in a dish. These endogenous behaviours are critical. It means that the connections from the breathing and suckling networks onto the motoneurons will be activated in a manner similar to what happens normally; i.e. we will monitor motor neuron activity while the breathing and suckling-related inputs are physiologically processed by the dendritic tree. Then, using optical methods to chemically turn off or excite different branches we will clarify the role of different dendrite branches in processing inspiratory and suckling inputs (i.e., do all dendrites have a similar role?). These experiments will provide new insight into neuronal integration and help clarify the basis for how the brain achieves such remarkable information processing.