Defining the neuromuscular mechanisms of vocal motor control

Abstract

The manner in which the brain sends commands to muscles to enact behavior is instrumental to our ability to interact with our environment. Moreover, the control of complex vocalizations is crucial for communicating with those around us. Therefore, in order to fully understand the neuromuscular mechanisms underlying vocal motor control, we must determine how the brain controls muscles and how those muscles’ activity translates into vocal behavior. The objective of this thesis is to quantify how patterns of vocal muscle activity are transformed into acoustic output, controlled by individual premotor neurons in the avian nucleus RA (robust nucleus of arcopallium), and modulated by spike timing of motor neurons, which directly activate muscle fibers. We first demonstrated that vocal muscles can both control multiple acoustic parameters, and do so in a context-dependent manner. We accomplished this goal by performing electromyographic recordings and targeted electrical stimulation of muscles during vocalization in addition to stimulating muscle in an ex vivo assay, in which the vocal organ is extracted and kept alive during the experiment. Next, we explored whether single premotor neurons control single or multiple muscles, with the implication that each scenario presents a different challenge to how the brain controls and modulates behavior. This experiment involved acutely recording from the brain and vocal muscles simultaneously and then using those recordings to determine functional connectivity. Finally, we showed how the brain uses spike timing in addition to spike rate to form a code that drives motor behavior. This was accomplished by recording single motor unit activity and stimulating respiratory muscles during quiet breathing. The work of this thesis elucidates how the brain controls vocal muscles and, subsequently, behavior, while laying a foundation for future studies to further define the functional relationship between the brain and muscles.