| Summary: | The descending motor control neurons of the larval zebrafish, including the reticulospinal cells, vestibulospinal cells, and the nucleus of the medial longitudinal fasciculus (nMLF), are the primary pathway by which locomotor information is transmitted to the spinal cord. They number approximately three hundred, in many cases are individually identifiable, and act as the sole pathway of information. This makes them amenable to precise study and characterization and allows us
to gain some insight into the control of locomotion. We characterize the activation patterns of the array of descending neurons during behavioral maneuvers such as slow swimming and routine turns in order to understand how these patterns of activation contribute to the diverse larval locomotor repertoire described over the past decade (Budick and O'Malley 2000; Thorsen et al., 2004; McElligott and O'Malley, 2005; Burgess and Granato, 2007). We use in vivo calcium imaging and
electrophysiology to identify neurons active in response to visual presentations of whole-field motion stimuli in a directionally sensitive manner, using the same stimuli which elicit the optomotor response (OMR) innate to larvae (Neuhauss et al., 1999). The speed of locomotion can also be modulated using the OMR, and we test activity patterns in one spinally-projecting midbrain nucleus, the nMLF, identified as active during presentations of forward OMR gratings. Firing rates and
calcium responses increase in this nucleus as grating speed increases, suggesting a role in the control of swimming speed. Electrical stimulation of the nMLF induces locomotor activity which can be modulated by increasing the stimulation frequency. Increasing stimulation frequency also correlates with increases in calcium activity in the nMLF. We suggest a role for the nMLF in the control of locomotion, by exciting spinal circuits responsible for rhythmic oscillation.
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