| Mauthner cell (M-cells) are large identifiable reticulospinal neurons found in taxa spanning the vertebrate clade, and their activation is associated with selective muscle activity and performance of startle behavior. Variability of responses has been shown to exist both between and within species (e.g. Meyers et al., 1998; Eaton and Emberley, 1991). In larval zebrafish stimulus-direction defendant variability within C-start startle performance exists and is associated with activity of cells that are serial homologs of the M-cell (Liu and Fetcho, 1999; O'Malley et al., 1996). Using, in vivo calcium imaging, single cell laser ablation and high-speed video recording of behavior, I show that descending excitatory interneurons, which may modulate variability through changes in their firing rates (Bhatt et al., 2007), receive input from sources outside of the Mauthner and homologous cells, and that their activity is not restricted to the M-cell associated initial C-bend. Between species M-cell activity can trigger either withdrawal or C-start behaviors with varying patterns of trunk muscle activity. To better understand interspecies variability, I have collected muscle activity and behavioral recordings of startle responses from two phylogenetically important species (lake sturgeon and rope fish) and compiled this data with the existing literature to generate evolutionary maps of startle behaviors and muscle activity patterns. I also survey the morphology of the Mauthner axon cap, a specialized structure that varies in presence and morphology across species (Stefanelli, 1980), but is known in goldfish to greatly modulate the M-cell's excitability (e.g. Faber and Korn, 1978). By comparing evolutionary maps of behavior, muscle activity, and cap morphology, I am able to hypothesize about the function of different cap morphologies. Presence of a composite (goldfish-like) cap may be needed to assure unilateral M-cell activity. In all cap morphologies extra-Mauthner reticulospinal cell activity and the presence and extent of inhibitory components of spinal circuits are important for determining behavior and muscle activity patterns. Together this data concludes that the neural circuitry responsible for the startle response of fishes is more complex than previously believed and may be used as a model for studying more elaborate processing schemes for the control of behavior. |
