title: Local Spinal Cord Circuits and Bilateral Mauthner Cell Activity Function Together to Drive Alternative Startle Behaviors.
abstract: The reticulospinal Mauthner cells (M-cells) of the startle circuit have been considered to be dedicated to one basic motor output and the C-type startle response in fish. The neural circuit underlying the C-start, a startle behavior in which the fish forms a "C"-shaped body bend has been described in depth in goldfish and zebrafish [1, 2] and is thought to occur in other species [3, 4]. However, previous research has shown that some species can perform a second type of startle called the S-start [5-7]. This startle response, in which the first movement creates an "S"-shaped body bend achieved with regional muscle activity on left and right sides, cannot be explained by M-cell circuit models. Here we use larval zebrafish to examine the S-start circuit. Since S-starts are elicited through tail stimulation [5-7] and ablating M-cells abolishes short-latency tail-elicited startles [8, 9], we hypothesized that M-cell activity was necessary for S-start generation. Our findings show that the M-cells fire simultaneously to generate the S-start. However, simultaneous M-cell spikes generated through direct current injection were not sufficient to generate S-starts. Through recordings of motoneurons, inhibitory interneurons, and sensory neurons, we uncover a mechanism for generating alternative startle behaviors; local sensory inputs drive inhibitory interneuron activity, which inhibits caudal motoneurons and pre-conditions their excitability prior to the arrival of M-cell spikes in the tail. We suggest that this motoneuron hyperpolarization can bias motor output to left or right sides, determining whether the fish performs a C-start or an S-start behavior.
authors.affiliation: Department of Organismal Biology and Anatomy, The University of Chicago, 1027 E. 57(th) Street, Chicago, IL 60637, USA.
authors.forename: Yen-Chyi
authors.initials: YC
authors.lastname: Liu
authors.affiliation: Department of Organismal Biology and Anatomy, The University of Chicago, 1027 E. 57(th) Street, Chicago, IL 60637, USA. Electronic address: mhale@uchicago.edu.
authors.forename: Melina E
authors.initials: ME
authors.lastname: Hale
mesh_terms: D000818:Animals; D007395:Interneurons / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009434:Neural Pathways / Q000502:physiology; D013216:Reflex, Startle / Q000502:physiology*; D011984:Sensory Receptor Cells / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*; D015027:Zebrafish / Q000502:physiology*
keywords: Mauthner cell; circuitry; descending input; escape behavior; hindbrain; motoneuron; motor control; preconditioning; sensory input; spinal cord; startle