Fig. 1

Fig. 1

Two types of V2a neuron distinguished by intrinsic properties. a Schematics illustrating multi-layer (top) and single-layer (bottom) models of connectivity underlying timing and amplitude control. + excitatory interneurons, – inhibitory interneurons, MN motor neurons, L left side, R right side. Arrows are glutamatergic connections and the resistor symbol denotes electrical coupling. b Top, schematic illustrating a larval zebrafish viewed from the side. Region at midbody (segments 10–16) where recordings were performed is shaded. Rostral, left; dorsal, up. Bottom, schematic illustrating muscle segments and each of the four identified primary motor neurons per spinal hemisegment (dotted circles) that innervate either epaxial (pI-II) or hypaxial (pIII-IV) muscle (horizonal blue or brown lines). hm, horizontal myoseptum. c Single confocal optical section from one spinal segment illustrating type I (purple arrowheads) and type II (green arrowheads) V2a neurons targeted for recordings. Soma positions are normalized to the dorsal and ventral aspects of spinal cord (d-v). Rostral is to the left. d Top, contrast inverted epifluorescent images of fills after electrophysiological recordings. Bottom, reconstructions illustrating the trajectory of the main axon. The descending axon of the type I V2a neuron continues out of the field of view (arrowhead). Scale to the left indicates normalized dorso-ventral soma position. e As in panel d, but for a bifurcating type II V2a neuron. In top image, black arrowheads mark axon bifurcation point and the continuation of the ascending axon out of the field of view. f Quantification of input resistance (Rin) and rheobase according to V2a neuron type. Type I V2a (n = 105), type II V2a (n = 145). Source data are provided in a Source Data file. g Top, instantaneous firing frequencies are plotted above the respective recordings from a type I V2a neuron. Bottom, examples of voltage deflections in response to a hyperpolarizing current step (–25 pA) and firing responses to current steps around threshold (left trace) and just above threshold (right trace). Voltage; I, current. Source data are provided in a Source Data file. h, As in panel g, but for a type II V2a neuron