Animals move over a range of speeds by using rhythmic networks of neurons located in the spinal cord. Here we use electrophysiology and in vivo imaging in larval zebrafish (Danio rerio) to reveal a systematic relationship between the location of a spinal neuron and the minimal swimming frequency at which the neuron is active. Ventral motor neurons and excitatory interneurons are rhythmically active at the lowest swimming frequencies, with increasingly more dorsal excitatory neurons engaged as swimming frequency rises. Inhibitory interneurons follow the opposite pattern. These inverted patterns of recruitment are independent of cell soma size among interneurons, but may be partly explained by concomitant dorso-ventral gradients in input resistance. Laser ablations of ventral, but not dorsal, excitatory interneurons perturb slow movements, supporting a behavioural role for the topography. Our results reveal an unexpected pattern of organization within zebrafish spinal cord that underlies the production of movements of varying speeds.

Segmentation of the vertebrate brain is most obvious in the hindbrain, where successive segments contain repeated neuronal types. One such set of three repeated reticulospinal neurons--the Mauthner cell, MiD2cm, and MiD3cm--is thought to produce different forms of the escape response that fish use to avoid predators. We used laser ablations in larval zebrafish to test the hypothesis that these segmental hindbrain cells form a functional group. Killing all three cells eliminated short-latency, high-performance escape responses to both head- and tail-directed stimuli. Killing just the Mauthner cell affected escapes from tail-directed but not from head-directed stimuli. These results reveal the contributions of one set of reticulospinal neurons to behavior and support the idea that serially repeated hindbrain neurons form functional groups.

The classic ‘size principle’ of motor control describes how increasingly forceful movements arise by the recruitment of motoneurons of progressively larger size and force output into the active pool. Here, we explore the activity of pools of spinal interneurons in larval zebrafish and find that increases in swimming speed are not associated with the simple addition of cells to the active pool. Instead, the recruitment of interneurons at faster speeds is accompanied by the silencing of those driving movements at slower speeds. This silencing occurs both between and within classes of rhythmically-active premotor excitatory interneurons. Thus, unlike motoneurons, there is a continuous shift in the set of cells driving the behavior, even though changes in the speed of the movements and the frequency of the motor pattern appear smoothly graded. We conclude that fundamentally different principles may underlie the recruitment of motoneuron and interneuron pools.

We report the development of aminergic neurons from 0-10 days postfertilization (dpf) in zebrafish (Danio rerio). This study was prompted by the lack of information regarding patterns of spinal aminergic innervation at early stages, when the fish are accessible to optical, genetic, and electrophysiological approaches toward understanding neural circuit function. Our findings suggest that aminergic populations with descending processes are among the first to appear during development. Descending aminergic fibers, revealed by antibodies to tyrosine hydroxylase (TH) and serotonin (5-hydroxytryptamine; 5-HT), innervate primarily the ventral (TH, 5-HT), but also the dorsal (5-HT) aspects of the spinal cord by 4 dpf, with the extent of innervation not changing markedly up to 10 dpf. By tracking the spatiotemporal expression of TH, 5-HT, and dopamine beta hydroxylase reactivity, we determined that these fibers likely originate from neurons in the posterior tuberculum (dopamine), the raphe region (5-HT) and, possibly, the locus coeruleus (noradrenaline). In addition, spinal neurons positive for 5-HT emerge between 1-2 dpf, with processes that appeared to descend along the ventrolateral cord for only 1-2 muscle segments. Their overall morphology distinguished these cells from previously described "VeMe" (ventromedial) interneurons, which are also located ventromedially, but have long, multisegmental descending processes. We confirmed the distinction between spinal serotonergic and VeMe interneurons using fish genetically labeled with green fluorescent protein. Our results suggest that the major aminergic systems described in adults are in place shortly after hatching, at a time when zebrafish are accessible to a battery of techniques to test neuronal function during behavior.