Fos-like immunohistochemical identification of neurons active during the startle response of the rainbow trout

A review of the neural basis underlying the acoustic startle response with a focus on recent developments in mammals

The startle response consists of whole-body muscle contractions, eye-blink, accelerated heart rate, and freezing in response to a strong, sudden stimulus. It is evolutionarily preserved and can be observed in any animal that can perceive sensory signals, indicating the important protective function of startle. Startle response measurements and its alterations have become a valuable tool for exploring sensorimotor processes and sensory gating, especially in the context of pathologies of psychiatric disorders. The last reviews on the neural substrates underlying acoustic startle were published around 20 years ago. Advancements in methods and techniques have since allowed new insights into acoustic startle mechanisms. This review is focused on the neural circuitry that drives the primary acoustic startle response in mammals. However, there have also been very successful efforts to identify the acoustic startle pathway in other vertebrates and invertebrates in the past decades, so at the end we briefly summarize these studies and comment on the similarities and differences between species.

Altered neuronal activity in the visual processing region of eye-fluke-infected fish

Fish, like most vertebrates, are dependent on vision to varying degrees for a variety of behaviours such as predator avoidance and foraging. Disruption of this key sensory system therefore should have some impact on the ability of fish to execute these tasks. Eye-flukes, such as Tylodelphys darbyi, often infect fish where they are known to inflict varying degrees of visual impairment. In New Zealand, T. darbyi infects the eyes of Gobiomorphus cotidianus, a freshwater fish, where it resides in the vitreous chamber between the lens and retina. Here, we investigate whether the presence of the parasite in the eye has an impact on neuronal information transfer using the c-Fos gene as a proxy for neuron activation. We hypothesized that the parasite would reduce visual information entering the eye and therefore result in lower c-Fos expression. Interestingly, however, c-Fos expression increased with T. darbyi intensity when fish were exposed to flashes of light. Our results suggest a mechanism for parasite-induced visual disruption when no obvious pathology is caused by infection. The more T. darbyi present the more visual stimuli the fish is presented with, and as such may experience difficulties in distinguishing various features of its external environment.

Verification of differentially expressed genes in relation to hydrostatic pressure in the brain of two wrasse species with high-tide preference in spawning

Fish that inhabit shallow water are exposed to periodic changes in tidal cues, including hydrostatic pressure (HP). The present study aimed at verifying differentially expressed genes (DEGs) in the brain of the threespot wrasse Halichoeres trimaculatus (tropical species) and the honbera wrasse Halichoeres tenuispinis (temperate species), both of which were exposed to HP at 30 kPa (possible high-tide stimuli in the field) or 1 kPa (low tide) for 3 or 6 h. A de novo assembly yielded 174,710 contigs (63,530 contigs were annotated) from the brain of threespot wrasse. Following RNA sequencing, quantitative PCR confirmed DEGs that were upregulated [AT atypical cadherin 2 (FAT2)] and downregulated [neuronal leucine-rich repeat protein 3 (LRRN3), dual specificity tyrosine phosphorylation-regulated kinase 1 (DYRK), mitogen-activated protein kinase kinase 1 (MAP2K1) and phosphoinositide 3 kinase (PI3K)]. The effect of HP on the transcription of these DEGs (except for MAP2K1) disappeared within 6 h, suggesting that HP is a transitory stimulus occurring at the beginning of the tidal cycle. Similar DEG transcription was observed in the brain of honbera wrasse maintained under HP for 6 h. In situ hybridization of the brain of the threespot wrasse revealed that strong signals of MPA2K1 were seen in the telencephalon, diencephalon and pituitary, whereas those of PI3K were seen in the telencephalon, diencephalon and medulla oblongata. This result suggests that these kinases are involved in sensory function (telencephalon), somatic and visceral function (medullar oblongata) and the neuroendocrine system (diencephalon and pituitary), all of which were related to changes in HP stimuli. Following HP exposure, the transcription of c-fos increased in the pituitary of honbera wrasse, suggesting that external stimuli directly or indirectly activate hormone synthesis at the hypothalamic-pituitary-gonadal axis. It is concluded that HP alters gene expression in relation to neural development and function in the central nervous system and plays a role in exerting tidal-related reproduction and feeding in wrasses.

Brain regions of marine medaka activated by acute and short-term ocean acidification

Altered behaviors have been reported in many marine fish following exposure to high CO₂ concentrations. However, the mechanistic link between elevated CO₂ and activation of brain regions in fish is unknown. Herein, we examined the relative quantification and location of c-Fos expression in marine medaka following acute (360 min) and short-term (7 d) exposure to CO₂-enriched water (1000 ppm and 1800 ppm CO₂). In the control and two treatment groups, pH was stable at 8.21, 7.92 and 7.64, respectively. After acute exposure to seawater acidified by enrichment with CO₂, there was a clear upregulation of c-Fos protein in the medaka brain (P < 0.05). c-Fos protein expression peaked after 120 min exposure in the two treatment groups and thereafter began to decline. There were marked increases in c-Fos-labeling in the ventricular and periventricular zones of the cerebral hemispheres and the medulla oblongata. After 1800 ppm CO₂ exposure for 7 d, medaka showed significant preference for dark zones during the initial 2 min period. c-Fos protein expression in the ventricular and periventricular zones of the diencephalon in medaka exposed to 1000 ppm and 1800 ppm CO₂ were 0.51 ± 0.10 and 1.34 ± 0.30, respectively, which were significantly higher than controls (P < 0.05). Highest doublecortin protein expression occurred in theventricular zones of the diencephalon and mesencephalon. These findings suggest that the ventricular and periventricular zones of the cerebral hemispheres and the medulla oblongata of marine medaka are involved in rapid acid-base regulation. Prolonged ocean acidification may induce cell mitosis and differentiation in the adult medaka brain.

Immediate early gene expression related to learning and retention of a visual discrimination task in bamboo sharks (Chiloscyllium griseum)

Using the expression of the immediate early gene (IEG) egr-1 as a neuronal activity marker, brain regions potentially involved in learning and long-term memory functions in the grey bamboo shark were assessed with respect to selected visual discrimination abilities. Immunocytochemistry revealed a significant up-regulation of egr-1 expression levels in a small region of the telencephalon of all trained sharks (i.e., 'early' and 'late learners', 'recallers') when compared to three control groups (i.e., 'controls', 'undisturbed swimmers', 'constant movers'). There was also a well-defined difference in egr-1 expression patterns between the three control groups. Additionally, some staining was observed in diencephalic and mesencephalic sections; however, staining here was weak and occurred only irregularly within and between groups. Therefore, it could have either resulted from unintentional cognitive or non-cognitive inducements (i.e., relating to the mental processes of perception, learning, memory, and judgment, as contrasted with emotional and volitional processes) rather than being a training effect. Present findings emphasize a relationship between the training conditions and the corresponding egr-1 expression levels found in the telencephalon of Chiloscyllium griseum. Results suggest important similarities in the neuronal plasticity and activity-dependent IEG expression of the elasmobranch brain with other vertebrate groups. The presence of the egr-1 gene seems to be evolutionarily conserved and may therefore be particularly useful for identifying functional neural responses within this group.

Cannabinoid modulation of zebrafish fear learning and its functional analysis investigated by c-Fos expression

It has been shown that zebrafish fear learning proceeds in the same way as reported for rodents. However, in zebrafish fear learning it is possible to substitute the use of electric shocks as unconditioned stimulus and utilize the inborn fear responses to the alarm substance Schreckstoff, instead. The skin extract Schreckstoff elicits typical fear reactions such as preferred bottom dwelling, swimming in a tighter shoal, erratic movements and freezing. This natural fear behavior can be transferred from Schreckstoff to any other sensory stimulus by associative conditioning (fear learning). We presented Schreckstoff simultaneously with a red light stimulus and tested the effectiveness of fear learning during memory retrieval. The two brain regions known to be relevant for learning in zebrafish are the medial and the lateral pallium of the dorsal telencephalon, both containing rich expressions of the endocannabinoid receptor CB1. To test the influence of the zebrafish endocannabinoid system on fear acquisition learning, an experimental group of ten fish was pretreated with the CB1 receptor agonist THC (Δ⁹-tetrahydrocannabinol; 100nM for 1h). We found that CB1 activation significantly inhibited acquisition of fear learning, possibly by impairing stimulus encoding processes in pallial areas. This was supported by analyzes of c-Fos expression in the brains of experimental animals. Schreckstoff exposure during fear acquisition learning and memory retrieval during red light presentation increased the number of labelled cells in pallial structures, but in no other brain region investigated (e.g. striatum, thalamus, and habenula). THC administration before fear conditioning significantly decreased c-Fos expression in these structures to a level similar to the control group without Schreckstoff experience, suggesting that Schreckstoff induced fear learning requires brain circuits restricted mainly to pallial regions of the dorsal telencephalon.

Dynamics of olfactory and hippocampal neurogenesis in adult sheep

Although adult neurogenesis has been conserved in higher vertebrates such as primates and humans, timing of generation, migration, and differentiation of new neurons appears to differ from that in rodents. Sheep could represent an alternative model to studying neurogenesis in primates because they possess a brain as large as a macaque monkey and have a similar life span. By using a marker of cell division, bromodeoxyuridine (BrdU), in combination with several markers, the maturation time of newborn cells in the dentate gyrus (DG) and the main olfactory bulb (MOB) was determined in sheep. In addition, to establish the origin of adult-born neurons in the MOB, an adeno-associated virus that infects neural cells in the ovine brain was injected into the subventricular zone (SVZ). A migratory stream was indicated from the SVZ up to the MOB, consisting of neuroblasts that formed chain-like structures. Results also showed a long neuronal maturation time in both the DG and the MOB, similar to that in primates. The first new neurons were observed at 1 month in the DG and at 3 months in the MOB after BrdU injections. Thus, maturation of adult-born cells in both the DG and the MOB is much longer than that in rodents and resembles that in nonhuman primates. This study points out the importance of studying the features of adult neurogenesis in models other than rodents, especially for translational research for human cellular therapy.

Activation of a multisensory, multifunctional nucleus in the zebrafish midbrain during diverse locomotor behaviors

Action potentials from the brain control the activity of spinal neural networks to produce, by as yet unknown mechanisms, a variety of motor behaviors. Particularly lacking are details on how identified descending neurons integrate diverse sensory inputs to generate specific locomotor patterns. We have examined the operations of the principal neurons in an intriguing midbrain nucleus, the nucleus of the medial longitudinal fasciculus (nMLF), in the larval zebrafish. The nMLF is the most rostral grouping of neurons that projects from the brain well into the spinal cord of teleost fishes, yet there is little direct physiological data available regarding its function. We report here that a distinct set of large, individually-identifiable neurons in nMLF (the MeL and MeM neurons) are activated by diverse sensory stimuli and contribute to distinct locomotor behaviors. Using in vivo confocal calcium imaging we observed that both photic and mechanical stimuli elicit calcium responses indicative of the firing of action potentials. Calcium responses were observed simultaneously with distinct swimming, turning and struggling movements of the larval trunk. While selectively contralateral responses were at times observed in response to a head-tap stimulus, these nMLF cells showed roughly similar numbers of bilateral responses. Calcium responses were observed at a range of latencies, suggesting involvement with both slow swimming patterns and the burst swimming component of the escape behavior. The MeL cells in particular were strongly activated during light-evoked slow swimming. The activation of MeL cells during the slow and burst forward swim gaits is consistent with their driving and/or coordinating the activity of slow and fast central pattern generators in spinal cord. As such, the MeL cells may help to shape a variety of larval behaviors including the optomotor response, escape swimming and prey capture.

Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

Control of visually guided behavior by distinct populations of spinal projection neurons

A basic question in the field of motor control is how different actions are represented by activity in spinal projection neurons. We used a new behavioral assay to identify visual stimuli that specifically drive basic motor patterns in zebrafish. These stimuli evoked consistent patterns of neural activity in the neurons projecting to the spinal cord, which we could map throughout the entire population using in vivo two-photon calcium imaging. We found that stimuli that drive distinct behaviors activated distinct subsets of projection neurons, consisting, in some cases, of just a few cells. This stands in contrast to the distributed activation seen for more complex behaviors. Furthermore, targeted cell by cell ablations of the neurons associated with evoked turns abolished the corresponding behavioral response. This description of the functional organization of the zebrafish motor system provides a framework for identifying the complete circuit underlying a vertebrate behavior.

Some voluntary C-bends may be Mauthner neuron initiated

Predatory fish sometimes capture a prey fish first by striking it from the side, allowing the predator to consume the stunned prey head first. The rapid body flexion that the predator uses to stun its prey is similar to the "C" shaped maneuver ("C-bend") that many fish species use when performing a C-start escape response. For most species, one of the two Mauthner neurons initiates the C-start and, together with other reticulospinal neurons, their activity determines the extent of the bend and the ultimate trajectory of the fish. Reported here is initial evidence of previously undescribed behaviors where goldfish strike an object while executing voluntary C-bends that are similar to their C-start escape responses. The overlapping distributions of turn durations, turn angles, and angular velocities suggest that at least some voluntary C-bends are initiated by the Mauthner neuron. This implies that the Mauthner neuron can be activated voluntarily in the absence of predator- or feeding-associated releasing stimuli.

Functional evidence for visuospatial coding in the Mauthner neuron

When startled by sound, goldfish make large turns away from a rostral stimulus and small responses away from caudal stimuli, suggesting that rostral startling stimuli recruit larger pools of reticulospinal neurons in the Brainstem Escape Network (BEN) than do caudal stimuli. Consistent with this idea, the zebrafish Mauthner (M-) cell fires when the fish is startled by tail-directed stimuli, but the M-cell homologues (MiD2cm and MiD3cm) are also recruited when the fish is startled by displacing the head. Because vision is known to modulate M-cell activity, a nonstartling, modulatory sensory 'signal' conveyed to the reticular formation may be stronger if the visual sensory image is from a rostral vs. caudal spatial location and could account for a differential neuron pool recruitment and response magnitude. In this study, electrophysiological recordings from cichlid Mauthner neurons showed that visual stimulation of the caudal retina (by a rostral cue) generates a depolarization that is about 1.5 times the amplitude of that generated by stimulation of the rostral retina (by a caudal cue). In behavioral testing, where fish were stimulated visually for 30 ms and then startled by sound, fish startled in the presence of a rostral visual stimulus performed larger amplitude and faster turns than when startled in the presence of a caudal visual stimulus. Thus, M-cell potentials might reflect the strength of visual input to the BEN in general. For a particular visual spatial location, the relative strength of descending visual input appears to contribute to a recruitment of a reticulospinal neuron population that generates a turn magnitude appropriate to the visual cue, and suggests that a retinotopic representation is preserved in the BEN.

Common sensory inputs and differential excitability of segmentally homologous reticulospinal neurons in the hindbrain

In the hindbrain of zebrafish and goldfish, reticulospinal (RS) neurons are arranged in seven segments, with segmental homologs in adjacent segments. The Mauthner cell (M-cell) in the fourth segment (r4) is known to trigger fast escape behavior. Its serial homologs, MiD2cm in r5 and MiD3cm in r6, are predicted to contribute to this behavior, which can be evoked by head-tap stimuli. However, little is known about their input-output properties. Therefore, we studied afferent projections from the auditory posterior eighth nerve (pVIIIn) and firing properties of MiD2cm and MiD3cm for comparison with the M-cell in adult goldfish. Labeling of RS neurons and the pVIIIn afferents with fluorescent tracers showed that the pVIIIn projected to r4-r6. Tone burst and electrical stimulation of the pVIIIn evoked EPSPs in the M-cell, MiD2cm, and MiD3cm. Stepwise depolarization typically elicited a single spike at the onset in the M-cell but repetitive spiking in MiD2cm and MiD3cm. This atypical property of the M-cell was mediated by dendrotoxin-I (DTX-I)-sensitive voltage-gated potassium channels together with recurrent inhibition, because combined application of DTX-I, strychnine, and bicuculline led to continuous repetitive firing in M-cells. The M-cell but not MiD2cm or MiD3cm expressed Kv1.2, a DTX-I-sensitive potassium channel subunit. Thus, the M-cell and its segmental homologs may sense common auditory information but send different outputs to the spinal circuits to control adaptive escape behavior.

Of lasers, mutants, and see-through brains: functional neuroanatomy in zebrafish

Behavioral functions are carried out by localized circuits in the brain. Although this modular principle is clearly established, the boundaries of modules, and sometimes even their existence, are still debated. Zebrafish might offer distinct advantages in localizing behaviors to discrete brain regions because of the ability to visualize, record from, and lesion precisely identified populations of neurons in the brain. In addition, genetic screens in zebrafish enable the isolation of mutations that disrupt neural pathways and/or behaviors, as an alternative lesioning technique with complementary strengths to laser ablations. For example, the Mauthner cell, a large identified neuron in the hindbrain, has been postulated to be both necessary and sufficient for the execution of escapes. We discuss in this review how experiments, using laser ablations, calcium imaging, and mutants have eroded this notion. Even in a simple behavior, such as escape, many parallel pathways appear to be involved with no single one being absolutely necessary. Lesion studies and the analysis of behavioral mutants are now also beginning to elucidate the functional architecture of the zebrafish visual system. Although still in an embryonic stage, the neuroanatomy of behaviors in zebrafish has a bright future.

Temporal constraints on visually directed C-start responses: behavioral and physiological correlates

To study the modulatory influences of visual information on Mauthner (M-) initiated C-start responses, short interval visual stimuli were presented to individual cichlid fish prior to being startled with a sound pulse. Because the axon of each Mauthner neuron activates trunk musculature contralateral to the soma, the initial direction of an ensuing startle response provides a behavioral measure of which cell has been driven closer to threshold by visual cues at the time the sound pulse causes one cell to fire. When an LED was illuminated on one side of the fish for 10 ms prior to a startling sound pulse, cichlids reliably turned toward the visual cue. At durations of 15 and 20 ms, fish turned away from the same stimulus. Thus, behavioral evidence suggests that the M-cell contralateral to a visual stimulus appears to be excited first but visual excitation of the M-cell ipsilateral to the visual stimulus follows and predominates. Consistent with the behavioral results, visually evoked excitatory potentials recorded intracellularly in the cichlid M-cells were complex, with initial PSPs showing latencies of about 11.6 ms from contralateral eye stimulation and 15.5 ms from stimulating the ipsilateral eye. PSP latencies in goldfish were longer and more similar for stimuli to the two eyes (about 22 ms). For contrast, sound-evoked PSPs begin within 2 ms. The relative long latencies for visually regulating M-cell function suggest that vision is most adaptive for biasing response direction prior to rather than during a predator's attack.

Probing neural circuits in the zebrafish: a suite of optical techniques

The ability to image neural activity in populations of neurons inside an intact animal, while obtaining single-cell or subcellular spatial resolution, has led to several advances in our understanding of vertebrate locomotor control. This result, first reported in a 1995 study of motoneurons in larval zebrafish, was the beginning of a series of technical developments that exploited the transparency and simplicity of the larval CNS. Presented here, in chronological fashion, is a suite of imaging techniques that have extended the ability to probe and optically dissect neural control systems. Included are methodological details pertaining to: (1). the in vivo optical recording of neural activity, (2). the optical dissection of complex neural architectures, and (3). additional fluorescence imaging-based techniques for the anatomical and physiological characterization of these systems. These approaches have provided insights into the descending neural control of escape and other locomotive behaviors, such as swimming and prey capture. The methods employed are discussed in relation to complementary and alternative imaging techniques, including, for example, the Nipkow disk confocal. While these methodologies focus on descending motor control in the larval zebrafish, the extension of such approaches to other neural systems is viewed as a promising and necessary step if neurobiologists are to bridge the gap between synaptic and brain region levels of analysis. The efficiency of optical techniques for surveying the cellular elements of intricate neural systems is of particular relevance because a comprehensive description of such elements is deemed necessary for a precise understanding of vertebrate neural architectures.

Visually guided injection of identified reticulospinal neurons in zebrafish: a survey of spinal arborization patterns

We report here the pattern of axonal branching for 11 descending cell types in the larval brainstem; eight of these cell types are individually identified neurons. Large numbers of brainstem neurons were retrogradely labeled in living larvae by injecting Texas-red dextran into caudal spinal cord. Subsequently, in each larva a single identified cell was injected in vivo with Alexa 488 dextran, using fluorescence microscopy to guide the injection pipette to the targeted cell. The filling of cells via pressure pulses revealed distinct and often extensive spinal axon collaterals for the different cell types. Previous fills of the Mauthner cell had revealed short, knob-like collaterals. In contrast, the two segmental homologs of the Mauthner cell, cells MiD2cm and MiD3cm, showed axon collaterals with extensive arbors recurring at regular intervals along nearly the full extent of spinal cord. Furthermore, the collaterals of MiD2cm crossed the midline at frequent intervals, yielding bilateral arbors that ran in the rostral-caudal direction. Other medullary reticulospinal cells, as well as cells of the nucleus of the medial longitudinal fasciculus (nMLF), also exhibited extensive spinal collaterals, although the patterns differed for each cell type. For example, nMLF cells had extensive collaterals in caudal medulla and far-rostral spinal cord, but these collaterals became sparse more caudally. Two cell types (CaD and RoL1) showed arbors projecting ventrally from a dorsally situated stem axon. Additional cell-specific features that seemed likely to be of physiological significance were observed. The rostral-caudal distribution of axon collaterals was of particular interest because of its implications for the descending control of the larva's locomotive repertoire. Because the same individual cell types can be identified from fish to fish, these anatomical observations can be directly linked to data obtained in other kinds of experiments. For example, 9 of the 11 cell types examined here have been shown to be active during escape behaviors.

Prey capture by larval zebrafish: evidence for fine axial motor control

Swimming and turning behaviors of larval zebrafish have been described kinematically, but prey capture behaviors are less well characterized. High-speed digital imaging was used to record the axial kinematics of larval zebrafish as they preyed upon paramecia and also during other types of swimming. In all types of swim bouts, a series of traveling waves of bending is observed and these bends propagate along the trunk in the rostral to caudal direction. The prey capture swim bouts appeared to be more complex than other swim patterns examined. In the capture swim bouts, the initial bends were of low amplitude and were most prominent at far-caudal locations during each individual traveling wave of bending. Later bends in the bout (occurring just prior to prey capture) appeared to originate more rostrally and were of larger amplitude. These changes in bending pattern during capture swims were accompanied by a marked increase in tail-beat frequency. Associated with these axial kinematics were changes in heading and an abrupt increase in velocity close to the moment of prey capture. These changing patterns of bending suggest precise, bend-to-bend, neural control over both the timing and the rostral-caudal locus of bending. This degree of 'fine axial motor control' has not previously been described in the teleost behavioral literature and is notable because it occurs in larval zebrafish, where descending control signals are funneled through the roughly three-hundred neurons that project from brain into spinal cord. These findings will necessitate a significant increase in the complexity of current models of descending motor control in fishes.

Evidence for a widespread brain stem escape network in larval zebrafish

Zebrafish escape behaviors, which typically consist of a C bend, a counter-turn, and a bout of rapid swimming, are initiated by firing of the Mauthner cell and two segmental homologs. However, after laser-ablation of the Mauthner cell and its homologs, escape-like behaviors still occur, albeit at a much longer latency. This might suggest that additional neurons contribute to this behavior. We therefore recorded the activity of other descending neurons in the brain stem using confocal imaging of cells retrogradely labeled with fluorescent calcium indicators. A large majority of identified descending neurons present in the larval zebrafish, including both ipsilaterally and contralaterally projecting reticulospinal neurons, as well as neurons from the nucleus of the medial longitudinal fasciculus, showed short-latency calcium responses after gentle taps to the head of the larva-a stimulus that reliably evokes an escape behavior. Previous studies had associated such in vivo calcium responses with the firing of action potentials, and because all responding cells have axons projecting into to spinal cord, this suggests that these cells are relaying escape-related information to spinal cord. Other identified neurons failed to show consistent calcium responses to escape-eliciting stimuli. In conjunction with previous lesion studies, these results indicate that the neural control systems for turning and swimming behaviors are widely distributed in the larval zebrafish brain stem. The degree of robustness or redundancy of this system has implications for the descending control of vertebrate locomotion.