Principles Governing Locomotion in Vertebrates: Lessons From Zebrafish

Abstract

Locomotor behaviors are critical for survival and enable animals to navigate their environment, find food and evade predators. The circuits in the brain and spinal cord that initiate and maintain such different modes of locomotion in vertebrates have been studied in numerous species for over a century. In recent decades, the zebrafish has emerged as one of the main model systems for the study of locomotion, owing to its experimental amenability, and work in zebrafish has revealed numerous new insights into locomotor circuit function. Here, we review the literature that has led to our current understanding of the neural circuits controlling swimming and escape in zebrafish. We highlight recent studies that have enriched our comprehension of key topics, such as the interactions between premotor excitatory interneurons (INs) and motoneurons (MNs), supraspinal and spinal circuits that coordinate escape maneuvers, and developmental changes in overall circuit composition. We also discuss roles for neuromodulators and sensory inputs in modifying the relative strengths of constituent circuit components to provide flexibility in zebrafish behavior, allowing the animal to accommodate changes in the environment. We aim to provide a coherent framework for understanding the circuitry in the brain and spinal cord of zebrafish that allows the animal to flexibly transition between different speeds, and modes, of locomotion.

Keywords: excitatory interneurons; motor behavior and motor control; neural networks; plasticity; spinal cord.

Figure 1

Known active neuron types and established connectivity within the spinal swim central pattern generator (CPG) network of larval zebrafish. (A) During slow swimming (<30 Hz), only the most ventral motoneurons (MNs) are active, which receive excitatory drive from ventrally-located circumferential descending (CiD) interneurons (INs; ventral V2a INs). Commissural excitatory INs multipolar commissural descendings (MCoDs), a subtype of V0v IN, also provide excitation to secondary motor neurons from the contralateral side. (B) During fast swimming (>30 Hz), the ventral MNs remain active but more dorsal secondary MNs and primary MNs also become recruited. Both the ventral CiD and MCoDs are actively de-recruited, whereas the dorsally-located CiDs and displaced dorsal CiDs (dis-CiD) become active. The displaced CiDs provide selective excitation to primary motor neurons. Note that several IN types known to be active during swimming are not displayed as their roles are less well defined.

Figure 2

The modular swim network in adult zebrafish. The IN and MN populations in the adult zebrafish are subdivided into three main types depending on the recruitment frequency: slow (red), intermediate (green) and fast (blue). This subdivision reflects the three distinct muscle types that in zebrafish are spatially segregated (right): the anaerobic “white” fibers (in blue), the aerobic “red” fibers (in red) and the intermediate fibers (in green). The main ipsilateral excitatory drive comes from the V2a population that is connected preferentially to MNs belonging to the same module through mixed chemical and electrical synapses. The V0v population in adult zebrafish also shows a differential speed-depended recruitment order, although its connectivity has not been yet defined. Bottom: MN pools are spatially defined, while V0v and V2a INs have lost their topographical organization during development.

Figure 3

The brainstem and spinal cord network in larval zebrafish for mediating a fast-onset C start response. (A) An acoustic stimulus from the left stimulates afferent hair cells, which provide direct excitation to the left M-cell (purple) via mixed chemical and electrical synaptic boutons. Afferent hair cells also provide convergent excitation to the ipsilateral M-cell via activation of spiral fiber neurons (SFNs). The afferent hair cells also stimulate a population of feedforward (FF) inhibitory INs, which mostly inhibit the contralateral M-cell and contralateral FF inhibitory neurons. As the M-cell spike propagates down its axon it stimulates cranial relay neurons (CRNs) via axon collaterals, which activate feedback (FB) inhibitory INs that prevent the M-cell from generating consecutive spikes. (B) When the M-cell axon reaches the spinal cord, it provides direct chemical excitation of primary MNs on the right side, as well as to excitatory CiD INs. CiD INs provide further excitation via mixed chemical and electrical synapses to primary and fast secondary MNs. Finally, the M-cell axon also rapidly excites inhibitory Commissural local INs (CoLos) via electrical synapses, which in turn inhibit various MNs and INs on the contralateral side. (C) The activation of this escape network results in strong contraction of fast musculature on the right side of the body, resulting in fish curving into the characteristic C-shape.

Figure 4

Modulation of adult swimming and escape circuits by endocannabinoids (eCBs). Swimming behavior is generated by speed dependent excitation of MNs by the spinal swim CPG network (gray connections). Activation of the escape network (purple connections) leads to temporary interruption of swim behavior. The escape command leads to a switch from slow to fast MN pools which is mediated by direct excitation of fast (blue) and indirect inhibition of slow MNs (red). The selection of escape over swimming behavior is positively modulated by eCB through enhancement of synaptic transmission.

Figure 5

Sensory modulation of swimming and escape circuits in larval zebrafish. (A) Pathways of Rohon-Beard (RB) neurons activity. Top: disynaptic cutaneous skin reflex. When the fish is at rest, an activation of RB sensory neurons results in swimming away from the stimulus through the excitation of commissural primary ascending (CoPA) INs. This mechanism is inhibited by during ongoing swimming. Bottom: during swimming, activation of RB sensory neurons act on the fast system increasing swimming frequency through the excitation of dorsal V2a INs. (B) Cerebrospinal fluid-contacting neurons (CSF-CNs) activity pathways. Top: lateral bending of the spinal cord during swimming activates CSF-CNs which respond inhibiting the MCoD (V0v) INs therefore stopping locomotion. Bottom: CSF-CNs are activated during escape behavior and act on the fast system inhibiting caudal primary MNs (CaP) and the CoPA INs, decreasing the swimming frequency. cc: central canal.