Locomotion is intimately linked to many behaviors, allowing for social interactions, spatial navigation, and hazard avoidance. Locomotor movements are seemingly simple and stereotyped, but the path to their execution is complex and entails many levels of integration to the final sequence of well-coordinated muscle activity along many body segments. The locomotor networks, called central pattern generator (CPG), must be endowed with mechanisms enabling precise tuning of the onset of locomotion, its speed, and coordination to match the intended behavioral outcome, such as slowly approaching a prey or rapidly escaping a predator. This is not a trivial task—it involves mechanisms ingrained within the connectivity of the CPG circuit combined with specific intrinsic neuronal properties. Research from our laboratory has revealed central organizational principles of the vertebrate locomotor networks.

 
Separate circuits for speed control 

How is the speed of movement encoded? The ability to vary the speed and force of our movements is essential for our survival. We move fast to avoid a threat or slow to approach a delicate object. This requires a fine control of the activity of motoneuron pools innervating muscles with slow, intermediate and fast contractile properties. Until recently, the prevailing view in motor control in general, and locomotion in particular, has been that the locomotor CPG consists of a single network that indiscriminately broadcasts the excitatory drive to all motoneurons. In this view, the order of motoneuron recruitment is dictated solely by the intrinsic properties of motoneurons according to the size principle. Our research has invalidated this long-held view and revealed that each motoneuron pool is driven by a dedicated type of excitatory V2a interneuron. This organization involves three V2a-MN circuit modules that are engaged individually or in combination to control the speed of locomotor movements from slow explorative to fast-speed locomotion. This ensemble circuit module organization endows the locomotor CPG with a gearshift mechanism to change the speed of locomotion, rather than relying only on acceleration based on the size principle as previously thought.

 
Multiple rhythm generating circuits

The switch from a quiescent state to the start of locomotion initially involves the generation of tonic commands from brain. The first relay for the transformation of these descending commands lies in the excitatory rhythm-generating interneuron populations, which represent the entry point to the locomotor CPG. Our research using adult zebrafish has identified the excitatory V2a interneurons, marked by the expression of the transcription factor Chx10, as the core rhythm-generating interneurons. Using a comprehensive mapping of synaptic connections, we revealed that V2a interneurons form three circuit modules interconnected by recurrent and hierarchical synaptic connections. Subpopulations of V2a interneurons are endowed with facultative pacemaker bursting properties, which allows V2a interneurons to pace the locomotor circuit at the appropriate speed. Indeed, the prevalence and intrinsic bursting frequency relates to V2a module affiliation and thus their recruitment speed. Thus, our studies have revealed fundamental insights into how locomotor circuit design and pacemaker properties work in tandem to provide an ignition and gear-shift mechanism to start locomotion and change speed.

 
Retrograde influence of motor neurons

Historically, the prevailing view in motor control was that motor neurons form the “final common pathway”; they serve purely as a relay of the final motor program generated by upstream interneuron circuits. Our recent results have changed this view and revealed an unforeseen influence of motor neurons on the spinal locomotor rhythm via electrical coupling to premotor excitatory interneurons. By combining whole-cell paired recordings, optogenetics and anatomy in adult zebrafish, we showed the existence of bidirectional electrical coupling between motor neuron dendrites and V2a interneuron axon terminals. This endows the motor neurons with the capacity, via backward propagation of electrical signals, to strongly impact the synaptic transmission and firing properties of the premotor rhythm-generating V2a interneurons and as a result influence the performance of the locomotor CPG. These results show that motor neurons are not a passive recipient of motor command but an integral component of the neural circuits for motor behavior.

 
Mechanisms for behavioral choice

Animals possess a rich behavioral repertoire and are endowed with decision-making mechanisms to select the most relevant behavior for a given context. Using adult zebrafish, we have examined the neural underpinnings of the context-dependent behavioral selection between escape and swimming, by recapitulating these two motor behaviors and the interaction between them in an in vitro preparation. The selection of escape over swimming is mediated by a switch between fast and slow motoneuron pools. The fast motoneuron pool underlying escape is engaged while the slow pool underlying swimming is disengaged via indirect inhibition that operates as a clutch to decouple these motoneurons from the premotor circuit. The onset of the escape, and the associated inhibition of swimming activity, are determined by a hardwired fast circuit. However, the threshold for initiation of escape and the extent of inhibition of swimming rely on wireless endocannabinoid neuromodulation acting in reverse to the wiring diagram. These results revealed a novel mechanism involving a hardwired circuit supplemented with endocannabinoid modulation that acts as a clutch to shift between slow and fast motoneuron pools and hence produce behavioral choice in vertebrates.