Design of mechanosensory feedback during undulatory locomotion to enhance speed and stability.

Undulatory locomotion relies on the propagation of a wave of excitation in the spinal cord leading to consequential activation of segmental skeletal muscles along the body. Although this process relies on self-generated oscillations of motor circuits in the spinal cord, mechanosensory feedback is crucial to entrain the underlying oscillatory activity and thereby, to enhance movement power and speed. This effect is achieved through directional projections of mechanosensory neurons either sensing stretching or compression of the trunk along the rostrocaudal axis.

Sensory Systems in the Peripheral and Central Nervous Systems Shape Host Response During Infections.

The function of sensory cells has been largely investigated in the field of neuroscience for how they report the physical and chemical changes of the environment ("exteroception") and of internal physiology ("interoception"). Investigations over the last century have largely focused on the morphological, electrical and receptor properties of sensory cells in the nervous system focusing on conscious perception of external cues or homeostatic regulation upon detection of internal cues. Research in the last decade has uncovered that sensory cells can often sense polymodal cues, such as mechanical, chemical, and/ or thermal.

CSF-contacting neurons respond to Streptococcus pneumoniae and promote host survival during central nervous system infection.

The pathogenic bacterium Streptococcus pneumoniae (S. pneumoniae) can invade the cerebrospinal fluid (CSF) and cause meningitis with devastating consequences. Whether and how sensory cells in the central nervous system (CNS) become activated during bacterial infection, as recently reported for the peripheral nervous system, is not known.

Spinal sensory neurons project onto the hindbrain to stabilize posture and enhance locomotor speed.

In the spinal cord, cerebrospinal fluid-contacting neurons (CSF-cNs) are GABAergic interoceptive sensory neurons that detect spinal curvature via a functional coupling with the Reissner fiber. This mechanosensory system has recently been found to be involved in spine morphogenesis and postural control but the underlying mechanisms are not fully understood. In zebrafish, CSF-cNs project an ascending and ipsilateral axon reaching two to six segments away.

Sensory Neurons Contacting the Cerebrospinal Fluid Require the Reissner Fiber to Detect Spinal Curvature In Vivo.

Recent evidence indicates active roles for the cerebrospinal fluid (CSF) on body axis development and morphogenesis of the spine, implying CSF-contacting neurons (CSF-cNs) in the spinal cord. CSF-cNs project a ciliated apical extension into the central canal that is enriched in the channel PKD2L1 and enables the detection of spinal curvature in a directional manner. Dorsolateral CSF-cNs ipsilaterally respond to lateral bending although ventral CSF-cNs respond to longitudinal bending.

Glia: A Gate Controlling Animal Behavior?

Although glia are known to have the potential to alter neuronal activity, their behavioral relevance is not well understood. A recent study has discovered that, when zebrafish give up on performing a visuo-motor task, glia integrate information from neuromodulatory neurons to stop motor output.

Taking a Big Step towards Understanding Locomotion.

Locomotion is generated by intrinsically oscillating circuits in the spinal cord that are modulated by information from the brain and periphery. In their seminal 1987 publication, Buchanan and Grillner provided for the first time evidence for excitatory spinal neurons receiving inputs from descending commands and sensory afferents, and synapsing onto motoneurons and commissural inhibitory interneurons. These critical findings established the circuit model for central pattern generators incorporating excitatory interneurons' role in the rhythm-production mechanism.

The Reissner Fiber in the Cerebrospinal Fluid Controls Morphogenesis of the Body Axis.

Organ development depends on the integration of coordinated long-range communication between cells. The cerebrospinal fluid composition and flow properties regulate several aspects of central nervous system development, including progenitor proliferation, neurogenesis, and migration [1-3]. One understudied component of the cerebrospinal fluid, described over a century ago in vertebrates, is the Reissner fiber.

Active mechanosensory feedback during locomotion in the zebrafish spinal cord.

The investigation of mechanosensory feedback to locomotion has been hindered by the challenge of recording neurons in motion. Genetic accessibility and optical transparency of zebrafish larvae provide means to revisit this question. Glutamatergic Rohon-Beard (RB) and GABAergic CSF-contacting neurons (CSF-cNs) are spinal mechanosensory neurons.

Locomotion: Control from the Periphery?

Studies of the neural control of locomotion have tended to focus on the modulation of motoneuron firing by premotor neurons; new work indicates that the regulation of synaptic transmission at the neuromuscular junction can also be important, revealing an inverse relationship between input resistance and synaptic output in motoneurons.

Intraspinal Sensory Neurons Provide Powerful Inhibition to Motor Circuits Ensuring Postural Control during Locomotion.

In the vertebrate spinal cord, cerebrospinal fluid-contacting neurons (CSF-cNs) are GABAergic neurons whose functions are only beginning to unfold. Recent evidence indicates that CSF-cNs detect local spinal bending and relay this mechanosensory feedback information to motor circuits, yet many CSF-cN targets remain unknown. Using optogenetics, patterned illumination, and in vivo electrophysiology, we show here that CSF-cNs provide somatic inhibition to fast motor neurons and excitatory sensory interneurons involved in the escape circuit.

Spinal sensory circuits in motion.

The role of sensory feedback in shaping locomotion has been long debated. Recent advances in genetics and behavior analysis revealed the importance of proprioceptive pathways in spinal circuits. The mechanisms underlying peripheral mechanosensation enabled to unravel the networks that feedback to spinal circuits in order to modulate locomotion.

Optimization of a Neurotoxin to Investigate the Contribution of Excitatory Interneurons to Speed Modulation In Vivo.

Precise control of speed during locomotion is essential for adaptation of behavior in different environmental contexts [1-4]. A central question in locomotion lies in understanding which neural populations set locomotor frequency during slow and fast regimes. Tackling this question in vivo requires additional non-invasive tools to silence large populations of neurons during active locomotion.

Locomotion: Electrical Coupling of Motor and Premotor Neurons.

A new study has found that zebrafish motor neurons can contribute to the generation of locomotor rhythms by feedback modulation of their premotor interneurons via gap junctions.

State-Dependent Modulation of Locomotion by GABAergic Spinal Sensory Neurons.

The cerebrospinal fluid (CSF) constitutes an interface through which chemical cues can reach and modulate the activity of neurons located at the epithelial boundary within the entire nervous system. Here, we investigate the role and functional connectivity of a class of GABAergic sensory neurons contacting the CSF in the vertebrate spinal cord and referred to as CSF-cNs. The remote activation of CSF-cNs was shown to trigger delayed slow locomotion in the zebrafish larva, suggesting that these cells modulate components of locomotor central pattern generators (CPGs).

Neuronal wiring: linking dendrite placement to synapse formation.

Understanding the processes that drive the formation of synapses between specific neurons within a circuit is critical to understanding how neural networks develop. A new study of synapse formation between motor neurons and pre-synaptic partners highlights the importance of dendrite placement.