Caenorhabditis elegans excitatory ventral cord motor neurons derive rhythm for body undulation

Abstract

The intrinsic oscillatory activity of central pattern generators underlies motor rhythm. We review and discuss recent findings that address the origin of Caenorhabditis elegans motor rhythm. These studies propose that the A- and mid-body B-class excitatory motor neurons at the ventral cord function as non-bursting intrinsic oscillators to underlie body undulation during reversal and forward movements, respectively. Proprioception entrains their intrinsic activities, allows phase-coupling between members of the same class motor neurons, and thereby facilitates directional propagation of undulations. Distinct pools of premotor interneurons project along the ventral nerve cord to innervate all members of the A- and B-class motor neurons, modulating their oscillations, as well as promoting their bi-directional coupling. The two motor sub-circuits, which consist of oscillators and descending inputs with distinct properties, form the structural base of dynamic rhythmicity and flexible partition of the forward and backward motor states. These results contribute to a continuous effort to establish a mechanistic and dynamic model of the C. elegans sensorimotor system. C. elegans exhibits rich sensorimotor functions despite a small neuron number. These findings implicate a circuit-level functional compression. By integrating the role of rhythm generation and proprioception into motor neurons, and the role of descending regulation of oscillators into premotor interneurons, this numerically simple nervous system can achieve a circuit infrastructure analogous to that of anatomically complex systems. C. elegans has manifested itself as a compact model to search for general principles of sensorimotor behaviours.This article is part of a discussion meeting issue 'Connectome to behaviour: modelling C. elegans at cellular resolution'.

Keywords: central pattern generators; functional compression; motor neuron; neuromechanical model; projection-premotor interneuron; proprioception.

Figure 1.

An integrative model for backward locomotion: local reversal oscillators are phase-coupled via proprioception, and dually regulated by descending inputs. (a) The A-class motor neurons exhibit intrinsic, oscillatory activities that are sufficient to drive backward movement. (Top panel) Calcium oscillation in the posterior A motor neuron was observed in animals where chemical synaptic transmission and all premotor interneurons were removed from the nervous system. Left, sample traces; right, raster plot of recording from multiple animals. (Bottom panel) A dissected ventral cord muscle preparation from an animal where all premotor interneurons were removed exhibited anterior A-class motor-dependent rhythmic postsynaptic currents (rPSCs) and action potential (AP) bursts, both denoted by red arrowheads. (b) The A-class motor neurons may use intrinsic proprioceptive properties to self-organize phase-coupling during backward movement. (Top panels) A comparison of calcium activities exhibited by posterior A-class motor neurons in an immobilized (left) and a freely moving (reversal) animal (right), where all premotor interneurons were removed from the nervous system. Movements strengthened both calcium oscillation of and phase-coupling among A-class motor neurons. (c) The AVA premotor interneurons provide descending inputs that dually regulate the A-class motor neuron's oscillation through a mixed gap junction and chemical synapse configuration. Gap-junction-mediated coupling between AVA and A-class motor neurons shunts their intrinsic oscillation, whereas chemical synapses allow optogenetically activated AVA to potentiate their oscillation. (d) A model: backward movement is driven by oscillation from a chain of distributed CPGs (the A-class motor neurons), phase-coupled by proprioceptive feedback and regulated by descending inputs. Figure panels adapted from [39,53].

Figure 2.

Multiple A-class motor neurons function as oscillators during backward movements. (a) Ablation of A-class motor neurons that reside in a restricted body segments did not prevent bending wave propagation at adjacent regions. (i) Schematics of the soma position of the A-class motor neurons, arbitrarily separated into the anterior, mid- and posterior segments. (ii) Representative kymographs of bending curvature along the body of animals missing A-class motor neurons in the anterior (left), mid- (centre) and posterior (right) body segments. (b) An animal with its mid-body trapped inside a microfluidic device continued to generate anteriorly propagating bending waves in the unrestrained anterior and posterior segment with different frequencies, as shown in a series of video frames (left), and by kymograph of time-varying curvature along the body (right) where vertical lines mark the anterior and posterior limits of the straight channel. Panel (a) adapted from [39]. See Materials and Methods.

Figure 3.

A biophysical model for forward locomotion that integrates local oscillators, proprioceptive couplings and descending inputs from premotor interneurons. (a) The B-class motor neurons in the mid-body exhibited rhythmic calcium activities upon optogenetic activation of premotor interneuron AVBs. Note that imaging experiments were carried out in mutants unc-13 (e51) where chemical synaptic transmission in the whole nervous system was largely abolished. (b) Local and directional (anterior to posterior) proprioceptive couplings propagate body undulations. When a mid-body region of a worm was constrained in a straight microfluidic channel, the posterior body region emerged from the channel would remain still and straight. Curvature kymograph showed that the bending waves could only propagate to the anterior limit of the channel. When dynamic curvature change in the worm mid-body was imposed by a pneumatic microfluidic device, rapid curvature changes and bending waves followed in the posterior body. (c) Descending inputs from AVB interneurons are required for mid-body oscillations. Curvature kymographs show that, in an AVB-ablated worm, the bending amplitude decayed monotonically towards the tail during forward locomotion. When an anterior body region of a wild-type worm was immobilized via optogenetic inhibition of B-class motor neurons or muscle cells, higher frequency and low-amplitude bending waves emerged from the mid-body. Ablating AVB premotor interneurons would abolish the mid-body bending waves. (d) Local body oscillators, proprioceptive coupling between B-class motor neurons and AVB-B gap junction coupling work synergistically to drive and propagate a coordinated undulatory wave from the head to the tail. When a strong and time-varying proprioceptive signal from an anterior body region is absent, AVB-B gap junction coupling induces mid-body high-frequency undulation. In the absence of AVB-B gap junction inputs, proprioceptive couplings are less effective in propagating bending waves, leading to rapidly decaying bending amplitude towards the tail. Figure panels adapted from [40,41].

Figure 4.

A high-voltage-gated calcium current UNC-2 underlies intrinsic membrane oscillation for oscillators for backward movement. (a,b) The decrease (lf) and increase (gf) of UNC-2 currents in A-class motor neurons led to decreased and increased amplitude and frequency of anterior A-motor neuron-dependent rhythmic rPSCs in dissected ventral cord muscle preparations, denoted by arrowheads (a), and reduced and increased frequency of calcium oscillation in a posterior A-class motor neuron DA9 (b). (c) These animals exhibited decreased and increased velocity during backward movement, as shown by representative bending curvature kymographs and the distribution of the instantaneous velocity of wild-type and unc-2 mutant animals. Note that all premotor interneurons and B-class motor neurons were removed from the nervous system in these animals, reiterating the sufficiency of an intrinsic activity of A-class motor neurons to drive cohesive anterior bending propagation and organized backward movement. Figure panels adapted from [39].

Figure 5.

UNC-2 may also underlie the activity of oscillators for forward movements. (a) The decrease (lf) and increase (gf) of UNC-2 activity led to velocity decrease and increase in both forward and backward movements. (i) Representative curvature kymographs in respective genetic background. Wild-type animal exhibits active movements consisting of anterior to posterior bending wave propagation, with occasional and short reverse movements. (ii) Histograms of instantaneous velocity distribution by animals of respective genotypes. Positive and negative values refer to forward and backward movements, respectively. (b) Forward velocity continued to exhibit an increase after the A-class motor neurons were ablated in unc-2(gf) animals, shown as the histogram of velocity distribution. Both lines of evidence support the idea that UNC-2 activity directly affects the forward circuit.

Figure 6.

Schematics of a model of C. elegans locomotion as dynamic coupling of multiple motor states. (a) Head oscillation and body undulation are separately controlled. Descending inputs and directional phase-couplings allow distributed local oscillators to drive body undulation during forward and backward locomotion, respectively. A mutually inhibitory motif is introduced to flexibly control the two motor programme sub-circuits. Head–body undulation can be bi-directionally coupled with the forward or backward body undulation to generate different motor programmes. (b) The spatial layout of descending projection-premotor interneurons, local motor neuron CPGs and proprioceptive couplings between motor neurons for body undulation that drive forward and backward movements.