From head to tail: A neuromechanical model of forward locomotion in C. elegans

Abstract

Introduction 1Behavior is grounded in the interaction between an organism's brain, its body, and its 2 environment. How simple neuronal circuits interact with their muscles and mechanical bodies 3 to generate behavior is not yet well understood. With 302 neurons and a near complete 4 reconstruction of the neural and muscle anatomy at the cellular level [1], C. elegans is an ideal 5 candidate organism to understand the neuromechanical basis of behavior. 6 Locomotion is essential to most living organisms. Since nearly the entire behavioral repertoire 7 of C. elegans is expressed through movement, understanding the neuromechanical basis of 8 locomotion is especially critical as a foundation upon which analyses of all other behaviors 9 must build. C. elegans locomotes in an undulatory fashion, generating thrust by propagating 10 dorsoventral bends along its body. Movement is generated by body wall muscles arranged 11 in staggered pairs along four bundles [2]. The anterior-most muscles are driven by a head 12 motorneuron circuit and the rest of the muscles are driven by motorneurons in the ventral 13 nerve cord (VNC). Although the nematode is not segmented, a statistical analysis of the VNC 14 motorneurons in relation to the position of the muscles they innervate revealed a repeating neural 15 unit [3]. Interestingly, while the repeating neural units in the VNC are inter-connected via a set of 16 chemical and electrical synapses, the head circuit is largely disconnected from the rest of the VNC 17 neural units. Motorneurons in both the head and the VNC circuit have been long postulated to 18 be mechanosensitive to stretch [1, 4, 5], and evidence in support of this has been shown recently 19 for the VNC [6]. Despite all of this anatomical knowledge, how the rhythmic pattern is generated 20 and propagated along the body during forward locomotion on agar is not yet well understood. 21A number of computational models of C. elegans locomotion have been proposed (see 22 reviews [7, 8, 9]). The model described in this paper differs from previous models in four 23 main ways. First, our model of the VNC incorporates the recent analysis of its repeating 24 structure [3]. Second, our model of stretch-receptors feedback takes into consideration recent 25 findings regarding the range and directionality of local body curvature on motoneurons [6]. Third, 26 our model takes into consideration the head motorneuron circuit, which had been largely ignored 27 in most models of locomotion, by either replicating an additional VNC unit or adding an oscillator 28 in the head. Finally, all current models have assumed specific mechanisms for how the rhythmic 29 movement is generated and propagated, with little systematic exploration of the possibilities. 30Here we present a model of forward locomotion grounded in the neurobiology, anatomy, and 31 physics of the worm. The model integrates a head motorneuron circuit based on hypotheses 32 postulated in the original "Mind of the Worm" paper [1] with a model of a repeating neural 33 unit in ...

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