Neuronal microcircuits for decision making in C. elegans

Abstract

The simplicity and genetic tractability of the nervous system of the nematode Caenorhabditis elegans make it an attractive system in which to seek biological mechanisms of decision making. Although work in this area remains at an early stage, four basic types paradigms of behavioral choice, a simple form of decision making, have now been demonstrated in C. elegans. A recent series of pioneering studies, combining genetics and molecular biology with new techniques such as microfluidics and calcium imaging in freely moving animals, has begun to elucidate the neuronal mechanisms underlying behavioral choice. The new research has focussed on choice behaviors in the context of habitat and resource localization, for which the neuronal circuit has been identified. Three main circuit motifs for behavioral choice have been identified. One motif is based mainly on changes in the strength of synaptic connections whereas the other two motifs are based on changes in the basal activity of an interneuron and the sensory neuron to which it is electrically coupled. Peptide signaling seems to play a prominent role in all three motifs, and it may be a general rule that concentrations of various peptides encode the internal states that influence behavioral decisions in C. elegans.

Nematode locomotion: dissecting the neuronal–environmental loop

“…To explore and navigate its environment, C. elegans integrates over a wide variety of physical and chemical cues [47,48]. On food, C. elegans mostly dwells in the same area, occasionally roaming to seek a better patch of food [49,50].…”

“…The inherently adaptive nature of sensory processing is in fact fundamental to achieving robust motor behavior. In nematodes this form of the neuronal-environmental loop manifests itself most clearly in navigation.To explore and navigate its environment, C. elegans integrates over a wide variety of physical and chemical cues [47,48]. On food, C. elegans mostly dwells in the same area, occasionally roaming to seek a better patch of food [49,50].…”

Sperm as microswimmers – navigation and sensing at the physical limit

“…Many cells and organisms also follow gradients of chemical or physical cues on periodic paths [82]. The sensory modalities used for navigation are as diverse as phototaxis, thermosensation, taste, olfaction, and electroreception; the organisms and cells include the nematode C. elegans, larvae from Drosophila and Platynereis, primordial germ cells of zebrafish, the alga Chlamydomonas, protists, and even some bacteria [83][84][85][86][87][88]. Diagram of a sperm cell navigating in a gradient of resact (blue).…”

The whole worm: brain–body–environment models of C. elegans

“…In order to orient, the rate of pirouette initiation is modulated by sensory activity, increasing or decreasing when the animal is moving in a favorable or unfavorable direction, respectively [50]. Some of the neuronal circuitry involved in klinokinesis has been studied, in particular the role of the command motor neurons that promote forward and reverse locomotion, since pirouettes occur during changes in the direction of locomotion [51,52]. A series of highly idealized computational models of klinokinesis have been developed [53][54][55][56].…”