Drosophila larval crawling is an attractive system to study patterned motor output at the level of animal behavior. Larval crawling consists of waves of muscle contractions generating forward or reverse locomotion. In addition, larvae undergo additional behaviors including head casts, turning, and feeding. It is likely that some neurons are used in all these behaviors (e.g. motor neurons), but the identity (or even existence) of neurons dedicated to specific aspects of behavior is unclear. To identify neurons that regulate specific aspects of larval locomotion, we performed a genetic screen to identify neurons that, when activated, could elicit distinct motor programs. We defined 10 phenotypic categories that could uniquely be evoked upon stimulation, and provide further in depth analysis of two of these categories to understand the origins of the evoked behaviors. We first identified the evolutionarily conserved Even-skipped+ interneuron phenotype (Eve/Evx). Activation or ablation of Eve+ interneurons disrupted bilaterally symmetric muscle contraction amplitude, without affecting left-right synchronous timing. TEM reconstruction places the Eve+ interneurons at the heart of a sensorimotor circuit capable of detecting and modifying body wall muscle contraction We then went on to identify a unique pair of descending neurons dubbed the ‘Mooncrawler’ descending neurons (McDNs) to be sufficient to generate reverse locomotion. We show that the McDNs are present at larval hatching, function during larval life, and are remodeled during metamorphosis while maintaining basic morphological features and neural functions necessary to generate backwards locomotion. Finally, using serial section Transmission Electron Microscopy (ssTEM) to map neural connections to upstream and downstream elements provides a mechanistic view of how sensory information is received by the McDNs and transmitted to the VNC motor system to perform backwards locomotion. Finally, we show that these McDNs are the same as those identified in recent work in Drosophila adults (Bidaye et al. 2014) to be sufficient to generate reverse locomotion. This dissertation includes previously published, co-authored material.

One central question of neuroscience asks how a neural system can generate the diversity of complex behaviors needed to meet the range of possible demands placed on an organism by an ever changing environment. In many cases, it appears that animals assemble complex behaviors by recombining sets of simpler behaviors known as behavioral primitives. The crawling behavior of the nematode worm Caenorhabditis elegans represents a classic example of such an approach since worms use the simple behaviors of forward and reverse locomotion to assemble more complex behaviors such as search and escape. The relative simplicity and well-described anatomy of the worm nervous system combined with a high degree of genetic tractability make C. elegans an attractive organism with which to study the neural circuits responsible for assembling behavioral primitives into complex behaviors. Unfortunately, difficulty probing the physiological properties of central synapses in C. elegans has left this opportunity largely unfulfilled. In this dissertation we address this challenge by developing techniques that combine whole-cell patch clamp recordings with optical stimulation of neurons. We do this using transgenic worms that express the light-sensitive ion channel Channelrhodopsin-2 (ChR2) in putative pre-synaptic neurons and fluorescent protein reporters in the post-synaptic neurons to be targeted by electrodes. We first apply this new approach to probe C. elegans circuitry in chapter II where we test for connectivity between nociceptive neurons known as ASH required for sensing aversive stimuli, and premotor neurons required for generating backward locomotion, known as AVA. In chapter III we extend our analysis of the C. elegans locomotory circuit to the premotor neurons required for generating forward locomotion, known as AVB. We identify inhibitory synaptic connectivity between ASH and AVB and between the two types of premotor neurons, AVA and AVB. Finally, we use our observations to develop a biophysical model of the locomotory circuit in which switching emerges from the attractor dynamics of the network. Primitive selection in C. elegans may thus represent an accessible system to test kinetic theories of decision making. This dissertation includes previously published co-authored material.