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“THE WORM” Caenorhabditis elegans as a model organism Caenorhabdi-what!?! • C. elegans is a nematode round worm. • Very small (1mm). • Naturally found in damp soil and rotting fruit. • Two sexes: hermaphrodite and male. • Grown in large quantities in labs. • Cultured on agar gel. • Eats bacteria (E. coli). Who’s cares about worms? • Excellent model organism for the study of: – Nervous systems* – Genetics (7K/20K genes shared with human. Fully sequenced) – Development (takes only 3 days, cell lineage tree is known) Anatomy Experimental techniques • • • • Microscopy Electrophysiology Calcium imaging Genetics... • Reconstruction • Laser ablation The worm vs. larger animals • Nervous system size: 80,000 100,000,000,000 302 • Neuron Complexity: – Neurons generally fire action potentials (“digital”). – The worm lacks the necessary ion channels, so the neurons do not spike. Instead, use graded potentials (“analogue”). What we know about it • Nervous system: – Invariant, all neurons identified by name. – Individual synaptic connections are mapped. – Role of many neurons known from ablations. • Genetics: – Genome has been fully sequenced. – Single gene mutants with known locus & phenotype. • Development: – Full sequence of cell divisions from egg to adult (cell lineage) has been mapped. C. elegans behaviour • Exhibits rich behaviours involving: – – – – Multiple sensory modalities (touch, smell, temperature). Learned associations (temperature and food preference). Current internal state (e.g. hunger). Locomotion* (central to all behaviours). • For example: – – – – Collective social behaviour (aggregation on food). Hunting food (if hungry). Threat avoidance (physical and chemical). Mating (male). Locomotion • Worm crawls on surface while lying on its side. • Forwards motion with reversals and turns. • Exhibits sinusoidal body wave. • Forwards motion achieved by propagating body wave from head to tail. • Muscles only allow bending in 2D (dorsal / ventral). Modelling locomotion, an interdisciplinary project • Our goal is to understand and model the worm’s forward locomotion. • This challenging project requires a group effort: – – – – Experimental biology (genetic, behavioural, ablations). Physics (mechanics of body/environment). Engineering (mechanical experiments, robotics). Computer science (data analysis, computational modelling). The locomotion system Minimum circuit • Identified by ablations. • One interneuron (AVB) provides “on” signal. • Gap junctions to fwd MNs: – 11 VB and 7 DB neurons • Few synaptic connections. • How are oscillations generated? • Stretch receptors sense body bending. A simple model • • • • Based on minimal circuit. Divided into 11 segments. Each contains two MNs. All receive current input from AVB. • Receive stretch input from local and posterior segment. • Sensory feedback is key mechanism. • Dorsal and ventral neuron compete to control segment bending. Gait adaptation • Worm locomotion generally studied on agar. • Gait is quite different when swimming in water. • Previous model can only reproduce crawling. • We wish to extend the model to both behaviours. • The gait change seems to depend on the changing “feel” of the environment. Body and environment • Worm locomotion is unusually dependent on sensory feedback loop. • This is dependent on the environment properties. • The neural model needs an embodiment in order to adapt to model gait adaptation. • We therefore want a physical model of the worm and the environment. “Intentions” Motor N.S. Muscles Sensory Neurons Body Environment Questions?