Features of C. Elegans Thermosensory System
Features of C. Elegans Thermosensory System

Summary/Abstract: To understand more complex neural systems, it is reasonable to start with analysing more simple ones. C. Elegans is a model organism for such analysis; both because of its mapped connectome and genome. Within C. Elegans, the simplest neural subsystem seems to be the thermosensory system. In this paper we analyse the neural pathway leading from thermosensory neurons down to locomotory neurons. Although rudimentary compared to mammals, neural system of nematode C. Elegans still remains to be elucidated by neuroscientific research. The Thermosensory system of the nematode is believed to have two major thermosensory neurons, AFDL and AFDR (that are also believed to have CO2-sensory function) and supportive sensory neurons involved in other modalities but also in thermosensation - AWCL, AWCR, ASIL and ASIR. A complete neural circuitry of the thermosensory circuit is not clear yet. Although the basic circuit is known, thermosensation in C. Elegans has many features and we do not know yet in detail how thermosensation works on the level of individual neurons and synapses. The thermosensory system of C. Elegans operates at several levels. Firstly, an animal remembers at what temperature it hatched and can adjust its thermotaxis accordingly. Secondly, the nematode exhibits three major forms of thermotaxis – moving from lower temperatures towards the preferred temperature, moving from higher temperatures towards the preferred temperature and isothermal tracking – remaining in the preferred temperature range. Thirdly, the preferred temperature is adaptable; if animals are starved at a certain temperature they will adjust their preferred temperature to a different level. Fourth ly, there seems to be different mechanism/circuit for extreme and for normal temperatures. Although C. Elegans is a relatively simple organism, its nervous system is still too complex for us to fully elucidate. By analysing one of the simplest C. Elegans subsystems - the thermosensory subsystem - we can still see several hurdles to be overcome before fully understanding this pathway. Researchers have been quite successful in identifying the neurons that contribute to the pathway (although there are still speculations that additional neurons may be temporarily recruited into the pathway). Although we know quite a lot about the thermosensory neurons (especially ADFL and ADFR) we are only beginning to learn about the function and the molecular underpinnings of interneurons in C. Elegans. And, at the same time, interneurons appear to be the part of the system where most of the neurocomputation happens. There are approximately 6000 synapses in the neural system of C. Elegans. Although the connectome of the worm has been mapped, we still do not know many features of individual neurons and their synapses. For example, it is still not clear what synapses are excitatory and which are inhibitory. Besides modelling studies, new experimental studies are necessary to establish the excitatory/inhibitory nature of interactions between respective neurotransmitters and their receptors.

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