The C. elegans nervous system simultaneously exhibits a minimalist simplicity and an astonishing complexity. The adult hermaphrodite nervous system comprises exactly 302 neurons, compared to 383 in the adult male. The identity and developmental history of each of these neurons have been completely described and are essentially identical between individuals. Moreover, through the painstaking work of John White and colleagues, the complete neuroanatomy of the adult hermaphrodite, including patterns of synaptic connectivity, has been reconstructed from serial electron micrographs (White et al., 1986). This "wiring diagram" for the C. elegans nervous system provides a substrate for understanding the neural control of behavior unrivaled in any system.
The worm nervous system is organized into several major ganglia in the head and tail (Hall et al., 2006; White et al., 1986). Sensory information is received through several classes of ciliated sensory neurons, mostly in the head, that synapse onto integrating interneurons. Most neural processing is carried out in the nerve ring, a circumferential neuropil that surrounds the isthmus of the pharynx. Locomotory behavior is controlled by the activity of a small set of command interneurons that regulate the function of several classes of motor neurons in the ventral nerve cord.
In the laboratory, C. elegans exhibits a variety of behaviors, including regulated forward and reverse locomotion, response to touch, temperature, food availability, and a large variety of chemosensory cues. As discussed below, hermaphrodite egg-laying and male mating are the primary sex-specific behaviors in this species. None of these behaviors is essential for laboratory viability, and all have been subjected to genetic analysis. Indeed, it is this tractability across the molecular, cellular, circuit, and systems levels that makes C. elegans such an attractive model for the genetic studies of behavior (reviewed by Rankin, 2002; Whittaker and Sternberg, 2004). The worm nervous system has also been shown to be capable of several forms of plasticity, including habituation to mechanical stimuli, adaptation to the presence of chemoattractants, and associative learning of chemosensory cues coupled to food availability and quality (Kuhara and Mori, 2006; Tomioka et al., 2006; Zhang et al., 2005; reviewed by Giles et al., 2006; Tsalik and Hobert, 2003). The study of non-sex-specific behavior in this organism has made many important contributions to understand the mechanisms by which genes control neural development, circuit function, and behavior; more information may be found in the excellent reviews on this topic (Bargmann, 2006; Goodman, 2006; O'Hagan and Chalfie, 2006; Sengupta, 2007; Von Stetina et al., 2006).
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