| Abstract: | The nervous system coordinates a sequence of muscle movements to give rise to animal behaviors. In complex invertebrates or lab-studied vertebrates, due to the large number of cells in their nervous systems and the complexities of their behaviors, it is difficult to address how circuits process information to direct each motor output of the behavior. In this dissertation, I used the Caenorhabditis elegans male copulation behavior as a model to address how a compact circuit coordinates different behavioral programs. Insertion of a male copulatory organ into a suitable mate is a conserved and necessary behavioral step for most terrestrial mating. However, the detailed molecular and cellular mechanisms for this distinct social interaction have not been elucidated in any animal. During mating, the C. elegans male cloaca is positioned over the hermaphrodite̕ s vulva as he attempts to insert his copulatory spicules repetitively. Rhythmic spicule thrusts cease when insertion is sensed. Circuit components consisting of sensory/motor neurons and sex muscles for these steps have been previously identified, but it was unclear how their outputs are integrated to generate a coordinated behavior. Here, I delineate cellular and molecular mechanisms that transduce sensory information into coordinated behavioral programs during male mating.I show that contraction of the male oblique muscles is required to sustain genital contact between the sexes. These muscles are innervated by the postcloacal sensilla (p.c.s.) sensory/motor neurons. The ionotropic AChRs that contain either the ACR-16 or the UNC-29 subunit mediate synaptic transmission at these neuromuscular junctions; and a Gαq-coupled muscarinic AChR, GAR-3, is likely to function presynaptically to enhance the ionotropic AChRs-mediated synaptic transmission. For spicules to rhythmically thrust during genital contact, activity of the oblique muscles and the gubernacular muscles is transmitted to the spicule protractor muscles instantaneously via gap junctions and causes shallow protractor contractions. The rhythmic protractor contractions eventually switch to sustained contraction, as the SPC sensory-motor neurons integrate information of spicule position at the vulva with inputs from the hook and cloacal sensilla. The ERG-like K plus channel, UNC-103 is likely to set a threshold requirement for integration of these inputs, so that sustained spicule protraction is not stimulated by fewer inputs. |
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