23 Mechanotransduction

Abstract

I. INTRODUCTION: THE NEURAL CIRCUIT FOR LOCOMOTION
Diverse mechanical stimuli are likely to be encountered constantly in Caenorhabditis elegans’ normal habitat, the soil. Therefore, it is not surprising that C. elegans utilizes mechanosensory neurons to regulate many of its behaviors. Touch regulates locomotion, foraging, egg laying, pharyngeal pumping, and defecation. Mechanosensory inputs not only give rise to simple reflexive avoidance behaviors, but also appear to control the overall activity of the animal. In this chapter, we describe the neural circuits for locomotion, foraging, and touch avoidance, their genetic analysis, and a molecular model for mechanosensory transduction. Since understanding regulation of locomotion by mechanical stimuli requires knowledge of the neural circuit for locomotion, we first describe this circuit with a focus on three key questions: How is the sinusoidal pattern of body bends generated? How are the antagonistic behaviors of forward and backward locomotion coordinated? How is the sinusoidal wave propagated along the body axis?

A. Musculature and Motor Neuron Innervation
The anatomy of the body wall muscles and of their synaptic inputs restricts locomotion to dorsal and ventral turns of the body. Locomotion consists of a sinusoidal pattern of alternating ventral and dorsal turns of the body musculature. The body wall muscles are organized into two dorsal rows and two ventral rows. Each row consists of 23 or 24 diploid mononucleate muscle cells arranged in an interleaved pattern (Sulston and Horvitz 1977). Dorsal and ventral body muscles are controlled by distinct classes of motor neurons. Five types of ventral cord...