Modulation of Central and Peripheral Rhythmicity in the Heartbeat System of the Leech

Abstract

Rhythmic movements are programmed by peripheral myogenic oscillators or central neural oscillators called pattern generators (Delcomyn, 1980; Roberts and Roberts, 1983). For peripherally programed rhythms, such as heartbeat in vertebrates, the muscles involved usually produce an endogenous polarization rhythm that drives their contractions. Myogenic oscillators have been amenable to experimental analysis because muscle is readily accessible to cellular techniques (Noble, 1979; Jewell and Ruegg, 1966). For centrally programed rhythms, such as walking, breathing, and chewing, the basic pattern of motor discharge underlying the rhythm can be produced by a part of the CNS that is isolated from all phasic sensory input. Thus a central pattern generator—a network of central neurons that requires at most tonic input to produce an oscillatory output—drives the coordinated pattern of rhythmic motor outflow. Over the past twenty years, we have developed an experimental understanding of how central pattern generators work. Most of this progress has occurred in a few favorable invertebrate preparations (Selverston and Miller, 1984; Getting, 1983; Calabrese and Peterson, 1983) where, owing to the restricted number of neurons in the CNS and the ease with which these neurons are identified and experimentally manipulated, a rigorous cellular approach is possible.