Impact of descending brain neurons on the control of stridulation, walking, and flight in orthoptera.

Abstract

Orthopteran insects (crickets, bushcrickets, and acridid grasshoppers) are preferred preparations for the study of the central nervous mechanisms that underlie behavior. Many of their behaviors are based on central rhythm-generating circuits located in the ganglia of the ventral nerve cord. Activities of these circuits must be coordinated and adapted to the behavioral context by sensory information, which can derive from proprioceptive or exteroceptive inputs. Information from various sensory modalities converges in yet unidentified "higher brain centers" that integrate and transform it into activity patterns across populations of descending brain neurons (DBNs). Transmission of "decisions" to the thoracic motor centers leads to adjustment of their functions in order to fit the sensory situation encountered. A number of unique DBNs has been identified by morphological and physiological criteria and their role in controlling aspects of specific behaviors has been the subject of various studies. Their functions range from "switch-like" transmitters of brain output to complex integration units for sensory inputs of various modalities and their appropriate insertion into the ongoing activities of the thoracic rhythm generators. This paper highlights some of the characteristics of DBNs by focussing on three motor behaviors: stridulation, a stereotyped behavior that seems to be mainly controlled by command-like DBNs; walking, a plastic behavior whose various parameters must continuously be adjusted to a changing sensory environment; and flight, in which the information for course corrections encoded for by different types of DBNs is transformed to match the rhythmic activity of the flight oscillators before it affects the respective motoneurons.