Modulation of activity in sensory neurons and wind-sensitive interneurons by cereal displacement in the cockroach

1. The cockroach Periplaneta americana can modify the sensory activity received by its central nervous system from the cerci, paired abdominal wind-responsive appendages. Medial displacement of the cerci produces a reduction in the number of sensory action potentials (AP's) elicited by a wind stimulus (Fig. 2) (Libersat et al. 1987; Goldstein and Camhi 1988). This movement occurs naturally, for example during flying. 2. This sensory reduction is present when measured as the integral of extracellularly recorded activity as well as when counting the number of AP's larger than a threshold voltage just larger than the background noise (Fig. 2C). 3. Histological results confirm prior physiological experiments suggesting that the reduction may be produced by mechanical forces on the sensory nerve, rather than synaptically (Fig. 4). 4. The wind-response of interneurons is significantly diminished by the sensory reduction when measured either extra- or intracellularly (Figs. 5, 6). Cells affected include identified ventral and dorsal giant interneurons (GI's), which carry directional information about wind from the abdominal cerci to the more anterior portions of the nervous system, and are involved in flying (Camhi 1980; Ritzmann 1984; Comer 1985). 5. The reduction in the interneuronal response was unaffected by the elimination of input from descending central pathways, and input from a cereal chordotonal organ that senses cereal position and inhibits some of the GI's (Fig. 5). Thus, the reduction in wind-evoked sensory activity can itself account for the modulation of interneuron activity. 6. The extracellularly recorded response of the nerve cord to wind was divided into: large action potentials (AP's) that included almost all of the GI AP's (Fig. 6B), and smaller AP's of unidentified cells. The activity of both small and large AP's is reduced by medial displacement of the cercus. However, the large AP's are significantly more reduced (Fig. 6). The small AP's comprise between one-half and two-thirds of the number of AP's recorded in response to wind. 7. The sensory reduction may serve to protect the circuitry of the exquisitely sensitive wind-sensitive escape system from habituation by the strong winds generated when the animal flies.