THE PHYSIOLOGY OF SENSORY CELLS IN THE VENTRAL SCOLOPARIUM OF THE STICK INSECT FEMORAL CHORDOTONAL ORGAN

The leg joints of invertebrates are governed by neural control loops that control their position and velocity during movements (for reviews, see Bassler, 1983, 1993). These neural control loops rely on sensory feedback about the position and velocity of the controlled leg joint. In invertebrates, this sensory feedback is provided by external (e.g. hair fields, hair rows) and/or internal sense organs (e.g. chordotonal organs). The femoral chordotonal organ (fCO) serves as the main proprioceptor in the control loop governing the femur-tibia (FT) joint of the insect leg. The fCO measures the position and movement of this joint (e.g. Bassler, 1965, 1993; Burns, 1974; Usherwood et al. 1968; Zill, 1985). Previous investigations have described the physiology of sensory cells within femoral chordotonal organs (e.g. stick insect, Hofmann et al. 1985; Hofmann and Koch, 1985; locust, Matheson, 1990; Matheson and Field, 1990). Numerous investigations have been undertaken into the central processing of sensory information provided by the fCO to gain an insight into the control of FT joint movement during different behavioural tasks, for example during resistance reflexes in the standing animal (locust, Burrows, 1987, 1988; Burrows et al. 1988; stick insect, Bassler, 1988; Buschges, 1989, 1990; Driesang and Buschges, 1993) or during active movements (stick insect, Bassler, 1988; Bassler and Buschges, 1990). Most previous studies have not, however, taken into account the morphological separation of the fCO into two distinct scoloparia in the legs of some species (stick insect, Fuller and Ernst, 1973; Hofmann et al. 1985; Hofmann and Koch, 1985; locust middle leg, Burns, 1974). It has been inferred that the whole fCO supplies position and velocity information about the FT joint. In contrast, recent studies of leg reflexes have shown that only its smaller scoloparium (Fig. 1A), containing approximately one-sixth of the total number of sensory neurones, provides the sensory information that is used by the FT control loop (locust, Field and Pfluger, 1989; stick insect, Kittmann and Schmitz, 1992). These studies did not show what types of sensory neurones are located in the ventral part of the fCO and thus contribute to the FT control loop. We have therefore investigated the physiology of sensory neurones that are located in the ventral scoloparium of the fCO.