ORIGINAL PAPER W. Stein á A. E. Sauer Physiology of vibration-sensitive afferents in the femoral chordotonal organ of the stick insect Accepted: 6 November 1998 Abstract The femoral chordotonal organ in orthopter- ans signals proprioceptive sensory information concerning the femur-tibia joint to the central nervous system. In the stick insect, 80 out of 500 aerents sense tibial position, velocity, or acceleration. It has been as- sumed that the other sensory cells in the chordotonal organ would serve as vibration detectors. Extracellular recordings from the femoral chordotonal organ nerve in fact revealed a sensitivity of the sense organ for vibra- tions with frequencies ranging from 10 Hz to 4 kHz, with a maximum sensitivity between 200 and 800 Hz. Single vibration-sensitive aerents responded to the same range of frequencies. Their spike activity depended on acceleration amplitude and displacement amplitude of the vibration stimulus. Additionally, 80% of the vi- bration-sensitive aerents received indirect presynaptic inputs from themselves or from other aerents of the femoral chordotonal organ, the amplitude of which depended on stimulus frequency and displacement am- plitude. They were associated with a decrease of input resistance in the aerent terminal. From the present in- vestigation we conclude that the femoral chordotonal organ of the stick insect is a bifunctional sensory organ that, on the one hand, measures position and movement of the tibia and, on the other hand, detects vibration of the tibia. Key words Femoral chordotonal organ á Presynaptic inhibition á Proprioception á Stick insect á Vibration Abbreviations fCO femoral chordotonal organ Introduction The femoral chordotonal organ (fCO) is a large prop- rioceptive sense organ in orthopteran insects that is known to respond to movements of the femur-tibia joint (stick insect: BaÈssler 1965; Hofmann et al. 1985; locust: Usherwood et al. 1968; Burns 1974). It provides sensory information about the femur-tibia angle for the central nervous system (stick insect: BuÈschges 1989, 1990; locust: Burrows 1988; Burrows et al. 1988). The morphology of this organ is described for legs in the locust (Burns 1974; Field and P¯uÈger 1989; Matheson 1990; Matheson and Field 1990) and the stick insect (FuÈ ller and Ernst 1973). The femoral chordotonal organ in the leg of the stick insect consists of a ventral sco- loparium with about 80 sensory cells and a dorsal scoloparium with about 420 sensory cells (Kittmann and Schmitz 1992). Previous investigations have de- scribed the response characteristics of the whole femoral chordotonal organ (stick insect: Hofmann et al. 1985; Hofmann and Koch 1985) and of its ventral scolopar- ium (Kittmann and Schmitz 1992; BuÈschges 1994). The sensory neurons in the latter encoded either position, velocity or acceleration of a tibial movement or a combination of two of these movement parameters. However, these investigations were carried out using ramp-and-hold stimuli to stimulate the receptor apo- deme of the fCO, the acceleration of which did not exceed 2 á 10 2 ms )2 (2 á 10 7 ° s )2 ). Extracellular recordings of the fCO nerve in locusts (Field and P¯uÈger 1989) indicated that the whole fCO might detect vibrations of even very small displacement amplitudes in a frequency range of 50±300 Hz. This sensitivity of the fCO to small high-frequency move- ments was also reported by Burns (1974). However, no study has systematically investigated the response of individual vibration-sensitive aerents in the fCO to repetitive stimuli over a wide range of frequencies. Par- ticularly, no information is available about the physi- ology of single vibration-sensitive aerents, their J Comp Physiol A (1999) 184: 253±263 Ó Springer-Verlag 1999 W. Stein á A.E. Sauer (&) Abteilung vergleichende Neurobiologie IV, UniversitaÈt Ulm, D-89069 Ulm, Germany e-mail: asauer@uni-ulm.de Fax: +49-731-5022629