Mechanism for range fractionation in chordotonal organs of Locusta migratoria (L) and Valanga sp. (Orthoptera : Acrididae)

The femoral chordotonal organ (FCO) of Locusta migratoria and Valanga sp. (Orthoptera : Acrididae) is a leg stretch receptor containing scolopophorous sensory neurones embedded in a ligament, which emerges distally from the body of the organ and connects to a distal apodeme. The ligament is pulled when the tibia is flexed. Thus the FCO bridges the femur-tibia joint. The ligament is divided into separate strands, each of which is composed of several or more, long attachment cells. These cells link individual scolopidia to the apodeme. An additional unloading strand connects the body of the FCO to a static point on the femoral integument. Because the strands are inserted along the apodeme in a sequential array, and because the unloading strand holds all the tension on the FCO when the ligament is relaxed (tibia extended), a mechanism for gradual, sequential uptake of tension by the ligament strands exists when the ligament is pulled (tibia flexed). This leads to a range fractionation of stretch-sensitive neurone responses during tibial flexion, and of relaxation-sensitive neurone responses during tibial extension. Observations on the ultrastructural distribution of desmosomes suggest that groups of attachment cells may be functionally connected and may collectively transmit force to specific groups of neurones.