An experimental and descriptive analysis of the responses and peripheral and central organisation of the locust metathoracic femoral chordotonal organ, a leg joint position and movement detector.

Abstract

Extracellular dye infusions, light microscopy, electron microscopy, and intracellular recording and staining have been used to physiologically and morphologically characterise individual sensory neurones of a locust joint stretch receptor, the metathoracic femoral chordotonal organ (mtFCO). A new population of receptor neurones is described, approximately doubling the number of neurones known in the mtFCO. These distally located neurones appear to represent a group previously thought to be missing in the hind leg. The positions of individually characterised neurones’ somata within the chordotonal organ are mapped in detail, and are shown to be related to aspects of each neurone's response to tibial movements. Neurones respond to position, velocity, or acceleration, or to combinations of these parameters. Both extension and relaxation sensitive neurones are found throughout the organ. Movement of the main distal attachment of the organ (the apodeme ligament) stimulates both types of receptors. Range fractionation of tonic and phasic responses, and hysteresis of tonic firing are described and illustrated in detail, and interpreted in terms of function. The central projections of mtFCO neurones are shown to differ between response types. Neurones within at least some response types also show gradations in their patterns of branching which can be related to the leg angle which causes their maximal tonic or phasic firing. This is similar to, but not the same as tonotopic mapping described previously for auditory chordotonal organs. A novel use of discriminant and cluster analyses to assist the description of neuronal morphology is presented.