Functions of a proprioceptive sense organ in freely moving insects: characteristics of reflexes elicited by stimulation of the locust metathoracic femoral chordotonal organ.

(1) The metathoracic femoral chordotonal organ is an identified joint angle receptor of the locust hindleg. In order to assess and quantify the functions of this sense organ in the control of posture, mechanical stimuli were applied to the main ligament of the receptor in freely standing locusts. These stimuli produced an afferent discharge that mimicked a sudden small (10-15 degree) change in the angle of the femoro-tibial joint of the hindleg. The reflex effects that resulted from afferent stimulation were monitored myographically in the tibial extensor and flexor muscles. The angle of the femoro-tibial joint at the onset of sensory stimulation was also recorded by still photography. (2) As previously reported, stimulation of the chordotonal organ in freely standing animals produced resistance reflexes in tibial muscles that opposed the apparent joint movement. However, we also found that, at certain joint angles, a different mode of reflex response was elicited in which motoneurons to the tibial flexor muscle fired in response to apparent movements in any direction. (3) In this study, characteristics of resistance reflexes in the tibial extensor muscle were analyzed quantitatively, as that muscle is innervated by only one slow excitatory motoneuron. The gain of the resistance reflex (ratio of the firing frequency during afferent stimulation versus the rate of activity prior to the stimulus) was quite high in all preparations, and represent a greater than two-fold increase in motoneuron frequency (mean 2.11 +/- 0.54 S.D.). The reflex gain was also highest at the lowest initial rates of motoneuron activity (circa 5 Hz) and declined for higher firing frequencies (maximum 35 Hz).(ABSTRACT TRUNCATED AT 250 WORDS)