Reflex Effects of the Femoral Chordotonal Organ Upon Leg Motor Neurones of the Locust

Abstract

ABSTRACT The femoral chordotonal organ (FCO) in a hind leg of a locust monitors the position and movement of the tibia about the femur. It consists of a group of sensory neurones embedded in connective tissue attached distally by two structures: the apodeme, which inserts close to the apodeme of the extensor tibiae muscle, and the flexor strand, which inserts at the base of the apodeme of the flexor tibiae muscle. The action of the apodeme and the flexor strand is reciprocal during movements of the tibia; the apodeme is stretched during flexion of the tibia whilst the flexor strand is relaxed. During extension, the apodeme is relaxed and the flexor strand is stretched. To analyse the reflex effects of this sense organ, all other sense organs of a hind leg were denervated. The apodeme of the FCO was then grasped between forceps, severed from its distal attachment site and its movements controlled by a function generator. The flexor strand remained intact and could be stimulated independently by moving the tibia. The different reflex effects mediated by the separate stimulation of the two components of the FCO were revealed by making intracellular recordings from the somata of leg motor neurones in the metathoracic ganglion. A movement stimulus to either component in a way that corresponded to tibial extension, excited flexor tibiae and inhibited extensor tibiae motor neurones. There was also an inter-joint effect whereby extension excited the depressor tarsi and inhibited the levator tarsi motor neurones. A flexion movement had the converse effects on these motor neurones. The effectiveness of the two components was dependent upon the velocity of the stimulus, the set position of the femoro-tibial joint at which the stimulus was applied, the initial direction of movement, and the activity of other neurones in the central nervous system. Slow motor neurones were depolarized more by low velocities of movement, whereas fast ones were depolarized more by high velocities. The two components produced their greatest effects at the set positions where they were most stretched ; thus the apodeme was most effective when the joint was flexed, and the flexor strand when it was extended. Elicited movements of the hind legs or apparently spontaneous changes of excitability enhanced or masked the typical response of the motor neurones to stimulation of the FCO, indicating that the effects of this sense organ are not to be viewed as rigid, but as modifiable in the context of the behaviour of the animal.