SENSORIMOTOR PATHWAYS INVOLVED IN INTERJOINT REFLEX ACTION OF AN INSECT LEG

Coordination of motor output between leg joints is crucial for the gen eration of posture and active movements in multijointed appendages of legged organisms, We investigated in the stick insect the information flow between the middle leg femoral chordotonal organ (fCO), which mea sures position and movement in the femur-tibia (FT) joint and the moto neuron pools supplying the next proximal leg joint, the coxa-trochante ral (CT) joint, In the inactive animal, elongation of the fCO (by flex ing the FT joint) induced a depolarization in eight of nine levator tr ochanteris motoneurons, with a suprathreshold activation of one to thr ee motoneurons. Motoneurons of the depressor trochanteris muscle were inhibited by fCO elongation. Relaxation signals, i.e., extension of th e FT joint, activated both levator and depressor motoneurons; i.e., bo th antagonistic muscles were coactivated, Monosynaptic as well as poly synaptic pathways contribute to interjoint reflex actions in the stick insect leg. fCO afferents were found to induce short latency EPSPs in levator motoneurons, providing evidence for direct connections betwee n fCO afferents and levator motoneurons. In addition, neuronal pathway s via intercalated interneurons were identified that transmit sensory information from the fCO onto levator and/or depressor motoneurons. Fi nally, we describe two kinds of alterations in interjoint reflex actio n: (a) With repetitive sensory stimulation, this interjoint reflex act ion shows a habituation-Iike decrease in strength, (b) In the actively moving animal, interjoint reflex action in response to fCO elongation , mimicking joint flexion, qualitatively remained the same sign, but w ith a marked increase in strength, indicating an increased influence o f sensory signals from the FT joint onto the adjacent CT joint in the active animal. (C) 1997 John Wiley & Sons, Inc.