MODULATION OF SENSORIMOTOR PATHWAYS ASSOCIATED WITH GAIN CHANGES IN APOSTURE-CONTROL NETWORK OF AN INSECT

The resistance reflex in the femur-tibia joint of stick insects shows a great variability in its strength which allows the animal to adapt t o different environmental requirements. This paper presents the modula tions in the neural reflex pathways which occur during an increase of the gain of the resistance reflex after tactile stimulation. The gain increase was associated with a short-term, reversible increase of slow extensor tibiae depolarization. Because membrane properties like rest ing potential and input resistance of this motoneuron remained unchang ed during the gain changes, the increase of depolarization appeared to result from an increase of stimulus-related inputs and thus was due t o modulations of the premotor neuronal network containing afferents of the femoral chordotonal organ and interneurons. However, no changes o f spike activity of sensory neurons and amount of their presynaptic in hibition was found during gain changes. In contrast, recordings from d ifferent types of identified premotor non-spiking interneurons demonst rated a correlation between the amplitude of stimulus-related inputs t o particular nonspiking interneurons and gain changes, while other non spiking interneurons appeared unaffected. Thus, an increase in gain of the resistance reflex must be due to a specific weighting of synapses between sense organ and particular non-spiking interneurons.