Modulation of sensorimotor pathways associated with gain changes in a posture-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 to different environmental requirements. This paper presents the modulations 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 resting potential and input resistance of this motoneuron remained unchanged during the gain changes, the increase of depolarization appeared to result from an increase of stimulus-related inputs and thus was due to modulations of the premotor neuronal network containing afferents of the femoral chordotonal organ and interneurons. However, no changes of spike activity of sensory neurons and amount of their presynaptic inhibition was found during gain changes. In contrast, recordings from different types of identified premotor non-spiking interneurons demonstrated a correlation between the amplitude of stimulus-related inputs to particular non-spiking 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. Accepted: 3 July 1998