Figure 11.

The effect of 5-HT on a CNS circuit. (a) shows two hypothetical models for the sensory-motor circuit. The first model is a monosynaptic model (left side) with sensory neurons directly making synaptic connections with motor neurons. The second model is a multisynaptic one in which the sensory neurons make synaptic connections with interneurons and then the interneurons make connections with the motor neurons. The two segmental nerves that innervate the last segment were cut and pulled into a suction electrode. The segmental nerves were stimulated at 40 Hz, 10 pulses, and the excitatory postsynaptic potentials (EPSPs) were being recorded from body wall muscle fiber 6 or 7. (b) The modulatory action of 5-HT on sensory-motor function in third-instar larvae. Application of 5-HT noticeably increased the frequency of evoked EPSPs. The far right traces are enlarged depictions of one evoked burst shown to the left.