Extrinsic modulation of the feeding network by the cerebral giant cells (CGCs). (A) Firing rates of a CGC recorded in a freely moving intact animal using fine wires attached to the cell body. The bottom traces show the instantaneous firing rate in spikes per minute (SPM). The CGCs fired rarely during quiescence (approximately 1 SPM, Ai), at higher rates during locomotion (approximately 7 SPM, Aii) but only when the CGCs fire at approximately 15 SPM (Aii) did the animal show any feeding behavior. These differences in firing rate were consistent in the eight animals that were recorded (mean rates plotted in Aiv). (B) Electrophysiological correlates of varying CGC firing rates. The firing of the CGCs were artificially set at firing rates that corresponded to those recorded in the intact animal and the feeding responses to SO stimulation tested by recording fictive feeding in a B10 motoneuron. At the highest rate of CGC firing (15 SPM, top trace), the SO could drive a fast rhythm that was equivalent to that recorded in food driven rhythms in the intact animal. With lower rates of CGC firing (7 SPM) the rhythm was much slower (middle trace) and in the absence of CGC firing very little fictive feeding activity could be observed apart from a few slow cycles at the beginning of SO stimulation (bottom trace). (C) The serotonin receptor antagonist, cinanserin, reversibly blocks the modulatory effects of CGC firing in a SO-driven feeding rhythm. The CGCs are the only neurons in the feeding network that are serotonergic. (D) The CGCs have monosynaptic (mono) and polysynaptic (poly) connections with CPG interneurons and motoneurons of the feeding network. These are excitatatory (bars) apart from the N3ps that have a dual inhibitory/excitatory (dot/bar) synaptic response to CGC stimulation. See Abbreviations for all definitions of neuron types.