The Postoral Brain Subesophageal Ganglion

Various functions have been ascribed to the subesophageal ganglion, including arousal prior to motor actions and sensory convergence from the brain. Generally, the three neuromeres of the subesophageal ganglion relate to the metameric identity of the mouthparts. Edgecomb and Murdock describe from flies that the labial neuromere of the subesophageal ganglion receives sensory axons from the dorsal cibarial organ, labellar sensilla, and labral sense organs and possibly some intersegmental inputs arrive from the tarsi. Stocker and Schorderet have supplied evidence from Drosophila that mechanosensory and gustatory chemo- (taste) receptors segregate out in the labral neuromere into discrete modality-specific zones. In honey bees, inputs from the mandibles invade the mandibular neuromere, which has been suggested to be absent in flies. However, it is unlikely that an entire neuromere has been eliminated and anatomical evidence for it is indeed present. In 1992 Shanbhag and Singh made the attractive suggestion that taste receptors segregate into chemospecific zones, because horseradish peroxidase uptake by the tips of functional species of receptors identified seven arborization areas in the subesophageal ganglion. Shanbhag and Singh suggested that these areas correspond to seven types of gustatory sensilla. These findings have not been contradicted and in larvae such receptor-specific zones appear to be substantiated by genetic markers of specific chemosen-sory axons to target neuropils. What is uncontroversial is that, at any neuromere of the subesophageal ganglion, neuropils are divided into a number of discrete synaptic regions. Some of these clearly belong to interneuron—motor neuron assemblages. In flies, local interneurons responding to sucrose reflect the discrete partitioning of lateral subesophageal neuropils.

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Eyes and Vision

Further Reading

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