THE PROBOSCIS EXTENSION RESPONSE IN THE BLACK BLOW FLY, PHORMIA REGINA MEIGEN: NEURAL CORRELATES AND REGULATION OF TARSAL TASTE THRESHOLD (NEUROSCIENCE, COBALT CHLORIDE, TRACT TRACING, MOTONEURONS)

ROBERT STANLEY EDGECOMB, Purdue University

Abstract

The central projections of axons from neurons innervating "largest" labellar hairs 1, 6 and 11 are limited to the subesophageal ganglion (SOG). Axons from hairs 1 and 11 project medially and have multiple contralateral branches. Axons from hair 6 remain in the ispsilateral hemiganglion and have one dorsal contralateral projection. The technique used for staining these sensory neurons often resulted in the staining of single axons. One cell type observed was common to each of the three labellar hairs, while the other types were unique for the hair stained. Sensory neurons from tarsal taste hairs project primarily to the ipsilateral side of the corresponding neuromere in the thoracico-abdominal ganglia, although a bundle of axons from each leg projects into the ipsilateral half of the SOG via the ventromedial region of the cervical connective. One fiber ascends to the SOG from each D5 hair and terminates in the same region as a branch of the cell common to all labellar hairs studied. Extension of the proboscis results from the contraction of three muscles--the protractor of the fulcrum (ProFul), the adductor of the apodeme (AdA), and the extensor of the haustellum (ExHst). The AdA and ExHst are served by four motoneurons having cell bodies in variable positions in the lateral soma layer of the SOG ipsilateral to the muscles innervated. The central projections of these motoneurons terminate primarily in the ipsilateral hemiganglion in the lateral and ventromedial neuropil. A few ventromedial branches project contralaterally. The ProFul is innervated by a single motoneuron. The central projections of this motoneuron follow a similar pattern to the other extension motoneurons running in the maxillary nerve, but have more ventromedial contralateral branches. Foregut stretch receptors carried by the recurrent nerve (RN) project anteriorly in the ipsi- and contralateral SOG at the level of the labrofrontal nerve. Transection of the RN in flies fed ad libitum, or in unfed flies subsequently administered a single meal, significantly lowered threshold, but to a level still significantly higher than unfed controls. Thus, although the RN plays an important role in the regulation of tarsal taste threshold in blowflies, other factors, in addition to these stretch receptors, affect threshold after feeding.

Degree

Ph.D.

Subject Area

Neurology

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