Characterization of tactile-sensitive interneurons in the abdominal ganglia of the cockroach, Periplaneta americana.

Tactile stimulation of an insect's abdomen evokes various behaviors including grooming and vigorous escape responses. We tested a sample of 37 tactile-sensitive abdominal interneurons for various morphological and physiological characteristics, including their ability to excite thoracic interneurons that are known to integrate wind information conducted by giant interneurons in the classical escape response. The results suggest that abdominal tactile-sensitive interneurons are heterogeneous both in anatomical and physiological properties. In general, these cells are very small interganglionic interneurons that respond to tactile stimulation at more than one abdominal segment. However, the larger population contained virtually all types of cells. Some projected anteriorly, others posteriorly, and still others projected in both directions. For most cells, the soma was on the side opposite to their axons, but in 24% of the cells it was on the same side. Patterns of dendritic arbors also varied among cells. However, tactile sensitivity was in general consistent with the morphological bias noted in dendritic branch patterns. We were able to document the existence of tactile abdominal interneurons that connect directly to thoracic interneurons involved in escape (TI[A]s). However, instances of demonstrated connectivity were rare. One cell that did show connectivity (AI652) was characterized in detail, and its properties were appropriate for conducting tactile signals in a directional escape system. The dendritic arbors were biased to the side that was ipsilateral to the cell's soma and axon. As a result, this cell's abdominal inputs and thoracic outputs are on the same side. This pattern is appropriate for generating the sensory fields recorded previously in TI(A)s. Its axon was located in the ventral median tract, which should bring it close to the integrating region of the TI(A)s.